neuropsychologia and preference for precision grasping predicts language lateralization

We investigated whether or not there is a relationship between hand preference for grasping and hemispheric dominance for language—and how each of these is related to other traditional measures of handedness. To do this we asked rightand left-handed participants to put together two different sets of 3D puzzles made out of big or very small LEGO© pieces. Participants were also given two self-reported handedness questionnaires, as well as tests of grip force and finger tapping speed. A language lateralization (dichotic listening) test was also administered. We found a positive correlation between hand use for preerebral asymmetries ichotic listening anguage lateralization eft-handers eft hemisphere cision grasping and language lateralization (i.e. the more participants used their right hand for grasping the small LEGO© pieces, the more language was lateralized to the left hemisphere). In addition, we identified two populations of left-handers according to their grasping performance: ‘left-right-handers’, who behaved exactly like right-handers; and ‘left-left-handers’ whose performance was the mirror image of that of right-handers. Finally, we found an increase in right-hand use when right-handers and ‘left-righte sma © n for v eaching and grasping handers’ had to pick up th hemisphere specializatio Handedness is one of the most salient traits of cerebral asymetry. Ninety percent of the human population could be classified s right-handed in that they tend to favour this hand for a range of asks, such as writing and using tools. In the case of more “basic” ehaviours, such as reaching out and picking up an object, howver, it would make sense (from an ecological and biomechanical erspective) to use the hand closer to the object rather than the referred hand. Yet, few studies have actually compared hand prefrence for visually guided grasping in leftand right-handers and hese have asked to pick up tools or objects one by one (Brown, oy, Rohr, & Bryden, 2006; Calvert & Bishop, 1998; Mamolo, Roy, ryden, & Rohr, 2004; Mamolo, Roy, Bryden, & Rohr, 2005; Mamolo, oy, Rohr, & Bryden, 2006). Recently however, we measured spontaneous hand preference n a ‘natural’ grasping task. We asked rightand left-handed particiants to put a puzzle together or to create different LEGO© models, s quickly and as accurately as possible, without any instruction bout which hand to use. We found that right-handers showed marked preference for their dominant hand when picking up bjects; as a population, left-handers, however, did not show this reference for the dominant hand and instead used their right and 50% of the time (Gonzalez, Whitwell, Morrissey, Ganel, & oodale, 2007). Closer examination of the left-handers’ behaviour ∗ Corresponding author at: Department of Kinesiology, University of Lethbridge, 401 University Dr., Lethbridge, AB T1K 3M4, Canada. E-mail address: claudia.gonzalez@uleth.ca (C.L.R. Gonzalez). 028-3932/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. oi:10.1016/j.neuropsychologia. 2009.07. 019 ll LEGO pieces. We discuss our results in relation to recent notions of leftisually guided actions and its relationship with the evolution of language. © 2009 Elsevier Ltd. All rights reserved. suggested that rather than using both hands equally often, some left-handers simply preferred to use their right hand whereas others preferred to use their left. But, why would some left-handers use their non-dominant right hand more often when picking up objects? It certainly did not seem to be related to the degree of left-handedness; all the left-handers in this earlier study, including those who used their right hand to pick things up, were strongly left-handed as measured by the self-reported handedness questionnaire. Indeed, even in the right-handed population there was variability in the hand that participants used to pick up the small objects. Some right-handed participants used their right hand nearly 100% of the time whereas others used it less, closer to 50% of the time. The present investigation was designed to explore the relationship between laterality in precision grasping and other measures of laterality. First, we asked rightand left-handed participants to put together different 3D models using big LEGO© pieces or small LEGO© pieces. We anticipated that grasps made towards the big objects would be made more often with the whole hand (4–5 digits) whereas those made towards the small objects would be made with a precision (thumb and index) or a tripod grasp (thumb, index and middle finger). In this way, we hoped to see if the degree of skill needed to grasp an object plays a role in determining which hand one chooses in order to pick it up. Secondly, we wondered if traditional measures of handedness (self-reported questionnaires and hand-performance tasks) would predict hand preferences for visually guided grasping. Some studies have shown, for example, that performance variables such as grip