LOCAL INTRACORTICAL CONNECTIONS IN THE CATS VISUAL-CORTEX - POSTNATAL-DEVELOPMENT AND PLASTICITY

1. We have studied the nature, development, and plasticity of local in tracortical interactions by examining the visual responses from pairs of cells in the visual cortex of anesthetized and paralyzed adult cats and kittens at postnatal age 4 wk. Simultaneous discharge from nearby cells was analyzed by the cross-correlation method to infer three typ es of neuronal interactions: monosynaptic excitation, monosynaptic inh ibition, and polysynaptic excitation. 2. Among cell pairs that exhibit correlated discharge, the nature of these interactions is similar in kittens and adults. For example, the mean monosynaptic delay is the sa me for cell pairs in the adult cat and the kitten: 0.8 ms. The primary difference in the distribution of neural interactions is the increase d prevalence of inhibitory interactions among adult cat cell pairs com pared with kitten cell pairs. However, in both age groups and in nearl y all laminae, polysynaptic interactions are the most commonly observe d type of interaction. 3. Neural interactions revealed by cross-correl ation analysis also were studied with respect to the receptive field p roperties of the cells. The hierarchical theory proposed by Hubel and Wiesel suggests that monosynaptic excitation should be seen from simpl e to complex cells. It further suggests that complex cells do not prov ide monosynaptic excitation either to simple or to other complex cells . However, we find no cases of monosynaptic excitation from simple to complex cells in the adult cat. Moreover, we find explicitly antihiera rchical connections, i.e., excitatory connections from complex to simp le cells, in both kittens and adults. The excitatory influence of comp lex cells on simple cell receptive field properties is not incorporate d into current models of receptive field structure formation. Our resu lts suggest that complex cells might have an important modulatory role in simple cell discharge. 4. Plasticity of intracortical synaptic con nections peaks with cross-correlograms. We examined Hebb's seminal hyp othesis regarding synaptic plasticity that the excitatory connections between two neurons should strengthen with simultaneous activity. To t est directly whether Hebbian plasticity can be rapidly induced by stim ulus-evoked activity, the strength of monosynaptic excitation between pairs of neurons was monitored during periods of visual stimulation la sting less than or equal to s2 h. Synaptic strength was computed by su mming the cross-correlogram bins containing the monosynaptic peak. Tra nsient increases in monosynaptic peak area are found among cell pairs linked by monosynaptic excitation after 8-15 min of visual stimulation . The duration and amplitude of these changes are similar in adult cat s and kittens. In about half of the cell pairs showing such transients , the increases in correlated discharge are not related to variations in the individual discharge rates of the two cells. 5. Changes in the amount of correlated discharge associated with monosynaptic excitation imply that the actual efficacy of the connections between cortical ce lls can increase during the course of visual stimulation in both adult cats and kittens. However, no sustained increases in correlated disch arge, as would be predicted a strictly Hebbian model of plasticity, ar e observed. Furthermore, there are no obvious differences between kitt ens, for which plasticity should be relatively high, and adult cats. T hese findings suggest that lasting Hebbian changes in synaptic efficac y may depend on additional inputs, such as attention-based modulation, that were not present in our preparation. It is also possible that th e time course required to cause clear alterations is considerably long er than what we were able to employ with the techniques this study.