Larger, smaller, odd or even? Task-specific effects of optokinetic stimulation on the mental number space

Previous studies have shown that number processing can induce spatial biases in perception and action
and can trigger the orienting of visuospatial attention. Few studies, however, have investigated how
spatial processing and visuospatial attention influences number processing. In the present study, we used
the optokinetic stimulation (OKS) technique to trigger eye movements and thus overt orienting of
visuospatial attention. Participants were asked to stare at OKS, while performing parity judgements
(Experiment 1) or number comparison (Experiment 2), two numerical tasks that differ in terms of
demands on magnitude processing. Numerical stimuli were acoustically presented, and participants
responded orally. We examined the effects of OKS direction (leftward or rightward) on number
processing. The results showed that rightward OKS abolished the classic number size effect (i.e., faster
reaction times for small than large numbers) in the comparison task, whereas the parity task was
unaffected by OKS direction. The effect of OKS highlights a link between visuospatial orienting and
processing of number magnitude that is complementary to the more established link between numerical
and visuospatial processing. We suggest that the bidirectional link between numbers and space is
embodied in the mechanisms subserving sensorimotor transformations for the control of eye movements
and spatial attention.