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LETTER TO THE EDITOR
; Dewar and Carey 2006; Haffenden et al. 2001). In other words, maximum grip aperture is typically scaled to the real—not the apparent—size of visual targets in these situations, reflecting the fact that the visuomotor transformations mediating skilled actions such as grasping make use of real-world metrics (Goodale and Milner 2004). In Gonzalez et al. (2006), we demonstrated that this insensitivity is much more apparent in the right hand than in the left—and that this is true in both right- and left-handers. On the basis of this finding, we suggested that the left hemisphere may play a special role in visuomotor control and that this left-lateralized control may also be present in most left-handers.
In putting forward this admittedly tentative idea, we did not mean to imply that the "motor command" center that Dr. Derakhshan refers to is organized in a similar fashion. We were referring entirely to those networks involved in transforming visual information into the required motor coordinates for action. These visuomotor networks, which appear to involve dorsal-stream areas within the intraparietal sulcus and the parieto-occipital junction, are anatomically distinct from those brain regions traditionally associated with language, praxis, and executive control (Goodale and Westwood 2004; Prado et al. 2005). This is not to say that the selection of particular motor outputs and the specification of the visual coordinates for action are not intimately related. Indeed, we discuss this possibility in the paper. Nevertheless, we recognize that such interactions do not require that all of these complex functions be represented in the left hemisphere and that the degree of interhemispheric interaction required to select, program, and control a visually guided action could vary enormously depending on the task and the individual performing it.
It is also worth mentioning that the task we used—picking up a small target embedded in an illusory display—was not a speeded one. Indeed, we wanted participants to choose their target on the basis of their perceptual impressions as to which one was the bigger (or smaller) of the two targets (which were of course identical in size on illusion trials). For this reason, reaction time would not have been a reliable measure of the lateralization of the "motor command" center. In point of fact, the reaction times for both hands were relatively long and highly variable and, as a consequence, did not show a reliable difference between the hands.
Again we emphasize that our demonstration of a right-hand "advantage" for visuomotor control in right- and left-handers says nothing about traditional notions of hemispheric dominance, either for language or for a motor command center. It does, however, suggest that lateralization of visuomotor control to the left hemisphere may be yet another functional asymmetry in the human brain that must be considered. Moreover, because the emergence of skilled visually guided limb movements almost certainly predates the emergence of speech and tool use, it is possible that the hemispheric organization of the functional networks underlying the visuomotor control also predate those implicated in language and praxis.
REFERENCES
Aglioti S, DeSouza JF, and Goodale MA. Size-contrast illusions deceive the eye but not the hand. Curr Biol 5: 679–685, 1995.[CrossRef][ISI][Medline]
Dewar MT and Carey DP. Visuomotor "immunity" to perceptual illusion: a mismatch of attentional demands cannot explain the perception-action dissociation. Neuropsychologia 44: 1501–1508, 2006.[CrossRef][ISI][Medline]
Gonzalez CLR, Ganel T, and Goodale MA. Hemispheric specialization for the visual control of action is independent of handedness. J Neurophysiol 95: 3496–3501, 2006.
Goodale MA and Milner AD. Sight Unseen: An Exploration of Conscious and Unconscious Vision. Oxford, UK: Oxford Univ. Press, 2004.
Goodale MA and Westwood DA. An evolving view of duplex vision: separate but interacting cortical pathways for perception and action. Curr Opin Neurobiol 14: 203–211, 2004.[CrossRef][ISI][Medline]
Haffenden AM, Schiff KC, and Goodale MA. The dissociation between perception and action in the Ebbinghaus illusion: nonillusory effects of pictorial cues on grasp. Curr Biol 11: 177–181, 2001.[CrossRef][ISI][Medline]
Prado J, Clavagnier S, Otzenberger H, Scheiber C, Kennedy H, and Perenin MT. Two cortical systems for reaching in central and peripheral vision. Neuron 48: 849–858, 2005.[CrossRef][ISI][Medline]
Claudia L. R. Gonzalez
Melvyn A. Goodale
CHR Group on Action and Perception
Department of Psychology,
University of Western Ontario,
London
Ontario N6A 5C2
Canada
Tzvi Ganel
Department of Behavioral Sciences,
Ben Gurion University of the Negev,
Beer-Sheva 84105
Israel
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