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Denise D. J. Grave, Eli Brenner, Jeroen B. J. Smeets; Pointing towards the Brentano illusion. Journal of Vision 2002;2(7):721. doi: 10.1167/2.7.721.
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© ARVO (1962-2015); The Authors (2016-present)
Perceptual judgements are often influenced by illusions that do not influence our performance in motor tasks. We assume that this is not because of a dissociation between visual processing for perceptual and motor tasks, but because illusions only produce biases in the analysis of certain aspects of the visual information. To test this assumption we investigated how the Brentano version of the Müller-Lyer illusion influences pointing. Subjects made open loop pointing movements from several starting positions (an endpoint of the illusion or a position outside the figure) to the center of the illusion. As soon as subjects moved their hand, the illusion and target point disappeared. When the movement stopped the illusion appeared again, with the target aligned with the position that the subject had reached. Our hypothesis was that the illusion will have a stronger influence on pointing when length information provides a useful cue about the distance to be moved. Thus a larger effect was expected in pointing movements from an endpoint of the figure to the center, than in movements from outside the figure. The results indeed show a bigger effect of the illusion on pointing movements from an endpoint of the figure to the center than for movements from outside the figure. An additional implication of this finding is that subjects do not only use visual information about the endpoint of the movement to guide their hand, but that they also use cues about the distance to be moved.
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