Evidence for Linear but Not Helical Automatic Representation of Pitch in the Human Auditory System.

The perceptual organization of pitch is frequently described as helical, with a monotonic dimension of pitch height and a circular dimension of pitch chroma, accounting for the repeating structure of the octave. Although the neural representation of pitch height is widely studied, the way in which pitch chroma representation is manifested in neural activity is currently debated. We tested the automaticity of pitch chroma processing using the MMN-an ERP component indexing automatic detection of deviations from auditory regularity. Musicians trained to classify pure or complex tones across four octaves, based on chroma-C versus G (21 participants, Experiment 1) or C versus F# (27, Experiment 2). Next, they were passively exposed to MMN protocols designed to test automatic detection of height and chroma deviations. Finally, in an "attend chroma" block, participants had to detect the chroma deviants in a sequence similar to the passive MMN sequence. The chroma deviant tones were accurately detected in the training and the attend chroma parts both for pure and complex tones, with a slightly better performance for complex tones. However, in the passive blocks, a significant MMN was found only to height deviations and complex tone chroma deviations, but not to pure tone chroma deviations, even for perfect performers in the active tasks. These results indicate that, although height is represented preattentively, chroma is not. Processing the musical dimension of chroma may require higher cognitive processes, such as attention and working memory.