Many studies show thatr temporal proximity plays an important role in cross-modal interactions (Fendrich & Corballis, 2001; Sekuler, Sekuler & Law. 1997). However, is it really sufficient to get the cross-modal effect? We presented random dots on a CRT monitor in synchronization with an auditory sequence to check whether the auditory stream segregation is indispensable for cross-modal interaction. Subjects were instructed to make a quick detection of a target in a rapidly changing sequence of visual stimuli. The sequence of visual stimuli consisted of quartets of dots, and the sequence of auditory stimuli consisted of quartets of tones, three low- and one high-pitched. The visual target was presented in synchronization with a high-pitched tone, while the other stimuli as visual distractors were synchronized with low-pitched tones. In order for subjects to recognize two sound streams, it was essential to repeat the auditory stimuli prior to displaying the dots. To examine the role of the repetition of the auditory sequence, we set four conditions defined by the audio-visual synchronization or asynchronization and by four low-pitched tones or three low-pitched plus one high-pitched tone. In the case where the subject could recognize the auditory stream segregation, his performance increased more significantly than in the case where he could not. We found that whether the visual target was synchronized with high-pitched tone or not, the latter high tone did not contribute to the detection of the visual target unless the auditory stream segregation could be recognized.Hence, it was important for the subject to recognize two separate streams of sounds (i.e., the auditory stream segregation). This suggests that the perceptual organization of sounds plays a critical role in the cross-modal effect.
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