Are perception and action affected differently by the Titchener circles illusion?

In the present study, we investigated the effects of the Titchener circles illusion in perception and action. In this illusion, two identical discs ca n be perceived as being different in size when one is surrounded by an annu lus of smaller circles and the other is surrounded by an annulus of larger circles. This classic size contrast illusion, known as Ebbinghaus or Titche ner Circles Illusion, has a strong perceptual effect. By contrast, it has r ecently been demonstrated that when subjects are required to pick up one of the discs, their grip aperture during reaching is largely appropriate to t he size of the target. This result has been considered as evidence of a cle ar dissociation between visual perception and visuomotor behaviour in the i ntact human brain. In this study, we suggest and investigate an alternative explanation for these results. We argue that, in a previous study, while p erception was subjected to the simultaneous influence of the large and smal l circles displays, in the grasping task only the annulus of circles surrou nding the target object was influential. We tested this hypothesis by requi ring 18 subjects to perceptually estimate and grasp a disc centred in a sin gle annulus of Titchener circles. The results showed that both the perceptu al estimation and the hand shaping while grasping the disc were similarly i nfluenced by the illusion. Moreover, the stronger the perceptual illusion, the greater the effect on the grip scaling. We discuss the results as evide nce of an interaction between the functional pathways for perception and ac tion in the intact human brain.