In any event the present

In any event, the present findings, coupled with Schenk’s (2012a) original observation that DF failed to scale her grasp to target width in the absence of any haptic feedback from the target, suggests that simple terminal information from contact with the object, rather than veridical haptic information about the object, is enough to keep the visuomotor networks in DF’s dorsal stream operating effectively – and that DF’s grip scaling, like that of healthy participants, chiefly relies on visual feedforward information. These findings are in agreement with the observation that intermittent haptic feedback from the goal object is sufficient to keep DF’s grip aperture tuned to the target’s visual width (Schenk, 2012a). Importantly, the present findings show that veridical haptic feedback about the target is not necessary to maintain grip scaling provided that the haptic and visual targets are coarsely co-localized (e.g. co-centred) and are highly similar in shape (e.g., cylinders that vary in diameter only, or simple rectangular and square blocks). Interestingly, if we accept that contact with the surface of the workspace constitutes terminal tactile feedback for target-directed grasps, then terminal tactile feedback can explain why DF continues to show significant grip scaling when reaching out to pick up 2D Efron shapes (Westwood et al., 2002). Terminal tactile feedback might influence two aspects of a target-directed grasp. First, it quinacrine might operate on top-down processes, minimizing cognitive supervision and preventing the participants from changing the way they approach the task. Second, terminal tactile feedback might operate on the bottom-up aspects of the programming of grasps. Presumably, contact with the visual target at the end of the grasping movement contributes spatial information about the width of the target and/or information about the timing of the finger contact with the target that the visuomotor system uses to update the programming of grip aperture for subsequent grasping movements.
In summary, the results of these experiments and our earlier work (e.g., Goodale et al., 1991; Whitwell et al., 2014) converge on the idea that DF’s spared visual control of grasping makes use of feedforward visual information in a manner similar to that in neurologically intact individuals. The results also suggest that the dorsal stream alone, without the help of form-processing areas in the ventral stream, is able to use tactile feedback about the width of the target to update the programming of grip aperture. Moreover, the clear dissociation between DF’s perceptual and visuomotor abilities in these experiments, coupled with evidence from other neuropsychological, neuroimaging, and neurophysiological studies (for review, see Goodale, 2011; Milner & Goodale, 2006, 2008; Westwood & Goodale, 2011), continues to provide strong support for the Two Visual Systems hypothesis. In short, the visual perception of objects relies on neural mechanisms that are to a large degree separate from those mediating the visual control of object-directed actions (Goodale & Milner, 1992).

This research was supported by grants from the Canadian Institutes for Health Research and the Canada Research Chair Program to MAG, post graduate doctoral awards from the Natural Sciences and Engineering Research Council of Canada and Ontario Graduate Scholarship Program to RLW, and an Early Career Award to CCP from the Wolfson Research Institute for Health and Wellbeing at Durham University. Finally, we extend a very special thank you to DF for her participation, interest, and patience throughout the test sessions.

The importance of vision to motor learning and control is somewhat obvious. Indeed, the seminal work of Woodworth (1899) proposed a role for vision in all aspects of movement – planning, control, and evaluation. Following from these original ideas, a multitude of behavioral studies that have affirmed Woodworth’s hypotheses and affirmed the role of vision in every aspect of motor learning and control.