Abstract
The perception of pitch is a widely studied and hotly debated topic in human hearing. Many of these studies combine functional imaging techniques with stimuli designed to disambiguate the percept of pitch from frequency information present in the stimulus. While useful in identifying potential “pitch centres” in cortex, the existence of truly pitch-responsive neurons requires single neuron-level measures that can only be undertaken in animal models. While a number of animals have been shown to be sensitive to pitch, few studies have addressed the location of cortical generators of pitch percepts in non-human models. The current study uses high-field functional magnetic resonance imaging (fMRI) of the feline brain in an attempt to identify regions of cortex that show increased activity in response to pitch-evoking stimuli. Cats were presented with iterated rippled noise (IRN) stimuli, narrowband noise stimuli with the same spectral profile but no perceivable pitch, and a processed IRN stimulus in which phase components were randomized to preserve slowly changing modulations in the absence of pitch (IRNo). Pitch-related activity was not observed to occur in either primary auditory cortex (A1) or the anterior auditory field (AAF) which comprise the core auditory cortex in cats. Rather, cortical areas surrounding the posterior ectosylvian sulcus responded preferentially to the IRN stimulus when compared to narrowband noise, with group analyses revealing bilateral activity centred in the posterior auditory field (PAF). This study demonstrates that fMRI is useful for identifying pitch-related processing in cat cortex, and identifies cortical areas that warrant further investigation. Moreover, we have taken the first steps in identifying a useful animal model for the study of pitch perception.
Highlights
Pitch is the perceptual correlate of sound frequency, and the way that pitch changes over time defines the melody of a musical piece
In order to determine the area of cat auditory cortex that selectively responds to stimuli with pitch, an iterated rippled noise (IRN) stimulus was contrasted against a narrowband noise stimulus that consisted of identical spectral content, but which did not elicit a pitch percept
At an individual-animal level, these clusters of significant BOLD activity encompassed multiple areas of the auditory cortex with peak levels of activity typically observed in areas occupying the banks of the posterior ectosylvian sulcus, including the posterior auditory field (PAF), ventral auditory field (VAF), and ventral posterior auditory field (VPAF)
Summary
Pitch is the perceptual correlate of sound frequency, and the way that pitch changes over time defines the melody of a musical piece. Through complex spectrotemporal processing, a singular pitch is perceived that corresponds to the fundamental of the complex source This pitch percept is the result of two pitch codes that are established in parallel at the level of the cochlea: a spectral code based on the site of maximal displacement along the tonotopically organized basilar membrane, and a temporal code based on the rate of action potential generation in afferent neurons that synapse at that characteristic place. While these cues are encoded at the auditory periphery, a growing literature suggests that the percept of pitch is first established cortically, at a point beyond primary auditory cortex (A1). These results have been supported by electrophysiological recordings in non-human primates that fail to find pitch-sensitive neurons in A1, instead localizing pitch processing to a cortical belt region along the anterolateral border of A1 in the marmoset [12] and to the lateral belt region in the macaque [13]
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