A letter-identification task, previously demonstrated to show activation of
the pulvinar nucleus of the thalamus with fluodeoxyglucose position emissi
on tomography, was administered to 20 normal volunteers. The letter to be d
etected could appear alone as a big stimulus or as a small one stimulus sur
rounded by flanking letters. To test for a hemispheric specialization for f
iltering processes, the stimuli were displayed horizontally, either in the
left or in the right hemifield, or vertically, either above or below the fi
xation point. In addition, to study the effect of cognitive processes on se
lective attention resources, we varied the feedback conditions, by deliveri
ng or not delivering a blue flash in cases of misses or mistakes. The resul
ts show a significant interaction between the type of stimulus (alone or su
rrounded by flankers) and the hemifield of presentation (left or right) onl
y in the condition where the subjects were presented stimuli horizontally w
ithout any feedback, In this condition, reaction times (RTs) were shorter i
n the left hemifield than in the right hemifield for single stimuli, wherea
s for stimuli surrounded by flankers, the opposite pattern was observed, th
at is, shorter RT in the right hemifield than in the left one. The present
findings suggest a hemispheric specialization for selective attention, in p
articular at the subcortical level. (C) 2000 Elsevier Science B.V. All righ
ts reserved.