Using the SSVEP to measure the SNARC-spatial attention effects in a parity judgment task

Abstract

Mental representation of numbers in the brain has been described as spatially organized on what is known as the "mental number line," with small numbers to the left and large numbers to the right. This representation leads to the "SNARC effect" (Spatial-Numerical Association of Response Codes), which refers to (1) improved behavioral performance for "congruent" conditions (e.g., left-handed response to small numbers) as compared to "incongruent" conditions (e.g., right-handed response to small numbers) and (2) perceiving numbers induces an automatic shift in covert spatial attention. Nonetheless, there is scarce physiological evidence for (or against) the prediction that number magnitude induces an automatic shift of spatial attention. Here we recorded EEG during the bimanual SNARC-parity judgment task (classifying numbers as odd or even) in attempt to find electrophysiological evidence for number-magnitude-induced automatic shifts of spatial attention. Attention was measured using the SSVEP (steady-state visual evoked potential) from four task-irrelevant flickering frequencies (two to the left: 15 and 20 Hz and two to the right: 24 and 17.14 Hz) in combination with an optimal spatial filtering method. EEG analysis was performed on the flicker frequencies closest to the fixation point: 20 and 24 Hz for the left and right visual hemifields, respectively. We replicated the expected behavioral patterns predicted by the SNARC effect, such that participants performed better for congruent conditions than on incongruent conditions. We observed significant changes in the SSVEP amplitude with respect to the baseline for the left (20 Hz) and right (24 Hz) flicker for both the congruent and incongruent conditions. Statistically significant differences between the congruent and incongruent conditions were larger for the congruent conditions for the flicker stimuli on the left. Together, these findings support the hypothesis that when numbers are part of the task, but their magnitude is not relevant, the SNARC-spatial attention effect is elicited as a cognitive effect resulting from the mental representation of numbers and its relationship with the space representation, more than merely a motor effect.