THE CONTRIBUTION OF INTRACORTICAL CONNECTIONS TO HORIZONTAL SPREAD OFACTIVITY IN THE NEOCORTEX AS REVEALED BY VOLTAGE-SENSITIVE DYES AND AFAST OPTICAL-RECORDING METHOD

Coronal slices from guinea-pig visual neocortex were stained with volt age-sensitive fluorescence dyes RH414 or RH795. Activity was evoked by electrical stimulation of either the white matter or layer 1. Emitted light intensity changes representing summated changes of membrane pot ential were recorded by a 10 x 10 photodiode array with a temporal res olution of 0.4 ms and a spatial resolution of 94 mum. The distribution and spread of activity in the horizontal direction was analysed. Foll owing stimulation of the white matter or layer I, two regions of activ ity were differentiated in the medio-lateral direction: a central regi on (approximately 1 mm wide) of high-amplitude activity close to the s timulation electrode and, distant from the stimulation electrode, peri pheral regions of low-amplitude activity. Central and peripheral regio ns differed in their rates of decline, their relative extent with stim ulation of different sites and within different layers. The total exte nt of non-synaptic evoked activity did not exceed that of the central region of high-amplitude activity. Along the extent of non-synaptic ac tivity, onset latencies of potentials were almost constant. Thus, acti vity of high amplitude in the central region was likely mediated by si multaneous activation of distributed afferent fibres. In contrast, no non-synaptic activity was found in peripheral regions. Therefore it is suggested that this low-amplitude activity was mediated without direc t afferent activation but via long-distance intracortical horizontal p athways. These pathways are known to terminate in layer III, and accor dingly latencies of responses in the periphery were shortest in upper cortical layers, whereas in the central region, latencies increased fr om lower to upper cortical layers.