Complex Behaviors Hard-Wired Into Primate Brains

Up until now, neuroscientists had assumed that the innate actions (hard-wiring) in primate brains were limited to simple movements. They believed that complex behaviors were all learned. But new studies are finding that a number of surprisingly complex behaviors appear to be hard-wired rather than learnt. Generally, 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.

The researchers, writing in the Proceedings of the National Academy of Sciences, report that they can elicit these complex behaviors by stimulating specific areas in the brain of a bush baby (Otolemur garnetti). Their results provide significant new support for the proposition that all primate brains, including humans, contain such an in-built repertoire of complex behaviors.

“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 lead researcher at Vanderbilt University. “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 added.

In the past, neuroscientists believed that an area called the primary motor cortex was responsible for innate behavior. When stimulated, this area triggered simple muscle movements. The fact that they were able to produce motions by only single muscles reinforced the idea that only simple movements were hard-wired into the 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 suggested that longer pulses might stimulate more complicated motions. Working with 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 decided to follow the Princeton researchers’ lead and try long-duration stimuli in the simpler brain of the bush baby. 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.

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