Listening to numbers affects visual and haptic bisection in healthy individuals and neglect patients

There is evidence that humans represent numbers in the form of a mental number line (MNL). Here we show that the MNL modulates the representation of visual and haptic space both in healthy individuals and right-brain-damaged patients, both with and without left unilateral spatial neglect (USN). Participants were asked to estimate the midpoint of visually or haptically explored rods while listening to task-irrelevant stimuli: a small digit ("2"), a large digit ("8"), or a non-numerical auditory stimulus ("blah"). In a control silent condition, the bisection error of USN patients was biased rightwards (namely, the marker of USN) only in the visual modality. Regardless of the direction of the bisection error committed in silent trials, listening to the small digit shifted the perceived midline leftwards, and listening to the large digit shifted the perceived midline rightwards, compared to a control condition in which a neutral syllable ("blah") was presented. The shift induced by listening to numbers occurred independently of the modality of response (i.e., both in vision and haptics), and in every group of participants. Interestingly, the effect of auditory numbers processing on space estimation was overall larger for haptically than for visually explored space in all participants. In conclusion, the present data show that listening to irrelevant numbers affects space perception also in patients with left USN, indicating that the spatial representation and attention processes disrupted by USN are not involved in these numerical magnitude-spatial effects.