Deric Bownds

Chapter 9

Acting Mind

Now we shift our emphasis from perception to action, to consider the nature of the body movements that make up our behaviors. This chapter starts by noting further how our minds, body actions, and environment are linked in a seamless whole and then considers how mind and consciousness might be defined from the point of view of movement. We next review some aspects of our experience of movement of which we are usually unaware, using several simple exercises to sense the hierarchical and habitual nature of the neuronal control of our motion, exercises that can also be used to refine and change that control. A further section takes a brief look at the neuronal pathways underlying action, and a final section considers how movement might be regulated by a Darwin Machine mechanism.

Action---The Interface of Mind and Environment

As we go about our daily lives, a significant fraction of our experience consists of preoccupation with the thoughts flowing through our heads, the chatter of our internal discourse. Occasionally, this noise may diminish, as though the brain is taking a brief time-out from constantly talking to itself. At such times, we can quietly notice the rest of our body and its experience as we move through our activities. Do you ever feel as though an "it" rather than an "I" is doing the moving? And do you ever wonder if this is what it would be like to be an animal, not self-conscious, moving through its terrain? In this state of mind, you probably notice many small signals from your body, and also from the environment, that you usually ignore. When you pass either an attractive or a menacing person, subtle somatic episodes or emotions associated with approach or avoidance appear and then disappear. If the menacing person suddenly says, "Got any spare change?" you note a more obvious tensing of the whole body and an increase in heartrate, as though it is preparing for flight. These multiple body flickerings 1 are present at all times, but normally you tune them out.

Such experiences reflect the processes going on as movement of your body leads to construction of a representation of the external world from the perturbations caused as that world touches it and showers it with odors, sounds, and light. It is easy to lose sight of this embodiment of our brain-mind because we spend most of the hours of our waking consciousness living in our heads---thinking about, reading about, and talking about ideas. Many of us usually tune out awareness of what is going on in the rest of our bodies as we move about and as our feelings change. The brain, however, does not tune out. Outside the narrow spotlight of our awareness, a massive and constant stream of information flows back and forth between brain and body. The global state of parasympathetic/sympathetic activation in the peripheral autonomic nervous system that innervates our viscera is being monitored, and the outcome affects the emotional balance between approach and avoidance in every passing moment.



It is our whole organism---body and brain---that interacts with our environment. Mind is about the rest of the moving body, about what to do with it, about where to move next.


The image conjured up by science fiction writers and philosophers of a "brain in a vat," isolated but otherwise identical to the normal brain, is unrealistic in more than just the obvious ways. The vat makers would have to reproduce the subtle and myriad sensory inputs to this brain, its responding actions on the body, and the brain's subsequent sensing of the effects of those actions---a formidable proposition! It is unlikely that there are autonomous symbolic brain structures that do not require, or are not regulated by, the exchange of information with the rest of the body. The body is an essential component of the content of our mind.

Acting on and perceiving the environment require constant modifications of body musculature. Goal-directed movements are constantly checked and refined as sensory receptors monitor their progress. The body is constantly changing itself in accordance with the visual world that is present and the actions being contemplated in it, part of a massive background calculation that yields each moment's answer to the question "Do I like or dislike, approach or avoid this situation?" Perceiving is thoroughly about acting, and acting is completely about perceiving, a rich relationship that can be obscured by the arbitrary separation of the two in most accounts and textbooks. 2 The self we all experience as we move through a day requires a continually reconstructed or updated image of our perceptions, feelings, and actions. What we think is going on "out there" is not just the photograph-like image we are most aware of. It is also a subtle symphony of brain representations of our viscera, our moving muscles, and the biochemical regulators of our hypothalamus.

Our minds are ensembles of physiological operations linking the muscular, endocrine, immune, and nervous systems (central, peripheral, voluntary, and autonomic). This ensemble that has evolved to guide action specific to our species (over geological time) and our culture (over the lifetime of the individual). Our mind-brain must be defined in a way that considers the whole organism, its movement, and its feelings. 3 It must also be described in terms of ongoing reciprocal interactions with the environment. Our actions change our environment, and those environmental changes in turn affect our actions in a continuous loop. This is the heart of cognition, and the explicit data storage and logical manipulation that are the focus of much cognitive neuroscience play the role of supporting this reciprocal interaction. Mind is much more than a filing cabinet or a logic machine. 4 And we need to keep all of this in mind as we try to achieve an understanding of action or movement sequences, whether spontaneous or learned.

