Pitch-period codes divide into delay-based vector codes and rate-based scalar codes

Evidence

Various schemes and neural mechanisms have been proposed for the encoding of pitch-related periodicity based on the temporal information carried by peripheral neurons. They have in common that they predict tuning in which a neuron is optimally responsive to a certain periodicity. Sounds that differ in pitch would activate different neurons; different sounds with the same pitch would activate the same neurons. Such schemes are referred to as vector codes (Churchland and Sejnowski 1992 ). Periodicity tuning is typically achieved by comparing spike trains with a delayed copy. The comparison involves multiplicative, subtractive, or additive neural interactions; and some source of delay from axons, cochlea, synapses, intrinsic membrane properties, etc. (reviewed by de Cheveigné 2005 ). From a physiological point of view, problems with this approach are that there is only limited evidence for such tuning in the brainstem and only over a limited range of periodicities; that no convincing delay mechanisms have been identified that cover a wide range of values (tens to tenths of ms); and that some of these schemes require elaborate (and biologically implausible) wiring. … The scheme proposed here dispenses with delays and suggests a scalar code, based on a property which is physiologically well documented. We surmise that entracking neurons at the brain stem level are not tuned to a particular periodicity but all code for a range of periodicities by their monotonic relationship between average firing rate and pitch-related period. — Dijk, Pim van, Physiology, Psychoacoustics and…, 4516

Categorization

• DEFINE: Defines the vector-code vs scalar-code distinction for pitch encoding, a key theoretical taxonomy.