Mapping mental number line in physical space: vertical and horizontal visual number line orientation in asymptomatic individuals with HIV.

Multiple studies have implicated frontostriatal dysfunction in human immunodeficiency virus (HIV) and described cognitive deficits with a focus on executive function and memory. Remarkably little is known about visuospatial and number processing in HIV, though these capacities are also supported by frontostriatal circuits and their parietal connections. We investigated the relation of numerical and spatial cognition in asymptomatic individuals with HIV and explored physical and mental number orientation using several modes of presentation and response: mental number line bisection, physical line bisection, and physical number line orientation on visually-presented horizontal and vertical number lines. The asymptomatic HIV+ group was significantly slower and produced more errors on visuospatial and number processing tasks than the HIV- group (n=37/group). Both groups showed significant correlations between number processing and visuospatial performance. These findings demonstrate that HIV-related brain damage early in the disease course can alter the spatial representation of numerical distance, providing evidence for disruption of frontostriatal circuits and their parietal projections underlying numerical processing in HIV. Besides disease-related effects, this study demonstrates that the physical number line preserves its numerical and spatial integrity in both the vertical and horizontal physical dimensions, as all participants exhibited underestimation of numerical distance, i.e. leftward bias (for horizontal presentation) and downward bias (for vertical). Our results also revealed a dissociation between the processing components of numerical distance and physical space, suggesting differentiation between the neural networks involved in number lines and physical line orientation.