Australian and Italian researchers have provided important new evidence for the cyclical nature of perception in an article published in Current Biology. They found that the sensitivity for detecting weak sounds is not constant in humans, but fluctuates regularly. The oscillation in sensitivity occurs separately for the two ears, first one then the other, switching every one-tenth of a second. This occurs so rapidly that we are normally unaware of it, but it can be detected in highly sensitive experiments. Oscillations have previously been reported for visual perception but not in audition. One reason for the failure to detect auditory oscillations previously is that the sensitivity of the two ears have been assessed together, whereas the new work shows that sensitivity alternates between the two ears, a subtle phenomenon missed by past studies. The new findings are important for many reasons. First, they show that oscillations are a general feature of perception, and not specific to vision. Second, the alternation in sensitivity between the two ears suggests that the brain samples input from each alternately, focusing attention on one ear at a time, allowing the brain to tag the ear of origin, important for many perceptual tasks. Third, the study showed that not only does sensory sensitivity alternate at a specific rhythm, but decision criterion also oscillates at a subtly different frequency.
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