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Stimulus Onsets and Offsets

     I returned from vacation to find an interesting E-mail
discussion on the importance of stimulus onset.  We have several
older studies (all published in Perception & Psychophysics) which
looked at different aspects of stimulus onset and offset.  In one
series of studies we were interested in the possible role played
by stimulus change in non-simultaneous masking.  The motivation
for the study was (1) the apparent salience of stimulus change in
the electrophysiological encoding of sensory information, (2) the
importance of spatial and temporal contours (change) in a number
of different sensory systems, and (3) the increased masking of
gated over continuous maskers (work of Rabb and Green).  In one
of our articles (P & P, 1975, 17, 445-449) we report on a
potential contribution of stimulus change to backward masking.
In a later study (P & P, 1980, 28, 547-549) we report on an
interaction between backward and forward masking based upon both
cuing and attentional properties of stimulus onsets and offsets.

     In our work on temporal order identification, we included an
evaluation of the abruptness of stimulus onset (P & P, 1988, 44,
257-271).  For stimuli with abrupt onsets (10 ms or less),
temporal order identification threshold is independent of
stimulus rise time.  For stimuli with more gradual onsets, the
temporal order identification threshold is a function of stimulus
rise time, thus paralleling findings reported for speech stimuli.
Finally, we also have looked at temporal order identification for
stimulus offset (P & P, 1983, 33, 54-62).

     I hope that some of this information is useful to you and
others interested in the perceptual organization of stimuli.  I
look forward to the continuing dialogue on this topic.

Dick Pastore