Visual System Case Studies

The purpose of this study was to support the dominant view that the visual system is functionally and anatomically dichotomized according to dissociations between the ventral- and dorsal-streams. To illustrate this hypothesis, James, Culham, Humphrey, Milner, & Goodale (2003) examined the case study of patient D.F., who “suffered severe bilateral damage to her occipitotemporal visual system […], while retaining the use of her occipitoparietal visual system” (James, Culham, Humphrey, Milner, & Goodale, 2003) following an anoxic episode. Lesions to the occipitotemporal system resulted in apperceptive visual agnosia, an “inability or marked difficulty in visually identifying an object or picture of an object as a result of impaired perceptual abilities” (Mendoza, n.d.), while leaving her ability to form guided actions intact. Moreover, in this study, two experiments were conducted that used functional MRI to examine activation in patient D.F.’s ventral stream during object recognition tasks, and dorsal stream while performing object-directed grasping tasks. The results obtained were then compared to those of healthy controls. Results from Experiment 1 suggest patient D.F. was able to make discriminations between intact color- and intact grayscale-drawings compared to scrambled color- and scrambled grayscale-drawings. However, area LOC, which was activated in healthy control participants during this task, fell within D.F.’s lesions site—the ventrolateral occipital region. Thus, D.F. produced no activation in this area. Instead, D.F., showed activation in other areas which suggests, “[her] capacity to recognize color and grayscale pictures of objects is dependent on spared regions of the ventral cortex, including the primary visual cortex and extensive areas of the lingual and fusiform gyri” (James et al., 2006). Research by Podrebarac (2013) supports this finding, suggesting, “visual shape information is processed within the LOC, while texture is processed in medial regions of the collateral sulcus (CoS)” (Podrebarac, 2013). Moreover, because “the perception of color in humans has been associated with activation of a ventromedial occipital area (in the collateral sulcus or lingual gyrus)” (Ungerleider & Haxby, 1994) this explains D.F.’s ability to discriminate color and texture despite sustaining damage to LOC. Moreover, these structures are acting like the LOC in healthy controls (James et al., 2006). Furthermore, albeit capable of processing color and texture, D.F.’s ability to discriminate between shape and size is impaired due to its functional location in the ventrolateral occipital region. In Experiment 2, patient D.F displayed bilateral activation in the same structures as healthy control participants; as was expected. However, unlike the healthy control participants, “D.F. did not show activation in the superior parieto-occipital sulcus, […which] has been shown to be activated during arm movement tasks” (James et al., 2006). MRI images revealed anatomical abnormalities outside of the ventral stream; however, despite this finding, researchers James et al. (2006) concluded from the experiments conducted that the structures affected remained functional. According to Pisella et al. (2006) “this may be partially explained by the reorganization of surrounding intact brain tissue” (Pisella, Binkofski, Lasek, Toni, & Rossetti, 2006). After sustaining damage, the brain could have shown plasticity; in which new neuronal connections were made from areas outside the damage site that took over some of the function that was impaired from the lesion. Moreover, this would indicate why D.F., was …show more content…
are compelling, however they are only suggestive of a single dissociation between the occipitotemporal and occipitoparietal visual systems. Moreover, to add weight to the findings from D.F.’s case study, similar research by Ungerleider & Haxby (1994) investigates the double dissociation of visual recognition and visuospatial performance in patients with lesions of occipitotemporal and occipitoparietal cortex, respectively (Ungerleider & Haxby, 1994). Empirical inquiries pertaining to a double dissociation compare the outcomes of patients with lesions to the occipitotemporal stream, as seen in patients like D.F. with visual agnosia; and patients with lesions to the occipitoparietal stream, such as patients with optic ataxia. For instance, optic ataxia results in impaired action—characterized by inappropriate reaching and grasping—while maintaining intact perception; meanwhile visual agnosia results in impaired perception, while maintaining intact action. As aforementioned the two visual stream deficits are characterized by reciprocity of impairment; functioning in one deficit that the other is impaired in—and vice versa. Therefore, “the reciprocal patterns of impairment found in optic ataxia and in visual agnosia have been considered as a functional double-dissociation” (Rossetti, Pisella, & Vighetto,