A Movement-Oriented View of Mind and Consciousness.

Starting with the first twitches of a developing embryo, the constant currency of animal life is action and movement. The innate movements of the embryo are the earliest building blocks of motor behavior. Those movements begin before the development of the nervous system that will ultimately control them, and they help shape the formation of that system. It is through testing the appropriateness of action that motor and sensory pathways develop and are coordinated. Most textbooks offer a description of the sort shown in part (a) of Figure 9-1 to represent the basic relationship between acting and sensing. A linear path from sensing to acting is portrayed, and the effect of action on subsequent perception, shown by the dashed line, is not emphasized.

Figure 9-1

(a) This conventional block diagram of body function emphasizes a one-way sequence from perception through analysis to action. (b) A more appropriate diagram shows a circular and continuous loop: Acting on the environment perturbs it, and perceiving the effect of that perturbation informs further action.

We could also present the sequence, in a subtly different but perhaps more useful way, as a circle or cycle wherein any point could be the start. For example, let's turn it around, as in part (b) of Figure 9-1, to make movement the primary step. Perception of the resulting perturbation of the environment then informs subsequent movement. In this view, we might consider consciousness a mechanism by which an organism initiates movement to acquire information about its present and past environment. 5 The perceived unity of our consciousness may have something to do with the fact that we are constantly generating coordinated movement patterns in the face of diverse sensory stimuli. Thus, in seeking the physical location of the neural correlates of consciousness, we might do well to look closely at the higher motor levels, located in frontal cortex and cingulate cortex.

Thinking can be viewed as the activity of deciding "what movements I make next." The most complex brains are found in animals that make the most complex motions. The fact that motor activity is commonly assumed to be a lower cortical function than those subserving "pure" thought reflects the tradition, found in many cultures, of separating mind and body and assuming that the latter is somehow lesser. But how can we say that the performance of an acrobat is less "intellectual" than that of a mathematician? Earlier in this century, investigators seriously discussed---but later dismissed---a motor theory of thought, which suggested that conscious thought correlated with brain activity underlying actual or imaged movement. 6 Now modern brain imaging has revealed that conscious planning and motor imagery engage the same brain areas, a result that makes such ideas more plausible (see below).



It is possible to view mental activity as a means to the end of executing actions, rather than depicting motor activity as something subsidiary, designed to satisfy the demands of the higher centers.


Kinesthetic Intelligence

An evolving kinesthetic intelligence may have led to our narrative self consciousness. The neurophysiologist William Calvin suggests that evolution of our motor skills, as in making and using tools and spears, established sequence-handling machinery in the premotor cortex that became Broca's area, which is crucial for generating the sequences of syllables used in talking to oneself and others. 7 Speaking is itself an intricate physical feat. From the point of view of the premotor cortex, articulating long strings of syllables might be a lot like climbing a rock face or pitching a baseball---or throwing a spear. Our fundamental kinesthetic experiences of up-down, right-left, inside-outside might provide a physical basis for the development of symbols and metaphors used in language (see Chapter 11 for more on this topic).

Consider the possibility that many of your conscious thoughts are associated with subtle acts of moving and sensing your musculature. Some experienced meditators report this to be the case. (Can you really think well without tensing your neck? Can you do math problems lying down?) We all know that feedback from muscle tension and limb position can affect our conscious feelings. Forming our facial muscles into the shape of a smile can actually lighten our mood, and making a frown can do the opposite. Calvin mentions the account of Oliver Sacks, who reported having his shoulder muscles electrically stimulated to generate the gesture of a shrug---whereupon he felt as though he wanted to shrug his shoulders to express nonchalance, as in "So?"

Action Repertoires

The consequence of a long developmental history that teaches effective patterns of acting and sensing is the construction of an elaborate hierarchy of neural agents that filter and process both output and input. Our brains are an archive of images or representations of both perceiving and acting, accumulated during our developmental history. 8 The activity of the brain reflects expectations set up by years of experience. We have a large array of motor repertoires---movement patterns built up from simpler subunits that we automatically execute, altering them through feedback if an intended action was not accomplished. 9 Luria, the famous Russian psychologist, has used the term kinetic melodies to describe such movement patterns. Many of us have such automatic routines for dressing, unlocking a door, or moving through a familiar house that we can be completely unaware of carrying out the actual movement sequences while we daydream about something else.

