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All you physiologists ...
This query is directed to those of you who are knowledgable
concerning the physiology of audition. We recently completed a
series of studies. Excerpts from our write-up are quoted below.
We end up by saying that we don't know of any physiological
studies that show how the phasic response of auditory neurons is
affected by the RATE of change in intensity of the signal (either
as an increase or a decrease). Can anybody help us? We also
welcome any other types of comments.
- Al Bregman
The experiments that we shall report were an attempt to
clarify a phenomenon that we observed in the laboratory. We were
experimenting with some software that could reverse the order of
samples in a digital sound file, a process analogous to playing a
tape backwards in a tape recorder. When a tone with a rapid rise
and a slow decay was played forward, it sounded like a clear
plucked-instrument tone. When played backward, however, it
sounded less definite. Even more interesting was the observation
that when a cluster of sudden-onset tones, highly overlapped in
time, were played in the forward order, they sounded distinct and
their order was clearly heard. However when the file was played
from back to front, the tones blended together and the temporal
order was unclear. Apparently this was due to the slow onsets of
the tones in the reverse direction. We decided to design formal
experiments to examine the role of abruptness of onset and offset
in these phenomena.
The findings concerning onset abruptness can be summarized
as follows: When clusters of pure tones of different frequencies
were highly overlapped in time, the ability to judge the order of
onset of the components was strongly affected by the suddenness
of their onsets (Experiment 1). As the onsets became more
abrupt, the ability to discriminate the order became more acute
up to and including the second fastest onset velocity used. At
the fastest one, there was a small decline in performance.
However, this decline could have been due to some disruptive
effect caused by the most abrupt onsets, and tended to disappear
with practice. Alternatively, it could have been a statistical
artifact. When we ourselves listened to the clusters, the
pitches of the more abrupt onset tones seemed more distinct than
those of tones with slower onsets.
When the onsets were synchronous but the offsets
asynchronous (Experiment 2), abruptness of change was again
important. More abrupt offsets made it easier to discriminate
the order in which the different pitches dropped out. However,
the ability to detect the order of offset was much worse than for
the order of onset. When we listened to the asynchronous offset
signals, the sudden offsets sounded like "plops" colored by the
pitch of the component that was dropping out.
Asking subjects to judge the quality and locations of
pitches in the stimuli gave results that tended to confirm our
own experiences. The rated "number of distinct sounds" declined
as the onsets or offsets became less abrupt. More distinct
pitches were heard at the onsets than at the offsets.
The rated "clarity" of pitches also declined smoothly as the
onsets became more gradual. The clarity of onsets was greater
than those of offsets. The mean differences between conditions
were less than for the ratings of "number of distinct sounds."
This may mean that there was some perceptual process other than
the one that asseses the pitches that was facilitated. This
process, revealing itself in the judgement of "number of distinct
pitches" may have played a role in the discrimination of the
order of onsets and offsets in the earlier experiments.
The judgements of "position of clear pitches" also showed
effects of the abruptness of change. Judgements of "clear
pitches at the beginning" or "clear pitches at the end" were
strongly associated with sudden onset conditions while the
response, "no clear pitches", was given primarily for the gradual
onsets or offsets.
It is interesting to recall that all the components were
within a critical band. When they were not asynchronous, it was
difficult to hear three different pitches in the mixture. Only
when a component changed in intensity did its pitch become
evident. This suggests that sudden amplitude changes cause a new
pitch analysis to be undertaken. The components that have
suddenly changed will dominate this analysis. Presumably the
other pure tones do not interfere with the computation of the new
pitch, despite being in the same critical band, because the
auditory system does not respond to them as strongly as to the
rapidly changing frequencies.
The fact that offsets also benefit from the suddenness of
the change suggests that Kubovy's name "onset-segregation effect"
may be too specific. Perhaps it is a "sudden-change" segregation
effect. In natural sounds, sudden onsets are more usual, being
caused by sudden impacts, as in the piano, or by sudden releases,
as in the guitar. Sudden drops in intensity would have to be
caused by a sudden stopping of a vibration, as at the end of the
word "hip". Although we distinguish sudden-onset sounds by a
name, "percussive", we have no comparable name for sudden-offset
sounds. The fact that sudden offsets do not give as clear
pitches as sudden onsets may be due to the fact that by the time
the auditory system is alerted to the sudden change, the energy
has greatly diminished and there is less left to analyze.
The well-known fact that many neurons in the auditory system
have a phasic response, responding more strongly to onsets than
to continuations of a sound could be the basis of the onset
effect that we observed. Presumably, the phasic response is tied
to the rate of onset of the sound. We are not, however, aware of
any physiological observations that show this. The offset effect
is also mysterious physiologically.