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Chapter 8
Perceiving Mind
This chapter, the beginning of
Part III of this book, launches our exploration of how the different agencies
of our minds work and how they are assembled into a coherent whole. These agencies
arise from the evolutionary and developmental histories discussed in Parts
I and II. We have noted many examples of the modularity of our brain functions
but have addressed only a fraction of the experimental evidence. This chapter
and Chapters 9 through 11 deal with our everyday experiences of sensing, acting,
having emotions, and using language, respectively. Breaking up the discussion
into these chunks helps us sort things out somewhat, even though these faculties
interact through so many feedback loops that to view them as separate is really
an oversimplification.
Ecology of Sensing and Acting
What animals like ourselves are
able to perceive is shaped by a long history of adaptation and co-evolution
with a particular environment, a particular ecology. We have receptors for
visible (to us) light between about 400 and 700 nanometers in wavelength. We
are unable to see patterns of ultraviolet light visible to the moth, we do
not have receptors for the infrared light that some snakes use to sense heat
given off by prey, and we do not have the acute long-distance vision of a hawk.
We cannot hear the sound of a dog whistle or follow the scent of an escaping
prisoner. Compared to most mammals, our sense of smell is quite poor. On an
evolutionary time scale, we, like other animal species, have developed sensing
and acting routines that are suitable for our particular physical and social
niches. From the flood of physical information that impinges on us---light,
sound, smell, and so on---we generate the perceptions we need to answer the
question "What do I do next?" In humans, as in the most simple animals,
perception serves action. The
two are so intimately linked that it is a bit arbitrary to present them in
two separate chapters.
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DESIGN NOTE: IMPORTANT POINT
The ultimate purpose of our perceptions
is to make it more likely that we will act in ways that help transmit our genes
to future generations. Perception serves action.
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Faculties such as visual perception
and movement are linked in a continuous loop both during the developmental
wiring of the brain and in the adult brain. Motion informs and refines vision,
which informs and refines motion. Vision has little meaning apart from its
use in behavior. One striking example of this has been provided by experiments
in which two groups of newborn kittens were treated differently. One
group could move about normally, and each of its members was harnessed to a
simple carriage and basket that contained a member of the second group. The
kittens in the second group had the same sort of visual input as the first,
but they were passive---unable to test the relationship between their own movement
and the environment. After a few weeks of this treatment, the first group behaved
normally, but those who had been carried around failed several visual tests,
such as avoiding steep drops. This experiment suggests that gaining knowledge
of how to act in response to what is seen requires testing by the visual guidance
of action and that such knowledge cannot be obtained just by passively extracting
visual features from the environment. The elaborate reciprocal testing among
developing brain, body, and external environment installs the filters and interpreters
of our sensory world that are described in this chapter.
Perception Focuses on Change
Perception involves much more than
just the brain, with its picture-like images and talking-to-oneself thoughts.
What we know of external reality we learn through the way that reality affects
our entire bodies. The outside world is registered in terms of the internal
modifications it causes as we move about, changing the state of our viscera,
our skin, and our autonomic nervous system, as well as our central nervous
system. Our brains register what is happening "out there" as a set
of rapidly flickering maps spread across our somatosensory and motor cortexes,
as well as less topographical representations of our viscera in the brainstem
and hypothalamus. One grounding reference for a sense of "self" appears
to be a constantly renewed primordial representation of the body in action.
Our sensory systems are most interested
in rapid changes in our environment, for it is these that we must respond to
in order to survive. Think of how quickly you react when an unexpected shadow
suddenly falls across your path. It is characteristic of our sensory receptors
that dramatic alterations in a stimulus, such as onset and offset, are detected
more easily than equal changes along a continuum. Recall the times when you
have turned on a three-way lamp and then turned if off again. The sequence
would have been something like: darkness -> 50 -> 100 -> 150
watts of light -> darkness. You notice dark -> 50
watts as a big effect; 50 -> 100 and 100 -> 150
seem much less; off is again a big effect. Similarly, you have a very clear
response to first touching your finger to your forearm. If you then apply a
slowly increasing steady pressure, and occasionally press harder briefly to
notice the amount of change you can just detect, the brief press must become
larger as the background pressure is increased. Each bit of stimulus has made
your skin receptors less responsive to the next bit of stimulus. The system "resets
itself" to a new level of sensitivity (in other words, it adapts) so that
its response is appropriate to the background.
Our perception of a stimulus is
seldom absolute but, rather, depends on what has gone before. For example,
if you move your hands among hot, cold, and tepid bowls of water, tepid feels
warm or cold depending on which direction you come from. (The same sort of
demonstration can be done with a set of three increasing weights. )
Although we are good at perceiving changes on the time scale of these moment-to-moment
behaviors, we usually don't notice changes that are much slower. You might
not be aware, for example, that air pollution in your city has increased slowly
over a period of several years, until a friend who visits after being away
for a long time says, "Boy, the air is a lot more polluted now than when
I left."
Perception Is Filtered and Directed
by Many Factors
Most of the sea of sound, odor,
gravity, and light in which we are immersed is known to our brain/body but
not explicitly perceived. What we are aware of perceiving is only a small fraction
of the information registered by our sensory organs and processed apart from
consciousness. In fact, imaging studies have revealed brain regions that are
responsive to novelty outside our awareness. Numerous
psychological studies have documented that unconscious perceptions can influence
subsequent cognitive and affective reactions. Our perceptions are formed as
primary sensations pass through an elaborate series of filters and interpreters
installed by our evolution and our individual development, as well as by the
attention, arousal, and expectations at hand. What
we perceive is a mixture of what is there and what we expect.
The filtering processes of our
brains help to guarantee that what gets through is relevant to the moment (when
we are hungry and we look for restaurants, not gas stations, as we drive along).
Anticipating a particular kind of stimulus actually increases blood flow to
the part of the brain that processes that sensory modality. Our
perceptions can be heightened or suppressed by powerful ascending pathways
that rise like a fountain from the brainstem and shower the cortex with arousal-regulating
neurotransmitters such as serotonin, acetylcholine, dopamine, norepinephrine
(also called noradrenaline), and histamine. Our
alertness and attention can be enhanced by drugs that stimulate norepinephrine
release in the brain. Brain
imaging studies reveal locations in the cortex that are components of networks
of attention and that also regulate orientation and vigilance.
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DESIGN NOTE: IMPORTANT POINT
Our stream of conscious awareness
is just a fraction of a larger intelligence that selects what perceptions make
it into awareness---an intelligence of which we are usually unaware.
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As an example of filters and censors
that can act on our vision and language, consider
being asked to finish the words S_X, SHI_, and F__K. If you reported six, shin,
and fork, was the censorship bias in the reception or the response? Probably
a mixture of both. Think of all the parts of your cortex that were lighting
up during this exercise! Or consider the "cocktail party effect." When
several conversations are going on around you, and you hear your name mentioned
in one, you attend to that conversation immediately and tune out the others.