Parallel Actions

With training, we can split ourselves into parallel steams of activity, becoming more muted versions of the split-brain patients mentioned in Chapter 7. One example of parallel streaming is the "automatic writing" that became a fad early in the 20th century. Gertrude Stein, working in William James's laboratory, trained herself to write down dictated words while reading something else. She recalled the reading but recalled nothing of the words she had written down by dictation. A similar kind of parallel processing happens in kinesic communication, or body language, which we discussed briefly in Chapter 5. We can be aware of, and consciously respond to, the verbal content of a discussion with someone, while at the same time unconsciously reacting to signals sent by that person's posture and tone of voice, and all the time we are making equally unconscious movements of our own. During acting and sensing, we are conscious of only a minute fraction of the total information flow. If this were not the case, we would be overwhelmed---like someone who tried to watch everything on 50 different cable TV channels at once or to read every message going over the Internet.

Movement Exercises That Reveal Underlying Mechanisms

You can become partly aware of some of the complex layers of activity underlying an action---some aspects of the kinetic melodies that underlie our daily acts. This requires that you be willing to slow down enough to engage the subtle sensing requested in the exercises that follow. You can experience what we see reflected by modern imaging and electrical recording experiments that monitor the brain's activity during the planning and execution of discrete actions. The first exercise deals with the planning that goes on before movement actually is executed. Both imagining and initiating a movement happen over a time scale of about a third of a second.



This exercise works best if someone is giving you these instructions, but you also can do it alone. Sit quietly with eyes your closed, letting both arms rest on the arms of your chair or on a desk. Try to relax your arms completely. Now imagine that someone is going to ask you to lift your right arm gently. Think about lifting your right arm, but don't actually do it. Then stop, and completely relax your right arm. Do you feel the "letting go" of the muscles you had already tensed slightly, the whole configuration you had prepared for lifting? It is really all there, planned before the gross movement occurs. Now repeat the experiment and actually go ahead and lift your arm. What do you feel?


MRI imaging shows that when subjects are asked to think about moving a hand, the premotor cortex and other areas adjacent to the motor cortex become most active; this indicates that there is a distinction between parts of the brain that prepare for movements and parts that carry them out. Just as imagining an object engages the same areas of visual cortex that actually viewing the object activates, so motor images engage regions that are active during actual movement. 10 Recent work has shown that motor imagery and motor execution involve activation of very similar cerebral structures at all stages of motor control, except that the final motor output is not expressed during motor imagery. 11

A second exercise deals with noting the various components of a movement pattern in more detail. Many of our habitual movement patterns contain extraneous contractions, and you can learn about this in an interesting way. You can sometimes sense more about what goes into a movement pattern by imagining that pattern than by actually carrying it out.



Repeat the previous arm-lifting exercise, but don't do the actual movements. Close your eyes and imagine yourself lifting your arm from the surface it is resting on. Gently note and feel all the muscles getting ready to lift the arm. Now note your neck muscles. Do you really need to tense them to lift your arm? No. The muscles that contract to lift your arms are down in your chest and back; the neck muscles are not needed. Imagine relaxing your neck while you lift your arm. Now, slowly, actually do the movement.


The subtle sensing that you are using in these exercises is at the core of some training techniques used by musicians, dancers, and athletes to refine their movement---techniques that tell us much about the relationship between moving and sensing. 12 They also enable us to detect chronic inappropriate movement patterns and retrain ourselves to move more efficiently. A further trick is to go beyond imagining a movement and to perform it very slowly and very gently (so that feedback can register to shape, in turn, the central program that is reading out). This works because our sensory receptors are most sensitive to stimuli of low to medium intensity. If a motion is made in a gross or more strenuous manner, the receptors don't report what is happening so effectively. In slow, gentle movements, one can sense and play with new configurations, instead of locking into a habitual pattern. If a task can be done by a group of, say, four muscles, but our habit is to let one muscle strongly predominate over the others, the brain listens to feedback from this one and ignores the rest. If appropriate use and interaction of these muscles are to be sensed, then small movements must be employed so that one senses tonus and stretch from all more or less equally. You can feel this by trying the following experiment.



Press your hand down hard on the arm of your chair, and note how much you can feel. Now release and shake out your arm and hand. Now press in the same way, but begin very gently and slowly---and see how much more you sense.


