Visual responses of area 18 neurons in awake, behaving monkey

Visual responses of cells in the posterior bank of the lunate sulcus, area 18 or V2, were studied in awake monkeys trained on a fixation task. Test stimuli included stationary and moving spots, slits and edges of white and colored light. Almost all area 18 cells would respond vigorously to appropriately specified test stimuli. Different cells were maximally sensitive to different stimulus parameters. We have divided area 18 cells into six groups, named descriptively according to the most relevant stimulus parameter for each: orientation, color, border, direction, spot, light inhibited. Orientation cells (85/238, 36%) were sensitive to the orientation of elongated, stationary stimuli, most responding over a range of ± 10-40° from optimal. These cells preferred a slit that was narrow relative to the total width of the receptive field. They responded about equally well to both directions of movement of a properly oriented stimulus and were indifferent to color. Color cells (39/238, 16%) responded with excitation to some colors and inhibition to others. All gave spatially coextensive color-opponent responses in a round or oval center. Some had suppressive surrounds which limited the size of a stimulus effective in eliciting one or both of the center responses. Border cells (33/238, 14%) responded best to a stimulus that filled an excitatory region without encroaching on a powerful suppressive flank. The excitatory region could be delimited by mapping with small spots. The flank was unresponsive to small spots but, nevertheless, had a powerful suppressive effect on excitatory responses. Direction cells (29/238, 12%) responded maximally to a stimulus moving in a specified direction across their receptive fields. Movement in the opposite direction elicited either no response or inhibition. These cells were indifferent to color, contrast, and leading-edge configuration of moving stimuli; some showed weaker responses to elongated stimuli. Spot cells (26/238, 11%) preferred a properly positioned small spot of light. Changing the position or increasing the size of an optimal stimulus weakened the response. The optimal stimulus was small relative to the total field extent. Spot cells responded equally well to different stimulus colors and to all directions of movement. Light-inhibited cells (26/238, 11%) were distinguished by high, regular, spontaneous activity which was reduced or abolished by light falling on their receptive fields. Field centers were round or oval; some fields had suppressive surrounds. These cells did not have color opponent properties. Stimuli were always presented on the fixation plane. Most cells responded as well to monocular stimulation through either eye as to binocular stimulation. Some orientation cells responded much better to binocular than to monocular stimulation.