Separate neural pathways for the visual analysis of object shape in perception and prehension

TMS can reveal contrasting functions of the dorsal and ventral visual processing streams

“…If LO has a specialisation for perceptual shape as neuropsychological (Goodale et al, 1994) and functional neuroimaging (Kourtzi & Kanwisher, 2001, Malach et al, 1995 results indicate, then it is reasonable to assume that some shape based processing must contribute to how the brain computes the distance discrimination. This reinforces the dissociation reported by Barolomeo and colleagues (2003) that when patients with neglect are asked to point to the middle of a screen or shape they are unimpaired, presumably as a result of the functional processing of shape in their intact ventral stream.…”

“…Our tasks did not require visuomotor manipulation of the visual target, unlike many of the neuropsychological dissociation studies of dorsal and ventral streams (Milner et al, 1991;Goodale et al, 1994). However they did require a motor action based on a cognitive perceptual decision.…”

Frames of Reference for Perception and Action in the Human Visual System

“…Thus, when she reaches out to grasp objects of different sizes, her hand opens wider mid-flight for larger objects than it does for smaller ones, just as it does in people with normal vision (26). Similarly, she rotates her hand and wrist quite normally when she reaches out to grasp objects in different orientations, and she places her fingers correctly on the boundaries of objects of different shapes (24,26). At the same time, she is quite unable to describe or distinguish between any of these objects when they are presented to her in discrimination tests.…”

“…Such patients cannot use visual information to rotate their hand or scale the opening of their fingers when reaching out to pick up an object, even though they have no difficulty describing the size or orientation of objects in that part of the visual field (24,35,48,49).…”

Grasping reveals visual misjudgements of shape

“…Similarly, misperceiving an object's size can influence grasping (Franz et al 2003), but the evidence is still under debate (Aglioti et al 1995;Smeets and Brenner 1999). Neurological evidence that perceiving shapes veridically is not crucial for grasping them correctly has been taken as a support for grasping having access to different, veridical, visual information about the object (Goodale et al 1994;Goodale and Milner 1992). Since different aspects of the visual information do not have to be mutually consistent (Smeets et al 2002), any difference between performance on two tasks could be explained by assuming that different aspects of the visual information are used for the two tasks.…”