Plasticity, Experience and Resource Allocation in Motor Cortex and Hypothalamus

Summary

Training kittens to avoid an “unsafe” visual stimulus by flexing one forearm has major effects on visual and somatic cortex adult organization (4). Here we show that two important interfaces to the world, motor cortex and the hypothalamus, are similarly affected. Punctuate stimulation of the motor cortex reveals a four-fold increase in the diameter of the area allocated to the control movement of the trained forearm relative to the untrained one. Some of the motor responses in the animals resembled elementary movements in comprising correct responses during training. Cellular responses in the hypothalamus showed a shift toward cells with selectivity for the trained forearm; some of these cells showed the additional characteristic of selective responsivity to the visual stimuli used in training. It appears that the partitioning of the motor cortex and the repertoire of responses available to it are substantially influenced by early experience. The access that sensory stimuli have to the hypothalamus is also modified, possibly changing the way in which the adult will later cope with demanding tasks. The results dramatically demonstrate that simple early experience exerts widespread effect on structures which will later prove critical in setting the limits of individual potential.

We have recently demonstrated that if an unusually large amount of information is presented to a patch of skin on one side of the body during development, a reallocation of resources takes place in somatosensory cortex such that the cortical area allocated to that patch of skin becomes many times larger than normal.

We have also shown that in this cortical area dendritic branching (1) and bundling (2) are considerably greater than in the corresponding area in the contralateral hemisphere where the untrained forearm is represented.

The procedure we used to bring about these changes consisted of a simple avoidance procedure in which a “safe” or an “unsafe” visual stimuli was presented to a kitten through goggles and the kitten was required to flex one of the forelegs to the “unsafe” stimulus or receive a mild shock on that foreleg. Such a discrete response was chosen to facilitate investigating the effect of the training on motor cortex to determine if a reallocation similar to that in somatosensory cortex had occurred. Penfield and others (3) have shown that parts of the body which have greater sensory and motor sophistication have larger cortical representations in general.

We show in this report that early experience has a significant effect on the final size of the representation areas within the motor cortex. Further, while the brain interacts with the outside world through the motor system, it exercises control over the internal milieu through an interface which is largely contained within the hypothalamus. We also show in this report that in the hypothalamus polymodal responses to visual and somatic stimuli have been modified in accordance to the stimuli used during training.