Laminar differences in spatiotemporal structure of simple cell receptive fields in cat area 17

Purpose. Many simple cells display gradients in response timing across their receptive fields {RF) (Movshon, Thompson & Tolhurst, 1978). These space-time (S-T) oriented RFs confer direction sensitivity to moving stimuli (McLean & Palmer, 1989). The degree of S-T RF orientation and DS tend to covary but much heterogeneity exists in this relationship (Reid, Soodak & Shapley, 1991); this has implications for models of DS. We have searched for sources of the heterogeneity. Methods. Cells' DS and S-T RF structure were measured using drifting and counterphasing sinewave gratings, respectively. S-T orientation for each cell was expressed as the slope of a line fit to the response temporal phase vs. stimulus spatial phase data (Albrecht & Geisler, 1991). Recording locations were reconstructed histologically. Results. There were no laminar differences in DS but clear differences existed in RF structure. Receptive fields of DS cells in layers 4B and 5A were more S-T oriented than those in layers 3, 4A and 6. The greatest differences existed between layers 4B/5A and 6. hi layers 4B and 5A, S-T orientation and DS were well correlated (r=0.78, m=0.8); in layer 6, S-T orientation values were uniformly low and not correlated with DS. Among DS-cells, little overlap existed between layers 4B/5A and 6 in S-T orientation values. Conclusions. First-order spatiotemporal RF mechanisms can account for much of the direction selectivity of cells in layers 4B and 5A, and some cells in 4A. Additional mechanisms are needed to account for DS in layer 6. High S-T orientation in layer 4/5A could be accounted for by inputs from lagged and nonlagged LGN afférents, which convey a large range of response timings to cortex (Saul & Humphrey, 3992).