CODING OF PITCH AT THE BRAINSTEM LEVEL

Pitch enables the perception of melody and plays a crucial role in speech communication. It also allows us to discern between simultaneous sound sources, such as two simultaneous speakers. According to (Plack et al., 2005), pitch is an attribute of sensation whose variation corresponds to music melody. The importance of pitch is acknowledged by prominent researchers: “Pitch perception is considered to represent the heart of hearing theory, and is, without doubt, the topic most discussed over the years" (Plomp, 2001).

Since pitch is not a physical but a perceptual attribute, it is a “label” the brain creates for sounds. By assigning this label, different frequency components are grouped together as belonging to one sound source. This label must obviously be based on, or coded by, activity patterns that are conveyed by the periphery to the auditory central nervous system. There is a large variety of stimuli that have been used in pitch research, from which we chose a limited class of stimuli: band-limited and broadband noise. We first investigate cues for monaural edge pitch (MEP). Previous psychoacoustic studies have shown that band-limited noise so-called MEP generates pitch near the edge frequency of the noise band (Klein and Hartmann, 1981). While this pitch is not very strong, this stimulus has the advantage that only few cues are available on which the pitch can be based. We describe a new temporal cue that is associated with the noise edge and may be the basis of MEP. Second, we investigate a specific hypothesis (Shamma and Klein, 2000) regarding the generation of harmonic templates, which are a fundamental component of pitch theory. This hypothesis proposes that harmonic templates emerge in the auditory system based on known physiological processes (cochlear filtering, rectification, lateral inhibition, temporal sharpening) followed by a process of coincidence detection. By collecting and analyzing responses from different types of neuronneurons at peripheral stages, combined with and further probing by computational models, we test the plausibility of this hypothesis. We conclude that harmonic templates are only generated when using a string of physiologically implausible assumptions.