Spatio-temporal summation of synaptic activity in visual cortical pyramidal cells in vitro

We investigated the spatio-temporal summation of excitatory postsynaptic potentials (EPSPs) in supragranular pyramidal cells of the rat extrastriate visual cortex. EPSPs were evoked orthodromically from different locations within the white matter (WM) via an 8-fold multi-electrode array. Stimuli were applied either sequentially from electrodes 1 to 8 or vice versa at defined interstimulus intervals (ISIs) or separately from each electrode. Maximum EPSP amplitudes were evoked from the WM just below the intracellularly recorded neuron. Even 800 microns lateral to this location, small EPSPs could be elicited. A sequential stimulation resulted in a large compound EPSP. In 79% (n = 34) of the cells tested, the compound response was non-directional and could be predicted from responses evoked by single stimulation electrodes. However, 21% (n = 9) of the neurons showed a non-linear spatial summation and a clear preference for the direction of the stimulation sequence. ISI-tuning curves revealed either a sharply tuned, a bandpass, a highpass or a lowpass characteristic for the non-directional as well as directional cells. This feature, together with the clear directional responses observed in some neurons, may be a correlate of the response preference to moving stimuli of cortical cells found in vivo.