Interactions between apparent motion rivalry in vision and touch

Introduction: In multistable perception, the brain alternates between several perceptual explanations of ambiguous sensory signals. Recent studies have demonstrated crossmodal interactions between ambiguous and unambiguous signals. However it is currently unknown whether multiple bistable processes can interact across the senses (Conrad et al., 2010; Pressnitzer and Hupe, 2006). Using the apparent motion quartet in vision and touch, this study investigated whether bistable perceptual processes for vision and touch are independent or influence each other when powerful cues of congruency are provided to facilitate visuotactile integration (Conrad et al., in press). Methods: When two visual flashes and/or tactile vibration pulses are presented alternately along the two diagonals of the rectangle, subjects’ percept vacillates between vertical and horizontal apparent motion in the visual and/or tactile modalities (Carter et al., 2008). Observers were presented with unisensory (visual/tactile), visuotactile spatially congruent and incongruent apparent motion quartets and reported their visual or tactile percepts. Results: Congruent stimulation induced pronounced visuotactile interactions as indicated by increased dominance times and %-bias for the percept already dominant under unisensory stimulation. Yet, the temporal dynamics did not converge for congruent stimulation. It depended also on subjects’ attentional focus and was generally slower for tactile than visual reports. Conclusion: Our results support Bayesian approaches to perceptual inference, where the probability of a perceptual interpretation is determined by combining a modality-specific prior with incoming visual and/or tactile evidence. Under congruent stimulation, joint evidence from both senses decelerates the rivalry dynamics by stabilizing the more likely perceptual interpretation. Importantly, the perceptual stabilization was specific to spatiotemporally congruent visuotactile stimulation indicating multisensory rather than cognitive bias mechanisms.

An ERP Investigation on Visuotactile Interactions in Peripersonal and Extrapersonal Space: Evidence for the Spatial Rule

The spatial rule of multisensory integration holds that cross-modal stimuli presented from the same spatial location result in enhanced multisensory integration. The present study investigated whether processing within the somatosensory cortex reflects the strength of cross-modal visuotactile interactions depending on the spatial relationship between visual and tactile stimuli. Visual stimuli were task-irrelevant and were presented simultaneously with touch in peripersonal and extrapersonal space, in the same or opposite hemispace with respect to the tactile stimuli. Participants directed their attention to one of their hands to detect infrequent tactile target stimuli at that hand while ignoring tactile targets at the unattended hand, all tactile nontarget stimuli, and any visual stimuli. Enhancement of ERPs recorded over and close to the somatosensory cortex was present as early as 100 msec after onset of stimuli (i.e., overlapping with the P100 component) when visual stimuli were presented next to the site of tactile stimulation (i.e., perihand space) compared to when these were presented at different locations in peripersonal or extrapersonal space. Therefore, this study provides electrophysiological support for the spatial rule of visual–tactile interaction in human participants. Importantly, these early cross-modal spatial effects occurred regardless of the locus of attention. In addition, and in line with previous research, we found attentional modulations of somatosensory processing only to be present in the time range of the N140 component and for longer latencies with an enhanced negativity for tactile stimuli at attended compared to unattended locations. Taken together, the pattern of the results from this study suggests that visuotactile spatial effects on somatosensory processing occur prior and independent of tactile–spatial attention.