Speech-independent production of communicative gestures: evidence from patients with complete callosal disconnection

Recent neuropsychological, psycholinguistic, and evolutionary theories on language and gesture associate communicative gesture production exclusively with left hemisphere language production. An argument for this approach is the finding that right-handers with left hemisphere language dominance prefer the right hand for communicative gestures. However, several studies have reported distinct patterns of hand preferences for different gesture types, such as deictics, batons, or physiographs, and this calls for an alternative hypothesis. We investigated hand preference and gesture types in spontaneous gesticulation during three semi-standardized interviews of three right-handed patients and one left-handed patient with complete callosal disconnection, all with left hemisphere dominance for praxis. Three of them, with left hemisphere language dominance, exhibited a reliable left-hand preference for spontaneous communicative gestures despite their left hand agraphia and apraxia. The fourth patient, with presumed bihemispheric language representation, revealed a consistent right-hand preference for gestures. All four patients displayed batons, tosses, and shrugs more often with the left hand/shoulder, but exhibited a right hand preference for pantomime gestures. We conclude that the hand preference for certain gesture types cannot be predicted by hemispheric dominance for language or by handedness. We found distinct hand preferences for specific gesture types. This suggests a conceptual specificity of the left and right hand gestures. We propose that left hand gestures are related to specialized right hemisphere functions, such as prosody or emotion, and that they are generated independently of left hemisphere language production. Our findings challenge the traditional neuropsychological and psycholinguistic view on communicative gesture production.