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Abstract:
Studies of patients with unilateral brain damage as well as
brain imaging of healthy individuals show that the right hemisphere
(RH) focuses on the global form of hierarchical visual stimuli and
the left hemisphere (LH) on its local elements. However, in
lateralized perceptual studies of normal subjects, a RH advantage
for global discrimination and a LH advantage for local
discrimination has been reported only once. In our first study,
normal subjects responded with manual go-no-go RTs, which avoids
spatial compatibility effects, to either of two targets that
appeared at either the global or the local level (divided
attention) of lateralized hierarchical letters. The stimuli
composed a few big (FB) or many small (MS) local letters, and were
followed by a pattern mask. Regardless of stimulus type, there was
a RH advantage for global targets and a LH advantage for local
targets. In a second study, subjects responded with vocal RTs to a
specified level (focused attention) of FB or MS stimuli, which
revealed a LH advantage for local discrimination in MS stimuli, but
no other asymmetries. In the context of prior literature, our
studies suggest that when each hemisphere of the normal brain is
free to focus on its preferred level, the preference overrides the
relative stimulus salience of the two levels. Future studies will
determine the relative importance of a divided-attention paradigm,
multiple targets, post-stimulus masking, and a go-no-go manual
response mode in eliciting RH global and LH local
advantages.
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