Differential effects of stimulus discriminability on processing speed at different stages of sensorimotor processing

Stahl, J., and Rammsayer, T.
Georg Elias Müller Instute for Psychology, University of Göttingen, Germany
E-mail: jstahl@uni-goettingen.de

It is well known that stimulus characteristics, such as brightness or intensity, effectively influence reaction time (RT). Usually, better discriminability of a response signal results in shorter RTs. Various chronophysiological measures such as N1 latency, P3 latency, stimulus-related (S-LRP) as well as response-related (R-LRP) lateralized readiness potentials, and EMG latency represent important research tools to systematically elucidate which stages of the information processing chain (sensory, cognitive, or motor processes or a combination of them) account for the effects of stimulus discriminability on RT. In the present study, the effect of stimulus discriminability was investigated in 48 participants using a visual two-alternative forced-choice RT task. High-discriminability stimuli were responded to more than 30 ms faster than low-discriminability stimuli. Analysis of event-related potentials revealed reliably shorter N1, P3, and S-LRP latencies, whereas R-LRP latency was not affected by stimulus discriminability. Furthermore, EMG latency, reflecting the time from onset of muscle activation to the onset of the overt motor response, was significantly shorter in the high- than in the low-discriminability condition. The overall pattern of results supports the notion that stimulus characteristics, such as discriminability, exert a major influence on speed of early sensory processing but do not affect speed of central motor processes such as motor preparation and/or response initiation. The differential effect of stimulus discriminability observed for R-LRP and EMG latencies points to the conclusion that peripheral motor processes rather than central ones may have contributed to faster responses obtained under high compared to low stimulus discriminability.