HEBBIAN-LIKE FUNCTIONAL PLASTICITY IN THE AUDITORY-CORTEX OF THE BEHAVING MONKEY

In this study, the necessary conditions, including those related to be havior, for lasting modifications to occur in correlated activity ('fu nctional plasticity') were examined in the behaving monkey. Previously , in-vitro studies of neuronal plasticity yielded important informatio n about possible mechanisms of synaptic plasticity, but could not be u sed to test their functionality in the intact, behaving brain. In-vivo studies usually focused on analysis of the responsiveness of single c ells, but did not examine interactions between pairs of neurons. In th is study, we combined the two approaches. This was achieved by recordi ng extracellularly and simultaneously the spike activity of several si ngle cells in the auditory cortex of the behaving monkey. The efficacy of neuronal interactions was estimated by measuring the correlation b etween firing times of pairs of single neurons. Using acoustic stimuli , a version of cellular conditioning was applied when the monkey perfo rmed an auditory discrimination task and when it did not. We found tha t: (i) functional plasticity is a function of the change in correlatio n, and not of the correlation or covariance per se, and (ii) functiona l plasticity depends critically on behavior. During behavior, an incre ase in the correlation caused a short-lasting strengthening of the neu ronal coupling efficacy, and a decrease caused a short-lasting weakeni ng. These findings indicate that neuronal plasticity in the auditory c ortex obeys a version of Hebb's associative rule under strong behavior al control, as predicted by Thorndike's ''Law of Effect''. (C) 1998 Pu blished by Elsevier Science Ltd. All rights reserved.