This study investigates whether interhemispheric interactions mediated by the corpus callosum play a role in orienting attention across the vertical meridian. Patients with complete or partial section of the corpus callosum participated in a spatial precueing task under conditions that required covert shifts of attention within or between the visual fields. Patients with complete callosal section demonstrated normal costs on invalid trials when the cue and target appeared in the same visual field. However, these patients were impaired on invalid trials in which attention had to be redirected across the vertical meridian. The between–within difference emerged only for patients with complete callosal section; it was not evident for a patient with section restricted to the anterior two-thirds of the callosum. Control experiments demonstrated that the deficit (1) is specific to shifts across the vertical meridian, (2) is not due to shifting between left and right hemispace, and (3) is related to the voluntary allocation of attention in response to the cue. These results suggest that interhemispheric communication, which is normally mediated by the posterior region of the corpus callosum, contributes to the efficient movement of attention between visual fields.

When observers are presented with hierarchical visual stimuli that contain incongruous coarse (“global”) and fine (“local”) pattern attributes, the global structure interferes with local pattern processing more than local structure interferes with global pattern processing. This effect is referred to as “global precedence.” The present experiments tested the hypothesis that global precedence depends on the presence of low spatial frequencies using stimuli constructed from “contrast balanced dots.” Stimuli composed of contrast balanced dots are largely devoid of low-frequency content. Choice reaction time to identify either the local or global pattern information was the dependent measure. Global precedence was found only for control stimuli that contained low spatial frequencies. In the absence of low-frequency information, local precedence was obtained. These findings suggest that global precedence is heavily dependent on the low spatial frequency content of the patterns.