Neuroanatomy of Unilateral Neglect
There is considerable evidence that damage to the inferior parietal lobule (IPL), more than any other brain region, produces the classic symptoms of unilateral neglect (Vallar & Perani, 1986). This region of the associative cortex, initially defined on the basis of late myelination during development, appears to be at the apex of a multistage cortical processing stream, and receives inputs from subcortical structures that carry occulomotor and attentional signals (Vallar & Perani, 1986). In fact, this region appears to be the primary cortical region governing attention and is known to integrate somatic and visual information as well as movement (Vallar & Perani, 1986). It is particularly important for localizing points in space and perceiving depth and motion (Vallar & Perani, 1986).The IPL is part of the ventral perceptual stream, but incorporates some new capacities, such as the ability to manipulate viewer-centered representations for tasks like mental rotation and map reading (Mattingley, 1999). Such abilities are thought to have been drafted from mechanisms evolving originally in the dorsal stream for the visual guidance of movement (Mattingley, 1999). Milner and Goodale hypothesize that neglect after inferior parietal damage is a purely perceptual disorder, and that visuomotor deficits (such as optic ataxia) arise only after dorsal stream damage to the superior parietal lobule (Mattingley, 1999).
The neuronal connections of the IPL have been documented in studies by Mesulam et al. (1981) and Robinson et al. (1986) through injection of anterograde or retrograde transport substances and determination of resulting visible projection patterns. Two types of cortico – cortico connections were seen (Robinson et al., 1986). Asanuma et al. (1985) also notes connections between the IPL and the medial pulvinar thought by Robinson et al. (1986) to play a role in attentional modulation. Overall, the IPL is well placed to integrate multiple sources of information needed to represent space and send the result of its constructions to other cortical areas (Asanuma et al., 1985).
Portions of the IPL, namely Brodmann’s area 40, exhibit right lateralized activation during both innocuous and painful stimulation, regardless of the side stimulated (Coghill et al., 2001). It can thus be inferred that components of information are processed, in part, by right lateralized systems ana logous to those that process visual spatial information arising from extrapersonal space (Coghill et al., 2001). Such right lateralized processing can account for the left sided neglect seen with injury to brain regions within the right cerebral hemisphere (Coghill et al., 2001).
The right parietal cortex mediates attention to both left and right halves of the body and extrapersonal space (Simon Frasier University). The right posterior parietal cortex is known to be involved in the manipulation of mental images (Brain Injury Resource Centre). Damage to the right parietal cortex has the most dramatic effect on drawing, resulting in images that are fragmented and characterized by the loss of spatial dimensions and the addition of extraneous lines (Brain Injury Resource Centre). Patients with parietal-lobe lesions to the rostral inferior parietal lobe of the right hemisphere demonstrate difficulties maintaining attention at a contralesional location (Maguire, 2002).
The left parietal hemisphere mediates attention only to the right side of the body and extrapersonal space (Simon Frasier University). The left posterior parietal cortex is involved in the production of mental images (Brain Injury Resource Centre). Drawing is affected to a lesser extent with left sided parietal damage. The act of drawing becomes slow and laborious, leading to a product that is coherent, albeit simple, exhibiting a gross lack of detail (Brain Injury Resource Centre). Lesions to the left parietal cortex tend to be compensated by the intact right parietal cortex, resulting in the association of disorders of spatial cognition to lesions of the right parietal lobe (Simon Frasier University).
The right parietal cortex mediates attention to both left and right halves of the body and extrapersonal space (Simon Frasier University). The right posterior parietal cortex is known to be involved in the manipulation of mental images (Brain Injury Resource Centre). Damage to the right parietal cortex has the most dramatic effect on drawing, resulting in images that are fragmented and characterized by the loss of spatial dimensions and the addition of extraneous lines (Brain Injury Resource Centre). Patients with parietal-lobe lesions to the rostral inferior parietal lobe of the right hemisphere demonstrate difficulties maintaining attention at a contralesional location (Maguire, 2002).
The left parietal hemisphere mediates attention only to the right side of the body and extrapersonal space (Simon Frasier University). The left posterior parietal cortex is involved in the production of mental images (Brain Injury Resource Centre). Drawing is affected to a lesser extent with left sided parietal damage. The act of drawing becomes slow and laborious, leading to a product that is coherent, albeit simple, exhibiting a gross lack of detail (Brain Injury Resource Centre). Lesions to the left parietal cortex tend to be compensated by the intact right parietal cortex, resulting in the association of disorders of spatial cognition to lesions of the right parietal lobe (Simon Frasier University).
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