SPATIOTEMPORAL SUMMATION OF SYNAPTIC ACTIVITY IN VISUAL CORTICAL PYRAMIDAL CELLS IN-VITRO

We investigated the spatio-temporal summation of excitatory postsynapt ic potentials (EPSPs) in supragranular pyramidal cells of the rat extr astriate visual cortex. EPSPs were evoked orthodromically from differe nt locations within the white matter (WM) via an 8-fold multi-electrod e array. Stimuli were applied either sequentially from electrodes 1 to 8 or vice versa at defined interstimulus intervals (ISIs) or separate ly from each electrode. Maximum EPSP amplitudes were evoked from the W M just below the intracellularly recorded neuron. Even 800 mu m latera l to this location, small EPSPs could be elicited. A sequential stimul ation resulted in a large compound EPSP. In 79% (n = 34) of the cells tested, the compound response was non-directional and could be predict ed from responses evoked by single stimulation electrodes. However, 21 % (n = 9) of the neurons showed a non-linear spatial summation and a c lear preference for the direction of the stimulation sequence. ISI-tun ing curves revealed either a sharply tuned, a bandpass, a highpass or a lowpass characteristic for the non-directional as well as directiona l cells. This feature, together with the clear directional responses o bserved in some neurons, may be a correlate of the response preference to moving stimuli of cortical cells found in vivo.