Role of contrast, lettercase, and viewing conditions in a lateralized word-naming task.

Abstract

Because of the asymmetric functional organization of the brain, the process of naming a visually presented word necessarily engages, in its end stage, the structures of the left anterior hemisphere, forcing information received by the right hemisphere to be transferred across the corpus callosum. On the basis of this functional organization, a task requiring speeded naming of a laterally presented word was designed to examine the nature of the information transferred from the right to the left hemisphere and the type of process initially performed by the right hemisphere before transfer. Three categories of variables, each affecting particular components of the stimulus were manipulated in a within-subject design: (1) the viewing conditions, with the target word presented under flicker masking, which desensitizes the transient low-frequency channels while increasing the sensitivity of the low-frequency sustained channels, with backward pattern masking, which interferes with the integration of high spatial frequencies, or without masking; (2) the contrast of the word stimulus, which determines stimulus energy; and (3) the lettercases, which influence the relative salience of the component and configural properties of the word. Analysis of latencies showed a right-visual-field main effect and interactions between visual field and contrast and between visual field and lettercase. Right visual field superiority increased with a reduction in stimulus energy, and the right hemisphere benefited more than the left hemisphere from the additional information provided by the configural property of the lowercase word. The viewing conditions influenced the strength of these effects but did not modify their general pattern. The results are discussed with respect to interhemispheric transfer of visual information and the processing competence of the cerebral hemispheres.