Psychophysiological evidence that the SNARC effect has its functional locus in a response selection stage

When participants judge the parity of visually presented digits, left-hand responses are faster for numerically small numbers, whereas right-hand responses are faster for large numbers [SNARC effect; S. Dehaene, S. Bossini, P. Giraux, The mental representation of parity and number magnitude. J. Exp. Psychol. Gen., 122, (1993) 371-396]. The present study aimed to find more direct evidence for the functional locus of this effect by recording brain waves while participants performed speeded parity judgments giving manual responses. Our results show clear and robust SNARC effects in the response-locked event-related potentials (ERPs) compared to the stimulus-locked ERPs, confirming that the SNARC effect arises during response-related rather than stimulus-related processing stages. Further analyses of lateralized readiness potentials strongly suggest that the SNARC effect begins to emerge in a response-related stage prior to response preparation and execution, more specifically, in a response selection stage.