Multisensory gain within and across hemispaces in simple and choice reaction time paradigms.

Recent results on the nature and limits of multisensory enhancement are inconsistent when stimuli are presented across spatial regions. We presented visual, tactile and visuotactile stimuli to participants in two speeded response tasks. Each unisensory stimulus was presented to either the left or right hemispace, and multisensory stimuli were presented as either aligned (e.g. visual right/tactile right) or misaligned (e.g. visual right/tactile left). The first task was a simple reaction time (SRT) paradigm where participants responded to all stimulations irrespective of spatial position. Results showed that multisensory gain and coactivation were the same for spatially aligned and misaligned visuotactile stimulation. In the second task, a choice reaction time (CRT) paradigm where participants responded to right-sided stimuli only, misaligned stimuli yielded slower reaction times. No difference in multisensory gain was found between the SRT and CRT tasks for aligned stimulation. Overall, the results suggest that when spatial information is task-irrelevant, multisensory integration of spatially aligned and misaligned stimuli is equivalent. However, manipulating task requirements can alter this effect.