The Effect of a Flashing Visual Stimulus on the Auditory Continuity Illusion

Article excerpt

The effect of a visual stimulus on the auditory continuity illusion was examined. Observers judged whether a tone that was repeatedly alternated with a band-pass noise was continuous or discontinuous. In most observers, a transient visual stimulus that was synchronized with the onset of the noise increased the limit of illusory continuity in terms of maximum noise duration and maximum tone level. The smaller the asynchrony between the noise onset and the visual stimulus onset, the larger the visual effect on this illusion. On the other hand, detection of a tone added to the noise was not enhanced by the visual stimulus. These results cannot be fully explained by the conventional theory that illusory continuity is created by the decomposition of peripheral excitation produced by the occluding sound.

The auditory continuity illusion is a compelling perceptual effect in which a sound (the inducee) interrupted momentarily by an extraneous sound (the inducer) is perceived as continuing through the inducer, even if the inducee is, in fact, physically absent during the presentation of the inducer (Houtgast, 1972; Miller & Licklider, 1950; Warren, Obusek, & Ackroff, 1972; see Warren, 1999, for a review). This phenomenon not only is a powerful illusion, but also represents an important ecological function of the auditory system that compensates for the effect of the masking by extraneous sounds often encountered in everyday situations. However, the mechanism underlying the auditory continuity illusion is not fully understood.

Some clues have been obtained concerning the processing stage at which the illusion is created. It has been shown that the continuity illusion involves frequency-selective processes. It occurs only when there is no decrease in excitation at the frequencies of the inducee during the presence of the inducer (Darwin, 2005; Houtgast, 1972; Warren et al., 1972). This masking potential rule indicates that peripheral filtering in the auditory system is involved in the process of creating the illusory continuity.

It has also been shown that the continuity illusion involves processes sensitive to binaural disparity. Kashino and Warren (1996) demonstrated that the illusion is inhibited when the interaural phase difference of the inducer and that of inducee are different and, as a consequence, the masking potential of the inducer is reduced due to the binaural masking level difference. This suggests that illusory continuity is produced after binaural convergence at the level of the brainstem nuclei. Darwin, Akeroyd, and Hukin (2002) also demonstrated the binaural contribution to the continuity illusion, using Huggins pitch.

There is also evidence that the continuity illusion involves processes that operate after the information from different frequency channels is integrated. Darwin (2005) examined the effect of simultaneous grouping of frequency components on the continuity illusion and demonstrated that judgments of auditory continuity are based on entire, simultaneously grouped objects, rather than solely determined by local criteria based on individual frequency channels.

Several studies have indicated that the process responsible for the auditory continuity illusion should operate before the perceptual attributes (e.g., loudness, pitch, etc.) of an auditory object are computed. For example, the apparent loudness of the inducer is reduced when continuity is perceived (McAdams, Botte, & Drake, 1998; Warren, Bashford, Healy, & Brubaker, 1994). Furthermore, continuity improves fundamental frequency (pitch) discrimination for unresolved complex tones (Plack & White, 2000).

Some studies in which a forced choice task has been employed have shown that the continuity illusion can impair performance (Petkov, O'Connor, & Suiter, 2003). This suggests that the continuity illusion is an automatic and compulsory process, free from the voluntary control of listeners. …