Abstract
Perceptual judgments about a sensory stimulus can be affected by simultaneous stimulation of another sensory modality. However it is not clear to what extent these cross-modal interactions are due to changes in sensory processing or to changes in the subjective decision about the output of that process. The present study explores how visual cues for motion affect the perception of motion in the tactile modality. Participants judged the direction (left vs right) of a textured surface moving briefly across the pad of their right index finger. This tactile motion was accompanied by the superimposed image of a textured surface that was either static or moving. Compared to the static image or non-coherent motion cues, coherent visual motion strongly biased the reported direction of tactile motion. We used the participants' confidence ratings for their tactile motion judgments to construct Receiver Operating Characteristic (ROC) curves, plotting hits against false-alarms. The presence of coherent visual motion skewed the ROC curve: the curve was positively skewed by the visual stimulus moving in the same direction as the tactile motion, and was negatively skewed by visual motion in the opposite direction. This distortion establishes that the visual motion cues changed the variance of the underlying distributions of sensory evidence for left vs right tactile motion. That is, the perceived tactile motion had less variability when accompanied by congruent visual motion than when accompanied by incongruent visual motion. Therefore, visual motion sharpened perception of congruent tactile motion and/or added noise to perception of incongruent tactile motion.
This research was funded by a grants from the Australian Research Council to JH and CC, and a long-term fellowship from the Human Frontiers Science Foundation to EA.