The Use of Iambic Pentameter in the

luminance-matched displays, pre-constancy information was relatively ambiguous with
respect to whether the target was present or absent. These displays were ambiguous
because the target had the same luminance as many of the distracters in the stimulus. In
luminance-unmatched displays, however, both the pre-constancy and post-constancy
information were unambiguous with respect to target presence. These displays were
unambiguous because the targets’ luminance did not match that of any of the distracters.
Participants were asked to look at the displays and report whether or not they saw a
lighter or darker square, relative to the medium distracters. If pre-constancy information
influenced visual search, reaction times should have been longer for the luminance-
matched trials. The results showed that reaction times were in fact longer for luminance-
matched trials. This suggests that pre-constancy features can affect performance in visual
search tasks.
Given that pre-constancy features can affect visual search, it is not unreasonable
to assume that these features might affect spatial grouping as well. Recall that Gulick and
Stake (1975) unintentionally generated this type of result with their study of grouping by
size constancy. Do other types of perceptual grouping operate along a time course? In
what follows, several experiments that have tested the time course of perceptual grouping
are described in detail.
Grouping by proximity and alignment. Do the Gestalt principles of grouping
require time to manifest? If so, do some principles manifest more rapidly than others?
Kurylo (1997) spoke to these questions by examining the time course of grouping by the
Gestalt principle of proximity and by alignment. Note that alignment is an instance of the
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