Filters and censors like these ones suggest the presence of a feedback loop
between cognitive processes of the frontal lobe and the more primary sensory
cortexes that are registering all of the input.
Our assumptions and expectations
can color our interpretation of what we hear and see. Consider the statements "I
went to the nudist play" versus "I went to the new display" or "the
prince of whales" versus "the prince of Wales." In this case,
context enables us to differentiate between phrases that sound identical. Figure
8-1 offers an example of a shifting visual interpretation. In seeing the object
shown on the left, we make the most simple assumption, that it is a hexagon,
unless prior expectations tell us that it is another view of the object on
the right.
Figure 8-1
Drawings that illustrate the distinction
between sensation and perception. The figure on the left is usually taken as
an octagon, but it could be interpreted as the cube on the right rotated to
a different position. The cube on the right, which is usually called the Necker
cube, can be seen as having its top surface receding either to the left or
to the right.
Distinguishing Between Sensation
and Perception
Look again at the object on the
right in Figure 8-1. Most people initially register this experience as looking
at the top of a square with the lower set of vertical lines defining a surface
that stands nearest the observer. However, if we stare at the figure long enough,
it will flip so that we seem to be looking up at the bottom of the box, with
the same vertical lines now defining its back side. Of course, neither the
object itself nor its image on our retinas is changing. This is only one of
many examples that can be used to demonstrate the distinction between sensation
(the direct sensory recording) and perception (what we take that recording
to mean). Recall that in Chapter 2 we discussed the evolutionary origins of
this distinction, as more complicated animals began to separate the pathways
of "what is happening to me" (sensation) from "what is happening
out there" (perception).
Such distinctions are now being
observed at the level of the firing of single nerve cells whose activity can
be correlated with the way an object is perceived. It
seems most likely that the rules for sorting out ambiguous visual images or
linguistic utterances are formulated during the postnatal development of our
brain circuitry. It is very
unlikely that an a priori set of rules that evolved as adaptations to our ancestral
paleolithic environment could anticipate and accommodate the visual and linguistic
ambiguities generated by our modern surroundings. Perception of three-dimensional
forms requires experience and learning. The
sophisticated scene and feature analyses carried out by our visual brains (discussed
below) are apparently constructed during development, as top-down processes
determine which work best in meeting changing environmental demands.
A more dramatic demonstration of
the brain's ability to generate perception distinct from sensation can be observed
in blind humans by attaching to their backs a patch driven by a video camera
that the subject controls, a camera whose output to the patch causes the stimulation
of multiple points on the skin. Each point represents one small area of the
image captured by the camera. Within hours, some subjects can learn to recognize
common objects such as cups and telephones, to point accurately in space, to
judge distance, and finally to use perspective and parallax to perceive external
objects in a stable three-dimensional world. The patterns projected onto the
skin develop no such "visual" content unless the individual is behaviorally
active, directing the video camera via head, hand, or body movements. After
a few hours, the person no longer interprets the skin sensations as being of
the body but projects them into the space being explored by the body-directed "gaze" of
the video camera. What develops is not necessarily understanding but a strategy
for responding appropriately. Test subjects do not locate objects as lying
up against their skin---any more than those of us with vision locate objects
as lying up against the retina of our eyes. Instead, they perceive objects
as being out there in space. Thus a tactile sensation of "what is happening
to me" is converted into a vision-like perception of "what is happening
out there." More recently
another form of sensory substitution, the conversion of visual images to sounds,
has proven partially successful.
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DESIGN NOTE: IMPORTANT POINT
A vivid demonstration of the plasticity
of sensation and perception comes from experiments in which blind subjects
can learn to "see with their skin."
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Perception of Spatial Relationships
Our brains use context to shape
our perception of relationships. Consider the example shown in Figure 8-2.
The two central circles are the same size, but they look different because
of the relative size of the surrounding circles. In this case, there is a fascinating
corollary: Although we consciously experience the central circles as differing
in size, if we are asked to pick up the center discs, our grip immediately
matches the true size of the objects. This demonstrates that the visual pathways
in our brain that are required for skilled actions are separate from those
that mediate our conscious perception. These
two pathways are described in more detail below.
Figure 8-2
Our subjective experience of the
size of the middle circle is influenced by the size of the surrounding circles.
Perceptions frequently do not correspond
to reality. Our stored images of familiar objects can be so strong that we
see or hear what we think should be present rather that what actually is present.
We (and all other animals) can act on the basis of caricatures of reality.
Just a few aspects of a stimulus are usually sufficient to enable us (and induce
us) to identify an object. (We are not unlike the male stickleback, a fish
that will attack any object that vaguely resembles the red belly of another
male.) We base our perception on best bets about what is out there. The point
of have perceptions is not to be a perfect physical measuring device; it is
to participate promptly in appropriate behaviors.
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DESIGN NOTE: IMPORTANT POINT
If you are ready to see a black
ace of spades or a red ace of hearts, when a red ace of spades is shown, you
are likely to see one of the two choices you have set for yourself rather than
the card actually before you.
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These examples illustrate that
sensing is seldom naive but, rather, depends on context and comparison with
a history of similar sensing. The
brain specializes in rough-and-ready detection of what's probably important,
not necessarily what is really there, and it is willing to make a lot of mistakes.
We can extrapolate from such simple examples to our more complicated behaviors.
Bias and hunches born of our history influence the scanning mechanisms we use
to watch out for changes or danger in the environment. In people who are constantly
apprehensive, those mechanisms seem to have taken on a chronic and inappropriate
negative bias. Even the most benign events are interpreted as threatening.
The converse of this situation is the Pollyanna syndrome, where negative input
is denied. Humans, as individuals and groups, frequently deny the relevance
of information they don't want to deal with. On a larger scale, we see this
response in the muted reactions that many people have to ethnic genocidal wars
and to the environmental and population crises.
Visual Systems
The five major sensory systems
(vision, olfaction, audition, sensation, and gustation) not only process separate
sensory modalities but also extensively interact with and influence each other.
For example, sound can alter our perception of visual motion, the auditory
cortex is activated during silent lip reading, and vision and touch are closely
linked in analyzing objects in our surroundings. We'll focus on vision here,
though. Humans are visual animals,
and at least 60 percent of our cerebral cortexes are involved in some aspect
of processing visual information. In the visual system we face directly the
question "Where does it all come together---where is the I?" We study
vision from the top down (psychological experiments) and from the bottom up
(electrophysiological experiments), and the area where the two have not yet
met provides some fascinating puzzles. Here we are touching on something not
fully understood: the link between neuronal firing and our conscious experience.
