Size-contrast illusions deceive the eye but not the hand

When we reach out to pick up an object, not only do we direct our moving limb towards the location of the object, but the opening between our fingers and thumb is scaled in flight to the object's size. Evidence obtained from patients with neurological disorders has shown that the visual processing underlying the calibration of grip aperture and other movement parameters during grasping is mediated by visual mechanisms located in the cerebral cortex that are quite distinct from those underlying the experiential perception of object size and other object features. Under appropriate conditions, such dissociations can also be observed in individuals with normal vision. Here we present evidence that the calibration of grasp is quite refractory to pictorial illusions that have large effects on perceptual judgements of size.

We used a variation of the familiar 'Titchener circles' illusion in which two target circles of equal size, each surrounded by a circular array of either smaller or larger circles, are presented side by side. Subjects typically report that the target circle surrounded by the array of smaller circles appears to be larger than the target surrounded by larger circles. In our test, two thin 'pokerchip' discs were used as the target circles. The relative size of the two discs was randomly varied so that on some trials the discs appeared perceptually different but were physically equivalent in size, and on other trials they were physically different but appeared perceptually equivalent. The perceptual judgements made by the 14 subjects in our experiment were strongly affected by this size-contrast illusion. However, when asked to pick up a disc, the scaling of the subjects grip aperture (measured opto-electronically before contact with the disc) was largely determined by the true size of the target disc and not its illusory size.

It would seem that the automatic and metrically accurate calibrations required for skilled actions are mediated by visual processes that are separate from those mediating our conscious experiential perception. Earlier studies on patients with neurological deficits suggest that these two types of processing may depend on quite separate, but interacting, visual pathways in the cerebral cortex.