Early frequency-specific contributions to serial-effects in audition

Recent stimuli affect the perception of current stimuli, referred to as serial effects. These effects were mainly studied in the visual modality, where it was suggested that perceptual biases towards previous stimuli (contraction) stems from high-level processing stages, and promotes object-level stability. We now asked whether high object-level stages underlie contraction also in the auditory modality. We administered a two-tone pitch discrimination task using both pure and complex tones. Both have pitch, but they are perceived as different timbre categories. Pitch contraction was observed to be largest between tones of the same timbre-category, in line with the object-level account. To decipher the role of early, frequency-specific, category-indifferent processing-stages we used complex tones with missing fundamental. They differ in their low-level frequency components yet have the same pitch. Hence, a high-level account predicts that pitch contraction will remain. Surprisingly, we observed no contraction to the missing fundamental frequency. Rather, pitch was contracted to the physically-present frequencies. Supporting the low-level contribution, we found that though attention enhances contraction, it is not necessary. These observations suggest that contraction bias is an inherent part of the various stages of the auditory hierarchy of sensory processing.