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brain regions that have spatial maps and indicate that these effects are not an effect on
general arousal. The fact that performance was worse in Experiment 2 than Experiment
1 is relatively uninformative because subjects performed a more difficult discrimination
task (deciding between two hands) as opposed to the simple detection task (on only one
hand) in Experiment 1.
Although there was not a correspondence between somatosensory frequency
and auditory frequency in the first two experiments, the onset of the electrical
cutaneous stimulus used in those experiments coincided with the onset of the auditory
stimulus and auditory-somatosensory interactions were observed. Unlike Experiments 1
and 2, Experiment 3 used piezoelectric Vibrotactile stimulation of much longer temporal
durations and specific frequencies. However, due to potential variations in mechanical
inertia of the stimulators and the skin, the mechanical deflection of the Vibrotactile
device may not have been precisely in phase with the sound. Nonetheless, the use of a
Vibrotactile stimulus allowed us to study auditory-tactile interactions in the frequency
domain, which is more precisely coded by the auditory and somatosensory systems, and
robust auditory-tactile interactions were measured. This suggests that there may be
three separate dimensions along which auditory-tactile multisensory integration may
occur (temporal synchrony, spatial concordance, and frequency concordance). In future
studies, it will be important to explore these dimensions. For instance, is the integration
between spatially congruent and frequency congruent auditory-tactile stimuli additive
and do harmonics of the sounds produce similar effects on Vibrotactile frequency
discrimination?