Comparison of the selectivity of postsynaptic potentials and spike responses in cat visual cortex

Intracellular recordings were made from neurons in the cat visual cortex (a rea 17) to compare the orientation and direction selectivities of the outpu t of a cell with those of the input the cell receives. The input to a cell was estimated from the PSPs (postsynaptic potentials) evoked by visual stim ulation, and the output estimated from the number of spikes generated durin g the same responses. For the whole sample, selectivity of the output of ce lls was significantly higher than selectivity of their input. Upon PSP to s pike transformation, the selectivity index was, on average, doubled. Howeve r, the degree of the selectivity improvement in individual cells was very d ifferent, varying from cases in which highly selective output was created f rom a poorly selective input and thus selectivity was greatly improved, to little or no improvement in other neurons. The improvement of selectivity w as not correlated with resting membrane potential, threshold for action pot ential generation, background discharge rate or amplitude of optimal PSP re sponse. Further, no systematic difference was found between simple and comp lex cells in the input-output relations, indicating that the 'tip of the ic eberg' effect on shaping the response selectivity was cell specific, but no t cell type specific. This supports the notion that multiple mechanisms are responsible for generation of the response selectivity, and that the contr ibution of any particular mechanism may vary from one cell to the other. Th e heterogeneity of the input-output relations in visual cortical cells coul d indicate different functions of cells in the cortical network; some cells are creating selectivity de novo, the function of other neurons probably b eing repetition and amplification of the selected signal and arrangement of the output of a whole column.