The Brentano illusion influences goal-directed movements of the left and right hand to the same extent

Recently, Gonzalez et al. (J Neurophys 95:3496-3501, 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 right-handed subjects make open-loop 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 illusion

Using a Stick Does Not Necessarily Alter Judged Distances or Reachability

Background It has been reported that participants judge an object to be closer after a stick has been used to touch it than after touching it with the hand. In this study we try to find out why this is so. Methodology We showed six participants a cylindrical object on a table. On separate trials (randomly intermixed) participants either estimated verbally how far the object is from their body or they touched a remembered location. Touching was done either with the hand or with a stick (in separate blocks). In three different sessions, participants touched either the object location or the location halfway to the object location. Verbal judgments were given either in centimeters or in terms of whether the object would be reachable with the hand. No differences in verbal distance judgments or touching responses were found between the blocks in which the stick or the hand was used. Conclusion Instead of finding out why the judged distance changes when using a tool, we found that using a stick does not necessarily alter judged distances or judgments about the reachability of objects

Why are saccades influenced by the Brentano illusion?

In the Brentano version of the Müller-Lyer illusion one part looks longer and the other looks shorter than it really is. We asked participants to make saccadic eye movements along these parts of the figure and between positions on the figure and a position outside the illusion. By showing that saccades from outside the figure are not influenced by the illusion, we demonstrate that the reason that saccades along the figure are influenced is that the incorrectly judged length is used to plan the amplitude of the saccade. This finding contradicts several current views on the use of visual information for action. We conclude that actions are influenced by visual illusions, but that such influences are only apparent if the action is guided by the attribute that is fooled by the illusion. © 2006 Springer-Verlag