Discrimination within and between hemifields: a new constraint on theories of attention.

Subjects were presented with two groups of characters and were to decide whether they were the same or different. The stimulus groups differed either by a single feature ("preattentive" trials) or by a conjunction of features ("attentive" trials). The two stimulus groups appeared at the corners of an imaginary square centered about the fixation point, falling either in the same or different hemifields. In two experiments, subjects evaluated both types of stimuli faster when they were presented in different hemifields than in the same hemifield. Subjects also compared pairs of single characters faster when they appeared in different hemifields. Finally, this different-hemifield advantage was eliminated when the characters appeared sequentially. These results indicated that two stimuli that appear simultaneously in the same hemifield engender either a competition for common processing structures or intrahemispheric inhibition.