(Non) interactions between numerical and visual space: evidence from unilateral neglect

The notion that in left neglect the pattern of error for visual and numerical lines (i.e., progressive rightward displacement of the midpoint as the interval size increases) are homologous is still debated. As first, here we further examined the degree of association between numerical errors and visual neglect by administering to thirty-two right brain-damaged patients a number bisection task and a battery to asses neglect. As second, we reasoned that if the two representation (i.e., numerical and visual) arehomologous, they should interact when concurrently activated (i.e., number processing should affect spatial orienting and viceversa). Thus, we analyzed whether the size of the numerical interval modulates both visual and numerical errors when visual and numerical space are activated at the same time. Eighteen patients displaying the aforementioned pattern of numerical error in the number bisection task were administered a visual-number bisection task, that consisted of two conditions: a) marking the midpoint of the empty space enclosed between two numerical extremes; b) Same as a), but here patients were asked to mark the midpoint by writing down the number that is numerically halfway between the two numerical extremes. The results showed that numerical errors are dissociable from visual neglect, and that the size of numerical intervals equally influenced numerical errors (both number bisection and visual-number bisection tasks), but not the visual errors (in visual-number bisection task), defined as the spatial position of the bisector. These findings suggest that attention shift along internal and external representations might be underpinned by different mechanisms.