Orientation sensitivity to graspable objects: an fMRI adaptation study.

It has been proposed that vision-for-perception and vision-for-action are subserved by distinct streams of visual processing, the ventral and dorsal stream, respectively [Milner, A. D., Goodale, M. A., 1995. The visual brain in action. Oxford University Press, Oxford]. Such a distinction has been supported by a recent functional magnetic resonance (fMR) adaptation study [Valyear, K. F., Culham, J. C., Sharif, N., Westwood, D., Goodale, M. A., 2006. A double dissociation between sensitivity to changes in object identity and object orientation in the ventral and dorsal visual streams: A human fMRI study. Neuropsychologia 44, 218-228], which demonstrated selectivity to object identity but not object orientation within the ventral stream, and selectivity to object orientation but not object identity within the dorsal stream. These results were interpreted as suggesting that changes to object identity (but not to orientation) would alter the representation of the stimulus in the perceptual/recognition system, whereas changes in object orientation (but not necessarily identity) would alter the coding of the stimulus within a visuomotor system concerned with behaviour such as grasping. If orientation sensitivity in the dorsal stream does reflect such a potential for action, then this sensitivity should be specific to graspable objects. Using an fMR adaptation paradigm, we presented participants with an image of either a graspable or non-graspable stimulus, followed by the same image in either the original orientation or its mirror image. One region within the dorsal stream, the lateral occipito-parietal junction (lOPJ), was shown to be sensitive to orientation changes for graspable stimuli; this region did not show orientation sensitivity for non-graspable stimuli. Thus, it appears that the sensitivity to orientation changes in this region is specific to graspable objects, presumably because such changes affect the affordances of graspable but not non-graspable objects.