Crossmodal modulation of visual and tactile numerosity judgments.

Nienke Weder, TNO Human Factors

Abstract
All day long we experience simultaneous multisensory signals. These usually originate from the same source. When concurrent series of brief signals in two different modalities do not match, the brain has a tendency to make the perceived number congruent. For instance, the perceived number of visual flashes can be altered by simultaneous presentation of an incongruent number of auditory beeps (Shams, 2000) or tactile taps (Violentyev, 2005). We investigated whether these effects also occur in the visual-to-tactile direction and tested whether the illusions reflect a robust perceptual effect or can be attributed to interference of the taps with the ability to perceive flashes. In two experiments participants counted either flashes or taps that were presented concurrently with task irrelevant signals in the other modality and rated their confidence in each response. We found significant illusion effects in the ‘count flashes’ experiment, but not in the ‘count taps’ experiment. We will discuss these results in the light of current models of modality appropriateness, information reliability and directed attention. The visual illusions also occurred in trials with a high confidence rating, which implies that participants are not aware of the illusion. Our findings confirm the robust perceptual nature of these crossmodal effects.

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