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Denise D. J. de Grave, Volker H. Franz, Karl R. Gegenfurtner; The coding of combined pointing movements and saccades in the Brentano illusion. Journal of Vision 2005;5(8):207. doi: https://doi.org/10.1167/5.8.207.
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For movements to a visual target either visual judgments of direction and distance (or length) of the required displacement can be used (vector coding), or the final position (position coding), or a combination of both. Earlier studies using the Brentano illusion (de Grave et al., VSS 2002) showed that pointing movements and saccades both use vector coding, however not to the same extent. The saccades relied more strongly on vector coding than the hand (pointing), leading to the conclusion that the eyes (saccades) and the hand (pointing) use different information for their movement. However, these studies do not rule out the possibility that eye and hand use the same information. In the pointing study correction saccades could have been made by the time the pointing movement was finished. This could explain the smaller effect on pointing if eye position information is used in pointing. In this study we tested whether combined saccades and pointing movements are influenced by the Brentano illusion to the same extent when the stimulus is only presented for 200 ms, so subjects could not make a corrective saccade. Subjects started with their index finger in the middle of a touchscreen and made saccades and pointing movements in four directions. Movements were always from an outer vertex of the Brentano illusion toward the middle vertex. We found an illusion effect of about 25% in pointing as well as in saccades when both movements are performed in the same task. This favours the interpretation that the same information is used in eye and hand movements.
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