when the stimulus ensemble comprises single context: a single frequency whose SPLs change at some point in time. Such single-context conditions show only assimilation effects described earlier; it takes dual context to produce differential contrast.
Clearly time--memory, on a relatively long scale; frame-of-reference-- is important in determining what we judge, how we judge it, and whether we judge two stimuli to be the same or different (see also Ward, 1987). Although we know a good deal about short-term sequential effects, we understand much less about the presumably longer term effects that can so influence perceptual judgments and psychophysical invariances. To locate processes that may pool effects over scores or hundred of trials requires appropriate experimental procedures. Perhaps the long-term process also has a "reset," such that major changes in the stimulus environment can act like typing "CLR" on a microcomputer, abrogating some of the recent contextual effects.
What does all this say about the processing and judgment of sensory and perceptual information? Perception--or, at least, perceptual judgment--is a dynamic process, one that depends not only on the current stimulus but on the possible stimuli that may occur and--what may be the same thing--on the recent and not-so-recent stimuli that have occurred. When we have an adequate theory as well as predictive models of psychophysical perception and judgment, we may be able to say whether it is just perceptual judgment--or whether it is perception itself--that exhibits this dynamism.
Preparation of this chapter was supported by NIH Grants NS21326 and RR05692.
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