The Auditory Continuity Illusion/Temporal Induction

["Richard M. Warren" <rmwarren@xxxxxxx>]

Dear List,

Questions concerning illusory auditory continuity (aka temporal 
induction) were raised by Fatima Husain.  Her first posting on October 
25 dealt with a question concerning instructions employed for an 
experiment involving the illusory continuity of a pitch glide 
interrupted by noise, and this has already been answered by others.  I 
wish to respond to her posting of October 28 concerning the basis for 
illusory continuity.

During the exchanges on the list, Al Bregman suggested that I might 
have something to contribute on the topic, and wrote “Dick, are you out 
there?”  Sorry for the tardy response, but I was tied up with several 
activities, and am just catching up with the postings.  I take issue 
with Fatima’s statement that “ I think that the continuity illusion is 
‘cortical,’ as opposed to say being auditory (anywhere in the central 
auditory processing system) or peripheral.”  There have been a number 
of experiments dealing with the stimuli and peripheral auditory 
stimulation response that must be considered for any model of illusory 
continuity.  It appears that a “temporal induction” can induce a 
perceptual synthesis of signal fragments that have been masked or 
replaced by brief (up to a few hundred ms) bursts of a louder sound.  
This louder inducer must be a potential masker of the signal fragment, 
and studies have indicated that a portion of its auditory 
representation is reallocated for the generation of contextually 
appropriate segments of a fainter signal.

Restoration of a portion of tonal frequency glides as employed by 
Fatima belongs to one of the three types of temporal induction; each of 
these has been studied in some detail:  Type 1 “homophonic induction” 
consists of the illusory continuity of the fainter of two alternating 
levels of the same sound (e.g., alternating levels of a 1,000 Hz tone 
that differ by at least 2 or 3 dB, alternating levels of white noise 
that differ by at least 0.5 dB); Type 2 “heterophonic induction” 
consists of the illusory continuity of the fainter of two alternating 
sounds that are qualitatively different (e.g., continuity of a 1,000 Hz 
tone alternating with a louder 1/3-octave band of noise centered on 
1,000 Hz), alternating levels of a 1,000 Hz tone with a louder 970 Hz 
tone); Type 3 “contextual catenation” also consists of the restoration 
of a contextually appropriate fragment of a signal as do the other two 
types, but unlike the others, the signal is dynamic, and the restored 
fragment differs from the preceding and following portions of the 
signal (e.g., the restored fragment of a tone glide as studied by 
Fatima, the phonemic restoration of portions of speech, or the missing 
notes of familiar melodies played on a piano).

In connection with the role played by the cortex in restoration, it 
should be kept in mind that different cortical loci and organizational 
rules are employed for the restoration of fragments of speech, familiar 
melodies, and pitch glides.  However, all three types of illusory 
continuity follow the same simple rule that the neural representation 
of the extraneous inducing sound include those units that would have 
responded to the absent segment.  There are also specific 
characteristics associated with the restoration of segments of 
particular sounds (e.g., the homophonic induction of tones, the 
phonemic restoration of speech) and these have provided new information 
about the processing of these signals when they are uninterrupted.

More information on this topic is available in Chapter 6 “Perceptual 
Restoration of Missing Sounds,” in my book, Auditory Perception:  A New 
Analysis and Synthesis (Cambridge University Press, 1999), which is 
devoted to a summary and discussion of the literature on illusory 
continuity, including contributions from our lab.  I believe the book 
is now out-of-print, but I have just signed a contract for a new 
edition with the same publisher.  It should appear next year.

Regards,

Dick

---------------------------------------------
Richard M. Warren
University of Wisconsin-Milwaukee
Department of Psychology
PO Box 413
Milwaukee, WI  53201-0413

Phone:  (414) 229-5328
Fax:      (414) 229-5219
Email:  rmwarren@xxxxxxx