Freezing effect in tactile perception: sound facilitates tactile identification by enhancing intensity but not duration.

Identification of a visual target can be enhanced by a simultaneously presented high tone embedded in a sequence of low tones. This is called "freezing effect" because it is as if the target display was frozen in time by the tone. Until now, however, it has not been known whether this sound facilitation effect exists for a target with modalities other than vision, such as tactility, and if so, what its underlying mechanism is. We demonstrate, for the first time, an audio-tactile freezing effect (Experiment 1). We use a method of constant stimuli in conjunction with a 2-AFC task to determine the point of subjective equality (PSE) of the duration (Experiment 2A) or intensity (Experiment 2B) of the tactile target. Results do not support the view that a high tone expands the duration of the tactile target, but rather that the tone enhances participants' subjective tactile intensity. When the tactile intensity of the target was increased to match the shift of PSE as in Experiment 2B, this increased intensity indeed improved identification, further suggesting that intensity enhancement is the mechanism (Experiment 3). The perceived tactile intensity enhancement by a sound indicates genuine multisensory integration.