1. The classically defined receptive field of a visual neuron is the area of visual space over which the cell responds to visual stimuli. It is well established, however, that the discharge produced by an optimal stimulus can be modulated by the presence of additional stimuli that by themselves do not produce any response. This study examines inhibitory influences that originate from areas located outside of the classical (i.e., excitatory) receptive field. Previous work has shown that for some cells the response to a properly oriented bar of light becomes attenuated when the bar extends beyond the receptive field, a phenomenon 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 available, however, about the relationship between end-and side-inhibition. We have examined the spatial organization and tuning characteristics of these inhibitory effects by recording extracellularly from single neurons in the cat's striate cortex (Area 17). 2. For each cortical neuron, length and width tuning curves were obtained with the use of rectangular patches of drifting sinusoidal gratings that have variable length 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 these 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 tuning curves for the dominant and nondominant eyes tend to be closely matched. 3. We also measured tuning characteristics of end- and side-inhibition. To obtain these data, the excitatory receptive field was stimulated with a grating patch having optimal orientation, spatial frequency, and size, whereas the end- or side-inhibitory regions were stimulated with patches of gratings that had a variable parameter (such as orientation). Results show that end- and side-inhibition tend to be strongest at the orientation and spatial frequency that yield maximal excitation. However, orientation and spatial frequency tuning curves for inhibition are considerably broader than those for excitation, suggesting that inhibition is mediated by a pool of neurons. This conclusion is further supported by the finding that the strength of end- and side-inhibition does not depend on the relative spatial phase between excitatory 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 population of neurons, located predominantly in Layer 6, that have very long receptive fields and exhibit pronounced side-inhibition. 5. To determine where end- and side-inhibition are generated in the visual pathway, we obtained dichoptic measurements of length and width tuning. For this purpose, an optimal patch of grating was confined within the excitatory 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 mediated dichoptically For three cells, inhibitory orientation and spatial frequency tuning curves were obtained dichoptically; these exhibit selectivity similar to that seen in monoptic tests. The strength of inhibition is not found to depend on the binocular (phase) disparity between inhibitory stimuli presented to the left and right eyes. Overall, these dichoptic results suggest that end- and side-inhibition are generated through intracortical inhibitory interactions between binocular neurons.
