Categorical+Perception

§1 Slogan: A phenomenon wherein a physical continuity is associated with a perceptual discontinuity, where the latter is a "category boundary". A "quantizing" of a perceived continuum.

§2 CP (shorthand for "categorical perception") was first discovered in the realm of speech perception, where the varying of a single dimension such as "rise time" of a consonant was seen to generate different categorical identities, e.g. "cha" vs "sha", and a case could be made that the transition from one category to the other was not "smooth". The evidence for the latter came in two flavors: an Identification Function and a Discrimination Function. The Identification Function demonstrated an unexpectedly sharp "boundary" on one side of which nearly all examples were categorized as "cha" and on the other side of which nearly all examples were categorized as "sha". The Discrimination Function revealed a "blip" such that "the same" amount of physical difference between a pair of examples was more easily perceived ("discriminated") when the examples were drawn from the area near the boundary than when both examples clearly fell on the same side of the boundary.

§3 Musical examples of CP? The same "rise time" distinction can be seen as marking a //timbral// difference, inviting the (//extremely// broad-brush) supposition that "timbre is categorically perceived". And there have been studies purporting to show that musical intervals are categorically perceived by musicians, although I am not aware that any of these studies bothered to obtain Discrimination Functions. (Yes, that does seem like a pretty egregious omission.)

§4 //Structure vs. expression//. One interesting way of conceptualizing the way categorical perception comes into play in ares of musical expression is by means of what might be called the "structure-vs-expression" distinction. Differences within between categories are "structural" differences" and are readily perceived. But differences within a category are not merely "thrown away"; rather they can be used for //expressive// purposes.  Almost indisputably, music that makes no use of this within-category latitude for expressive variation sounds mechanical and "unmusical"; depending on how the latitude is used, it can distinguish the novice from the virtuoso.  How to characterize the way the virtuoso makes use of this within-category latitude ("dirt") remains a mystery, although one well worth investigating.