These exercises---imagining a movement or carrying it out very gently and slowly---can be a basis for retraining muscle movement patterns, for laying down new procedural memories and their corresponding long-term changes in the wiring of our brains. This is relevant to more people than just dancers and athletes. As we age and begin to have more difficulty jumping over fences and getting into the back seats of compact cars, it is in part because we waste so much energy and effort. We usually learn movement patterns when we are younger and more energetic, and we refine them only until they are "good enough." Only later do we discover that they really don't work very well. Retraining to refine our inefficient movement patterns can enable us to enjoy our customary activities for a longer time.

Neuronal Pathways Underlying Action

How are appropriate action and sensing linked?--by specialized committees of neurons on many levels of organization, from the very simple to the very complex. Committees of such agents in the spinal cord can direct walking in the absence of the brain, but the higher centers are required for purposeful action. These committees, which often have overlapping memberships, can organize themselves if they are given some feedback about how well they are doing. Acting and sensing are linked by many feedback loops. Motor nerves descending from brain to spinal cord send branches to ascending sensory pathways, and these branches serve to adjust sensory bias or communicate an expected sensory input from the about-to-be-ordered movement so that it can be compared to what actually happens. This is the system that permits motor learning by trial and error. 13 Think about learning how to write when you were very young. Can you remember how hard it was to get your hand to make fine motor movements---how your letters were very large and crude at first? Years of variation (trying different muscle movements) and then selection (noting which movements resulted in the best and smallest letters), went into establishing the stereotyped movements you now use when you write a letter by hand.

To exert motor control, our brains approximately reverse the transformations that we discussed in Chapter 8. Instead of building up an image of what is outside, we "realize," or carry out, the image of a desired action on the outside world. 14 This process has to be instructed by the "what" and "where" systems of the temporal and parietal lobes mentioned in Chapter 8. 15 These systems define the world in which the movement is occurring. World-centered and person-centered coordinates of movement must be established on the basis of maps of the environment in the parietal cortex and the hippocampus, which are themselves built from sensory input. These coordinates are used by the supplementary motor areas and the premotor cortex as they prepare instructions for the primary motor cortex and other movement centers in the brain. It is important to distinguish movement kinematics, the sequence of positions that a limb is expected to occupy at different times, from movement dynamics, the actual control of the movement plan as it is carried out under differing environmental conditions.



Whereas perception builds up an internal representation of the external world, action starts with an internal image of the desired outcome of a movement and then proceeds to execute that movement.


It is presently thought that the supplementary motor area contains an action programming subsystem that sends coordinates of each component movement, in the proper order, to an instruction generation subsystem associated with the premotor cortex, which then instructs a movement execution subsystem of the primary motor cortex. 16 The location of these cortical areas is shown in Figure 9-2. The complexity of these processes is revealed by the variety of apraxias (breakdowns in skilled movements) that can be caused by brain lesions. Patients with ideomotor apraxia are able to perform a sequence of movements, but they have difficulty with the individual components. Individuals with ideational apraxia can perform an activity such as lighting a candle if the lights go out, but they are unable to follow an instruction to do so. Disorders sometimes focus on specific activities, as in dressing apraxia and oral apraxia (inability to make skilled gestures with facial muscles.) The agraphias, a series of specific disruptions in the ability to write, can occur in the presence or absence of the ability to read. Some of these disorders reflect problems in producing the movements per se, and others are caused by problems in language processing.

Figure 9-2

Cortical areas involved in the planning and execution of movement. Imaging experiments reveal that very simple habitual movements activate mainly the primary motor cortex. Activity in the supplementary motor cortex increases during more complex movements, whether they are actually carried out or are just mentally rehearsed without execution.

A massive cable of axons carries instructions from motor cortex to motor neurons in the spinal cord that direct fine movements of our limbs, hands, and fingers. This direct pathway from cortex to the spinal cord appears first in mammals, and increasingly detailed motor and sensory maps of the body appear in the somatosensory and motor cortexes of higher mammals and primates. The human cortex differs strikingly from that of the chimpanzee in that much larger sensory and motor areas are devoted to the face and hands. Dominance of our motor activity by one cortex, usually the left, is much more striking than in monkeys and apes. We also go to greater lengths in employing one arm and hand, usually the left, to support and provide context for fine movements, while the other hand, usually the right, carries them out.

