Is the S-opponent chromatic sub-system sluggish?

Abstract

The S-opponent pathway has a reputation for being sluggish relative to the L/M-opponent pathway. Cottaris and De Valois [Nature 395 (1998) 896] claim that S-opponent signals are available in Macaque V1 only after 96-135 ms whereas L/M-opponent signals are available after 68-95 ms. Our experiments tested whether this large latency difference could be observed psychophysically. We measured reaction times to S/(L + M) and L/(L + M) increments. Both the equiluminant plane and the tritan line were empirically determined and we used spatio-temporal luminance noise to mask luminance cues. An adaptive staircase progressed according to observers' performance on a "go, no-go" task and provided concomitant estimates of threshold and of reaction time. When brief stimuli are confined to chromatic channels and presented at equivalent (threshold) levels and when latency is estimated from visually triggered reaction times, we find that the difference between the L/M-opponent and S-opponent sub-systems is, at most, 20-30 ms.