The numerosity of any set of discrete elements can be depicted by a genuinely abstract number representation, irrespective of whether they are presented in the visual or auditory modality. The accumulator model predicts that no cost should apply for comparing numerosities within and across modalities. However, in behavioral studies, some inconsistencies have been apparent in the performance of number comparisons among different modalities. In this study, we tested whether and how numerical comparisons of visual, auditory, and cross-modal presentations would differ under adequate control of stimulus presentation. We measured the Weber fractions and points of subjective equality of numerical discrimination in visual, auditory, and cross-modal conditions. The results demonstrated differences between the performances in visual and auditory conditions, such that numerical discrimination of an auditory sequence was more precise than that of a visual sequence. The performance of cross-modal trials lay between performance levels in the visual and auditory conditions. Moreover, the number of visual stimuli was overestimated as compared to that of auditory stimuli. Our findings imply that the process of approximate numerical representation is complex and involves multiple stages, including accumulation and decision processes.

We can tap in synchrony with a simple sequence of auditory tones, but our taps generally precede it by several tens of milliseconds("negative mean asynchrony"). A simple sequence of sounds sometimes causes auditory perceptual organization different from the physical sequence. However little is known about interactions between the auditory perceptual organization and sensorimotor synchronization. The present study aimed at investigating effects of the auditory stream segregation (one stream or two) on the synchronized tapping. Participants listened to a sequence of two different auditory tones presented alternately, and were asked to tap in synchrony with one group of tones (ie, high tones or low) with index fingers. After that, they were also asked whether the tones were perceived one or two streams. The results showed that negative mean asynchrony was caused both in perceived one stream and two. However, the amplitude of the asynchrony was smaller when they perceived the tones as one stream than as two. This indicates that auditory perceptual organization affects sensorimotor synchronization even if auditory tones were presented with the same timing. When we synchronize bodily with auditory tones as in the case of a motor rehabilitation, we have to take account of auditory perceptual organization.

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