The cortical system for voluntary movements is layered on top of two other levels of movement control: descending systems of the brainstem and the spinal cord. Medial (toward the center) brainstem pathways are important in combining somatosensory, visual, and vestibular (balance) information to control posture and the axial core muscles that must position themselves to support movement of the limbs. Lateral (toward the edges) brainstem pathways control distal muscles of the limbs and are important for goal-directed action. Thus it is the medial pathways that are most involved when your posture is automatically adjusting itself against gravity as you walk or turn, and the lateral pathways come into play when you reach out and pluck fruit off a tree. The spinal cord, the lowest level of motor control, contains local circuits that carry out rhythmic motor patterns, such as walking or running, and also reflexive withdrawals from aversive stimuli. These local spinal circuits are modulated by the higher centers.

The three-level hierarchy of cortex, brainstem, and spinal cord is regulated by two further subcortical systems that report back to the cortex via the thalamus (see Figure 9-3). These are the basal ganglia and the cerebellum. The function of these complex structures is not well understood. The basal ganglia are a complex set of nuclei that receive inputs from all cortical areas. Until recently it was thought that these nuclei sent information back mainly to the motor planning areas of the frontal cortex, but recent work has shown they make specific closed loops with virtually all cortical areas. Electrical recording and brain imaging studies suggest that basal ganglia structures are involved in establishing the context for motor behaviors and also some purely cognitive operations. Activity is observed that corresponds to motivations, expectations, or rewards. Diseases of the basal ganglia cause motor abnormalities such as involuntary movements and loss of spontaneous movements. One of the nuclei, the substantia nigra (Latin for "black stuff") is dark because its neurons contain a derivative of the neurotransmitter dopamine. It is the death of these dopaminergic neurons that causes Parkinson's disease. Abnormality in the dopamine system of another center, the caudate nucleus, appears to be associated with the faulty neurological braking that causes the chronic tic disorder Tourette's syndrome. 17



Although the function of the basal ganglia is not well understood, several disorders result from abnormalities of these areas. Parkinson's disease results from the loss of dopaminergic neurons in the substantia nigra, and Tourette's syndrome can be traced to abnormalities in the caudate nucleus.


The cerebellum receives sensory input that enables it to compare actual performance with intended movements and to compensate when things do not go according to plan. Lesions here disrupt balance and coordination. A recent interpretation is that the cerebellum is not activated by the control of movement per se but rather serves as an error-checking device during motor, perceptual, and cognitive performances, processing sensory data related to those activities. 18 There is strong evidence that the cerebellum is involved in motor learning, because such learning can be blocked by lesions in one of its three main subdivisions. 19 Genetic manipulation of mice to remove a particular form of the postsynaptic receptor acted on by glutamic acid can make them deficient in one type of cerebellum-dependent motor learning. 20 The cerebellum receives input from the periphery as well as from the sensory and motor cortexes and sends output mainly to the motor regions of the brainstem and cerebral cortex. The cerebellum's input and output happen at a fixed rate, 10 times a second, so we actually move in a somewhat jerky way at about this frequency. 21 There is accumulating evidence that the cerebellum does much more than just effect motor coordination and control. Different regions of it become active during different cognitive tasks (matching verbs with nouns, for example). 22

Figure 9-3

Modulation of the motor control hierarchy of cortex, brainstem and spinal cord by subcortical systems. Most cortical regions send information to both the basal ganglia and the cerebellum. After being processed by the lower structures, this information is fed back via the thalamus to those same cortical regions. It may also be sent to other targets. The basal ganglia seem to deal mainly with the motivation and context for action, whereas the cerebellum monitors the status of action. Recent experiments implicate both the basal ganglia and the cerebellum in higher cognitive functions such as language.

If this is beginning to sound complicated, it is. We know much less about motor systems than about the visual system (this is why textbooks typically devote twice as much space to sensation as to motion). The descriptions of motor pathways that we have, of the last ten or so layers of synapses before a muscle contraction, are not as detailed (or as simple) as the description we can give of the first ten or so synapses of sensory input in the visual system. Recently, however, progress has been made in understanding the coordination of movement sequences at the cortical level. 23 Electrical recordings from single neurons in the primary motor cortex of monkeys show that firing can correlate with several different aspects of a movement: its direction, its force, or the visual or other stimuli that are used as the instruction to move. 24 In a monkey trained to move its arm in the direction of a visual stimulus such as a point of light or a spiral, the firing of cells that calculate the movement can be detected just before the actual motion begins. 25 Although large areas of motor cortex are organized into areas of spatial representation (legs, arms, torso, head, and so on), finer-scale maps seem to represent combinations of muscles used in coordinated actions (for example, the muscles of the torso, shoulder, upper arm, and lower arm that must be activated in a precise and smooth sequence for the animal to reach out and grasp an object). 26

