The mental number line modulates visual cortical excitability

The mental number line has been shown to exert an influence on the visuo-spatial allocation of attention, with presentation of numbers from the low and high ends of the mental number line inducing covert shifts of spatial attention to the left and right side of visual space, respectively. In the present study we used transcranial magnetic stimulation (TMS) to investigate whether this attentional modulation influences excitability of the early visual cortex. By using the phosphene threshold as a measure of visual cortical excitability, we show (in 10 subjects) that number priming modulates excitability of the early visual cortex in a topographic fashion: low numbers, associated with left side of space, increase the excitability of the right early visual cortex (the stimulation of which induces phosphenes in the left hemifield) and decrease the excitability of the left early visual cortex (the stimulation of which induces phosphenes in the right hemifield). The opposite pattern of results was observed for high numbers. Our results suggest that the attentional shifts induced by the mental number line are manifested at the earliest cortical stages of visual processing.