John Byers started thinking about the brain and play almost by accident. A zoologist at the University of Idaho, Byers had spent years studying the playful antics of deer, pronghorn antelopes and the wild mountain goats called ibex. He knew that play was risky — he had observed ibex kids falling off steep cliffs as they romped — and at first he thought maybe the animals were taking such risks because the motor training helped them get in physical shape for adulthood. But something about this idea troubled him. Play can be exercise, he reasoned, but it was of too short duration to lead to long-term fitness or build muscle tone.
Byers preferred an alternate theory. In almost every species studied, a graph of playfulness looked like an inverted U, increasing during the juvenile period and then falling off around puberty, after which time most animals don’t play much anymore. One winter afternoon in 1993, Byers was roaming the stacks at the University of Idaho library, flipping through books the way you do when you’re not quite sure what you’re looking for. One book contained a graph of the growth curve of one important region of the brain, the cerebellum, over the juvenile period in the mouse. The growth curve of the mouse cerebellum was nearly identical to the curve of mouse playfulness.
‘‘It was like a light went on in my head,’’ Byers told me from Washington, D.C., where he is temporarily working at the National Science Foundation. ‘‘I wasn’t thinking specifically about play, but I sort of had a long-term interest in behavioral development.’’ And there it was: a chart that made it look as if rates of play in mice synchronized almost perfectly with growth rates in one critical region of the brain, the area that coordinates movements originating in other parts of the brain.
Intrigued, Byers enlisted the help of a graduate student, Curt Walker, who looked through the scientific literature on cerebellum development in rats and cats. ‘‘Then we compared those rates to what was known about the rates of play in those species,’’ Byers said. ‘‘And rats and cats showed the same relationship as mice: a match between when they were playing and when the cerebellum was growing.’’
The synchrony suggested a few things to Byers: that play might be related to growth of the cerebellum, since they both peak at about the same time; that there is a sensitive period in brain growth, during which time it’s important for an animal to get the brain-growth stimulation of play; and that the cerebellum needs the whole-body movements of play to achieve its ultimate configuration.
This opened up new lines of research, as neuroscientists tried to pinpoint just where in the brain play had its most prominent effects — which gets to the heart of the question of what might be lost when children do not get enough play. Most of this work has been done in rats. Sergio Pellis, a neuroscientist at the University of Lethbridge in Alberta, Canada, is one of these investigators. He studies how brain damage in rats affects play behavior, and whether the relationship works in reverse: that is, not only whether brain-damaged rats play abnormally but also whether play-deprived rats develop abnormalities in their brains. Pellis’s research indicates that the relationship might indeed work in both directions. In a set of experiments conducted last year, Pellis and his colleagues raised 12 female rats from the time they were weaned until puberty under one of two conditions. In the control group, each rat was caged with three other female juveniles. In the experimental group, each rat was caged with three female adults. Pellis knew from previous studies that the rats caged with adults would not play, since adult rats rarely play with juveniles, even their own offspring. They would get all the other normal social experiences the control rats had — grooming, nuzzling, touching, sniffing — but they would not get play. His hypothesis was that the brains in the experimental rats would reflect their play-deprived youth, especially in the region known as the prefrontal cortex.
At puberty the rats were euthanized so the scientists could look at their brains. What Pellis and his collaborators found was the first direct evidence of a neurological effect of play deprivation. In the experimental group — the rats raised in a play-deprived environment — they found a more immature pattern of neuronal connections in the medial prefrontal cortex. (This is distant from the cerebellum; it is part of the cerebrum, which constitutes the bulk of the mammalian brain.) Rats, like other mammals, are born with an overabundance of cortical brain cells; as the animal matures, feedback from the environment leads to the pruning and selective elimination of these excess cells, branchings and connections. Play is thought to be one of the environmental influences that help in the pruning — and, this research showed, play deprivation interferes with it.
Figuring out what these findings mean in terms of function involves a certain amount of conjecture. Pellis interprets his observation of a more tangled, immature medial prefrontal cortex in play-deprived rats to mean that the rat will be less able to make subtle adjustments to the social world. But maybe the necessary pruning can happen later in life, through other feedback mechanisms having little to do with play. Maybe there were already compensatory changes happening elsewhere in the brains of these young rats where no one had thought to look. Current research in Pellis’s lab, in which the brain is damaged first and the rat’s playing ability is measured afterward, seems to confirm that the medial prefrontal cortex has an important role in play. But the exact nature of its action is still not clear.
Check back next week for the continuation of this great article on "Play"