LENGTH AND WIDTH TUNING OF NEURONS IN THE CATS PRIMARY VISUAL-CORTEX

1. The classically defined receptive field of a visual neuron is the a rea of visual space over which the cell responds to visual stimuli. It is well established, however, that the discharge produced by an optim al stimulus can be modulated by the presence of additional stimuli tha t by themselves do not produce any response. This study examines inhib itory influences that originate from areas located outside of the clas sical (i.e., excitatory) receptive field. Previous work has shown that for some cells the response to a properly oriented bar of light becom es attenuated when the bar extends beyond the receptive field, a pheno menon known as end-inhibition (or length tuning). Analogously, it has been shown that increasing the number of cycles of a drifting grating stimulus may also inhibit the firing of some cells, an effect known as side-inhibition (or width tuning). Very little information is availab le, however, about the relationship between end-and side-inhibition. W e have examined the spatial organization and tuning characteristics of these inhibitory effects by recording extracellularly from single neu rons in the cat's striate cortex (Area 17). 2. For each cortical neuro n, length and width tuning curves were obtained with the use of rectan gular patches of drifting sinusoidal gratings that have variable lengt h and width. Results from 82 cells show that the strengths of end- and side-inhibition tend to be correlated. Most cells that exhibit clear end-inhibition also show a similar degree of side-inhibition. For thes e cells, the excitatory receptive field is surrounded on all sides by inhibitory zones. Some cells exhibit only end- or side-inhibition, but not both. Data for 28 binocular cells show that length and width tuni ng curves for the dominant and nondominant eyes tend to be closely mat ched. 3. We also measured tuning characteristics of end- and side-inhi bition. To obtain these data, the excitatory receptive field was stimu lated with a grating patch having optimal orientation, spatial frequen cy, and size, whereas the end- or side-inhibitory regions were stimula ted with patches of gratings that had a variable parameter (such as or ientation). Results show that end- and side-inhibition tend to be stro ngest at the orientation and spatial frequency that yield maximal exci tation. However, orientation and spatial frequency tuning curves for i nhibition are considerably broader than those for excitation, suggesti ng that inhibition is mediated by a pool of neurons. This conclusion i s further supported by the finding that the strength of end- and side- inhibition does not depend on the relative spatial phase between excit atory and inhibitory grating stimuli. 4. Laminar analysis reveals that end- and side-inhibited neurons are found in all layers of the cortex . The only laminar specialization observed involves a distinct populat ion of neurons, located predominantly in Layer 6, that have very long receptive fields and exhibit pronounced side-inhibition. 5. To determi ne where end- and side-inhibition are generated in the visual pathway, we obtained dichoptic measurements of length and width tuning. For th is purpose, an optimal patch of grating was confined within the excita tory receptive field of one eye, whereas the inhibitory regions of the other eye were stimulated with grating patches of variable length or width. Results from 13 cells show that end- and side-inhibition are me diated dichoptically For three cells, inhibitory orientation and spati al frequency tuning curves were obtained dichoptically; these exhibit selectivity similar to that seen in monoptic tests. The strength of in hibition is not found to depend on the binocular (phase) disparity bet ween inhibitory stimuli presented to the left and right eyes. Overall, these dichoptic results suggest that end- and side-inhibition are gen erated through intracortical inhibitory interactions between binocular neurons.