While patients with OA show impaired obstacle avoidance, several different patient populations show preserved avoidance. Most notably, when patient D.F. – who suffers profound VFA after localized damage to her ventral stream – was asked to perform the same task as described above, her hand path trajectories were sensitive to the position of obstacles (Rice et al., 2006). Thus, despite impairments in the ability to consciously report the obstacle locations, D.F. nonetheless took those obstacles into account when performing visually guided reach actions. It is worth mentioning that D.F. also shows preserved obstacle avoidance during locomotion (Patla & Goodale, 1996). The question of obstacles and locomotion is another area of active research (e.g. Fink, Foo, & Warren, 2007; Marigold, Weerdesteyn, Patla, & Duysens, 2007) but for succinctness I restrict this review and the review of behavioural work (see section 1.7) to reaching and grasping tasks. Other evidence of preserved obstacle avoidance in neuropsychological patients comes from studies of patients with hemi-spatial neglect (McIntosh, McClements, Dijkerman, Birchall, & Milner, 2004; McIntosh, McClements, Schindler et al., 2004; Milner & McIntosh, 2004). Patients who suffer from visuospatial neglect have difficulty directing their attention to one of their visual hemi-fields. This disorder almost always results from right hemisphere brain damage leading to neglect of the left visual field. In some cases, this disorder can be accompanied by a symptom known as visual extinction, where two simultaneously presented stimuli, one to each visual field, will result in the contralesional stimuli (again, almost always on the left side) not being perceived. Despite these problems attending to and perceiving stimuli in contralesional space, neglect patients, including those with extinction, show sensitivity to obstacles (McIntosh, McClements, Dijkerman et al., 2004; McIntosh, McClements, Schindler et al., 2004; Milner & McIntosh, 2004). Specifically, on the same task used for the VFA and OA patients, neglect patients show patterns of deviation to obstacle asymmetries that match control subjects (McIntosh, McClements, Dijkerman et al., 2004; Milner & McIntosh, 2004). In a related test with a patient who showed visual extinction, obstacles could be presented alone or in pairs (always symmetrical when paired). When presented in pairs this sometimes led to the patient reporting only one obstacle; their reach trajectory,
however, was identical regardless of whether they perceived both obstacles or not (McIntosh, McClements, Schindler et al., 2004; Milner & McIntosh, 2004). This sensitivity to obstacles without conscious awareness again provides evidence for dorsal stream control over obstacle avoidance. One additional experiment demonstrating
preserved obstacle avoidance in a neuropsychological population actually comes from the study described above with the unilateral OA patients (Rice et al., 2008). As was noted above, when asked to perform the obstacle task under normal circumstances, avoidance was impaired in the patients with OA. However, when the researchers introduced a 5s delay between the presentation of the obstacles and the cue to reach (note during the delay, vision was absent, as it was during the reach in the non-delay task) the avoidance performance of the OA patients returned to normal. Introducing a delay without visual feedback is thought to shift the representation of the workspace from brain areas in the dorsal stream – which code, in real-time, the visuomotor relevance of objects – to brain areas in the ventral stream – which are responsible for the perceptual encoding of objects that are retrieved (and stored) in memory (Milner, Paulignan, Dijkerman, Michel, & Jeannerod, 1999). Thus, when the workspace representation was shifted from the dorsal stream to the ventral stream in this study of OA patients (Rice et al., 2008) a sensitivity to obstacle position emerged.
Finally, one recent study that we conducted (Striemer, Chapman, & Goodale, 2009) also speaks to the representation of obstacles by the dorsal stream. In this study, we tested a patient with extensive damage to his primary visual cortex in the right hemisphere, resulting in a dense left visual field hemianopia. We used a task similar to the one described above (see Figure 1.8a) but included trials where only a single object appeared (at each of the four different locations). In addition, we asked our participant to provide a verbal report of the location of any obstacles he saw after each reach trial was completed. In the first experiment, the participant performed the task without a delay between the visual presentation of objects and the reach. In this immediate reach condition, our patient showed a preserved sensitivity to obstacle position in both his good right visual field and his blind left visual field (where he never reported seeing objects). In a second experiment, however, in which we introduced a 2-s no-vision delay between the
was observed only when the obstacles were located in the right visual field. The patient showed no sensitivity to obstacles located in his left (blind) field. Taken together, these results suggest that in this patient, visual input into the dorsal stream from his blind field was somehow mediating his real-time obstacle avoidance while visual input to his ventral stream from his blind field was not available, leading to a failure in the avoidance of obstacles in delay. But, if vision flows through occipital cortex before being separating into the parallel dorsal and ventral processing pathways, why do we see any preserved