Dorsal and ventral visual stream contributions to perception-action interactions during pointing.

The Ebbinghaus illusion, in which a central circle surrounded by large circles appears to be smaller than a central circle surrounded by small circles, affects the speed of pointing movements. When the central circle appears to be big, pointing movements directed towards it are faster than when the central circle appears to be small. This effect could be due to an interaction between ventral stream processing associated with determining relative object size and dorsal stream processing associated with sensorimotor output. Alternatively, the dorsal stream alone could mediate the effect via the transformation of object shape representations into motor output within the parietal lobe. Finally, ventral stream processing could be integrated into motor output through projections to the prefrontal cortex and subsequently to the motor areas of the cortex, thus bypassing the dorsal stream. These three alternatives were tested by disrupting either the ventral or dorsal stream processing using transcranial magnetic stimulation (TMS) while subjects made pointing movements as quickly and accurately as possible to the central target circles within the Ebbinghaus illusion display. The relative changes in reaction time, movement speed, and movement accuracy for small versus large appearing target circles were compared when TMS was delivered over each site as well as at a control site (SMA). The results showed that TMS over either the dorsal or ventral stream but not the SMA reduced the influence of the illusion on the pointing movement speed but did not affect reaction time or movement accuracy. A second control experiment was completed in which TMS was delivered during pointing movements to target circles of physically different sizes that were not surrounded by either large or small circles. This allowed us to determined whether the effect we observed in the main experiment was due specifically to the relative size information contained within the illusory display and the effect this has on the preparation of pointing responses or to an influence on basic perceptual and sensorimotor processes occurring within the ventral and dorsal streams, respectively. The results showed that the affect on pointing movement speed was still present with dorsal but not ventral stream stimulation. Taken together, this evidence suggests that the ventral stream contributes to pointing movements based on relative object size information via its projections to the prefrontal areas and not necessarily through interactions with the dorsal stream.