Frequency-tagging EEG reveals instruction-driven magnitude integration using the numerical distance effect

While humans can readily access the common magnitude of various codes such as digits or dot sets, it is not yet clear whether this integration occurs spontaneously, or only when involved in explicit magnitude processing. We addressed this question by examining the neural distance effect, a robust marker of magnitude processing, with a frequency-tagging approach. By varying the instructions given to participants, we compared spontaneous processing of visually presented numerosities to explicitly oriented processing of magnitude or parity. Electrophysiological responses were recorded while participants were viewing rapid sequences of a base numerosity presented at 6 Hz (e.g., “2”) in randomly mixed codes: digits, number words, canonical dots and fingers. A deviant numerosity either close (e.g., “3”) or distant (e.g., “8”) from the base was inserted every five items. We observed clear discrimination responses of the deviant numerosity despite its code variation. The distance effect (larger responses when base/deviant are distant than close) was present when participants were explicitly oriented to magnitude and parity, but not in simple viewing. We thus argue that abstract magnitude processing is only partly spontaneous. It indeed requires sufficient cognitive resources to be engaged in numerical processing, but occurs whatever semantic aspect of numbers is activated