Old speech phenomena don't die they just become morphed into neuroscience studies.
The phenomenon of categorical perception appears to be riding the coattails of the resurgence of interest in motor theories of speech perception. Back in the motor theory heyday, categorical perception was all the rage. Listeners appeared to perceive speech sounds differently from non-speech sounds, i.e., categorically, and this was taken as evidence for the motoric nature of the speech perception process. The argument was something like this... Acoustic signals vary continuously. Articulatory patterns are categorical (/b/ is always produced bilabially). Perception mirrors the categorical nature of articulation. Therefore we perceive speech via our motor system.
Problems with this view quickly arose. Non-human, and therefore non-speaking, animals such as chinchillas and quail, were found to exhibit categorical perception for speech sounds. Babies too, who hadn't yet acquired the ability to articulate speech, also exhibited categorical perception. Categorical perception of non-speech sounds was also demonstrated. Further, perception of speech sounds was found to be continuous if listeners were asked to rate how well a stimulus represented a given category rather than asking them to make a binary decision.
Interest in categorical perception (CP) faded -- except in neuroscience where the pace of CP studies seems to be accelerating. Here's just a few from this year:
Möttönen R, Watkins KE. Motor representations of articulators contribute to
categorical perception of speech sounds. J Neurosci. 2009 Aug 5;29(31):9819-25.
Salminen NH, Tiitinen H, May PJ. Modeling the categorical perception of speech
sounds: A step toward biological plausibility. Cogn Affect Behav Neurosci. 2009
Clifford A, Franklin A, Davies IR, Holmes A. Electrophysiological markers of categorical perception of color in 7-month old infants. Brain Cogn. 2009
Prather JF, Nowicki S, Anderson RC, Peters S, Mooney R. Neural correlates of categorical perception in learned vocal communication. Nat Neurosci. 2009 Feb;12(2):221-8.
I hinted previously that the failure to use signal detection analysis methods in the context of categorical perception studies may have contaminated the whole field of CP research. Lori Holt recently pointed me to a paper by Schouten et al. 2003, provocatively titled "The End of Categorical Perception as We Know It". The point of the paper is exactly was I was hinting at: perception only looks categorical because of inherent bias in the tasks used to measure it.
The traditional categorical-perception experiment measures the bias inherent in the discrimination task
(Schouten et al. 2003, p. 71)
Here's another interesting quote from this paper:
Despite an auspicious beginning with a clear experimental definition ... categorical perception has in practice remained an ill-defined or even undefined concept, which could be used to underpin a variety of sometimes mutually exclusive claims, for example for or against the motor theory (p. 72)
This is an interesting paper that is worth a close look. But back to bias...
Let me illustrate very simply using some categorical perception data that I pulled from the literature. The graph below shows real data from a CP experiment using a GA-DA continuum. The task is explicitly categorical: subjects are asked to decide whether a stimulus is an example of GA or DA. This is not a good task to determine whether subjects perceive speech sounds categorically because it forces them to categorize. As Schouten et al. put it, "... if the nature of the task compels subjects to use a labelling strategy, categorical perception will be pretty much a foregone conclusion" (p. 77). Nonetheless, use of d-prime measures shows a rather different picture to standard measures. The vertical access is proportion of GA responses, and the horizontal axis is the various stimuli along the continuum. Perception looks nicely categorical.
Now plot the same data in d-prime units. To do this you can calculate d' for each pair of adjacent stimuli (how well are Ss discriminating Stim1 from Stim2, Stim2 from Stim3, etc.). Plotted here is cumulative d'. We should see discontinuities in the cumulative d'. Instead we see a more continuous function.
Have a look at the papers by Lori Holt and Andrew Lotto that I highlighted in a previous post as well as the Schouten et al. paper for more critical views on the nature of categorical perception. Then there's always long-time CP skeptic Dominic Massaro. His work on the topic is also worth a look.
What are the implications for neuroscience studies of speech perception? Well, if CP is nothing more than task effects and/or subject bias, then by using CP paradigms to map speech perception systems, all that is being mapped is task strategies and/or subject bias. No wonder all these studies find effects in the frontal lobe!
Schouten, B. (2003). The end of categorical perception as we know it Speech Communication, 41 (1), 71-80 DOI: 10.1016/S0167-6393(02)00094-8