Recordings from the ventral premotor cortex of the monkey have demonstrated the existence of neurons that fire both when the monkey grasps or manipulates an object and when it observes the experimenter making a similar action. These neurons, which are sometimes called mirror neurons, appear to be part of a system that matches observed events to similar internally generated actions, and thus they form a link between observer and actor. Imaging studies on humans show corresponding activation of premotor cortical areas both when an action is executed and when another person is observed carrying out the same action. 27 These mimetic phenomena may represent an observation-execution matching system, an information exchange mechanism that underlies the more complicated mimetic and linguistic human intelligences that we discussed in Chapter 5.

Movement Generation by Darwin Machines

How might the hierarchy of subroutines that go into making our movement be regulated and refined, especially those supporting movements that are very rapid? Reaching, throwing, and kicking are ballistic movements carried out so fast that there isn't time for feedback from sensory receptors. The minimum time for a nerve signal to make the round trip from our arm to our brain and back to our arm is about 110 milliseconds. A dart throw is over in about 120 milliseconds, so there is no time for us to receive and process feedback on how things are going. The instructions for the sequential muscle movements must be stored and then "read out" in some way. You got a sense of this "get set" storage in the first self-experiment in this chapter.

The neurophysiologist William Calvin describes a movement sequence as analogous to a row of railroad cars in a train, where each car represents a component of the movement. Each component could consist, for example, of a particular set of nerve cell firings that cause movement of a subset of the muscles engaged in the total movement sequence. How is the best movement sequence (arrangement of railroad cars) chosen? 28 Imagine that many trains are lined up in parallel, each with a slightly different sequence of cars (movement instructions), as in a railroad yard that contains many parallel train tracks. (Figure 9-4 illustrates the oversimplified case of three tracks.) All of these tracks converge on a track that exits from the train yard (the movement sequence actually performed). Now, let each train be tested for how effectively its sequence of cars works. Let those trains that worked best duplicate themselves, displacing some trains that were less effective. Repeat the cycle until identical trains are running on every track. Finally, put all these tracks to work together to provide refined control for a series of motions. (Another analogy might be that of a number of people starting to sing randomly but eventually coming together into a chorus.) We are essentially describing yet another version of the Darwin Machine.



Movement may be controlled by the application to neural networks of a version of the same abstract Darwin Machine that is used on the longer time scales of species evolution and organismal development. In all of these contexts, variation and selection act together as though to attain a goal.


Figure 9-4

A Darwin Machine model for generating and refining a movement sequence. Let each symbol represent one stage in an overall movement sequence. (a) These three sequences of symbols represent parallel but slightly different routes to generating the overall movement. Each sequence is executed and "graded" in terms of its effectiveness in the current environment and by comparison with memories of similar past sequences. This grade is indicated by the number at the left of each sequence. (b) A sequence that is ranked as less effective can be replaced by a duplicate of a more successful sequence (arrow). Sometimes a slight alteration that occurs during this duplication might produce an even more effective sequence. (c) Repetition of this process many times finally generates a unified "chorus" of parallel and similar sequences that, when executed together, can control a complicated movement with great precision. Of course, the brain would use a great many more sequences than the three illustrated here.

Why do we suggest that there are many trains carrying out the same sequence of instructions? Because for any one train by itself, there is a problem. The range within which a nerve cell may fire after it is triggered is from 1 to 10 milliseconds. Calvin points out that this is too imprecise even for such an activity as throwing a rock at a stationary prey 4 meters away. There is an 11-millisecond launch window in which a rock can be thrown to hit a rabbit at 4 meters. A rock released too early will overshoot; a rock released too late will land in front of the rabbit. The launch window shrinks to 1.4 milliseconds if the target is 8 meters away. How do we guarantee a launch within a 1-millisecond interval using nerve cells that have a 1- to 10-millisecond "jitter"? Part of the answer may be that trains with identical sequences of cars can emerge simultaneously from the marshaling yard and have their firing times averaged. When the firings of many motor neurons are averaged, the motor effect is much more reliable than that elicited by the firing of any of the individual neurons. (Think of an engineer who wants to measure the time at which midnight occurs and has a 100 different clocks, each of which is off by a few seconds. Noting when half of the clocks have struck midnight gives a much more accurate estimate than listening to just one.)

