Abstract
At the physical level, the experience of pitch has a single determinant: the repetition rate of a waveform in the acoustic signal. Yet, psychologists describe pitch as composed of two perceptual dimensions, height and chroma. Chroma accounts for octave equivalence, whereby sounds with fundamental frequencies at a 1:2 ratio are perceived as sharing the same pitch. A current controversy debates whether chroma is a basic perceptual property dependent on biological constraints or a higher-order cognitive construct shaped by culture. Here, we used high-density electroencephalography (EEG) and time-resolved multivariate pattern analyses to characterize pitch processing in humans at 3 months of age. We found that, when exposed to repetitive sequences of orchestral tones, infants encode two separate pitch-related dimensions automatically and with divergent dynamics. Namely, our classifiers isolated height-specific information from the neural signal rapidly after the onset of the auditory sequences. Beyond approximately 600 ms, the performance of pitch height decoders fell to chance level and did not recover. In contrast, neural patterns displaying octave equivalence were retrieved later in the trial, over multiple time windows throughout the unfolding of the auditory sequence, and after sequence offset. Overall, this study reveals that very early in human development, the pitch of naturally rich tones is processed over two distinct encoding stages, capturing not only their absolute height but also their relative position in the octave. We speculate that separate encoding mechanisms reflect distinct functional roles carried by the two dimensions.