LAMINAR DIFFERENCES IN THE SPATIOTEMPORAL STRUCTURE OF SIMPLE CELL RECEPTIVE-FIELDS IN CAT AREA-17

Previous studies of cat visual cortex have shown that the spatiotempor al (S-T) structure of simple cell receptive fields correlates with dir ection selectivity. However, great heterogeneity exists in the relatio nship and this has implications for models. Here we report a laminar b asis fbr some of the heterogeneity. S-T structure and direction select ivity were measured in 101 cells using stationary counterphasing and d rifting gratings, respectively. Two procedures were used to assess S-T structure and its relation to direction selectivity. In the first, th e S-T orientations of receptive fields were quantified by fitting resp onse temporal phase versus stimulus spatial phase data. In the second procedure, conventional linear predictions of direction selectivity we re computed from the amplitudes and phases of responses to stationary gratings. Extracellular recording locations were reconstructed histolo gically. Among direction-selective cells, S-T orientation was greatest in layer 4B and it correlated well (r = 0.76) with direction selectiv ity. In layer 6, S-T orientation was uniformly low, overlapping little with layer 4B, and it was not correlated with directional tuning. Lay er 4A was intermediate in S-T orientation and its relation (r = 0.46) to direction selectivity. The same laminar patterns were observed usin g conventional linear predictions. The patterns do not reflect laminar differences in direction selectivity since the layers were equivalent in directional tuning. We also evaluated a model of linear spatiotemp oral summation followed by a static nonlinear amplification (exponent model) to account for direction selectivity. The values of the exponen ts were estimated from differences between linearly predicted and actu al amplitude modulations to counterphasing gratings. Comparing these e xponents with another exponent-that required to obtain perfect matches between linearly predicted and measured directional tuning-indicates that an exponent model largely accounts for direction selectivity in m ost cells in layer 4, particularly layer 4B, but not in layer 6. Dynam ic nonlinearities seem essential for cells in layer 6. We suggest that these laminar differences may partly reflect the differential involve ment of geniculocortical and intracortical mechanisms.