Not only does vision interact with
our other senses, but it also forms an ensemble with movements of body muscles
in which perception and action are linked. Vision involves much more than passive
perception. Contractions of muscles that move the iris, rotate the eyes, control
the face muscles around the eyes, contact neck muscles, and bring about other
somatic movements can all be integral components of the experience of vision.
Self-Experiment: A Look at Vision
and Its Muscular Correlates
We can use a simple exercise to
illustrate the linkage of vision to muscle movements and tension. The following
is best done if you can have someone else read the instructions to you, but
it will probably work if you read through the instructions yourself, paying
attention to the places where you are asked to pause before continuing. Take
it slowly; don't rush.
Start by trying to focus and concentrate
very hard on the following line, imagining that you will have to reproduce
it from memory and that you are given only 10 seconds to learn and remember
the characters:
#*$%)@!?
Now, stop. Move your forehead muscles.
Are you frowning? Expand and contract the muscles around your eyes. What is
their position now, and what was it when you were looking at the characters?
Open and shut your jaw gently. Are the muscles loose or slightly clenched?
Shrug your shoulders up and down. Are they relaxed or slightly elevated? Move
your head slightly forward and back to stretch the muscles along the back of
your neck. How tense are they? Now note your breathing. Take a deep breath,
let it out, and see what pace of breathing seems natural as you resume.
You are likely to note numerous
muscle contractions of which you are not usually aware. These patterns of contraction
go along with your paying close, focused attention to a stimulus such as the
characters you memorized.
Now, let's move on to another set
of instructions, but try to remember the patterns of muscle tension that you
just experienced. Read the following three instructions several times so that
you can remember them, but do not read further. When you have the instructions
in mind, you can continue with the experiment.
1. Rub your hands together to warm
them, and then place your palms lightly over your eyes. Leave them there for
at least 30 seconds. Let your eye muscles and breathing relax.
2. Place your hands in your lap,
and, in your mind's eye, imagine a horizontal bar and slowly look back and
forth from one end of the bar to the other. Then imagine a vertical bar, and
do the same.
3. After several minutes of doing
this, slowly open your eyes to receive the whole visual world without reaching
out to focus on any particular item.
Once you have read these steps
several times, do what they say before reading any further! Only then should
you continue reading.
Now that you have taken all three
steps, consider this question: How does your visual experience right now compare
with what you were feeling just after you were asked you to focus on and remember
the characters in the previous exercise? Are you more aware of the edges of
your visual world? Does your vision feel more "open" than before?
Check through the muscles you reviewed above. Do they feel harder or softer?
Some individuals who have completed
this exercise with spoken instructions have reported that their visual world
was very fuzzy, or even disappeared entirely, for a moment. This is similar
to the experience most of us have had while listening to a boring sermon or
lecture: We notice with a start that the visual scene was gone for a moment.
If our eyes become completely still, the image of the external world on our
retinas becomes stationary and we are temporarily blind. Seeing requires rapid
saccadic movement of the eye muscles---that is, a constant darting about of
the eyes that causes corresponding shifts of the visual field on the retinas.
Visual sensing requires scanning and noting differences. We imagine we see
a whole visual scene, but in fact our brains are attending to a series of rapid
snapshots taken at each point where the eye rests as it shifts about. The point
of the imagined movements along the horizontal and vertical bars was to get
you to relax the chronic tonus of the muscles that usually move the eyes, in
an effort to neutralize this scanning mechanism.
Perhaps you noted a difference
between your focused vision at the start of the exercise and your more open
vision at the end. A check of the muscles mentioned above would then illustrate
some of the muscular correlates of vision focused on the details of a single
object versus vision opened up to take in the whole visual world. In the former
case, light is directed toward a small central region of our retinas called
the fovea, which contains a high concentration of color-sensitive cone cells
and is responsible for high-resolution vision (see Figure 8-3, below). In the
latter case, the periphery of the retina is also engaged to take in the whole
visual world, including its edges. Foveal vision is often correlated with the
tensing of facial muscles and the contraction of flexor muscles along the front
of the torso. The relaxing exercises were meant to engage input from more of
the retina, letting more of the peripheral visual field that stimulates the
edges of the retina be accessible to notice. This more open vision can correlate
with relaxation of facial muscles and the front flexor muscles of the torso.
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DESIGN NOTE: LONG IMPORTANT POINT
Some athletes who must keep track
of complicated movements over their entire visual field report cultivating
a more open, or holistic, seeing of the sort we are getting at here. If instead,
they focus, they must move their heads to see as much, and the act of focusing
inhibits the flexibility of somatic muscle movement. This kind of visual experience
may account for some athletes reporting that they enter a trance ("a zone")
during periods of peak performance. The feeling is that an entity largely outside
of conscious awareness has taken over and is running the show.
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Our Visual Brain
Now let's consider our visual brain,
the subject of the most intense contemporary research in cognitive neuroscience. Visual
sensation and perception are the best understood of our mental faculties. This
system manages the ordinary but amazing feat of keeping our perceptions of
external objects constant as we move about, even as the size, shape, color,
and brightness of the objects projected onto our retinas is constantly changing.
The visual system must receive a flickering, two-dimensional image from the
retinas and render this image into some sort of three-dimensional version that
can guide appropriate action.
Our elaborate visual cortexes are
the latest installment in the evolution of seeing. They follow upon the transitions
we discussed earlier: from light-sensitive patches of membrane to image-forming
eyes that send information to complex neural networks. Studying
these cortexes in the cat and macaque monkey has revealed much of what we know
about our own central visual pathways. Although we have been evolving separately
from the macaque monkey for more than 15 million years, and thus must be cautious
in comparing our visual brains, many areas of functional specialization seem
to be similar in both.
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DESIGN NOTE: IMPORTANT POINT
Humans are primarily visual animals,
relying much less than most other vertebrates on touch, smell, and hearing.
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The visual system has been studied
more intensively than other parts of the brain for several reasons. For one
thing, it is relatively easy to describe exactly the input to the visual system---the
form, intensity, motion, and color of light that hits our eyes and is then
transmitted to our retinas. (It is much harder to describe the quantity of
an odor we inhale and to pinpoint exactly where it acts.) Another reason why
interest has focused on the visual system is that the anatomy of the structures
involved in visual processing is better understood than that of any other part
of the brain. A series of relay stations can be followed from the retina to
deep in the brain. At each stage of the relay, one can sample the activities
of adjacent nerve cells and find that they are spread out in a topographical
representation of the external world. In the occipital cortex, for example,
this representation has become distorted, as though you had taken the retinal
image on a piece of paper and crumpled it up to make it fit on the foldings
and twistings of the cortex.
The next section offers a brief
description of visual pathways and the processing of visual information in
parallel streams that deal with different aspects of visual images, such as
form, motion, and color. This will set the stage for us to consider of how
visual perception is accomplished.