Calvin further proposes that a cerebral Darwin Machine might underlie our ability to carry a small-scale model of external reality in our heads, all the while continuing to test it against various alternatives, continuously revising our assessment of which is best, predicting future situations, and so on. His suggestion is that a random sequencer arranges templates into parallel strings, each of which is "graded" by memories of how similar strings performed in the past and weighted for emphasis by the present environment. Dozens or hundreds of such sequences might be tried out simultaneously, with the most appropriate new generation shaping up in milliseconds, thus initiating insightful action without overt trial and error. It is interesting that the shaping-up or selective mechanisms appear to be damaged in some patients with frontal lobe lesions. These patients can spin scenarios of action but cannot easily choose among them.

Ideas that flow through our heads could be selected as candidates for storage in long-term memory by similar mechanisms. Most of the candidate sequences would be held in our unconscious short-term memory and thrown away after use. Transfer to long-term memory would be necessary to establish the ability to recall such sequences. Only those that survived a thorough reality test would be retained. The sequences---at least those involved in the short-term generation of language---might contain about seven chunks of information each. (We can hold about seven digits, such as a phone number, in short-term memory long enough to repeat them or dial a phone.) This could be why, when deciding what to say next, we plan ahead by no more than about half a dozen chunks, words, or concepts. We usually don't know how our sentences are going to end when we start them.


Trying to understand action is a more daunting task---and probably a more complex one---than we face with perceptions, the subject of Chapter 8. Our self conscious cognitions rest on a vast background of moving muscles and viscera that are continually monitored as the organism moves about in its physical and social environment. The core event of our daily activities is a continuous feedback cycle in which we initiate movements of these muscles and viscera to acquire information about, and change, our internal and external environments, and perception of these changes instructs further action. Our habitual movements are regulated by a vast repertoire of automatic movement routines learned during development. These are orchestrated by a complex array of cortical mechanisms that proceed from an image of an intended action to its execution. Electrical recordings of brain cell activities and imaging studies reveal some of building blocks of this movement control---a collaborating hierarchy that extends from cortex to subcortical, brainstem, and spinal cord mechanisms. Specific brain lesions can impair different aspects of the sequences involved in planning, instruction generation, and execution of movement. One model suggests that the generation and refinement of movement utilize a Darwin Machine mechanism in which more successful sets of instructions are duplicated at the expense of less useful ones.

The reasons that an array of nerve cells might have for firing in the supplementary motor or premotor cortex are much more numerous than those that affect an array of cells in the visual cortex. The visual cells, even though they are subject to feedback from higher centers, have the fairly definite job of analyzing information about visual sensory stimuli, whereas the motor cells may participate in the resolution of many decisions about why and how a reaction to a particular visual stimulus is taking place. Goals, memories, and intentions have more influence on actions than on perceptions. Further, as we consider the constant feedback between action and perception that links our brain, body, and world together, yet another set of actors must be integrated into our description: the array of emotions that provide rapid response systems and that also bias and pattern many of our behaviors. These emotions are the subject of the next chapter.

Questions for Thought

1. The first section of this chapter reexamines the idea of an extended definition of mind, first mentioned in Chapter 1, a definition encompassing body and environment. This can be contrasted with a definition that restricts the idea of mind to the activities of the brain. What are the merits of these two approaches? Which do you think is more useful?

2. This chapter gives several examples of our ability to carry out several sensory-motor sequences, or kinetic melodies, at the same time. Some we are conscious of, some not. Please continue the thinking about selves that we started in Chapter 7 and suggest how you might incorporate such information into your idea of what a "self" is.

3. Electrophysiological experiments have shown that nerve cell firing in preparation for a movement can begin slightly before our conscious awareness of intending to make that movement. How can you reconcile such information with the assumption that consciousness plays a role in controlling our actions?

4. Can you think of experiments that might test the Darwin Machine model that Calvin has suggested underlies complex action sequences (including sequences of thought)? For example, could the complex sequence of speaking language provide a context for testing this idea?

Suggestions for Further General Reading

Clark, A. 1997. Being There. Cambridge, MA: M.I.T. Press. An extended discussion of how mind must be defined in terms of an acting body and its interactions with the external environment.

Damasio, A.R. 1994. Descartes' Error---Emotion, Reason, and the Human Brain. New York: Putnam's. The first section of this chapter draws on Damasio's Chapter 10, "The Body-Minded Brain."

