Using eye tracking to study numerical cognition: the case of the ratio effect

In both behavioural and brain-imaging studies, numerical magnitude comparison tasks have been used to glean insights into the processing and representation of numerical magnitude. The present study examined the extent to which eye movement data can be used to investigate the neurocognitive processes underlying numerical magnitude processing. Twenty-two participants performed a numerical comparison task (deciding which of two Arabic numerals represents the larger numerical magnitude) while eye tracking data was recorded. The ratio between numbers (smaller/larger) was manipulated and ranged from 0.11 to 0.89. Consistent with previous reaction time and accuracy studies, the present results demonstrated significant main effects of ratio on the number of fixations, as well as a significant main effect of correct (numerically larger) versus incorrect (numerically smaller) number on the duration of fixations. Furthermore, data from the present investigation also revealed that participants made significantly more saccades between the two numbers for large relative to small ratio trials. Moreover, the ratio effects on eye movements were uncorrelated with the effect of numerical ratio on reaction times, suggesting that eye tracking measures of number comparison may tap into a different level of numerical magnitude processing than reaction time measures do.