The capacity of attention and simultaneous perception of objects: a group study of Huntington's disease patients

Using a whole report-paradigm based on [Bundesen, C. (1990). A theory of visual attention. Psychological Review, 97, 523-547; Bundesen, C. (1998). A computational theory of visual attention. Philosophical Transactions of the Royal Society of London B, Biological Sciences, 353, 1271-1281] theory of visual attention (TVA), [Finke, K., Bublak, P., Dose, M., Müller, H. J., & Schneider, W. X. (2006). Parameter-based assessment of spatial and non-spatial attentional deficits in Huntington's disease. Brain, 129, 1137-1151] demonstrated profound reductions in perceptual processing speed and visual working memory (WM) storage capacity in Huntington's disease (HD) patients. A comparably severe impairment of visual processing capacity has previously been reported for two simultanagnosia patients [Duncan, J., Bundesen, C., Olson, A., Humphreys, G., Ward, R., Kyllingsbaek, S., van Raamsdonk, M., Rorden, C., & Chavda, S. (2003). Attentional functions in dorsal and ventral simultanagnosia. Cognitive Neuropsychology, 20, 675-702]. To investigate whether such a deficit does also prevail in HD, the simultaneous perception of visual objects was tested in 10 HD patients under free viewing conditions and without time constraints. Objects were presented under four different conditions: (i) single, (ii) multiple adjacent, (iii) multiple embedded, and (iv) multiple overlapping. The dependent measure was the percentage of identification failures. Performance was compared to that of 15 healthy subjects matched for age, education, gender and general mental ability. For HD patients, the percentage of errors in the various testing conditions was examined for correlations with the TVA parameters of visuo-perceptual processing speed and WM storage capacity. These parameters were estimated using verbal whole report of briefly presented letters. TVA permits the two parameters to be estimated mathematically independently and relatively unaffected by any motor deficits present in HD. The identification error rate was substantially increased in HD patients, compared to control subjects, in the overlapping-figures subtest. This deficit was significantly and negatively correlated with processing speed, whereas there was no correlation with WM storage capacity. These results demonstrate the presence of deficits in simultaneous perception in HD, related to a severe reduction in perceptual processing speed. The results are discussed with respect to a dopamine mediated decline of cortical cholinergic activation, diminishing the number of visual objects that can be simultaneously represented within the visual processing system.