Hemispheric differences in dichaptic scanning of verbal and spatial material by adult males and females.

An experimental design based on the combination of dichaptic presentation associated to the Posner's paradigm was adopted to investigate laterality effects for verbal and spatial non-linguistic stimuli in male and female adult normal subjects. In a grapheme similarity judgment task based on "Name Identity" a right hand/left hemisphere advantage was found. Conversely, laterality effects were neither observed when the task involved "Perceptually Identical" or "Different" letter pairs. In a further experiment, the same methodology was adopted to verify hemispheric effects with spatial non-linguistic material, and a significant advantage for the left hand/right hemisphere was observed. Contrary to many previous studies, no gender or gender x task effects have been detected in both experiments. The present results suggest the existence, also in the tactile domain, of a direct link between input type and the linguistic or non-linguistic processing to which the two hemispheres are devoted. The overall pattern of data seriously hampers Witelson's original hypothesis that letter stimuli presented in the tactile modality are primarily processed as spatial stimuli, and are therefore dependent on the right hemisphere functioning.