Auditory Perception: Laurel and Yanny Together at Last

Controlling audibility with noise for online experiments using sound

Online auditory experiments use the sound delivery equipment of each participant, with no practical way to calibrate sound level or frequency response. Here, a method is proposed to control sensation level across frequencies: embedding stimuli in threshold-equalizing noise. In a cohort of 100 online participants, noise could equate detection thresholds from 125 to 4000 Hz. Equalization was successful even for participants with atypical thresholds in quiet, due either to poor quality equipment or unreported hearing loss. Moreover, audibility in quiet was highly variable, as overall level was uncalibrated, but variability was much reduced with noise. Use cases are discussed.

An implicit representation of stimulus ambiguity in pupil size

To guide behavior, perceptual systems must operate on intrinsically ambiguous sensory input. Observers are usually able to acknowledge the uncertainty of their perception, but in some cases, they critically fail to do so. Here, we show that a physiological correlate of ambiguity can be found in pupil dilation even when the observer is not aware of such ambiguity. We used a well-known auditory ambiguous stimulus, known as the tritone paradox, which can induce the perception of an upward or downward pitch shift within the same individual. In two experiments, behavioral responses showed that listeners could not explicitly access the ambiguity in this stimulus, even though their responses varied from trial to trial. However, pupil dilation was larger for the more ambiguous cases. The ambiguity of the stimulus for each listener was indexed by the entropy of behavioral responses, and this entropy was also a significant predictor of pupil size. In particular, entropy explained additional variation in pupil size independent of the explicit judgment of confidence in the specific situation that we investigated, in which the two measures were decoupled. Our data thus suggest that stimulus ambiguity is implicitly represented in the brain even without explicit awareness of this ambiguity.