Sensory sluggishness dissociates saccadic, manual, and perceptual responses: an S-cone study.

Sensory information travels to visual and motor areas via several distinct pathways, some of them being fast-like the achromatic magnocellular and retinotectal routes-and others slower-those carrying chromatic signals, in particular S-opponent signals. It is debated whether common visual processing stages are used for different types of responses, such as initiating saccadic or manual responses or making perceptual judgments. The present paper casts new light on this question by comparing the participation of fast and slow pathways across these responses. In the first experiment, we measured manual and saccadic reaction times to luminance and S-cone signals, equated in detectability for each participant and presented on either sides of fixation. Our results show that both manual and saccadic responses are slower for S-cone stimuli. Most interestingly, this reaction time difference was twice as large for saccadic responses as for manual responses, suggesting that saccades rely more on the fast signals, not supported by S-cone stimuli, than do manual responses. In a second experiment, our participants performed temporal order judgments on pairs of luminance and S-cone stimuli. Our results show no evidence of perceived time discrepancy between the two signals, which may imply that perceptual judgments utilize different signals from either manual or saccadic responses.