THE BRAIN FROM TOP TO BOTTOM

Making a Voluntary Movement

When you cross a street, you unconsciously modulate the length of your last few strides as you reach the other side, so that when you get to the curb, you are just the right distance to step up onto the sidewalk easily.

Stability in the basic motor pattern for walking is a necessary condition for the expression of such modulations, whether they are reflexive or cognitive. Patients with neurological disorders that disturb the stability of their stride have much more trouble in making such adjustments to their environment.

Reaction time is the delay between the time that a signal is given and the time that an action is taken in response to it. We see fast reaction times, for example, when a hockey goalie blocks a sizzling slapshot, or when someone catches a vase as it is falling off the table. We often say such people have "good reflexes", but that description isn't really accurate. A reflex is an involuntary, stereotypical reaction, whereas these responses by people with very short reaction times are voluntary movements adapted to particular situations.

Through training, people can reduce their reaction times by teaching their nervous systems to anticipate such things as the possible path of a hockey puck or a baseball. But this improvement in reaction time for a specific task cannot be generalized to all activities. For instance, if a ball rolls out in front of a car, the time that it takes the driver to hit the brakes will be just the same (about one second) regardless of whether that person is a star goalie or someone who is not athletic at all. Also, some substances, such as alcohol, increase reaction time in all human beings, whereas others, such as caffeine, reduce it slightly.

MENTAL IMAGING OF BODY MOVEMENTS

The brain's first and foremost function is to enable your organism to act in its environment so as to ensure its survival. But this environment is constantly changing, and your actions must constantly adapt to it. To learn a new action, you must repeat it a certain number of times to encode it in your procedural memory. This repetition is what is commonly called training or practicing.

But there is another way to improve your execution of a movement once you have memorized the general set of gestures involved, and that is by simply rehearsing the movement mentally. This process, known as "mental imaging", is commonly used by top athletes to try to shave those precious fractions of a second off their times. Downhill skiers, for example, mentally rehearse their entire path down the race course before they start their descent. They see every turn, feel their body pass over every bump, and mentally perform all the manoeuvres required to race their way down the hill.

People who thus imagine themselves performing a movement activate the same representations in their brains as they would when actually planning and controlling this action.The results of many experiments strongly indicate that when we imagine an action and when we actually perform it, the areas of the brain involved are quite similar.

This phenomenon whereby imagining an action activates the same brain areas as actually performing it may even apply to watching an action being performed by someone else. In other words, simply watching someone perform an action increases the activity in those parts of your brain that would normally be activated when you were performing this action yourself. The discovery of mirror neurons in the mid-1990s provided a cellular basis for this phenomenon. These neurons are activated both when you perform an action and when you see someone else performing it.

For example, a tennis fan who closely watches the actions of a champion player during a tennis match experiences brain stimuli that perfectly match that player's muscle activity. The next time the fan goes to play tennis, he or she will find it easier to recall and replicate these movements.

We also know that when you imagine yourself performing an action, it is your premotor cortex that is especially active. When you imagine someone else performing this same action, the activity in your premotor cortex is accompanied by activity in your right parietal cortex—an area that has been specifically found to play a role in distinguishing between the self and others.

The attribution of the origin of the action to yourself or to someone else might thus undergo specific alterations that could contribute to disturbances in self-recognition such as those seen in schizophrenia, for example.

There are two possible perspectives that you can adopt in mental imaging. From the outside or external perspective, you imagine something as if you were watching it on a video. Hence this perspective is intrinsically more visual. In contrast, from the first-person or internal perspective, you imagine events as if you were there yourself and seeing them through your own eyes. This latter form of imaging involves more kinesthetic sensations (sensations inside your muscles, joints, and tendons).

In fact, to be effective, mental imaging must incorporate the appropriate sensory traits for the action you are trying to perfect. A basketball player who is imagining herself dribbling will try to visualize the ball, her teammates, and her opponents. She will try to hear the ball hitting the backboard and feel her feet pounding the boards. Athletes who participate in more individual events, such as diving and weightlifting, must try to imagine all the physical and kinesthetic sensations associated with those movements.