Contrasting the dorsal and ventral visual systems: Guidance of movement versus decision making

It is widely accepted that the ventral visual pathways are involved in the identification of objects and the dorsal visual pathways in the visual guid ance of reaching and grasping movements. But there are also situations, suc h as in a choice reaction time task, in which the subjects must select betw een actions on the basis of visual cues, This paper uses brain imaging to e xplore the pathways that are involved. Studies using PET and fMRI show that when subjects learn which actions are appropriate given the visual context , there are learning-related increases in the inferotemporal cortex and the ventral prefrontal cortex to which it projects. An event-related fMRI stud y shows that the activity in the inferotemporal cortex is time-locked to th e presentation of the visual cue and the activity in the ventral prefrontal cortex to the response. Finally two PET studies directly compare the dorsa l and ventral systems. In the second of these the subjects either move thei r finger on a moving target or identify the direction of movement and press one of two buttons to report the direction. When the subjects report the d irection there is activity in the middle temporal. gyrus and ventral prefro ntal cortex. It is suggested that, when subjects must consciously identify the context and decide on the appropriate action, ventral pathways are invo lved. (C) 2001 Academic Press.