A Detailed Model of the Primary Visual Pathway in the Cat: Comparison of Afferent Excitatory and lntracorticallnhibitory Connection Schemes for Orientation Selectivity

In order to arrive at a quantitative understanding of the dy-namics of cortical neuronal networks, we simulated a de-tailed model of the primary visual pathway of the adult cat. This computer model comprises a sox so patch of the visual field at a retinal eccentricity of 4.S0 and includes 2048 ON-and OFF-center retinal p-ganglion cells, 8192 geniculate X-cells, and 4096 simple cells in layer IV in area 17. The neurons are implemented as improved integrate-and-fire units. Cortical receptive fields are determined by the pattern of afferent convergence and by inhibitory intracortical con-nections. Orientation columns are implemented continuously with a realistic receptive field scatter and jitter in the pre-ferred orientations. We first show that realistic ON-OFF-responses, orienta-tion selectivity, velocity low-pass behavior, null response, and responses to spot stimuli can be obtained with an ap-propriate alignment of geniculate neurons converging onto the cortical simple cell (Hubel and Wiesel, 1962) and in the absence of intracortical connections. However, the average receptive field elongation (length to width) required to obtain realistic orientation tuning is 4.0, much higher than the av-erage observed elongation. This strongly argues for addi-tional intracortical mechanisms sharpening orientation se-lectivity. In the second stage, we simulated five different inhibitory intracortical connection patterns (random, local, sparse-lo-cal, circular, and cross-orientation) in order to investigate the connection specificity necessary to achieve orientation tuning. Inhibitory connection schemes were superimposed onto Hubel and Wiesel-type receptive fields with an elon-gation of 1. 78. Cross-orientation inhibition gave rise to dif-ferent horizontal and vertical orientation tuning curves, something not observed experimentally. A combination o