Cognitive neuroscience and neuropsychology 1507 Hemispheric dominance for mental rotation: it is a matter of time Branka Milivojevic a,b , Jeff P. Hamm b and Michael C. Corballis b Mental rotation is often said to be a right-hemisphere function. The evidence for this claim, however, remains equivocal. To investigate whether right-hemispheric superiority relates to speed of processing, we measured the latencies and amplitudes of individual participant’s mental-rotation event-related potential effects over the parietal electrodes. The results suggest that increases in parietal negativity begin around 400 ms after stimulus onset, and continue until 550 ms over the right hemisphere, and 610 ms over the left hemisphere. The effect of orientation on event-related potential amplitudes during those times do not differ between hemispheres. These results indicate that the lateralization effect is primarily related to timing, rather than the extent of cortical involvement. NeuroReport 20:1507–1512  c 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins. NeuroReport 2009, 20:1507–1512 Keywords: EEG, event-related potential, evoked potentials, hemispheric asymmetries, mental rotation, right hemisphere, right parietal, visual-evoked sensory potential a Department of Psychology, University of Glasgow, Glasgow, Scotland, UK and b Research Centre for Cognitive Neuroscience, University of Auckland, Auckland, New Zealand Correspondence to Dr Branka Milivojevic, PhD, Department of Psychology, University of Glasgow, 58 Hillhead Street, Glasgow G12 8QB, Scotland, UK Tel: + 44 (0) 141 330 6165; fax: + 44 141 330 4606; e-mail: branka.mili@gmail.com Received 15 April 2009 accepted 20 May 2009 Introduction Mental rotation (MR) refers to imagined rotation of an object from one orientation to another. It is often cited as the prototypical higher-level visuospatial cognitive operation and, as such, it has been suggested that MR is more dependent on processes in the right than in the left hemisphere (e.g. Ref. [1]). The empirical evidence for this greater right-hemisphere involvement supposition is currently equivocal; although some studies have re- ported evidence for a right-hemispheric superiority for MR [2–4], others have reported bilateral involvement [5–8] or even a left-hemispheric superiority [9,10]. One account for mixed lateralization findings is that the hemispheric dominance for MR is dependent on the strategy that an individual uses to perform the task [11]; the right hemisphere is recruited when the participants use a holistic strategy and rotate the image as a whole; the left hemisphere is recruited when the participants use an analytic strategy and rotate the image in a piecemeal manner. In addition to individual preference, the choice of stimuli can also affect the strategy used for MR; piecemeal processing is elicited by complex unfamiliar shapes, such as torus shapes, requiring rotation in depth [12]; holistic processing is elicited by familiar two-dimensional stimuli, such as alphanumeric characters, requiring rotation in the picture plane [13]. However, even the studies that have used rotated mirror-normal letter decision task, devised by Cooper and Shepard [14], have failed to yield consistent results in neuroimaging studies, whereas some studies report selective MR-related activation in the right hemisphere [2], some in the left hemisphere [10] and others reporting bilateral involvement [7]. Corballis [1] suggested that some of the conflicting results reported in the literature could be explained as relative superiority of the right hemisphere in terms of the rate of MR. That is, both hemispheres can perform the operation, but the right hemisphere can do so more quickly. If this is the case, then superior temporal resolution of the event-related potentials (ERPs) would allow for the relative timing of the MR effects over the left and the right hemispheres to be examined. The ERP correlate of MR is an increase in parietal negativity approximately between 350 and 800 ms post- stimulus onset [15,16]. The amplitude of this slow parietal negativity increases monotonically as a function of the angular disparity of stimuli from the upright position, mirroring the reaction time results and suggest- ing that this ERP component is closely tied to the neurophysiological operations underlying MR [17]. Although the spatial resolution of ERPs is relatively poor in comparison with functional MRI (fMRI) and PET, the question of hemispheric lateralization can be addressed with high-density ERPs if the cortical generators of the activity are sufficiently distant from the midline, as seems The research was carried out at the Research Centre for Cognitive Neuroscience, University of Auckland, Auckland, New Zealand. 0959-4965  c 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins DOI: 10.1097/WNR.0b013e32832ea6fd Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.