AbstractIn a simple reaction time (RT) task, subjects must respond to a single stimulus with a response that is known in advance. However, RTs can differ depending on the stimulus modality whereby auditory RTs are typically faster than visual. Furthermore, combining modalities results in RTs that are faster than either stimulus alone produces. The mechanisms underlying this effect, known as intersensory facilitation, are currently unclear. Recent studies have hypothesized that activation related to initiation can sum under certain circumstances (e.g. startle). Of interest in the current study was whether a model of additive initiation-related activation could explain the intersensory facilitation effect. Twelve participants performed a RT task requiring a targeted wrist extension following either a visual go-signal, an auditory signal, or a combination of both with a varying stimulus onset asynchrony (SOA) (e.g. auditory stimulus presented either 0, 25, 50, 75, 100, or 125 ms after visual go-signal). Electromyography (EMG) from the wrist prime movers, as well as displacement was recorded from all participants. As expected, RTs were shorter when both stimulus modalities were presented concurrently. Importantly, as the SOA increased, the observed results closely fit RTs predicted by a model involving additive initiation slopes. Specifically, stimulus presentation times resulting in the largest amount of activation overlap resulted in the greatest relative RT speeding. These results indicate that the neural signals arising from the differing stimuli may converge at a common structure in the brain responsible for response initiation.
Acknowledgments: Supported by the Natural Sciences and Engineering Research Council of Canada (NSERC)