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Critical Analysis.docx

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University of Toronto Scarborough
Andy Lee

Critical Analysis Ventral occipital lesions impair object recognition but not object-directed grasping: an fMRI study Submitted to: Dr. Andy Lee By: Anum Khuzaima Aslam INTRODUCTION There are two primary pathways in the nervous system which process information about object perception and object location. These have been identified by Ungerleider and Mishkin as ventral and dorsal pathways respectively. One of the most famous cases of damage in ventral stream pathway was observed in patient D.F which resulted in visual form agnosia. Structural fMRI revealed D.F suffered damage particularly in ventrolateral regions of occipital lobe with dorsal and primary visual cortex intact. In this fMRI study patient D.F along with healthy subjects participated in two separate experiments which assessed their performance on tasks designed to measure ventral or dorsal function (Thomas et al 2003). Experiment 1 was designed to assess ventral stream pathway by the use of two dimensional objects of varying color, line drawings or grayscale images. For evaluating dorsal stream pathway simple three dimensional objects were used in a grasping task. In experiment 1 healthy subject were able to identify all intact objects however D.F’s performance on line drawings was significantly poor with less than 10% response rate. When structural fMRI was examined healthy participants had significant activation in lateral occipital cortex (LOC) while identifying line drawings, greyscale images and colored objects. Such activation was absent in D.F’s occipital region making it difficult to recognize line drawings. There was some overlap between activation patterns of healthy subjects with D.F in intraparietal and fusiform gyrus however high levels of activation in peristriate cortex were exclusive to D.F (Thomas et al 2003). In experiment 2 object grasping task D.F along with her healthy counterparts displayed prominent activation in dorsal-stream pathway. Even though her reaching and grasping of objects was fairly unaffected there was no activation in superior parieto-occipital sulcus which was activated in healthy subjects during reaching and grasping task. This may be the result of reorganization of function due to neuroplasticity post brain injury (Thomas et al 2003). The results from this fMRI experiment provide converging evidence to previously gathered data on visual form agnosia. It further provides evidence for an independent relationship between dorsal and ventral projections which allow for object location and object perception respectively. The pattern of activation as observed in fMRI reveal an inactivation of LOC in D.F when presented with line drawings making her unable to identify images (Thomas et al 2003). CRITIQUE The researchers in this study were able to provide converging evidence to support implication of ventral stream pathway in object recognition and further alluded to activation of dorsal stream pathway in object location. The high resolution of fMRI in this study enabled researchers to identify precise locale of brain lesion in D.F despite diffuse damage across ventrolateral region in occipital lobe. In the past visual form agnosia has been studied by researchers Milner et al (1991) in patient, Mr. S. Like D.F, Mr. S suffered from severe object recognition problems without sensory or intellectual impairment (Milner et al 1991). Moreover, they suffered similar challenges in distinguishing simple geometric shapes. Since Mr. S was poor at copying images he was diagnosed with apperceptive rather than associative agnosia. In t
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