by E. Paul Zehr
Related to the supplementary motor area, but even more upstream, is the premotor area. In the premotor area, more abstract features of movement than muscle selection and coordination are planned. For instance, preparation for a general direction of movement or response to a certain cue. So, if Batman is perched above an alleyway waiting to swoop down on Oswald Chesterfield Cobblepot (a.k.a. the Penguin) as the bird-brained bad guy emerges from a side door, Batman is really waiting for a cue. As soon as the door opens, activity in Batman’s premotor cortex will signal him to strike. Of course, in order for us to see these he would have to be OK with wearing the necessary scalp electrodes under his cowl. In some movements changes in electrical activity within the brain can be detected almost a second before movement occurs.
This premovement brain activity was originally discovered by German scientists Hans Kornhuber and Luder Deecke in 1965. They recorded electrical activity from the supplementary and premotor areas as people made voluntary finger flexion movements. This discovery was the first hard evidence of detectable brain activity that was directly related to voluntary movement. It was called a “readiness potential” (in German, Bereitschaftspotential) and has been studied a great deal ever since.
This activity is curious for two main reasons. Events in the nervous system operate on a millisecond (thousandths of a second) time-scale. So, a change in brain activity directly related to movement but happening before movement actually occurs is very puzzling. Studies have shown that these potentials occur not only before movement is detectable but also before a person is even aware that he or she will move! That means we could record these changes in Batman’s brain activity before he even started thinking about swooping down on the Penguin. This kind of observation has provided much study for scientists trying to investigate the issue of what “voluntary” really means and what “conscious will” is, but all this is a bit beyond our focus here.
If we now move even farther upstream, we come to a fork that goes in one direction to the cerebellum and in the other to the basal ganglia (have another look at Figure 7.2). Both of these VP advisors for movement provide support to the CEO for producing movement. The information that they convey and the kind of support they provide to the cortex are not the same. Generally we could say that the cerebellum is a bit more related to sensation and body movement itself, while the basal ganglia is a bit more related to internally generated movements.
When the Batbrain Doesn’t Function as It Should
We know a lot about how certain parts of the brain work from older research in clinical neurology. Through many years of observation, neurologists were able to piece together certain deficits in movement that some patients had and correlate those deficits with obvious damage to the brain (for example, from a gunshot wound) or upon dissection in autopsy.
For the cerebellum, sadly, gunshot wounds played a very important role in explaining just what this brain area does. The Irish neurologist Sir Gordon M. Holmes (1876–1965) was a pioneer in this work. During World War I, Holmes saw many soldiers with head injuries from gunshot, and this spurred on his study of the cerebellum and its functions related to balance and movement control. The cerebellum is heavily involved in assisting ongoing movement and coordination of the moving limbs and posture of the body. The cerebellum, although it takes up only about one-tenth of the volume of the brain, has more than 50% of the neurons.
Combining this with the dramatic suppressive effect that alcohol has on the function of the nervous system provides the underlying basis for the roadside “checkpoint” tests that the police will use when they pull over a driver under suspicion of driving while intoxicated. The tasks of standing upright with feet together and eyes closed, of reaching out and touching the tip of the nose with the index finger, and of walking a straight line all require cerebellar input and regulation. That is, the cerebellum in its VP role gives very important advice to the cortex in these tasks. The “sleepiness” of cerebellar neurons from alcohol consumption makes these kinds of tests very relevant for inferring impairment. Of course, any inferences require confirmation by breath or blood analysis. To relate this back to our analogy of motor control relationships in the brain, we could consider this an example of when the advisor provides very poor advice that may or may not be ignored by a truculent president!
The basal ganglia, the clumps of neurons at the base of the brain, are very important in helping to initiate or trigger movement. Following on from the description of cerebellar activity above, a main role for the basal ganglia can be best understood when they aren’t functioning properly. An exallent example of this is in the serious neurodegenerative disorder known as Parkinson’s disease. James Parkinson (1755–1824) was an English physician who wrote a book describing a “shaking palsy” based on his experiences with many of the patients he had seen in his career. The “shaking palsy” was so named because of the obvious tremor that Parkinson’s patients demonstrate even when no movement is attempted.
Other characteristics of Parkinson’s disease include slowness of movement and a difficulty in starting movements. Parkinson’s disease arises because of problems with the part of the complicated neural circuitry that keeps the basal ganglia functioning and that is supported by neurotransmitters. The cells that produce the neurotransmitter dopamine slowly die off. Because these cells function as a supply route within the basal ganglia, the overall function of the basal ganglia slowly degenerates and weakens. The upshot is that the basal ganglia eventually fail to provide the needed advice for voluntary movement and therefore movement becomes more and more difficult to perform. Sadly, this is often paired with unintentional and unwanted twitching movements.
