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Re: By any other name...

Hi Bruno and List,

Yes, the sound is actually there, in some sense, though not
necessarily exactly as it was before.  Take the example of a pure tone
in which a short gap has been filled with white noise.  If the noise
is intense enough (in the frequency region of the tone) so that there
is no drop in the activity of neurons that would normally signal the
tone, we will hear the tone continue through the noise.  Is it really
there acoustically?  Well, yes and no.  Yes there is energy in the
critical band defined by the tone's frequency, but unlike the tone,
its phase and amplitude are all over the place.  Remember that any
noise band _nominally_ contains every frequency component of its
spectrum.  But in the short run, the noise contains only an
approximation of any particular component.  However, the auditory
system accepts the wobbling, warbling frequency information in the
noise as a continuation of the tone, provided that the noise-filled
gap is not too long.  Dick Warren has shown that you can measure how
long the tone is heard in the noise when the gap becomes longer.

A more interesting case is when the gap-containing sound is itself a
flat band of noise (say 1000-1500 Hz), and the noise inserted into the
gap has a much wider flat spectrum (say 100-3000 Hz),.  In this case,
one could say that the properties of the narrow-band noise and the
corresponding band in the wide-band noise correspond very well.  In
fact the stimulus could just as easily have been made by starting with
a long burst of the narrow band noise, not inserting a gap into it at
all, but just adding a short burst containing 100-1000 Hz noise and
1500-3000 Hz noise at the temporal midpoint of the signal.  In this
case, it is clear that the narrow-band noise is really there.
However, although the same sound can be made in two different ways, it
can only be heard in one way, as a narrow band of noise continuing
through a wider-band interruption.  You can never hear it as a narrow
band, followed by a wide band, followed by a narrow band (assuming
that the wide-band interruption is not too long).

This favoring of the hearing of a continuing sound is an example of
what I have described as the "old-plus-new" heuristic, which can be
spelled out as follows:  "Whenever a spectrum changes, so as to become
more complex or louder, especially if the change is sudden, the
auditory system should determine whether this changed spectrum could
consist of a continuation of an old sound with the addition of a new
one.  If so, it should hear it that way.  The properties of the new
sound can be determined by calculating what would have had to be added
to the old sound to obtain the changed spectrum."

This is a valid strategy in a world in which sounds overlap in time.
The old-plus-new heuristic is a powerful way not only of establishing
the continuation of sounds through interruptions, but at getting a
good peek at a new sound at the moment that it enters a mixture.

If you are interested, you can read more about these ideas in my book,
Auditory Scene Analysis, 1990, pp.344-380.

Best wishes,

Albert S. Bregman, Emeritus Professor
Psychology Department, McGill University
1205 Docteur Penfield Avenue
Montreal, QC, Canada H3A 1B1.
    Tel: (514) 398-6103
    Fax: (514) 398-4896

On 3/22/07, Bruno Repp <repp@xxxxxxxxxxxxxxxx> wrote:
Dear Richard:

There is a philosophical (or methodological?) problem I've had with
this effect for a long time: If, as you say, "the interrupting louder
sound stimulates the same peripheral receptors that would have been
stimulated if the sound had indeed been present", what proves that
the sound is actually absent?


>The auditory continuity phenomenon has been the subject of several
>communications earlier this month, and several names of people
>associated with this illusion were mentioned.  Massimo Grassi
>correctly stated that Vicario's name belongs on the list.  He did
>indeed observe the effect in 1960, naming it "L'effetto tunnel
>acustico."  But Miller and Licklider seem to have been the first
>discoverers in 1950.  Several other investigators, unaware of the
>earlier publications, made their own independent discoveries.  This
>led to a multiplicity of terms describing the effect including
>"picket fence effect," "auditory figure ground effect," and more
>recently, "auditory continuity effect," "auditory induction," and
>"temporal induction."
>The communications this month seem to have limited this phenomenon
>to the illusory continuity of steady-state tones and tone glides
>through interruptions by a louder noise.  But this phenomenon is
>much broader:  portions of any sound can be restored if the
>interrupting louder sound stimulates the same peripheral receptors
>that would have been stimulated if the sound had indeed been
>present.  In everyday life this effect represents a sophisticated
>process that can restore portions of signals (including speech) if
>they have actually been masked.  This is accomplished by
>reallocating a portion of the neural representation of the louder
>interrupting sound for the perceptual synthesis of the fainter
>signal.  In support of this mechanism, it had been shown that when
>illusory restoration of the fainter sound (either a tone or speech)
>occurred, it was accompanied by a decrease in the loudness of the
>interrupting sound [R.M. Warren et al., 1994, Auditory induction:
>Reciprocal changes in alternating sounds.  Perception and
>Psychophysics, 55, 313-322].
>For a review of the literature, see Chapter 6 "Perception of missing
>sounds" in R.M. Warren, 1999, Auditory Perception: A New Analysis
>and Synthesis, New York:  Cambridge University Press (a third
>edition is now in production by Cambridge).

-- Bruno H. Repp Haskins Laboratories 300 George Street New Haven, CT 06511-6624 Tel. (203) 865-6163, ext. 236 Fax (203) 865-8963 http://www.haskins.yale.edu/staff/repp.html

NOTE: I am at Rutgers University, Newark, two days each week,
usually Wednesday and Friday, and don't read my
Haskins e-mail on those days. To reach me at Rutgers, send
e-mail to <repp@xxxxxxxxxxxxxxxxxxxxxx>.