Visual Information Is Processed
in Parallel Streams
The lens of our eye projects an
inverted image of the external world onto the retina, the thin film of nerve
cells at the back of the eye (see Figure 8-3). This film contains millions
of light-sensitive photoreceptor cells much like the grains in a photographic
emulsion, and the pattern of their excitation corresponds precisely to the
patterns being viewed. The photoreceptor cells then "talk" to a second
layer of cells (bipolar cells), which in turn talk to a third layer of cells
(ganglion cells) whose axons form the optic nerve that carries visual information
toward the center of our brains. This nerve is a bundle or cable containing
about a million ganglion cell axons. Electrical recordings show that each of
these axons responds best to discrete small spots of light (or darkness); thus
they are sometimes called spot detectors. Such recordings also show that the
ganglion cells from which these axons derive specialize to report different
kinds of stimuli.
Figure 8-3
Anatomy of the human eye and retina.
(a) Like the lens of a camera, the lens of the eye casts an inverted image
of external objects onto the retina. The image of the object on which we are
most directly focusing falls on a central part of the retina called the fovea,
which contains color-sensitive photoreceptors (cones) and is most important
in daylight vision. The scene surrounding this central object projects to more
peripheral parts of the retina that contain fewer cones and more rod photoreceptors.
Rod cells are important in vision in dim light. (b) The retina itself is a
thin tissue consisting of three layers of nerve cells, and light passes through
these layers before actually striking the photoreceptors. Axons of the ganglion
cells form the optic nerve, which carries information from retina to brain.
Also illustrated are a few of the cells that can transmit information sideways
in the retina.
Some ganglion cells with relatively
large cell bodies (magnocellular cells, M) are especially sensitive to changes
in a light stimulus and ultimately feed information to parts of the visual
cortex that specialize in analyzing motion, location, size, and spatial relationships.
Other ganglion cells with smaller cell bodies (parvocellular cells, P) specialize
in color and high contrast, and their information is finally used by cortical
areas that are involved in object recognition. The distinction between M and
P pathways arises at the first information relay between photoreceptor cells
and bipolar cells and then persists into the visual areas of the brain. You
will encounter further references to M and P pathways later in this chapter.
As the optic nerves from both eyes
travel toward the center of the head, they follow the pathway outlined in Figure
8-4, meeting each other in a crossing called the optic chiasm and then proceeding
via the lateral geniculate nucleus to the V1 areas of the right and left hemispheres.
(Look back at the view of these pathways shown in Figure 7-1.) Area V1 of the
left visual cortex, which reports on the right visual world, and area V1 of
the right visual cortex, which reports on the left visual world, would be just
under your left and right forefingers if you placed them at the middle of the
back of your head at about the level of the top of your ears. Recall from Chapter
7 that the two halves of our visual world are reconnected and integrated by
means of the corpus callosum, the large bundle of nerve fibers that communicates
information between the two hemispheres. We have already noted that cutting
this bundle in some patients with intractable epilepsy caused them to become
individuals who see the two halves of their visual world with two different "selves."
Figure 8-4
Information flows from retina to
brain. Axons that report from the left sides of our two retinas, and thus relay
information about the right-hand side of our visual world (our right visual
field), proceed back toward our left hemispheres, where they connect to the
left lateral geniculate nucleus, a part of the thalamus that is the brain's
first nerve relay station for visual input. The axons that report from the
right sides of our retinas (our left visual field) do the opposite, sending
information about the left visual field to the right lateral geniculate nucleus.
Alternating layers of the lateral geniculate structures get input from one
or the other eye, and each of the layers is specialized to project information
for either the M or the P pathway onto the visual cortex. Information
about the left and right halves of our visual world remains segregated as the
two lateral geniculate nuclei then send information back to area V1 of the
occipital cortex, the primary visual cortex. In this drawing, the optic nerves
and lateral geniculate nuclei are shown larger than their actual sizes for
clarity.
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DESIGN NOTE: LONG IMPORTANT POINT
Certain developmental dyslexics
who do poorly in tests that require rapid visual processing show abnormalities
in the lateral geniculate magnocellular layers that process fast, low-contrast
visual information. Also, imaging studies of such individuals show reduced
activity in higher visual centers that process motion information. Some dyslexics
have difficulty not only with rapid visual stimuli but also in processing sounds,
which suggests a more general deficit in processing of rapidly changing sensory
stimuli.
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Form, Motion, and Color
Axons that have arrived at area
V1 from the lateral geniculate nucleus make connections on neurons in the middle
of the six cell layers of the cortex (at layer 4), and at this point they are
still spot detectors, responding best to small spots of light or darkness just
as ganglion cells of the retina and cells in the lateral geniculate nucleus
do. However, a fascinating transformation can be observed by exploring the
activity of cells in nearby layers of the cortex. An electrode inserted perpendicular
to the cortex (into layers either higher or lower than layer 4) reveals cells
that fire best not in response to a spot of light but, rather, when a small
patch of retina is stimulated by a line of one particular orientation. Although
the exact wiring that turns spot detectors into "line detectors" is
not known, a simple hunch is probably the correct one. If a number of spot
detectors arranged in a straight line (that is, cells that are ultimately getting
information from a line of receptor cells on the retina) all "reported" to
a single following cell, they would constitute a line detector. As the electrode
that found the line detector is moved up and down through the cortical layers,
it encounters cells with more complex properties. Some cells respond best not
to lines of light but to dark-light edges; some distinguish whether those edges
are moving from right to left or from left to right. Others respond best to
right-angle corners between light and dark.
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DESIGN NOTE: IMPORTANT POINT
As we move into the cortex, we
find cells that respond to more complex stimuli. Instead of being sensitive
to spots, cells fire in response to lines of a particular orientation, movement
from a specific direction, and edges of light or dark. The components of our
visual world are being constructed one piece at a time.
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The plot thickens as the electrode
is removed and inserted into an adjacent area of cortex. Now the preferred
orientation for the line detectors has shifted slightly, as well as that for
the more complicated cells. Moving across the cortex and lowering the recording
electrode, we encounter a series of "orientation columns," each about
20--50 microns (micrometers) across, and each column's orientation preference
is slightly different from the preferred orientation of the preceding column.
These columns are driven mainly by one eye, but after the electrode has sampled
across the cortex for 200--500 microns, columns are encountered that prefer
the other eye. Thus mapping the surface of the visual cortex reveals stripes
or bands of cortex 200--500 microns across that are driven mainly by one or
the other eye. These orientation and occular dominance columns are shown in
Figure 8-5. Finally, the cortex contains islands of cells that respond to the
color of the stimulus.
The surface of area V1 of the cortex,
then, is a rich mosaic of partially segregated subcompartments that contain,
for each spot on the retina and in the visual world, cells whose firing corresponds
to all possible orientations of the stimulus, whether the stimulus is presented
to the right or left visual world, whether it is colored, whether the stimulus
is rapid with low light contrast or slow with high contrast, whether the disparity
of the same stimulus on the two retinas gives clues about its distance or depth,
and so on.
