Authors: Denise D J de Grave Eli Brenner Jeroen B J Smeets
Publish Date: 2008/11/15
Volume: 193, Issue: 3, Pages: 421-427
Abstract
Recently Gonzalez et al J Neurophys 953496–3501 2006 reported that movements with the left hand are more susceptible to visual size illusions than are those with the right hand We hypothesized that this might be because proprioceptive information about the position of the left hand is less precise If so the difference between the hands should become clearer when vision of the hand is removed so that subjects can only rely on proprioception to locate their hand We tested whether this was so by letting righthanded subjects make openloop pointing movements within an illusory context with and without vision of their hand On average the illusion influenced the left and the right hand to the same extent irrespective of the visibility of the hand There were some systematic differences between the hands within certain regions of space but these were not consistent across subjects We conclude that there is no fundamental difference between the hands in susceptibility to the Brentano illusionWhen investigating the effect of visual illusions on visuomotor performance tasks are normally performed with the dominant hand For most people this is the right one Gonzalez et al 2006 examined how visual illusions affect grasping movements with the left and the right hand in both left and righthanded subjects They showed subjects small objects which they had to pick up from within an illusory context The illusion affected maximum grip aperture more when grasping movements were performed with the left hand than when they were performed with the right hand irrespective of the subject’s handednessAlthough this seems a rather strange result one may not simply expect lefthanded subjects to behave in a mirrorsymmetric manner to righthanded ones When left and righthanded subjects are asked to grasp small objects with a precision grip righthanded subjects prefer to use their dominant hand whereas lefthanded subjects use their right hand about half of the time Gonzalez et al 2007 see also Stins et al 2001 Several other studies have also shown differences in performance between the arms reviewed in Goble and Brown 2008 Important for the present study is the finding of Van Beers et al 1998 who demonstrated that the nonvisual proprioceptive information about the position of the hand is not only more precise for the right hand for righthanded subjects but also for lefthanded onesThe asymmetry in proprioceptive accuracy is the basis of a possible explanation for the larger effect of illusions on the left hand A lower precision in proprioception for the left than the right hand means that egocentric information about the location of the left hand is less precise than egocentric information about the location of the right hand As subjects are likely to rely less on imprecise information van Beers et al 2002 they will rely more on other sources of information such as movement distance when using their left hand and therefore be more strongly influenced by illusions of length de Grave et al 2004Whether this can account for the influence on grip aperture in the grasping study of Gonzalez et al 2006 depends on how one thinks that the effect of context items on grip aperture arises The effect of the illusion implies that we must reject the idea Smeets and Brenner 1999 that egocentric information about the positions of the contact points is the only information that is used A first option is that allocentric information about the object’s size which is influenced by the illusion is also used Jeannerod 1981 Alternatively elements in the surrounding context may be regarded as obstacles that subjects have to take care not to collide with Biegstraaten et al 2007 de Grave et al 2005 Haffenden and Goodale 2000 Both giving size more consideration and taking more care to avoid obstacles when moving with the left hand would be consistent with the effect of the illusory context having more influence on the less precise left hand than on the more precise right handIf the difference in proprioceptive precision underlies the difference in illusion effects between the hands the latter difference should increase when subjects rely more on proprioception rather than vision to localize the hand In this study we investigate whether this is so by comparing the effect of an illusion when visibility of the hand is removed with that when the hand remains visible When the hand is invisible subjects cannot rely on visual information regarding the position of their hand but have to use proprioceptive information which is less precise for the left hand Since grasping is known to be influenced by factors such as posture Cuijpers et al 2004 and the consistency of haptic feedback Cuijpers et al 2008 we chose to use a simpler pointing task on a graphics tablet We chose the Brentano illusion because it is known to affect pointing movements considerably but still much less than it affects size judgments so we can be sure that we will be able to detect small changes in the effect of the illusionThe methods are very similar to two conditions of an earlier experiment in which subjects had to move a pen to a target that disappeared as soon as the pen started to move de Grave et al 2004 In the first condition the hand was continuously visible whereas in the second condition the hand was invisible throughout the experiment The critical difference with the study of de Grave et al 2004 is that subjects performed each condition twice once with their left hand and once with their right handTen subjects took part in this study All subjects performed both conditions They all had normal or correctedtonormal vision and were righthanded according to the Edinburgh Handedness Inventory score Oldfield 1971 One subject had a score of 80 the score for all the others was 100 The study was approved by the ethics committee of the Faculty of Human Movement Science
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