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I've read with interest the various models and theories that have
followed my brief description and query concerning the basis for dichotic
verbal transformations, since they cannot be handled by conventional
theories of speech perception. However, as Al has observed on 20 May
"... none of them has addressed Dick's major observation. How could one
'binding' of properties (say the voice at the left ear) habituate
independently of the other binding when it involves the same word?"
Pierre on 20 May stated that "... I think that the phenomenon has not been
described in sufficient detail to draw any conclusion." I agree with
Pierre, and believe that speculation concerning the basis for this
illusion can be sharpened if I describe some of the published
quantitative aspects of this effect.
But before doing so, lest it be thought that I have not given
considerable thought to the problems raised by this illusion (and indeed,
that thought did not precede the experiments), I would like to express my
ideas concerning this and similar issues. Unlike intelligently designed
systems and models, biological systems often achieve amazing results
through mechanisms that are neither the most simple or elegant, but
reveal themselves through experiments (consider transduction by the inner
ear vs. a microphone). A second point is that there is often more than
one mechanism available for perceptual analysis of sensory data. This
has been brought home to me once again by our experiments on the verbal
organization of repeated sequences of steady-state vowels in which
the high-frequency and low-frequency components undergo independent
verbal organization [Warren, Healy, & Chalikia, JASA, 1996, 100, 2452-2461].
Now for the nitty-gritty of verbal transformations. First, the basic
illusion: verbal transformations occur while listening to a repeated
sequence of speech sounds. The rate of illusory changes
(transformations) is equivalent for monaural and diotic presentations.
The stimuli can be isochronous sequences of steady-state vowels (item
durations 30 - 100 ms), syllables, lexical items, phrases, or short
sentences. While there is considerable individual variability, for a
monosyllabic word repeated twice/sec, there are about 5 - 10 changes/min
with young adults (18 - 25 yr). The rate of changes is more rapid for
children (8 yr), and dramatically less for the elderly (62 - 86 yr) --
about 1/5 that of young adults. The forms heard with young adults are
often neologisms and, as also reported for jargon aphasia, follow the
phonotactic rules for phonemic clustering in English. In addition, the
forms for young adults almost always consist of syllables actually
occurring in English (question -- is this also true in jargon aphasia?).
Young children occasionally violate phonotactic rules, and the elderly
almost always restrict their responses to lexical items. One further
finding: with young adults, reducing the repetition rate by introducing
pauses between repetitions results in a proportional decrease in the rate
of verbal transformations (i.e., the same number of repetitions produces
the same number of changes).
With this out of the way, let's consider dichotic verbal transformations
reported with the same repeated word delivered to each ear twice/sec at
an interaural asynchrony of exactly 1/2 of the repetition period. The
independent changes to other words and syllables occur at the same rate
at each ear (no right-ear advantage). The changes occur asynchronously
at each ear, and the concurrent forms at any given time are generally
independent of each other. When instructed to monitor changes on both
sides, the combined rate of change at the two ears is much less than the
rate occurring with either monaural or diotic stimulation. When
listeners are instructed to monitor only one ear with the dichotic
stimulus, the rate at that ear is about the same as the combined rate
when both ears are monitored.
One further point related to construction of an appropriate model -- it
is not only habituation or satiation that occurs at each side -- there
are also independent shifts in the criteria for acceptable tokens by the
nodes, templates, cell assemblies, or whatever, that are responsible for
the recognition of verbal forms. Changes occur when salience of a
perceived form is exceeded by the salience of a new form resulting from a
criterion shift [see Warren, "Criterion shift rule and perceptual
homeostasis", Psych. Rev., 1985, 92, 574-584].
A final item: to address a question raised by Al, Jim Bashford has used
a repeating word "calculate" (800 ms duration) highpassed and lowpassed
at 1,500 Hz and delayed one version relative to the other. They were
both presented diotically, and when there was an asynchrony of 100 ms or
more between the two versions, as Al anticipated, independent verbal
transformations were heard by each of three people in the lab.