Interaction of perceptual grouping and crossmodal temporal capture in tactile apparent-motion.

Previous studies have shown that in tasks requiring participants to
report the direction of apparent motion, task-irrelevant mono-beeps
can "capture" visual motion perception when the beeps occur
temporally close to the visual stimuli. However, the contributions
of the relative timing of multimodal events and the event
structure, modulating uni- and/or crossmodal perceptual grouping,
remain unclear. To examine this question and extend the
investigation to the tactile modality, the current experiments
presented tactile two-tap apparent-motion streams, with an SOA of
400 ms between successive, left-/right-hand middle-finger taps,
accompanied by task-irrelevant, non-spatial auditory stimuli. The
streams were shown for 90 seconds, and participants' task was to
continuously report the perceived (left- or rightward) direction of
tactile motion. In Experiment 1, each tactile stimulus was paired
with an auditory beep, though odd-numbered taps were paired with an
asynchronous beep, with audiotactile SOAs ranging from -75 ms to 75
ms. Perceived direction of tactile motion varied systematically
with audiotactile SOA, indicative of a temporal-capture effect. In
Experiment 2, two audiotactile SOAs--one short (75 ms), one long
(325 ms)--were compared. The long-SOA condition preserved the
crossmodal event structure (so the temporal-capture dynamics should
have been similar to that in Experiment 1), but both beeps now
occurred temporally close to the taps on one side (even-numbered
taps). The two SOAs were found to produce opposite modulations of
apparent motion, indicative of an influence of crossmodal grouping.
In Experiment 3, only odd-numbered, but not even-numbered, taps
were paired with auditory beeps. This abolished the
temporal-capture effect and, instead, a dominant percept of
apparent motion from the audiotactile side to the tactile-only side
was observed independently of the SOA variation. These findings
suggest that asymmetric crossmodal grouping leads to an attentional
modulation of apparent motion, which inhibits crossmodal
temporal-capture effects.