Thomasson, R; (2006) Visual cognition after hemispherectomy: a neuropsychological study. Doctoral thesis , UCL (University College London).

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Abstract

The detrimental effects of medically intractable seizures on the developing brain have been previously documented, and a wealth of literature provides support for surgical intervention. The removal of a cerebral hemisphere (hemispherectomy) may be considered the most radical of these interventions, yet there is often no further detriment to general intellectual function or language after surgery. The aims of these studies were to evaluate visual cognitive abilities after hemispherectomy for intractable childhood epilepsy, to establish whether any differences were apparent between patients according to side of hemispheric removal, and to determine the relationship between age at seizure onset and subsequent cognitive outcome. Twenty two (12 left, 10 right) hemispherectomised patients took part in these studies. The possibility of dissociation between general intelligence levels and visual cognitive ability in the lone hemisphere was addressed by using control participants (n = 19) matched to individual patients according to chronological age, gender and general non-verbal reasoning ability. Neuropsychological and experimental measurements were used to assess visual cognitive functions including stereopsis, visual attention, face processing, spatial abilities and construction. Impaired performance, particularly on more complex tasks was found in all groups, and was consistent with generic reduction of cognitive function. These difficulties occurred regardless of side of injury, though visual recall and recognition may be selectively disadvantaged in the lone left hemisphere and warrants further study. Hemispherectomised patients and their controls differed on measures of visual search and memory for complex designs, suggesting some limitations of function in the lone hemisphere may occur above and beyond generalised reduction of cognitive function. Results were broadly consistent with theories of equipotentiality and interactive specialisation, whereby the unfolding of a hemisphere specific architectural blueprint is disrupted by cerebral injury, enabling a range of functions to develop in either hemisphere, albeit with limitations that appear to be a product of generic reduction in cognitive function and lack of processing space.

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