Cortical mechanisms for pitch representation.

Pitch is critical to the perception of music and speech and for sound-source segregation, and is one of a number of perceptual attributes that can be associated with a sound object. An understanding of the neural mechanisms for pitch perception is needed to explain important aspects of human perception, but work on neural substrates uses different techniques in different species and it is a moot point whether 'universal' pitch mechanisms might exist. In these reviews we examine critically the extent to which this might be the case. Pitch perception requires neural mechanisms to create a representation corresponding to the percept from sensory cues in the frequency and time domains. Oxenham (2012) will consider the fundamental question of the relationship between sensory properties and perceived pitch that constrains what might be legitimately called a pitch mapping in brain experiments. Sensory cues that are necessary for pitch perception are represented in the ascending auditory pathway and cortex. We consider here the distinct but related question: what are the neural correlates of the pitch percept itself? Cortical mechanisms for pitch representation are implicated by studies at three different levels of neural organization. We take a broad perspective here that includes single-neuron recording in ferret and marmoset (Wang and Walker, 2012), local field-potential recordings from neural ensembles in humans (Kumar and Schönwiesner, 2012), measurement of the fMRI BOLD response related to neural ensemble activity in macaques and humans (Griffiths and Hall, 2012), and the modeling of interactions between neural ensembles in humans (Kumar and Schönwiesner, 2012). Pitch mappings are first considered at the level of neurons that represent the pitch percept rather than the frequency-or time-structure of the stimulus. Such representation is arguably similar to the coding of perceived color rather than the frequency of light in certain visual neurons (Zeki, 1973). From first principles, a single pitch value might be represented by activity in single neurons that are tuned to a given pitch value or as an ensemble property of a group of less well tuned neu-rons. Neurons in marmoset auditory cortex (Bendor and Wang, 2005) can show tuning to the pitch of a complex stimulus that is similar to that of a pure tone consistent with representing perceived pitch. Those neurons are precisely tuned. Neurons in ferret auditory cortex (Walker et al., 2011) can represent pitch information in parallel with information about spatial location and timbre (defined as sound perceptual attributes distinct …