The Contribution of Intracortical Connections to Horizontal Spread of Activity in the Neocortex as Revealed by Voltage Sensitive Dyes and a Fast Optical Recording Method

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Abstract
Coronal slices from guinea-pig visual neocortex were stained with voltage-sensitive fluorescence dyes RH414 or RH795. Activity was evoked by electrical stimulation of either the white matter or layer I. Emitted light intensity changes representing summated changes of membrane potential were recorded by a 10 x 10 photodiode array with a temporal resolution of 0.4 ms and a spatial resolution of 94 microns. The distribution and spread of activity in the horizontal direction was analysed. Following stimulation of the white matter or layer I, two regions of activity were differentiated in the medio-lateral direction: a central region (approximately 1 mm wide) of high-amplitude activity close to the stimulation electrode and, distant from the stimulation electrode, peripheral regions of low-amplitude activity. Central and peripheral regions differed in their rates of decline, their relative extent with stimulation of different sites and within different layers. The total extent of non-synaptic evoked activity did not exceed that of the central region of high-amplitude activity. Along the extent of non-synaptic activity, onset latencies of potentials were almost constant. Thus, activity of high amplitude in the central region was likely mediated by simultaneous 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 direct afferent activation but via long-distance intracortical horizontal pathways. These pathways are known to terminate in layer III, and accordingly latencies of responses in the periphery were shortest in upper cortical layers, whereas in the central region, latencies increased from lower to upper cortical layers.