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Re: By any other name...

The discussions of illusory continuity observed when gaps in a signal have been filled with a louder sound that were sparked by my March 21 email have been fascinating, and I’ll give some of my comments and observations for whatever they are worth. Bruno Repp’s “philosophical (or methodological?) problem” of whether or not the signal is present in the noise-filled gap has been thoroughly vetted, so I have nothing to add to that discussion.

Yoshitaka Nakajima’s report of the illusory continuity of the longer of two crossing frequency glides of different durations that shared “a short silent gap (40 ms or less) at their crossing point” is interesting. The gap of 40 ms is much briefer than the 200 or 300 ms usually employed in continuity experiments. Perhaps the broadband spectral splatter accompanying the rapid offset of the tone glides may have facilitated the illusion. However, this does not address why it is the longer rather than the shorter glide that is perceived as continuous.

The effects of gap duration are important. At gap durations greater than 100 ms the higher amplitude sound must be capable of masking the fainter for illusory continuity to be heard. Gaps in a steady-state tone can be spanned when filled by a louder broadband noise for up to about 300 ms [Warren, Obusek, and Ackroff, 1972, Auditory induction: Perceptual synthesis of absent sounds, Science, 164, 586-587]. Noise-filled gaps up to about 500 ms can be spanned for a tone glide [Dannenbring, 1976, Canadian Journal of Psychology, 30, 99-114]. There is a striking difference in the continuity limit for tone and for noise: A fainter narrow band noise (1/3-octave centered at 1-kHz) interrupted by a louder broader band noise (0.5-2 kHz) can maintain illusory continuity for durations as long as 50 seconds [Warren et al., 1972].

The term “auditory continuity” used to describe the illusion can be misleading, since it limits the illusion to complete closure of gaps in the signal. However, it has been reported [Warren et al., 1994, Auditory induction: Reciprocal changes in alternating sounds. Perception & Psychophysics, 55, 313-322] that, when contiguous pure tones having different frequencies and amplitudes are alternated, conditions can be chosen to produce a continuum of illusory lengthening, with closure as an easily perceived endpoint. The extent of illusory lengthening is accompanied by a proportional illusory decrease in apparent amplitude of the louder inducing tone. For this (and other reasons) I prefer the more general term “induction” to “continuity.”

Al Bregman in his email of 22 March considered the perception of auditory induction/continuity to be an example of an “old-plus-new heuristic” which he 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 similar, but represents a somewhat different approach than my consideration that auditory induction/continuity represents the reversal of masking that employs a contextually determined reallocation of a portion of the neural representation of the louder sound for the perceptual synthesis of the fainter. This difference in approach has led to Al’s hypothesis that phonemic restoration represents a “schema-driven stream segregation” that follows his gestalt principles for auditory organization and does not involve reallocation of a portion of the interrupting noise (Bregman, Auditory Scene Analysis, MIT Press, 1990). However, Warren et al. (1994) demonstrated that the loudness of the interrupting sound does decrease when phonemic restoration occurs, as with other types of illusory continuity.

By studying illusory continuity of dynamic signals such as tone glides and speech (phonemic restorations), it is possible to examine and to discover the special rules underlying the perceptual organization of these sounds, and a considerable literature has developed for both tone glides and speech. However, if we shift our attention to the higher amplitude sound inducing continuity, some interesting (and puzzling) phenomena are observed. This is best studied using the simplest condition: alternating levels of the same sound. The information gained using this “homophonic” continuity can inform us about the neurophysiological coding of amplitude level (i.e., loudness). Since this research is not widely known, I’ll describe it briefly. Take, for example, the case of alternating levels of a 1-kHz pure tone, each lasting 200 ms. When the higher level tone is fixed at 70 dB, and the lower (apparently continuous) level is raised in a step-wise fashion, the apparent loudness of the higher (70 dB) level decreases (as measured by matching with a comparison tone). When the level difference is one or two dB a dramatic change occurs: Illusory continuity and reallocation cease, and the veridical perception of alternating levels of the same pure tone is heard by all. In addition, when the amplitude difference approaches the alternation threshold, listeners may hear the 70 dB level as a pulsed atonal jangling sound superimposed on the continuous tone.

Noise behaves differently. When alternating levels of broadband noise are substituted for the 1-kHz tones, the effects observed for the higher amplitude sound differ, indicating differences in the mechanisms for loudness coding. As the apparently continuous lower level noise approaches that of the pulsing fixed level noise, the pulsing noise appears to become fainter. But listeners never hear two alternating levels of broadband noise. Even when the difference in the 200 ms noise levels is less than 1 dB, the higher amplitude noise is heard (paradoxically) as a very faint pulsed noise superimposed upon a much louder continuous noise.

Jim Bashford and I are mulling over some speculations concerning mechanisms. These are mainly based on the general principle that illusory continuity involves reallocation of a portion of the neural representation of the higher amplitude interrupting sound to synthesize the missing portions of the fainter sound. This would leave a diminished residue corresponding to the higher amplitude sound. Does the atonal residue that can be heard with alternating tones correspond to the perception of neural coding that represents an increase above the continuous level? Why does homophonic continuity with noise differ from that with tone? Any suggestions concerning the reciprocal interaction of two levels of the same sound would be appreciated.