Friday, February 26, 2010

Evidence for a geometric brain module suggested by William's syndrome defect

Context from the introduction by Lakusta et al.
When rats, human toddlers, or adults are disoriented in a chamber, they search for targets using geometric properties of the layout, often ignoring quite salient nongeometric cues. This pattern has led scientists to hypothesize that reorientation in animals (including humans) is guided by a cognitive module that engages geometric properties of layouts such as the lengths of surfaces, the angles of their intersections, and geometric sense (i.e., “left-” and “right-ness”), but does not engage nongeometric information such as surface color.
The authors find that geometric reorientation abilities are specifically impaired in Williams Syndrome patients. (Williams syndrome results from a microdeletion in chromosome 7q11.23. People with the syndrome have mild to moderate retardation and highly selective but severe impairment in a range of spatial tasks that normally engage parietal and other dorsal stream functions of the brain.)  Here is their abstract:
The capacity to reorient in one’s environment is a fundamental part of the spatial cognitive systems of both humans and nonhuman species. Abundant literature has shown that human adults and toddlers, rats, chicks, and fish accomplish reorientation through the construction and use of geometric representations of surrounding layouts, including the lengths of surfaces and their intersection. Does the development of this reorientation system rely on specific genes and their action in brain development? We tested reorientation in individuals who have Williams syndrome (WS), a genetic disorder that results in abnormalities of hippocampal and parietal areas of the brain known to be involved in reorientation. We found that in a rectangular chamber devoid of surface feature information, WS individuals do not use the geometry of the chamber to reorient, failing to find a hidden object. The failure among people with WS cannot be explained by more general deficits in visual-spatial working memory, as the same individuals performed at ceiling in a similar task in which they were not disoriented. We also found that performance among people with WS improves in a rectangular chamber with one blue wall, suggesting that some individuals with WS can use the blue wall feature to locate the hidden object. These results show that the geometric system used for reorientation in humans can be selectively damaged by specific genetic and neural abnormalities in humans.

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