Cross-sensory inhibition or unisensory facilitation: A potential neural architecture of modality switch effects

In a simple reaction time task in which auditory and visual stimuli are presented in random sequence alone (A or V) or together (AV), there is a so-called reaction time (RT) cost on trials in which sensory modality switches (A→V) compared to when it repeats (A→A). This is always true for unisensory trials, whereas RTs to AV stimuli preceded by unisensory stimuli are statistically comparable with the Repeat condition (AV→AV). Neural facilitation for Repeat trials or neural inhibition for Switch trials could both account for these effects. Here we used a neural network model (Multisensory Integration with Crossed Inhibitory Dynamics (MICID) model) to test the ability of these two distinct mechanisms, inhibition and facilitation, to produce the specific patterns of behavior that we see experimentally, modeling switch and repeat trials as well as the influence of the interval between the present and the previous trial. The model results are consistent with an inhibitory account in which there is competition between the different sensory modalities, instead of a facilitation account in which the preceding stimulus sensitizes the neural system to its particular sensory modality. Moreover, the model shows that multisensory integration can explain the results in case of multisensory stimuli, where the preceding stimulus has little effect. This is due to faster dynamics for multisensory facilitation compared to cross-sensory inhibition. These findings link the cognitive framework delineated by the empirical results to a plausible neural implementation.