Figure 8-5
Orientation and ocular dominance
columns of the primary visual cortex, area V1. For simplicity, this drawing
represents the domains as rectangular in shape, but they actually are arranged
in pinwheel configurations.
Area V1 in the back of the cortex
acts as a post office, sending information forward to an adjacent area of cortex,
V2, and then (both directly and via V2) to the areas V3, V4, and V5 that occupy
folds, or crenulations, in the cortex a small distance toward the front of
the brain (Figure 8-6). Lesions in area V1, usually caused by strokes, result
in a complete block of the ability to acquire visual information consciously.
Both electrical recordings and brain imaging studies show that areas V3, V4,
and V5 deal with distinctively different aspects of the visual input. Area
V5 (which is also referred to as the MT area) is most active in response to
moving stimuli. Brain lesions in this area can cause akinetopsia, the inability
to distinguish moving forms, without loss of color or form perception. Area
V5 responds poorly to moving stimuli in dyslexic subjects who show deficits
in reading and phonological awareness. A
small area adjacent to V5, called MST, processes the visual motion, or optic
flow, that results from movement of the observer through its environment.
******************
DESIGN NOTE: LONG SELF-EXPERIMENT
Recall sitting in a stationary
train as another train slowly pulled past on an adjacent track and then feeling
that your own train was moving backward after the other train passed. This
is called a motion after-effect illusion. You may also have looked at a waterfall
for a period of time and then, on looking to the side, observed that trees
in the adjacent forest appeared to be moving upward. When normal subjects are
presented with this motion after-effect, known as the waterfall illusion, they
report seeing motion even though the stimulus is stationary, and at the same
time their V5 areas are very active. This
is an intriguing result, because it begins to provide us with a neural correlate
of one of our subjective experiences.
***************
Figure 8-6
Location of different visual areas,
shown using the macaque monkey brain, wherein they have been mapped with the
most precision.
Area V4 is most reactive to form-with-color
input, such as a Mondrian painting. Lesions in this area can cause achromatopsia,
the inability to distinguish between colors while form perception remains intact.
Firing of neurons in area V4 can be correlated with visual attention directed
either toward an area of space or toward a particular object. Surgical
excision of area V4 in the rhesus monkey causes deficits in the recognition
of objects that have been transformed either in their size, degree of occlusion,
or amount of contour information provided. In
general, increasingly complex functions are spared as lesions occur in consecutively
higher parts of the human visual system. These
areas correspond to several dissociable levels of conscious vision: phenomenal
vision, object vision, and object recognition.
It seems that visual information
goes from the post offices of areas V1 and V2 into a variety of pigeonholes
where it is analyzed piecemeal. There are at least 32 visual areas in the macaque
monkey, comprising approximately 50 percent of the cortex, and about half of
these contain maps of the external world that are reported by the retina. As
information flows toward association cortex and the front of the brain, visual
information mixes with other sensory input (such as sound and somatosensory
stimuli) and motor systems. There is such a melding of visual input with resulting
motor output that it is difficult to say where it all comes together before
action is taken. Action can be initiated well before visual processing is complete.
As information flows from sensory input toward motor output, we would expect
to find stages at which responses to stimuli began to correspond to the task
about to be performed, rather than to the sensory input itself. Indeed, the
activity of cells in the posterior parietal cortex of monkeys can correspond
to actions about to be performed.
The "What" and "Where" systems.
The prevailing idea is that the
distinction between magnocellular (M) and parvocellular (P) pathways persists
as information flows from area V1 toward higher visual centers. The
M pathway carries information about where a visual stimulus is and about its
spatial relationships, size, and motion. This pathway projects from V3 (dynamic
form) and V5 (motion) in a dorsal information stream toward the parietal lobe,
where motor and somatosensory cortices are located (Figure 8-7). Cells in this
stream respond rapidly to changes in stimuli, are not sensitive to color, have
low resolution, report information from relatively large areas of the retina,
and are very sensitive to changes in contrast (light intensity changes at edges).
This is what one might expect for a system that is ultimately responsible for
rapid guidance of muscular movements in response to visual stimuli. It is probably
most appropriate to view this dorsal pathway as a visuomotor control pathway,
not just a sensory or perceptual pathway. There is no clear demarcation of
where vision ends and action begins.
Figure 8-7
Dorsal (where) and ventral (what)
streams of information.
The P pathway proceeds mainly from
V3 and V4 in the direction of the temporal lobe (the inferotemporal cortex),
sending a ventral stream of information that is most concerned with what a
stimulus is. Cells in this pathway have high resolution (that is, they report
information from a very small area of the retina), respond to color, are slower
than M cells in responding to changes in stimuli, and are not very sensitive
to changes in contrast. These are properties that seem appropriate for object
recognition, and in fact, a part of the temporal cortex appears to be specialized
for the detection of faces (see below).
The separation of these two major
streams of information suggests that information about the identity of objects
can be computed separately from the information needed to react to that stimulus.
Occipital-parietal-temporal association areas might call up various memories
relevant to a stimulus (such as the fact that apples can be used to make pies),
and decisions related to the stimulus would also involve the frontal lobes.
The dissociation of object-processing and spatial-processing domains persists
into the prefrontal cortex in primates.
***************
DESIGN NOTE: IMPORTANT POINT
The ventral "what" system
(temporal lobe) might tell you that an object is an apple; the dorsal "where" system
(parietal lobe) would tell you where it is and how to shape your hand to pick
it up.
***************
The distinction between dorsal
and ventral streams has been studied most carefully in macaque monkeys, with
their approximately 30 different visual areas. Imaging studies have so far
differentiated ten human cortical visual areas with corresponding properties.
Of course, the distinctions between the M and P, parietal and temporal, where
and what pathways are surely not so clear and tidy as the simplified picture
being presented here. There is segregation of information, but these pathways
also interact continuously. This account does not cover further information
streams that may sketch boundaries and paint in their color, nor does it describe
how different information streams might converge and recombine.
Visual Pathways Outside the Cortex
Axons from the retina also pass
to at least ten other regions, including the pretectum and superior colliculus
of the midbrain, areas that are important in regulating eye movements guided
by visual input. Visual projections such as these, that do not pass through
the cortex, may underlie the phenomenon of blindsight. Subjects with lesions
in the primary visual areas have scotomas, or blind zones, in their visual
fields. They cannot verbally report on visual stimuli in these areas, but when
asked to guess whether a stick (that they "can't see") is held horizontally
or vertically, they answer correctly. And they can direct their hand to where
a point of light is flashed in their blind area. This
is an example of sensation being flawed while perception is partially intact.
