Parameter-based assessment of spatial and non-spatial attentional deficits in Huntington's disease

A major challenge for neuropsychological research on Huntington's disease is the identification of biomarkers for the disease at the level of cognitive functions. Given that cortical-striatal-thalamic circuits are particularly vulnerable, possible markers loading functionally on these brain regions should be particularly significant. We investigated whether parametric values derived from a 'theory of visual attention' (TVA) can serve that purpose. They are derived as mathematically independent, quantitative measures of attentional components, and the tasks require only non-speeded vocal responses. As such, the methodology seems well suited for testing patients with motor problems and general cognitive decline. Accumulating neuroanatomical evidence suggests that striatal atrophy in Huntington's disease is asymmetrical with a more pronounced left-sided degeneration. We applied a partial-report paradigm to analyse whether this results in a pathological (leftward) bias of the spatial distribution of attention. In partial report, red target letters are presented either alone or accompanied by either a second target or a green distractor letter presented in the same or in the opposite hemi-field. Since basal ganglia lesions have also been shown to cause spatially non-lateralized impairments, that is, reduced perceptual processing speed and visual working memory (WM) storage capacity within both hemi-fields, we tested possible reductions in these parameters with a whole-report paradigm. Here, columns of five red or green letters are briefly presented and the subject has to report as many as possible. Eighteen patients and 18 matched control subjects performed a partial- and a whole-report task with briefly presented letter displays. In partial report, Huntington's disease patients demonstrated a pathological bias, indicating increased attentional weighting to the left hemi-field. The extent of lateralization was strongly related to age at onset and to the number of cytosine-adenine-guanine (CAG) triplet repeats on gene IT15. In contrast, the extent of lateralization was not related to disease progression as reflected by the duration of the disease since onset of the first symptoms. In whole report, the non-lateralized attentional parameters processing speed and visual WM storage capacity were reduced bilaterally in both hemi-fields. The extent of the reduction was related to the disease duration since onset, whereas no significant correlation with CAG repeats or age at onset was found. Laterality of attentional weighting may, therefore, represent a possible trait marker reflecting the intensity of the pathogenic mechanisms, while the reduction of visual processing speed and storage capacity may be state markers for the stage of disease progression.