The Continuity Illusion: Vowel Perception, Frequency Modulation, And
Hearing Backwards In Time

R.P. Carlyon, J.M. Deeks, D. Norris, and S. Butterfield MRC Cognition
& Brain Sciences Unit, 15 Chaucer Rd., Cambridge CB2 2EF, England.

When a 'target' is turned off and then resumed a short time later, it
can be heard as continuous, provided that the silent interval is
filled by another sound that would have masked the target if it had
actually remained uninterrupted. Hence both the level and frequency
content of the 'inducing sound' are crucial. We performed a series of
experiments investigating this 'continuity illusion' and its
relationship to other aspects of auditory processing. In one study, we
generated four different vowels, each consisting of two formants (F1
and F2). When the two formants were presented simultaneously,
identification performance was very good. In a second condition, they
were alternated for one second, so that F1 and F2 were never present
at the same time; the duration of each formant presentation was 100 or
200 ms. Performance in this condition was close to chance. In a third
condition, the F1s and F2s still alternated, but the silent intervals
in each formant region were filled by noise bursts. The same noise
burst was used to fill the gaps for all the F2s used, and its level
was set in a preliminary experiment to induce the illusion of
continuity for all F2s presented in isolation, and to fail to do so
for all F1s. Similarly, the noise used to fill all F1 gaps induced
continuity for all F1s in isolation, but for no F2s. Performance in
this condition was substantially better compared to the condition with
no noise, and to other conditions in which noise was added only to the
F1 or F2 gaps. This demonstrates that the neural mechanisms
responsible for vowel perception receive input from those underlying
the continuity illusion. A second study investigated the finding that,
when a frequency modulated (FM) tone is interrupted, and that
interruption filled by noise, listeners not only hear the tone as
continuous, but also hear the modulation continue through the
noise. We wondered whether the phase of FM would be preserved during
the illusion. To test this, we asked subjects to discriminate between
two stimuli, both of which consisted of two portions of a 1-kHz tone
modulated at a rate of 5 Hz, and separated by a 200-ms interval filled
by noise. The level and frequency content of the noise were sufficient
to induce the continuity illusion. In one of the two sounds the FM
phase was the same after the noise as it would have been if the tone
had been uninterrupted. Subjects could not discriminate between this
sound and one in which the FM phase after the noise was shifted by
180°. This shows that FM phase is not preserved in the illusion, and
demonstrates a paradoxical percept: subjects hear a modulation as
continuous, but do not notice what would be an obvious phase reversal
in that modulation. Finally, we presented listeners with a 300-ms
wideband noise, which was immediately followed (without interruption)
by a 300-ms narrowband noise. When asked to adjust the duration of a
second narrowband noise presented 500 ms later, they adjusted it to a
duration of about 370 ms. This is consistent with the onset of the
first narrowband noise being perceived as occurring before the end of
the wideband noise. We will present additional data investigating this
explanation. If correct, it is an example of 'hearing backwards in
time': a subsequent sound (narrowband noise) affects what is heard
before the end of a preceding sound (wideband noise).