Direct visual pathways for reaching movements in the macaque monkey.

The brain seems to process the location of objects faster than their intrinsic features, such as size, when these parameters are used to guide action. To uncover a potential anatomical substrate of these different processing speeds, we investigated in the monkey the pathways linking extrastriate visual cortex with the dorsal premotor area, a frontal area known to be involved in visually guided reaching movements. Retrogradely transported anatomical tracers were injected at physiologically defined sites and the distribution of labelled cells was examined in the ipsilateral cortex. We found a projection to the dorsal premotor cortex from the parieto-occipital area (PO). This area receives direct projections from the primary visual cortex (V1), and is part of the dorsal visual stream involved in the processing of spatial information. No direct projections to the dorsal premotor cortex arise from the ventral visual areas, thought to process object features. Our finding provides evidence for direct pathways from the dorsal visual stream to the dorsal premotor cortex and supports the view that the location of objects is processed faster by the brain than their intrinsic features.