Fixation locations when grasping partly occluded objects Department of Experimental Psychology, Justus-Liebig-Universität Giessen, Giessen, Germany ,& Research Institute MOVE, Faculty of Human Movement Sciences, VU University Amsterdam, Amsterdam, The Netherlands Denise D. J. de Grave Department of Experimental Psychology, Justus-Liebig-Universitat Giessen, Giessen, Germany Constanze Hesse TNO, Soesterberg, The Netherlands Anne-Marie Brouwer Department of Experimental Psychology, Justus-Liebig-Universitat Giessen, Giessen, Germany Volker H. Franz When grasping an object, subjects tend to look at the contact positions of the digits (A. M. Brouwer, V. H. Franz, D. Kerzel, & K. R. Gegenfurtner, 2005; R. S. Johansson, G. Westling, A. Bäckström, & J. R. Flanagan, 2001). However, these contact positions are not always visible due to occlusion. Subjects might look at occluded parts to determine the location of the contact positions based on extrapolated information. On the other hand, subjects might avoid looking at occluded parts since no object information can be gathered there. To find out where subjects fixate when grasping occluded objects, we let them grasp flat shapes with the index finger and thumb at predefined contact positions. Either the contact position of the thumb or the finger or both was occluded. In a control condition, a part of the object that does not involve the contact positions was occluded. The results showed that subjects did look at occluded object parts, suggesting that they used extrapolated object information for grasping. Additionally, they preferred to look in the direction of the index finger. When the contact position of the index finger was occluded, this tendency was inhibited. Thus, an occluder does not prevent fixations on occluded object parts, but it does affect fixation locations especially in conditions where the preferred fixation location is occluded. Keywords: fixations, saccades, eye movements, occlusion, grasping Citation: de Grave, D. D. J., Hesse, C., Brouwer, A.-M., & Franz, V. H. (2008). Fixation locations when grasping partly occluded objects. Journal of Vision, 8(7):5, 1–11, http://journalofvision.org/8/7/5/, doi:10.1167/8.7.5. Introduction Fixation locations during visuomotor actions Several studies have examined eye movements during goal-directed hand movements. When pointing towards an object (either with their hand or with a pointing device), subjects tend to look at the position at which they are pointing. This suggests a tight coupling between gaze and pointing. Indeed, this link has been found in several studies. By looking at the target object, errors in pointing movements are reduced (Bock, 1986; Enright, 1995; Henriques, Klier, Smith, Lowy, & Crawford, 1998; Medendorp & Crawford, 2002; Neggers & Bekkering, 1999). Furthermore, if target presentation is delayed (Frens & Erkelens, 1991) or the target position has to remembered (Admiraal, Keijsers, & Gielen, 2003), the direction and the variability of the errors in pointing movements are correlated with those in gaze. When making a combined eye–hand movement towards a target that changes position during the saccade (double step paradigm), Bekkering, Adam, van den Aarssen, Kingma, and Whiting (1995) showed that the eye as well as the hand adapts to the new position of the target. These studies suggest that the same information is used to drive the hand and the eye or that they use a common motor command (Bekkering, Abrams, & Pratt, 1995; Biguer, Jeannerod, & Prablanc, 1982; Biguer, Prablanc, & Jeannerod, 1984; Bock, 1986; Reina & Schwartz, 2003). The coupling between eye and hand is not always so tight. Several experiments using visual illusions found Journal of Vision (2008) 8(7):5, 1–11 http://journalofvision.org/8/7/5/ 1 doi: 10.1167/8.7.5 Received August 10, 2007; published May 20, 2008 ISSN 1534-7362 * ARVO