Thinking of how badly things can go wrong when the advisory roles of the cerebellum and basal ganglia are disrupted speaks to how finely tuned the nervous system is in someone like Batman. Movements are called upon with great ease and produced according to desired specifications. Looking at Figure 7.2, it is easy to notice that the command for movement has to be relayed to the spinal cord—that is, to the level of the local district managers and agents in the field. It is at this level that the actual output command to make muscles become active occurs. The neurons at this level are really concerned with implementing the movements brought into action by the spinal cord motor neurons. These are the cells that actually command muscles to contract and that we discussed in Chapter 4. As such, all inputs eventually funnel (the actual physiological term is “converge”) onto the motor neurons, which Sherrington called “the final common pathway” for movement control. The neurons in the spinal cord are also charged with adapting the motor output based on what is occurring—that is, making use of feedback—during the movements that are already under way.
For example, lots of information about what your arms and legs are doing during movement comes from the movement of skin across joints, the stretch of your muscles, and the tension in your tendons. Together this information about the movement of the body parts is called “proprioception.” This term means perception of our body and our movements. It is contrasted with the perception or sensing of what is happening on the outside of our bodies.
It appears that the Batman writers (in the guise of the Penguin) know the importance of proprioception for motor control. In “Hidden Agendas” (Detective Comics #598–600, 1989), Oswald Cobblepot is shown using a fancy device—not grounded in any actual science, in case you are wondering—called a “synaptic field disruptor.” He sets it to “full blast” so he can incapacitate and control the bad guy goon Moraga. When I read this story I really wished that I had at least one piece of equipment in my own lab that had a full-blast setting, but no such luck. The best I could do was “maximum output.” In addition to telling the spinal cord and the supraspinal structures like the cerebellum and cerebral cortex about movement and the state of the arms and legs, sensory information carried in proprioceptive pathways can also be used to cause immediate and rapid corrections to movement.
At the level o
f spinal cord motor control, we have therefore entered the territory of what constitutes “reflex” control. This is the level of the agents in the field indicated back in Figure 7.2. The term “reflex” is actually a fairly problematic one. It seems like it should be very straightforward, though, since we hear expressions like “knee-jerk response” as a way to describe a predictable response to something or a “reflex reaction” used to describe a fast response in sports. And many Batman stories describe him as responding with “instant reflexes.” Can reflexes be instant? (No, they can’t, as we’ll explore in a bit.) What is a reflex really? If you pick up a dictionary, you might find that a reflex is defined as “an automatic or involuntary response to a stimulus.”
The earliest known reference that I have found (and, mind you, I have looked pretty hard!) is from David Hartley, who in 1748 described “reflex actions.” A reflex seems to be something that reflects or is a consequence of something else. Even before 1748, the great natural philosophers Jean Fernel (1497–1558) and René Descartes (1598–1650) described similar concepts without using the word “reflex.” My favorite definition of a reflex is something that is “best defined as responses evoked with great probability by particular stimuli,” which was articulated by Vernon Brooks in 1985.
So a reflex is something that can be fairly automatic and can happen very rapidly. Why is that? Reflexes are so fast because they typically operate in defined (yet also quite complex) terms around sets of neurons that provide positive or negative feedback (think back to Chapter 3). The basic outline of the negative feedback for reflexes is shown in Figure 7.3. The flowchart shows the idea that the stretching of muscles activates receptors in the muscle itself. These receptors send nerve impulses (action potentials) back to the spinal cord. In the spinal cord they activate the motoneurons that are in the same muscle. This means that the feedback from the stretch makes the muscle contract more. This might seem weird, but if you think it through, it is a good way to remove the stress. The stress is stretch of the muscle. If the muscle contracts and the body part (the foot in this case) moves, then the effect of the stretch is reduced. It really is negative feedback. That is what the big stop sign with the X in it indicates in Figure 7.3.
To give some more context, let’s think about a reflex pathway that you use every day. Have you ever misjudged the depth of a step while walking upstairs? Usually, your foot is placed far enough forward on a step so that when you put your weight onto that leg, your ankle doesn’t give and you are able to keep stepping with no interruption. Sometimes, however, we don’t get one of our feet far enough into the step when the weight shift onto that leg occurs. When that happens the ankle gets overstretched.
You have little spindle-shaped receptors (propriocepters, such as those the Penguin was trying to disrupt earlier) in your muscles. It is their job to tell the spinal cord and brain about the length of the muscles—from which the nervous system computes joint angle—and how fast changes in muscle length are occurring—from which the nervous system computes speed of movement. When activated by stretch, these muscle spindle receptors send a signal back to the motor neurons in the spinal cord causing them to strongly and rapidly increase contraction levels. This is cleverly termed the “stretch reflex,” by the way, and it is rapid because there are no other synaptic connections that need to be made. Not all reflexes work with this few synapses, but the idea is basically the same anyway, as is shown in Figure 7.4.
Figure 7.3. Feedback concept in reflexes. The X shows that the response of muscle contractions stops the stimulus.
Figure 7.4. Basic spinal reflex. Courtesy David Collins.
