Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Dr. Melvyn Goodale


Skilled arm and hand movments designed to obtain and manipulate objects (prehension) is one of the defining features of primates. According to the two visual system hypothesis (TVSH) vision can be parsed into two systems: (1) the ventral ‘stream’ of the occipital and inferotemporal cortex which services visual perception and other cognitive functions and (2) the ‘dorsal stream’ of the occipital and posterior parietal cortex which services skilled, goal-directed actions such as prehension. A cornerstone of the TVSH is the ‘perception-action’ dissociation observed in patient DF who suffers from visual form agnosia following bilateral damage to her ventral stream. DF cannot discriminate amongst objects on the basis of their visual form. Remarkably, however, her hand preshapes in-flight to suit the sizes of the goal objects she fails to discriminate amongst when she reaches out to pick them up; That is, unless she is denied the opportunity to touch the object at the end of her reach. This latter finding has led some to question the TVSH, advancing an alternative account that is centered on visuo-haptic calibration. The current work examines this alternative view. First, the validity of the measurements that have underlined this line of investigation is tested, rejecting some measures while affirming others. Next, the visuo-haptic calibration account is tested and ultimately rejected on the basis of four key pieces of evidence: Haptics and vision need not correlate to show DF’s ‘perception-action’ dissociation; Haptic input does not potentiate DF’s deficit in visual form perception; DF’s grasp kinematics are normal as long as she is provided a target proxy; and denying tactile feedback induces a shift in grasp kinematics away from natural grasps and towards pantomimed (simulated) ones in normally-sighted populations.