| Summary: | It is easier to determine the identity of a pair of mixed-case letters when they appear on opposite
sides of fixation (across-display) than when presented on the same side (within-display). This
across-display advantage is taken as evidence by some that the hemispheres can each process letter
identity independently (computational complexity theory, Banich, 1998). One unexplored feature
of the across-display advantage is that it typically occurs in conjunction with a left-field advantage.
If a within- display advantage is obtained instead, it typically occurs in conjunction with a right-field
advantage. One reason for studying this relationship further is because both field (left vs. right)
and display (across vs. within) advantages may index hemispheric differences in attentional
orienting. According to the asymmetrical orienting hypothesis, the left hemisphere orients to right
space, generating both right-field and within-display advantages for some tasks. The right
hemisphere orients most efficiently to left space, generating a weak left-field advantage, but it can
also orient to right space, contributing to an across-display advantage in other tasks. These
interpretations were tested in a total of 8 experiments, involving 242 participants. Experiments 1
&2 confirmed the correlation between left-field and across-display advantages in mixed-case letter
rmtching. Experiment 3 revealed similar left-field and across-display advantages in same-case letter
matching when a task irrelevant dimension (color) was added. Experiments 4-6 tested matching
tasks and response modes favoring the left hemisphere (localization, color reporting, rmtching on
rhyme) in an effort to dissociate the field and display effects. This was unsuccessful. Finally, in
Experiments 7 and 8, participants were instructed to mentally compare letters in a strict order.
This lead to a mimicking of the standard data pattern, or its elimination, depending on which
instructions were followed. These findings do not support either theory because strategic effects
should not affect parallel processing or hemispheric differences in attentional orienting. The
implications are discussed with regard to hemispheric interactions, attentional orienting, and the
more general problem of estabfohing priority in visual processing. A qualitative model is
presented to account for the data, involving both top-down strategic biases (left-to-right
comparison) and bottom-up processing biases (opposite-direction orienting).
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