Minsky, M. 1986. The Society of Mind. New York: Simon & Schuster. Minsky's book has interesting descriptions of the sorts of agents that are necessary to underlie and support action: hierarchies of stable subassemblies.

Calvin, W.H. 1990. The Cerebral Symphony. New York: Bantam. The material on Darwin Machine models of movement is drawn from Chapters 10 and 11 of Calvin's book.

\eoc\Reading on More Advanced or Specialized Topics

Crammond, D. 1997. Motor imagery: Never in your wildest dream. Trends in Neuroscience 20:54--57. This article describes experiments that monitor brain activity during the imagining of movement as well during its actual execution.

Gazzaniga, M.S., ed. 1995. The Cognitive Neurosciences. Cambridge, MA: M.I.T. Press. Part IV of this volume is devoted to motor control.

Bear, M., Connors, B., & Paradiso, M. 1996. Neuroscience: Exploring the Brain. Baltimore, MD: Williams & Wilkins. Chapters 13 and 14 of this book review spinal cord and brain control of movement.

Kosslyn, S.M., & Koenig, O. 1992. Wet Mind---The New Cognitive Neuroscience. New York: Free Press. Chapter 7 discusses brain control of movement.

1. Damasio, 1994, refers to these flickerings as "somatic markers.'

2. Luria, 1973, Ch. 8, 9.

3. Mind described in terms of its embodiment is central to the theories of Damasio, 1994; Lakoff and Johnson, 1980; Johnson, 1987; Varela et al, 1992;and Edelman, 1992. The first section of this chapter is drawn largely from Ch. 10 of Damasio.

4. These points are made by Clark, 1997.

5. Cotterill, 1995.

6. Richie, 1936.

7. Calvin, W.H., 1990.

8. See Clark, 1997, Chapter 2, for a discussion of mind as controller versus mind as mirror.

9. Luria, 1973, coined the term "kinetic melodies." Minsky's book, The Society of Mind , 1986, has interesting descriptions of the sorts of agents that are required to underlie and support action, hierarchies of stable subassemblies. A central idea in the architecture of complexity is that stable subassemblies evolved because the failure rate of a system built from such assemblies is lower (Simon, 1969).

10. Jeannerod and Decety, 1995. See Jeannerod, 1994, for an extended discussion of motor representations, with peer commentary.

11. Crammond, 1997.

12. Feldenkrais, 1972

13. There is debate on how much ongoing sensory information is used by stereotyped motor programs, described by Pennisi, 1996.

14. Discussions of motor control can be found in Kandel et al, 1991, and Bear et al. 1996. A section of the volume edited by Gazzinaga, 1995, is on motor systems. Current Opinion in Neurobiology devotes one issue each year to reviews of neural control.

15. Neuronal activity reflecting movement intention is found in the posterior parietal cortex (Snyder et al, 1997; Carpenter, 1997.) This appears to be the main area in which the interface between "sensory" and "motor" processing occurs.

16. Kosslyn and Koenig, 1992, Ch. 7. See also Roland and Zilles, 1996, for functions and structures of the motor cortices in humans.

17. Wolf et al, 1996.

18. Gao et al., 1996.

19. Raymond et al., 1996, review evidence for the cerebellum as a neuronal learning machine.

20. Aiba et al., 1994.

21. Welsh et al., 1995

22. See Fiez, 1996; Thach, 1996; Middleton and Strick, 1998; for reviews of evidence for involvement of the cerebellum in cognition. Schmahmann and Sherman, 1998, characterize cognitive impairment associated with cerebellar lesions.

23. Reviews of adaptive nerve networks regulating movement are provided by Katz, 1996, and Morton and Chiel, 1994.

24. Graziano and Gross, 1998, describe multiple coordinate systems used to guide movement, each attached to a different part of the body.

25. Schwartz, A.B., 1994. Carpenter et al., 1999, show that neurons in the motor cortex not only code movment but also help recognize and remember the sequence of events in time, as a prelude to movement.

26. Barinaga, 1995.

27. For discussion of brain systems in monkeys and humans that are active both in carrying out actions and in observing them in others see Rizzolatti and Arbib, 1998; Hari et al, 1998; Decety and Grezes, 1999. For a discussion of how these systems might underlie a self/other system, or self consciousness, see Georgieff and Jeannerod, 1998.

28. The material concerning Darwin Machines for movement is taken from Calvin, 1990, Chapters 10 and 11

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