God Soul Mind Brain

to the motion of an object looming at the monkey, invading its personal space. Some cells were sensitive to touch on the monkey’s fur, or to sounds nearby in the room. STP was definitely not exclusively visual.

    Among the strange cells they found in STP, one type of neuron became active when the monkey looked at a person walking. The cells were sensitive to “biological motion.” They were prowl cells. A person wheeled past on a cart was no good; nor was a person walking in a baggy gown. A specific biological motion of limbs triggered this type of neuron. Other biological motion cells seemed to be sensitive to other kinds of limb movements.

    Our ability to recognize biological motion is another example of an overlooked skill. We are so good at it, and it comes so automatically, that we take it for granted. But we are good at it because, like monkeys, we have an elaborate brain mechanism dedicated to the task. The importance of biological motion was demonstrated particularly beautifully by Johansson. He fixed small lights on the limbs and bodies of dancers, put the dancers in a dark room, and filmed them walking, running, dancing, riding bicycles, and so on. He then showed the films to other people. The result was sensational. There is something about the biological prowl of human limbs that we instantly recognize. On the basis of a few points of light wiggling around a screen, any normal person can say, “That’s easy, that’s a person running,” or, “That’s a person riding a bicycle,” or even, “That’s two people dancing, swirling around each other.” But if the film is played upside down, the response is likely to be, “That’s a bunch of lights jiggling randomly.” Our detectors for biological motion are evidently so expert that a few points of light, moving in the right way, evokes in our minds an instantaneous perception of a body. The gestures, the intent, the meaning of the movements all appear obvious.

    One can guess that this machinery evolved early and is probably shared by a great range of species. For example, an antelope sees a lion. By the exact prowl of the lion’s limb, by its body language, the antelope can sense whether the lion is hunting or just passing by. Even in this simple example, the antelope is computing something about the inner state of the lion in order to predict its behavior. Perceiving biological motion is fundamentally about constructing a perceptual model of another intentional agent.

    Another socially relevant type of neuron in area STP, the gaze neuron, was discovered by Perrett and his colleagues. (Perrett is known to give scientific lectures while wearing neon hair and a superhero outfit.) Gaze cells are sensitive to the direction of someone else’s gaze. If you show a picture of a pair of eyes to a monkey, the gaze neurons will become active depending on whether those eyes are looking to the right or left. Some gaze neurons signal one direction of gaze, some signal another direction. Monitoring someone else’s gaze is at the heart of social perception. It allows us to guess at whether someone is listening to us, noticing us, attending to that other person or that passing car, aware of this or that object. If the eyes are the windows to the soul, then a neuronal machine for monitoring gaze direction goes a long way toward computing a model of someone else’s soul.

    Some of the other properties of STP, less thoroughly studied, may also play into the same story of social perception. For example, many neurons in STP respond to an object looming in toward the body, invading the monkey’s personal space. Nothing works so well to activate these cells as a person looming toward the monkey’s face.

    Personal space plays a large role in the social behavior of monkeys and humans. In the 1950s, Hediger, a naturalist and zoo curator, studied personal space by observing the behavior of a range of animal species. In the 1960s, Hall extended the work to human interaction. Hall observed that humans are exquisitely sensitive to interpersonal spacing. We maintain a margin of safety around us (the size of a comfortable margin of safety varies from culture to culture) and we judge other people’s mood and aggression partly by how they respect or violate that margin of safety. The looming-sensitive neurons discovered in STP could plausibly be part of the underlying mechanism for assessing personal space.

    The brain’s special sensitivity to objects looming into