******************
DESIGN NOTE: SELF-EXPERIMENT
You can get a feel for what blindsight
might be like by performing a simple exercise. Try looking about the room and
then closing your eyes. Visual sensation will cease, but for a while at least,
your knowledge of the room will persist, and you will be able to reach in the
right direction for something. This is not surprising. But imagine what it
would be like if you were to keep your eyes permanently closed and still had
continuously updated knowledge of the position and shapes of objects as though
you had just closed your eyes. You would feel that your perception was not
legitimate, much as a patient with blindsight does, because you would not have
ongoing involvement in the sensation of seeing.
***************
Blindsight can be produced in normal
subjects by a clever visual stimulus that places a square-shaped array of paired
dots moving past each other in differing locations within a larger square array
of similarly paired dots moving at right angles to the first. The dots are
so closely superimposed that an observer does not consciously perceive two
groups of dots. Still, subjects "guess" the location of the smaller
set of moving dots correctly. The
stimulus is one that is projected only poorly from area V1 to higher visual
areas, which suggests that activation of these higher areas is required for
subjective awareness of a stimulus. This
correct performance in the absence of awareness might utilize subcortical pathways.
There are many other examples, in both normal and brain-damaged subjects, demonstrating
nonconscious or implicit knowledge of stimuli that cannot, however, be perceived
explicitly or recollected consciously. This
observation suggests that, in addition to the modules for explicit sensing
and reporting, other distinct modules capable of sensory processing can still
do their work when the first set of modules is out of order.
Ensembles of Cells Encode Faces
and Other Icons
Are there specific dedicated cells
whose firing corresponds to a single complex image (such as your grandmother's
face)? Apparently not. It is clear that cells in our inferior temporal cortex
are active in facial recognition, but stimuli are represented by patterns across
ensembles of cells, not by the firing of single cells. Face-selective
cells are found to be members of ensembles for coding faces rather than detectors
for a particular face. In the
superior temporal sulcus, different populations of cells, located in patches
3--5 millimeters across, are selective for different views of the face and
head. These cells may support a parallel analysis of different characteristic
views of the head that combine to support view-independent (object-centered)
recognition. Some neurons in the inferior temporal cortex can learn to respond
to all views of previously unfamiliar objects, but most respond to only one
view. Activities of these cells
may be important as "social attention" signals, indicating where
other persons are directing their attention. Such a process is required for
social interactions.
Activation of face-sensitive cells
can correlate with activity in both the hippocampus (declarative memory) and
the amygdala (involved in emotional memory). Positron emission tomography (PET)
scan studies show that large areas of temporal and occipital cortex are active
during the processing of facial information. When
subjects are asked to visualize internally the face of a famous person, the
face area of the temporal cortex becomes more active. Magnetic resonance imaging
(MRI) studies using binocular rivalry show how activity in the face area can
correlate with visual awareness: A subject whose left eye is shown a picture
of a face and whose right eye is shown a picture of a house reports switching---experiencing
one and then the other every few seconds. During intervals in which the face
is reported, the face area becomes relatively more active, and when the house
is reported, the region surrounding the hippocampus that is involved in spatial
recognition becomes more active.
******************
DESIGN NOTE: LONG IMPORTANT POINT
Brain lesions, particularly to
the right hemisphere's temporal lobe, can cause face-recognition impairments,
or prosopagnosias, while sparing the ability to recognize most other objects. Such
a case is described by Oliver Sacks in his book The Man Who Mistook His Wife
for a Hat. The man in question had lost his ability to recognize faces, so
he had to use recollection of the dress his wife was wearing to recognize her
at a party.
***************
Electrical recordings from monkey
cortex have recently shown that columns of cells in the inferior temporal cortex
are selective for a range of similar stimuli. One intriguing set of experiments
started with a large collection of toy animals, vegetables, and natural objects
as visual stimuli and then progressively simplified and abstracted the features
necessary for eliciting the strongest responses of single neurons. Columns
responsive to colored bars, hand shapes, ovals with gradients of luminosity,
face shapes, T-shapes and other iconic stimuli were found.
The inferior temporal cortex appears
to use ensemble coding, with no one cell representing the detailed information
being processed. Rather, the world we perceive emerges from the coordinated
firing of sets of interconnected neurons. Examples
of ensemble coding occur at all levels of the visual system. Color perception
is an emergent property of a "committee" of photoreceptors. Depth
perception is an emergent property of disparity-detecting cells in area V4
whose firing reflects how the position of an object on the retina changes as
its distance from the eye changes. Individual cells are not sharply tuned for
objects, say 8.0--8.2 meters away, but we can make this distinction by averaging
the responses of large numbers of similar cells. Such estimates improve with
the square root of the number of cells involved (such that quadrupling the
number of cells doubles the precision). Similarly, any single cell in area
V5 might not track the velocity of a visual stimulus with great accuracy, but
the average response of a large number of these cells connected together might
be very accurate indeed. We can resolve spatial differences in images on our
retina that are smaller than the separation of the photoreceptors, presumably
because the reports of many receptors on the retinal position of a stimulus
are averaged.
Information Streams Flow Forward,
Backward, and Sideways
Work on the visual system is refining
the simplistic classical distinction, still widely used in teaching physiology,
between what goes into the brain (sensations, perceptions) and what comes out
(actions). Lurking behind this idea was the incorrect metaphor of a clear chain
of command: an input stream, then someplace where "it all comes together
for decision making," and then an output stream. The initial stages of
visual and other sensory input are indeed hierarchical and involve mainly the
flow of information from peripheral receptors into the brain. (For example,
our brains do not send information back to our retinas.) Information flow in
the final stages of motor output also is mainly one-way. In between these more
peripheral functions, however, visual information is flowing in the dorsal
and ventral streams mentioned above into 30--50 different areas where it mixes
with signals from other sensory areas and motor areas, with lots of cross-talk
and feedback.
Even at the early stage of the
lateral geniculate nucleus, which is usually described as a simple relay station
passing information from retina to cortex, feedback connections predominate.
The lateral geniculate nucleus gets only 20 percent of its input from the retina;
the other 80 percent is feedback from the visual cortex and input from lower
centers in the brains such as those that control eye movement. Recordings from
the lateral geniculate body or area V1 in live animals show that the responses
are very context-sensitive, changing with body tilt or posture or with auditory
stimulation. Resonating neuronal ensembles link vision, posture, sound, and
so on. Meaning resides in complex patterns of activity among the numerous units
that make up these networks.
******************
DESIGN NOTE: IMPORTANT POINT
There are so many reciprocal and
cooperative interactions between visual and other systems that the situation
is better described as a cocktail party than as a chain of command.
