Long-range suppressive interactions between S-cone and luminance channels.

Surround suppression (SS) refers to a reduction in the effective stimulus contrast in one visual location produced by a stimulus presented in an adjacent location. This type of suppression is tuned for orientation and spatial frequency and is thought to be a cortical process. In this paper we used psychophysical measurements to determine whether S-cone-driven signals are affected by surround suppression and, if so, whether S-cone and achromatic signals interact at spatially-remote locations. Our results revealed three important aspects of surround suppression. Firstly, we show that S-cone probes are suppressed by simultaneous S-cone contrast surrounds and that this suppression has the characteristics of a cortical mechanism. Secondly, we show that when probes and surrounds are presented simultaneously, there are no suppressive interactions between S-cone and luminance stimuli. Finally, we demonstrate that this apparent independence is an artifact of signal timing: when the S-cone components of the stimuli precede the luminance components by approximately 40 ms, we find a significant interaction between the two pathways. The amplitude of this interaction depends critically upon the relative onset times of the two components. These results indicate that some component of surround suppression depends on neural computations that occur after the S- and luminance pathways are combined in striate cortex. In addition, the strong dependence of the magnitude of surround suppression on temporal ordering suggests that much of the effect is driven by transient signals.