Spatial numerical bias in acute vestibular neuritis

Number magnitude is often represented spatially in the mind by a mental number line, on which small numbers are located to the left of space and large numbers to the right. As vestibular dysfunction can affect aspects of spatial cognition, we wondered whether patients with acute vestibular loss would show a directional bias along the mental number line.

We gave 18 patients with vestibular neuritis (VN) (eight left VN, ten right; mean age 54 years, range 31-75 years; four females) and 15 normal age- and education-matched controls (mean age 47 years, range 26-75 years; 11 females) a mental number bisection task.

The patients with left VN underestimated the middle number (mean sum of signed errors -3.3, SE 1.5), while patients with right VN overestimated it (mean 1.9, SE 0.7). The direction of effect aligned with the direction of slow phase velocity. The results for the normal controls fell in between the two patient groups, and represented an underestimate of the middle number (mean -1.5, SE 0.8). In the patients, the effect was greater without visual fixation and in the acute stage compared to 1 or 2 weeks later. The error rates were similar across all groups and conditions.

Our results show that acute vestibular loss produces a temporary directional bias in numerical processing that is only present in the absence of visual cues. The effect is similar to that seen in patients with visuospatial neglect.