Are parity and magnitude status of Arabic digits processed automatically? An EEG study using the fast periodic visual stimulation

Many studies have shown that humans can easily extract numerical characteristics of single digits such as numerical magnitude and parity status. We investigated whether spontaneous processing of magnitude or parity status can be observed when participants are passively presented with sequences of briefly displayed Arabic digits. We assessed the parity processing by presenting seven odd digit numbers followed by one even digit (and reverse) with a sinusoidal contrast modulation at a frequency of 10HZ in one-minute sequences. The same paradigm and frequencies were used to investigate magnitude processing (i.e. seven digits smaller than five followed by one digit larger than five; and reverse) and control condition (i.e. sequence of 1-4-6-9 followed by 2-3-7 or 8). We observed a strong EEG activation on right parietal electrodes and a weaker activation on left parietal electrodes in all conditions. Left and right activations were stronger in the parity than in the other conditions, reflecting an automatic retrieval of parity information conveyed by the Arabic digits. The weaker activation during the magnitude task could reflect a more complicated access of the information corresponding to magnitude status. Right activations during the control task could be due to the fact that subjects can quickly learn to categorize numbers arbitrarily. These neuronal activation patterns are consistent with the neuro-imaging literature describing the localization of basic numerical processing. Our findings indicate that magnitude and parity status are extracted automatically from Arabic digits, even when numerical stimuli are presented without instructions at a high presentation rate.