In this second lecture, we focus on pitch perception, an essential feature of musical perception. It is one of the most studied features of auditory perception. The mechanisms of pitch detection are largely determined by cochlear physiology.
We began by reviewing the psychoacoustic data on pitch perception. The sensation of pitch was introduced using as an example the complex harmonic sounds emitted by musical instruments, and their spectro-temporal representation. After recalling Joseph Fourier's decomposition of a periodic sound as a superposition of harmonic sounds (sounds with multiple frequencies of the same fundamental frequency), we discussed the pitch invariance properties of complex harmonic sounds (with respect to changes in sound intensity or other acoustic perturbations), which enable us to follow the melodic line of a piece as the variation of the fundamental frequency as a function of time. One of the remarkable features of this invariance is the phenomenon of extracting the missing fundamental frequency, which we illustrated with examples of synthetic sounds.
We then turned to the psychoacoustic characterization of pitch sensation, based on the determination of auditory "critical bands" and the essential distinction between harmonics resolved and unresolved by the cochlear filter. On the basis of this characterization, two main pitch detection mechanisms were discussed: "temporal" extraction involving the temporal coding of sounds already detectable in the auditory nerve, and "spectral" extraction involving cochlear tonotopy. These two mechanisms are distinguished by different invariance properties: spectral pitch extraction is largely insensitive to phase manipulations of harmonics, but is disturbed by harmonicity defects; conversely, temporal extraction is sensitive to phase manipulations and not very sensitive to spectrum distortions. These differences are best highlighted by analyzing the characteristics of pitch detection in relation to the resolved or unresolved character of harmonics.
In the second part, we addressed the central aspects of pitch perception. Current research focuses on understanding how pitch is encoded in the auditory cortex. Two recent papers by Daniel Bendor, Michael Osmanski, Xiaoqin Wang, and others (Bendor, Wang, 2005; Bendor et al., 2012; Song et al., 2016), were discussed, which provide the first evidence of neurons specifically detecting pitch within auditory cortex. These studies shed light on the debate over the temporal and spectral mechanisms of pitch extraction, showing the existence of different types of neurons in the auditory cortex that are sensitive to pitch, but whose response properties are consistent with either mechanism. This work provides the first experimental demonstration of the existence of a pitch center in the primary auditory cortex, located at its anterolateral border in a region coding for low sound frequencies.
