Grasping Visual Illusions Complementary Evidence For Two Visual Streams

Further evidence for a dissociated processing of visual information derives from healthy human subjects. Several studies revealed a significant impact of visual illusions on perceptual estimates of size and/or location while grasping and pointing movements were unaffected (Aglioti et al., 1995; Bridgeman et al., 2000; Danckert et al., 2002; Haffenden et al., 2001; Meegan et al., 2004). However, despite numerous findings in favor of such a dissociation, subsequent studies revealed a more inconsistent view. Some authors did not find a comparable dissociation or found at least a somewhat smaller but nevertheless significant effect on actions (Daprati and Gentilucci, 1997; Elliott and Lee, 1995; Franz, 2003; Franz et al., 2000, 2003; Gentilucci et al., 1996; Pavani et al., 1999). Interestingly, the measure typically used to demonstrate effects on grasping movements in these studies has been the maximum grip aperture (MGA). Undoubtedly, the MGA appears to be a straightforward measure of grasping performance. However, in search of potential effects of illusions (i.e. perceptual context cues) on reaching and grasping, other parameters of grasping might be taken into account such as velocity and force. Some authors indeed revealed effects of visual illusions on these kinematic parameters (Brenner and Smeets, 1996;

Jackson and Shaw, 2000; van Donkelaar, 1999). Actually, the discussion is quite controversial. Evidence in favor of as well as against an influence of visual illusions on grasping has been reported (for review: Bruno, 2001; Carey, 2001; Franz, 2001; Goodale and Westwood, 2004; Milner and Dyde, 2003; Plodowski and Jackson, 2001).

One of the most valuable contributions to the debate proposes the use of different spatial attributes of a certain object during estimation and grasping tasks. Following this line of evidence, estimation relies more on size and extend information, whereas grasping is guided by discrete target positions for each finger at the respective object. It seems as if visual illusions typically exert a different influence on these different spatial attributes. Thus, the observed behavioral dissociations would not represent a divergence between perception and action but between different spatial properties used for the execution of the respective tasks (Smeets et al., 2002). Milner and Dyde (2003) on the other hand have suggested a differentiation between illusions which affect different levels of visual processing. They found a differential impact of the rod-and-frame illusion and the simultaneous-tilt illusions (Dyde and Milner, 2002). Whereas the first is assumed to be based on contextual information, the latter might to be due to local interactions within the visual field mediated by inhibitory connections in V1 (Milner and Dyde, 2003). Just recently, the work of Bruce Bridgeman and Paul Dassonville added further controversial evidence to this field of research. They explored the impact of the Roelofs effect on goal-directed hand movements and perceptual estimations of stimulus positions (Bridgeman et al., 2000; Bridgeman et al., 1997; Dassonville and Bala, 2004; Dassonville et al., 2004). The observed dissociation of the Roelofs effect on pointing and estimation has previously been assumed to be in line with the two visual streams theory (Bridgeman et al., 1997). But in their most recent work both authors interpret their findings in a very different way. The dissociation between action and perception found for this illusory change of target position might be indirectly mediated by an underlying common process involved in action control and perception. From their latest results they conclude that a shift of the subjective body midline within one and the same egocentric spatial frame induced by the Roelofs effect exerts a different impact on the accuracy of perceptual estimations and immediate goal-directed movements (Dassonville and Bala, 2004; Dassonville et al., 2004). However, while their data suggest a simple common mechanism to explain different outcomes for motor control and perception, it does not rule out dual visual processing per se. If we assume, in agreement with Milner and Goodale (1995), that the proposed midline shift is mediated by and affecting only the ventral (cognitive or perceptual) visual stream, the results of the Dassonville and Bridgeman groups fit nicely to the two visual streams theory. Furthermore, up to now it remains unclear whether their results can be generalized to other visual illusions as well.

Interestingly, it is also unclear how the performance of patients suffering from optic ataxia or visual form agnosia is affected by visual illusions. As far as we know, illusions such as e.g. the Muller-Lyer or the Ebbinghaus illusion have not been investigated in patients with these disorders. A recent study reported that patients with visual agnosia were not prone to a size-weight illusion (larger objects are felt to be lighter in comparison to smaller objects of the same physical weight). When executing the same task without visual information, i.e. when retrieving the size of the objects from kinesthetic input only, the patients showed the same illusion effect than controls (Dijkerman et al., 2004). However, although this experiment revealed a clear dissociation between visual and kinesthetic processing, it is unclear whether these patients would incorporate the illusory visual information into motor behavior or not.

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