Subject: Re: An Auditory Illusion From: Al Bregman <bregman(at)hebb.psych.mcgill.ca> Date: Tue, 29 Apr 1997 18:00:00 -0400On Mon, 28 Apr 1997, Richard M Warren wrote: > The question I have concerns a puzzling variant of verbal transformations > that occurs when each ear hears the same repeated word dichotically with > the delay of 1/2 the repetition period separating the repeating > statements in each ear -- the illusory changes occur independently on > each side, so that different forms can be heard simultaneously at each > ear. ... the question is: If there is a single speech processor, how > can the same stimulus be heard asynchronously as two forms that are > sometimes dramatically different? Dear Dick The phenomenon implies that at least some part of the "speech processor" is not serial. It seems that the stage responsible for extracting phonetic information from the acoustic signal can carry this out in parallel on two or more signals. However, since all the energy enters the ears mixed together, what is a "signal"? I imagine that a "signal" is whatever the auditory system has grouped into a single stream. That is, extraction of phonetic categories goes on in parallel in concurrent streams. This parallel extraction can be done on the basis of spatial location, as in the demonstration described in your e-mail. But I would also expect, since pitch is a good basis for the segregation of speech streams, that if you replicated the experiment with a man's and a woman's voice both coming from the same location, you would also see independence in the verbal transformation effect (VTE) in the two voices. Perhaps low-pass filtering and high-pass filtering of a man's voice would also create a pair of signals that, if looped asynchronously, would display independence of VTE. There is presumably a serial (capacity-limited) process that operates on the interpreted phonemes, and possibly on other interpretations that have been extracted in parallel. The serial process may be the one that synthesizes a final meaning based on the parallel-processed features. Evidence for a serial stage is that we cannot understand two concurrent conversations in any detail. What about the counter-argument that a SERIAL processor could keep track of two streams of speech by rapidly alternating back and forth between them? I think that this could be ruled out as a mechanism. The single speech processor would have to not ONLY alternate back and forth, but would have to "get tired" of phonetic interpretations independently for the two signals. This seems unlikely. I think, then, that your experiments on VTE force us to the conclusion that the speech processor is not a homogeneous serial mechanism. It seems likely that there is a stage that is preattentive and operates on concurrent auditory streams in parallel. Regards, Al ---------------------------------------------------------------------- Albert S. Bregman, Professor, Dept of Psychology, McGill University 1205 Docteur Penfield Avenue, Montreal, Quebec, Canada H3A 1B1. Phone: +1 514-398-6103 Fax: -4896 Email: bregman(at)hebb.psych.mcgill.ca Lab Web Page: http://www.psych.mcgill.ca/labs/auditory/laboratory.html ----------------------------------------------------------------------