***************
The projection of a later stage
of processing back onto an earlier one is referred to as top-down, re-entry,
or re-entrant processing. This re-entrant input is often more diffuse than
the incoming information was. Thus the forward projections of V1 and V2 to
V3 and V5 are very specific; thin stripes of V2 cells project to V4, whereas
thick stripes of cells project to V3 and V5. Yet the return (re-entrant) input
to V1 and V2 from the specialized visual areas is very broad. This input apparently
allows V1 and V2, which are largely devoted to simple line detectors, to represent
complex features that require feedback from higher cortical areas. Thus correlates
of attention and figure-ground separation have been found in recordings from
single cells and small groups of cells in areas V1 and V2. One
example comes from cells that respond to the illusory edges of the invisible
triangle (the Kanizsa triangle) shown in Figure 8-8. Cells in areas V1 and
V2 that respond to an illusory contour or line at a border where no contrast
exists must be receiving feedback from higher visual areas that have defined
the whole figure.
Figure 8-8
The Kazinski triangle. Cells are
found in areas V1 and V2 that can respond to the illusory edges formed by these
shapes.
The effect of top-down processing
can be readily observed in both monkeys and humans when attention is focused
on a particular aspect of a stimulus, such as its color, motion, or identity.
Higher centers in the frontal lobes apparently feed back to enhance the activity
of cells and areas dealing with that aspect of the stimulus, while suppressing
others. Thus
instructions to pay attention to the motion, shape, or color of a stimulus
enhance the activity of V5, V4, and the temporal lobe "what" pathway,
respectively.
Another example of a correlation
between a sophisticated visual perception and neural activity comes from neurons
in areas V1, V2, and V4 whose activity follows perception during binocular
rivalry experiments. Monkeys were trained to report their perceived orientation
of a grating stimulus by pressing levers, and then gratings of different orientations
were shown to the two eyes. Many cells, particularly in area V4, showed activity
that correlated with the perceptual dominance of one, and suppression of the
other, stimulus. When recordings
moved on to the inferior temporal cortex, activity of nearly all the cells
reflected the dominant perception.
A linking of perception and action
at the single-cell level has also been observed. Monkeys
can be trained to respond behaviorally to a set of light points moving together
in a fixed direction in a field also occupied by randomly moving points (the
human observer doesn't notice the coherent movement of the subset; everything
just seems to be random). Activity in the V5 region is recorded that corresponds
to this discrimination. Microstimulation of the circuit that encodes this direction
of motion elicits the trained behavioral response (without stimulating the
retina). This work has been carried even further to show that the responses
of some cells in the frontal lobe hold information about intended behavioral
movement before it is actually carried out.
Re-entrant, or top-down, processing
presumably underlies the activation of areas V1 and V2 that is observed in
magnetic resonance and PET imaging studies when a visual stimulus is recalled. The
areas activated by a visual stimulus can increase their activity during mental
recall of the same stimulus. Brain imaging studies show that both motor areas
and area V1 can become active when subjects are asked to rotate imagined visual
objects. Numerous fields in parietal, temporal, and frontal cortex also become
active during manipulation of images. PET
studies show differential recruitment of the temporal ("what") and
parietal ("where") pathways when subjects are asked to imagine an
object or to imagine tracing a pathway between two familiar street locations.
Damage to the right frontal lobes can compromise tasks that involve mental
imagery in the (imagined) left visual field.
******************
DESIGN NOTE: LONG IMPORTANT POINT
The fact that knowledge stored
in higher centers can activate primary visual cortex might partially explain
why our visual perception can be biased to report what we want to see. In the
fascinating case of Charles Bonnet syndrome, damage to early parts of the visual
pathway results in a blind spot in some part of the visual field. Subjects
report vivid visual hallucinations confined entirely to the region of this
blind spot. This suggests that the hallucinations may be generated by feedback
from higher cortical centers---feedback that can no longer be compared with
what is actually present in the subject's visual world.
***************
As one might expect, the "what" and "where" systems
are selectively compromised by damage in the temporal and parietal lobes, respectively.
For example, a car accident victim with a lesion between the part of the brain
that compiles images coming from the eyes and the library of images in memory
could not recognize what he saw but could still draw on his library of images.
He could not name an asparagus shown to him but could draw an asparagus when
asked. His ability to recognize human faces was unaffected by the brain damage,
which implies a separate system for face recognition. What
emerges from all these facts is a picture not only of the bottom-up flow of
information but also of top-down projection of higher brain areas back to earlier
visual areas.
A Binding Process Underlies Visual
Perception
How are different aspects of the
visual stimulus organized into perception? This is called the binding problem.
Complex visual images are being processed by parallel pathways that specialize
in color, form, motion, solidity, and so on. How does the brain associate the
work being independently done by separate cortical regions? Different properties
such as shape, color, and motion must be "bound" to the objects they
characterize. An object and its motion must be distinguished from its background.
These problems are among the most crucial in cognitive neuroscience. The
binding problem is not just a theoretical worry. Some patients with bilateral
parietal lobe damage miscombine colors and shapes and are unable to judge relative
or absolute visual locations. Spatial information associated with the dorsal "where" pathway
has been compromised.
Our visual consciousness starts
building as our eyes dart about over the surface of a viewed object to generate
a series of very rapid, focused snapshots. These are the saccadic eye movements
mentioned in the discussion of the first self-experiment in this chapter. The
information gathered during a saccade is delivered to a scanning and processing
mechanism in the visual cortex; the process is complete in about 50 milliseconds.
This mechanism registers distinctive boundaries that are created by elementary
properties of brightness, color, or line orientation. We rapidly and automatically
process the property of subjective closure of forms. A
pop-out effect occurs for letters and digits: A letter target is detected more
efficiently among digits than among letters, and vice versa. Because letter
recognition and digit recognition are not innate, it is likely that the customary
separate groupings of letters and digits in the environment leads to their
separate encoding. For postal workers who routinely see letters and digits
together, letters do not pop out so much from digits.
******************
DESIGN NOTE: LONG IMPORTANT POINT
Our experience of a continuous
visual world is an illusion. It could be arranged, by attaching a tracking
device to your cornea, that small fragments of a text (from a whole written
page) are displayed only during the brief times during which you are actually
directing a saccade toward their position on the page. An observer not connected
to this device would see only transient flickers of word fragments darting
about the page, but you would experience a continuous page of text.
***************
The scanning system encodes useful
elementary properties of the overall scene (such as texture, color, orientation,
size, and direction) into feature maps in different brain regions. An
abstracting function may then compose key aspects of the image that distinguish
the object of attention from its surround---a master or saliency map is formed.
Thus the early stages of the visual pathway provide a faithful representation
of the retinal image, whereas later stages of processing hold edited representations
of the visual world modified to suit the immediate interest and goals of the
viewer. If a key part needs
further examination, then the brain refers to the individual feature maps.
