Intracortical transmission of specific evoked activity across the cerebral cortex.

Abstract

The intracortical transmission of abnormally generated activity, like spreading depression and the direct cortical response, across the cortex is well known. In the experiments described here, the tangential transmission of a more normal type of cortical activity, the short latency response evoked by contralateral forepaw stimulation, was investigated. No evidence for tangential transmission was found from surface recordings. Various methods, such as passing polarizing currents, intravenous injection and topical application of strychnine, were used to alter the state of cortical excitation in order to induce a change in tangential transmission; none was found. The boundaries of the contralateral forepaw receiving area (CRA) appeared to be rigid, since the responses could not be induced beyond the perimeter by these manipulations of the cortex. Evoked responses in the centre of the CRA were shown to be unaffected by the excitation state of the cortex 2 mm or more away. The data support the hypothesis that the neural activity represented by the short latency surface evoked response is not transmitted laterally across the cortex and does not appear to be influenced by activity, beyond a range of about 2 mm or possibly less. The effectiveness of the methods used to alter cortical excitation, and to detect a spread of the evoked activity are discussed.