Neuropsychologia 47 (2009) 1621–1626 Contents lists available at ScienceDirect Neuropsychologia journal homepage: www.elsevier.com/locate/neuropsychologia Vision in the palm of your hand Liana E. Brown a , Brendan F. Morrissey b,c,d , Melvyn A. Goodale b,c,d,∗ a Department of Psychology, Trent University, Peterborough, Ontario, Canada K9J 7B8 b Department of Psychology, University of Western Ontario, London, Ontario, Canada N6A 5C2 c Department of Physiology and Pharmacology, University of Western Ontario, London, Ontario, Canada N6A 5C2 d CIHR Group on Action and Perception, University of Western Ontario, London, Ontario, Canada N6A 5C2 article info Article history: Received 23 May 2008 Received in revised form 11 August 2008 Accepted 21 November 2008 Available online 28 November 2008 Keywords: Target location Visuomotor control Tactile resolution Multisensory processing Pointing abstract Here we show that pointing movements made to visual targets projected onto the palm of the hand are more precise and accurate than those made to targets projected onto back of the hand. This advantage may be related to the fact that the number of cortical bimodal neurons coding both visual and tactile stimuli increases with tactile receptor density, which is known to be higher in glabrous than in hairy skin. © 2008 Elsevier Ltd. All rights reserved. The hand and face are among the most densely innervated regions of the body with respect to touch. There are a greater num- ber of tactile receptors embedded in the skin of the hand and face than in other regions of the body (Edin, Essick, Trulsson, & Olsson, 1995) and there is a greater proportion of somatosensory cortex devoted to these areas relative to other body parts (Penfield & Boldrey, 1937). As a result, the skin of the hand and face has superior tactile perceptual resolution as evaluated by two-point discrimina- tion tests (Weinstein, 1968). Neurophysiological studies in the macaque monkey have revealed neurons in different brain regions that respond both to visual and tactile stimulation with overlapping receptive fields (RFs). These bimodal neurons have been found in the dorsal half of the ventral premotor cortex [PMv; a.k.a. the polysensory zone (PZ); Graziano & Gandhi, 2000], the ventral intraparietal sulcus (VIP), and the putamen (Graziano, Yap, & Gross, 1994). The skin and pericuta- neous space near the hands and face are represented more densely by these bimodal neurons than are the skin and surrounding space of other parts of the body. In short, there appears to be a correspon- dence between tactile receptor density and bimodal-cell density. Body parts with higher tactile receptor density have greater visual- ∗ Corresponding author at: CIHR Group on Action and Perception, University of Western Ontario, London, Ontario, Canada N6A 5C2. Tel.: +1 519 661 2070; fax: +1 519 661 3961. E-mail address: mgoodale@uwo.ca (M.A. Goodale). tactile bimodal representation than body parts with lower tactile density and perceptual resolution. An interesting feature of visual-tactile neurons is that they can be recruited by a visual stimulus presented alone (i.e. without any accompanying tactile stimulation) on or near a body part, primarily the hand or face. Thus, the question arises as to whether or not the ‘quality’ of the representation of a purely visual stimulus presented on the skin is correlated with the tactile resolution of that skin. In other words, high bimodal-cell density associated with skin regions with high tactile receptor density could mean that visual stimuli that appear on or near these regions would be represented more robustly than visual stimuli presented on or near skin regions with low tactile receptor density. To test this possibility, we asked healthy undergraduates to per- form visually guided pointing movements with their right hand to targets presented on their palm or on the back of their left hand. We chose this task to measure target quality because we know that measures of pointing accuracy and precision are sen- sitive to the location of targets defined using tactile and noxious stimuli (Koltzenburg, Handwerker, & Torebjork, 1993; Moore & Schady, 1995). Moreover, it seems likely that bimodal neurons evolved for the control of visually guided movements in concert with tactile information (e.g. Graziano & Cooke, 2006). Palmar glabrous skin has an estimated mechanoreceptor density of approx- imately 70 units/cm 2 , providing this surface with high tactile acuity (Johansson & Vallbo, 1979). In contrast, the less sensitive hairy skin on the back of the hand has an estimated tactile receptor density of less than 5 units/cm 2 (Macefield, 1998). If the relationship between 0028-3932/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2008.11.021