PLASTICITY FOLLOWING NEONATAL VISUAL-CORTEX DAMAGE IN CATS

We have used the cat visual system as a model system to investigate ho w remaining areas of the brain are able to take over functions that ar e lost following brain damage and why neonates show better behavioral recovery than adults. Anatomical studies with both anterograde and ret rograde tracing methods reveal an increased projection from retina thr ough thalamus to the posteromedial lateral suprasylvian (PMLS) extrast riate visual area of cortex in the damaged hemisphere of cats with a n eonatal visual cortex (areas 17, 18, and 19; VC) lesion. No such enhan ced projection is seen after an adult lesion. In addition, single-cell neurophysiological studies indicate that physiological compensation i s present in PMLS cortex after a neonatal VC lesion but not after an a dult lesion. The physiological compensation replaces (or maintains) pr operties that are characteristic of PMLS neurons; there is little or n o improvement to replace the superior spatial properties of striate co rtex (or areas 18 or 19) neurons that were lost. Immunohistochemical s tudies of the possible roles of neuronal growth factors in the compens ation indicate that low- and high-affinity receptors are present that would allow several neurotrophins to influence the normal retina throu ghout life. Furthermore, these receptors are upregulated transneuronal ly following neonatal VC damage and thus could play a role in lesion-i nduced changes in the retina and its central projections. Ongoing stud ies are continuing to examine the presence of neurotrophins and their receptors in the retina and brain during normal development and after VC damage. In addition, studies of the effects of administering neuron al growth factors are underway to determine whether compensation for V C damage can be improved in neonates or even be produced in adults.