How are salient features bound
together into a master map? One suggestion is that visual attention may be
mediated by subcortical structures such as the pulvinar, claustrum, and superior
colliculus, as well as by the prefrontal cortex. Bursts
of correlated action potentials in these structures may represent the saliency
map. Evidence suggests that neurons in the visual cortex that are activated
by the same object in the world tend to discharge synchronously. According
to this theory, separate populations of cells---say, those responding to aspects
of the color, shape, texture, motion, smell, and sound of a stimulus like a
bus, along with those that hold memories of buses experienced in the past---all
send out nervous impulses at the same rate for a fraction of a second. As the
neurons all fire together, sometimes oscillating about a central frequency,
the perception of the bus is created in the network. The
binding is a matter not of where but of when. Thus the synthesis of the visual
image in the brain would depend not only on the simultaneous activity of cells
in the different specialized visual areas but also on the temporal synchrony
of their responses. This synchronous
activity would include areas such as V1 and V2 with which the specialized visual
areas are reciprocally connected. To
link visual perception and action, it would be necessary to synchronize also
with motor areas of the cortex, and such synchronization has been observed. Synchronization
of the firings of large ensembles of neurons could be important whether or
not those firings were occurring in a rhythmic or oscillatory pattern. (Investigators'
excitement about the joining of nerve cells in rhythmic oscillations being
a possible answer to the binding problem has been muted somewhat by the discovery
that the oscillations measured in the visual areas of the cat brain are not
observed in visual areas of the monkey brain.)
The visual percept might not reside
in any given visual area, even if that area is critical for certain features
of that visual image, but might rather result from ongoing activity in several
connected visual areas. It seems likely that the simultaneous activity of many
visual areas is necessary for conscious visual perception. Stimuli
do not reach visual awareness unless all these areas are stimulated, even if
signals bypass area V1 to reach the specialized visual areas indirectly, as
is observed in blindsight.
Neural Correlates of Visual Consciousness
A goal of current research is to
determine the neural correlates of our experience of seeing---our visual consciousness.
It seems most likely that visual awareness and planning involve the frontal
cortexes, which receive connections from the higher, specialized visual areas
such as V4 and V5, but not directly from area V1. This
argues against activity in area V1 by itself being the neural correlate of
images we are aware of. It
is possible that there are specific neurons, normally dependent on input from
many areas, whose firing gives rise to the current content of our visual (or
other) consciousness. If this is the case, one might be able---with some not-yet-invented
technique---to stimulate these cells directly and generate in a human subject
the same subjective experience that is associated with their normal input and
firing.
Summary
We are at the threshold of understanding
how the array of modular processors that first transform and analyze different
aspects of our sensory world---such as visual motion, form, depth, and color---proceed
to interact and generate our conscious experience of a visual percept. Progress
in understanding the perceptual worlds of hearing, tasting, smelling, and body
movements is not so advanced. The great variety of line detectors, shape detectors,
icon detectors, figure-surround discriminators, and so on represent parts of
the solution to the central problem posed by the physical environments in which
the visual brain evolved: how to process visual information in a way that leads
to effective action in the world, action ultimately in the service of reproduction.
Sensory information coming into our brains is arranged hierarchically at first
but soon merges into regions of association cortex that specialize in "what" or "where" information---regions
that blend sensory modalities and mix with motor, premotor, and prefrontal
areas. These higher areas send information back to earlier stages of processing,
and cross-talk between different information streams contributes to the binding
together of percepts. The initiation of action, then, has a fuzzy origin that
becomes more structured and hierarchical as actual movement repertoires are
executed.
Higher vertebrates in particular
use the past as well as the present in interpreting current sensory input.
What has been seen in the past can influence what is seen in the present. Experience
during development of the brain and as in adulthood leads to the assembling
of a whole repertoire of assumptions and interpretations that have proved useful
in the past. These most often facilitate perception and actions, but they also
can bias and confuse our responses if a past interpretation is not appropriate
to the present. As we will see in the next chapter, systems that regulate body
movement also acquire training and habits, which, though they are indispensable
for skilled action, can also impede appropriate responses to novel situations.
Questions for Thought
1. One approach to understanding
perception is to ask, "What kind of machine would a good physicist or
engineer build to render inside the head an accurate representation of the
sights, sounds, and so on out there in the real world?" Another approach
is to say, "Forget the omniscient engineer. What perceptual mechanisms
have in fact been cobbled together during evolution to guide action in the
real world, in the service of passing on our genes?" What do you think
are the merits of each of these two approaches? Do they suggest different research
strategies?
2. Many experiments demonstrate
that most of our perceptions are not naive but are informed by a previous history
of sensing that imposes biases and interpretations on what we think we perceive.
Why do you think our brains would have been designed this way? What are the
advantages and disadvantages? How might you try to determine whether such biases
are learned or innate?
3. Would a lesion in your right
inferior temporal lobe be expected to influence how you pick up an object with
your left hand? If so, how?
4. Visual information flows from
higher visual areas back to lower visual areas such as V1. What are some ideas
about the role such top-down processing might play?
Suggestions for Further General
Reading
Ornstein, R. 1991. The Evolution
of Consciousness. New York: Prentice Hall. Several examples of the relativity
of perception cited in this chapter were taken from this book, which is an
engaging account of some of the topics covered in this and subsequent chapters.
Goleman, D. 1986. Vital Lies, Simple
Truths: The Psychology of Self Deception. New York: Simon & Schuster. An
accessible account of psychological studies that demonstrate how perceptual
filters, self-deceptions, self-images, and the like are constructed. Several
examples used in this chapter were taken from this book.
Crick, F. 1994. The Astonishing
Hypothesis: The Scientific Search for the Soul. New York: Scribner. This book,
like the book by Zeki, cited below, gives a description of information processing
by the visual system.
Pinker, S. 1997. How the Mind Works.
New York: Norton. Chapter 4 of this book includes an interesting discussion
of how the visual mind solves problems of shape, space, and perspective.
Reading on More Advanced or Specialized
Topics
Zeki, S. 1993. A Vision of the
Brain. Oxford, England: Blackwell Scientific Publications.
Gazzaniga, M.S. (ed). 1995. The
Cognitive Neurosciences. Cambridge, MA: M.I.T. Press.
The next two volumes are collections
of brief review articles that cover essentially all of the areas of modern
cognitive neuroscience. They have material relevant to all the chapters in
Part III of this book. Together with Gazzaniga's cognitive neuroscience textbook
and the citations on the Web site for this book, they provide more detailed
primary references on the observations reported in this chapter.
Squire, L.R., & Kosslyn, S.M.
1998. Findings and Current Opinion in Cognitive Neuroscience. Cambridge, MA:
M.I.T. Press.
Gazzaniga, M.S., Ivry, R.B., & Mangun,
G.R. 1998. Cognitive Neuroscience---The Biology of the Mind. New York: Norton.
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