Lateral differences in visual processing: relative vs exclusive hemispheric specialization.

In some cases, lateral differences reveal cerebral hemispheric asymmetry in cognitive operations. Such asymmetry can indicate the absolute dominance of one hemisphere, the "dominant" hemisphere managing the task whatever the hemisphere of entry. Consequently, a commissural transfer is needed for stimuli sent to the non-dominance as only relative: the hemisphere receiving the stimulus is able to manage it, but the dominant one can do it more accurately and/or faster than the other. These two models should be able to be distinguished by means of a correlational study and a study of the stimulated hemifield X responding hand interaction. The latter is based on the assumption of contralateral control of slight distal movements. These two methods were applied to data derived from 24 right-handed normals who were given three tasks for which right visual field superiority was expected: word/non-word discrimination, vowel/consonant classification of letters, and odd/even categorization of numbers. The stimuli were displayed centrally or laterally for 150 ms. The data indicated that the right-field superiority for words resulted from a relative dominance, that the right-field advantage for numbers resulted from an absolute dominance, and that no asymmetry emerged for letters.