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Visual control of action but not perception requires analytical processing of object shape


The visual perception of object shape depends on ‘holistic’1,2,3,4 processing in which a given dimension cannot be perceptually isolated from the other dimensions of the object. The visual control of action (such as grasping an object), however, which is mediated by cortical areas that are largely independent of those mediating conscious perception5,6,7,8, must take into account only the most action-relevant dimension of an object without being misled by other non-relevant object features. Here we report the results of two experiments showing that vision for perception and vision for action deal with objects in a fundamentally different manner. We tested participants' ability to make perceptual judgements of the width of different rectangular objects or to grasp them across their width, while in both cases ignoring length9,10. Participants could not ignore length when making perceptual judgements of width but they could completely ignore length when grasping the same objects. These results suggest that in situations in which the elementary dimensions of an object's shape are perceived in a holistic manner, the same dimensions are treated analytically when a visually guided action is directed at that same object.

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Figure 1: Tasks used in experiments one and two.
Figure 2: Effects of irrelevant variations in length on the perception of object width and on object-directed grasping (across the object's width).
Figure 3: Effects of irrelevant variations in length on the reaction time for simulated grasping.
Figure 4: Effects of length on grip scaling for width in simulated grasping and real grasping in the filtering condition.


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We thank J. Pomerantz, M. Morgan, S. Köhler and P. Gribble for comments on the manuscript, and G. Króliczak for his help. This research was supported by grants to M.A.G. from the Canadian Institutes of Health Research and the Canada Research Chairs Program.

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Ganel, T., Goodale, M. Visual control of action but not perception requires analytical processing of object shape. Nature 426, 664–667 (2003).

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