Ventral and dorsal stream contributions to the online control of immediate and delayed grasping: a TMS approach.

According to Milner and Goodale's theory of the two visual streams, the dorsal (action) stream controls actions in real-time, whereas the ventral (perceptual) stream stores longer-term information for object identification. By this account, the dorsal stream subserves actions carried out immediately. However, when a delay is required before the response, the ventral (perceptual) stream is recruited. Indeed, a neuroimaging study from our lab has found reactivation of an area within the ventral stream, the lateral occipital (LO) cortex, at the time of action even when no visual stimulus was present. To tease apart the contribution of specific areas within the dorsal and ventral streams to the online control of grasping under immediate and delayed conditions, we used transcranial magnetic stimulation (TMS) to the anterior intraparietal sulcus (aIPS) and to LO. We show that while TMS to aIPS affected grasp under both immediate and delayed conditions, TMS to LO influenced grasp only under delayed movement conditions. The effects of TMS were restricted to early movement kinematics (i.e. within 300 ms) due to the transient nature of TMS, which was always delivered simultaneous with movement onset. We discuss the implications of our findings in relation to interactions between the dorsal and ventral streams.