Returning to the example of overstretching because of foot misplacement on the stairs, one of two things would occur. You may have, in your lifetime, experienced both of them. First, the best-case scenario is that the stretch reflex increase in muscle activity at the ankle is large enough to help overcome the stretch, and you carry on stepping. Only after a few seconds have passed will you likely even recollect that something odd happened while you were going up the stairs. That is good; your reflexes did their job well. Second, the worst-case scenario is that the reflex correction is too weak or the stair possibly too slippery, and your foot slides right off the step. This likely would send you crashing forward and force you to either grab the railing (if there is one), or the higher steps (if you can), or to violently bash your shin into the next step (often this is the outcome).
Instant Reflexes
So, can you train reflexes? Well, more highly trained and physically fit people have shorter movement times. Movement time means (again, don’t be surprised by this) the overall time taken for a movement to occur. It is often used in the context of evaluating how fast someone can respond to something, for example, if you had to decide when to move to a target when given a “go” signal. Movement time is the elapsed time from the onset of the signal to when you actually complete the movement. It is composed of your reaction to the signal (your ability to detect when to move, your choice to move, and your neural command to your muscles) and your actual physical movement. With training, movement times can be much reduced. This is particularly true in martial arts where maximum speed (“ballistic”) movements are common.
This improvement is not largely due to a change in the reaction time. It is instead due to a faster physical movement. So, while the responses of Batman are very fast, his reflexes aren’t instant! However, some kinds of reflexes, like the muscle stretch ones we have been talking about, do become stronger and more reliable with training. Scientists would say that excitability in reflex pathways is increased. This again makes sense because the whole nervous system is trying to work together and reinforce movement. So, it would stand to reason that feedback would improve after more training. This is really the case for the sort of strength and power and martial arts training that Batman has to do.
Teaching an Old Bat New Tricks
Now that we have a general understanding of how Batman’s movements are produced, let’s explore how he actually learns new skilled movements. This is the process usually described as motor learning. To begin, take a minute to think about what it was like for you to learn how to do some special sport or movement skill. Maybe it was learning how to swing a golf club, to throw a spiral with a football, or to hit a baseball. What was it like while you were learning? What kind of mistakes did you make? Probably many! It was also probably very frustrating. While you were learning how to hit that ball, many changes were taking place in your body. These changes aren’t the ones related to your increasing frustration levels, by the way, but there is a link to those too. What I am talking about are actual changes in how your brain and spinal cord—the central nervous system, or CNS—were working to make your muscles do what needed to be done. Scientists who study the nervous system call these changes plasticity because it gets at the idea of “changeability.” When thinking of Batman we are really starting to ask how it is he learned the skills to become such a proficient martial artist and dark knight avenger. What changes actually occurred in the CNS of the Caped Crusader when he did all his training? Why do we get better at motor skills when we practice them over and over again?
To achieve the high level of crime-fighting skill that we all know Batman has attained, he would have had to move through three levels of motor learning: cognitive, associative, and automatic. Keeping the basic idea of progression that I described earlier, we will refer to our protagonist as Bruce in the cognitive stage, the Bat-Man in the associative stage, and Batman in the automatic stage. Details on these stages can be found in Figure 7.5. The first level, cognitive, is where someone is just learning how to perform a physical skill. Let’s take doing a front kick, for example. While in the cognitive level, Bruce Wayne would use information on how the skill should be performed to develop a motor plan, also called a motor program. This plan represents the basic organization of what needs to be accomplished to actually do the task. At this stage, a lot of conscious thought would go into th
e movements, and Bruce would have to pay close attention to the sequence of motions he must perform. For the front kick, that sequence of motions might be lifting the knee, pulling up the ankle, kicking by extending the knee, and then pulling the toes up and back while hitting the target.
Figure 7.5. Stages of Batman’s motor learning. Modified from Fitts and Posner (1967).
At this stage Bruce would keep in mind the big picture of the skill and be unable to deal with small details of the movement or cope with performing the movement in different settings—like when under attack, for instance. Bruce would likely not get that much specific feedback from his teachers at this stage. Henri Ducard, a main teacher of Bruce Wayne and one who was central to the “Blind Justice” story arc (Detective Comics #598–600, 1989), would know to let Bruce work on developing his motor programs for movement. Without establishing some kind of program for a task such as a front kick, it would be difficult in the next stage to refine that program.
The next stage is the associative one. Here, the Bat-Man would begin to concentrate on a more refined approach to his skills. Instead of just worrying about picking up his foot to do his kick, he would begin to do things like timing the movements within the kick and refining his motions. When to pull the toes up, when to extend the knee exactly, and how to time those actions most efficiently would be his main focus. The Bat-Man would be in this stage for a long time and would have a lot to gain from feedback. During his front kick practice, Ducard might urge the Bat-Man to quickly pick up his knee by pulling through his hip and trying to fling out his leg to strike a target. At this level, the Bat-Man would gradually learn to more efficiently perform the skills and deal with the environment in which he is training. The Bat-Man would not need to attend to every aspect of the performance at this stage and would be fine-tuning his skills.