items belonging to a particular category also shared the same brightness, a condition
called "correlated physical dimension present" (Merikle, 1980, p. 287). The correlated
physical dimension was absent on the other half of the occasions.
Subjects were instructed to recall letters only, digits only, or as many items as possible
(regardless of category membership). The finding of interest is the observation that
partial report was superior to whole report, despite the fact that the selection criterion for
partial report was category information, both in the presence and in the absence of the
correlated physical dimension. The conclusion was drawn that selection by category
under the partial-report situation was possible. Consequently, "iconic memory might not
be precategorical in nature" (Merikle, 1980, p. 288).
The validity of Merikle's (1980) conclusion can be questioned because the partial-report
cue was given to the subjects before stimulus presentation. Experiment 2 was designed to
test the perceptual grouping explanation of the basic sensory findings.
Method
Subjects. Six of the 12 subjects in Experiment I were given an honorarium to participate
in Experiment 2.
Apparatus. The apparatus was the same as that used in Experiment 1.
Materials. There were three kinds of stimulus items: the ensemble of letters of the
alphabet used in Experiment 1, the digits 2 through 9, and some common symbols. The
ensemble of symbols was: $, ?, #, Q, %, and *. Three items were selected randomly,
without replacement, from each of the three kinds of stimuli on every trial.
Three types of stimulus displays were prepared for Experiment 2. Examples of
"square-good gestalt," "square-poor gestalt," and "column" cards can be found in Figure
5. A "square-good gestalt" card is one in which the three items from a particular category
(e.g., letters) form a triangle in the display, whereas the rest of the six items from the
other two categories are distributed randomly in the display. A "square-good gestalt" card
subtends a visual angle of 2° 14' in width as well as in height
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