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The Auditory Continuity Illusion/Temporal Induction

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.



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