When you grab a piece of food and put it in your mouth, when you smile in response to the smile of a passerby or squint and grimace in anger, the complex pattern of movements that you make may be hard-wired into your brain. Scientists have long known that many of the behaviors of lower organisms are innate. In the insect world, for example, instinctive behaviors predominate. Birds have a larger repertoire of fixed behaviors than dogs.
In primates, voluntary or learned behavior predominates, so neuroscientists have assumed that the hard-wiring in primate brains is limited to simple movements and complex behaviors are all learned.
Now, however, studies are finding that a number of surprisingly complex behaviors appear to be built into the brains of primates as well.
These are “biologically significant” behaviors that appear likely to improve the primate’s ability to survive and reproduce. They include aggressive facial patterns, defensive forelimb movements, and hand-to-mouth and reaching-and-grasping movements.
Vanderbilt researchers, writing this week in the Proceedings of the National Academy of Sciences Online Early Edition, report that they can elicit these complex behaviors by stimulating specific areas in the brain of a small nocturnal primate called the Galago or bush baby (Otolemur garnetti). Their results provide significant new support for the proposition that all primate brains, including our own, contain such a repertoire of innate complex behaviors.
“We have now seen this feature in the brain of an Old World monkey and New World prosimian. The fact that it appears in the brains of two such divergent primates suggests that this form of organization evolved very early in the development of primates. That, in turn, suggests that it is characteristic of all primate brains, including the human brain,” says Jon Kaas, the head of the research group, Distinguished Professor of Psychology at Vanderbilt University and investigator at the Vanderbilt Kennedy Center for Research on Human Development.
“These results explain why certain behaviors – such as defensive and aggressive movements, smiling and grasping food – are so similar around the world. It is because the instructions for these movements are built-in and not learned,” he adds.
Over the last 20 years, neuroscientists have identified an area called the primary motor cortex, which, when stimulated, triggers simple muscle movements. The fact that they were able to produce only motions by single muscles and other simple movements reinforced the idea that only simple movements were hard-wired into primate brain circuitry.
Then, last year Michael Graziano at Princeton University pointed out that previous stimulation experiments in the motor cortex – the area that controls bodily motions – had been done using very short electrical pulses that last less than a half-second. He further suggested that longer pulses might stimulate more complicated motions. Working with alert macaques, he and his colleagues found that applying such long-duration signals did in fact elicit several of these complex behaviors, much as they had predicted.
Kaas and his colleagues, research assistant professor Iwona Stepniewska and doctoral student Pei-Chun Fang, decided to follow the Princeton researchers’ lead and try long-duration stimuli in the simpler brain of the Galago. When they did, they also found that this type of stimuli triggered complex behaviors. In fact, they were able to stimulate a larger number of complex movements than the Princeton group had reported, including aggressive facial patterns, defensive forelimb movements, and hand-to-mouth and reaching-and-grasping movements.
The Princeton researchers stimulated areas in the motor cortex. The Vanderbilt researchers found that they could also elicit these behaviors by stimulating a nearby area of the brain called the posterior parietal cortex. This area is heavily interconnected with the motor cortex and had previously been associated with transforming data from the eyes and other senses into a spatial map of the surrounding environment. The new findings reveal that this brain area also plays an important role in complex, innate behaviors.