Identifying conceptual neural responses to symbolic numerals

Neural processing of numerical concepts may be measured in humans automatically, without a related numerical task. However, the extent to which neural responses to symbolic numbers are due to physical stimulus confounds independently of conceptual representations remains unknown. Here, we targeted conceptual responses to parity (even vs. odd), using an electroencephalographic (EEG) frequency-tagging approach with a symmetry/asymmetry paradigm. Fifty second sequences of Arabic numerals (2-9) were presented at 7.5 Hz; odd and even numbers were alternated, so that differential responses to parity would be captured at 3.75 Hz (7.5 Hz/2). Parity responses were probed with four different stimulus sets, increasing in intra-numeral stimulus variability. Moreover, two control conditions were tested for each stimulus set, comprised of non-conceptual numeral alternations (strong control, for small inter-group physical differences: 2,3,6,7 vs. 4,5,8 and 9; weak control, for large physical differences: 2,4,5,7 vs. 3,6,8,9). Significant asymmetrical responses at 3.75 Hz were found over the occipitotemporal cortex to all conditions, thus even for arbitrary numeral groups. The weak control condition elicited the largest response in the stimulus set with the lowest level of variability (1 font). Only in the stimulus set with the highest level of variability (20 hand-drawn, colored exemplars per numeral) did the response to parity surpass both control conditions. These findings show that physical differences across small sets of Arabic numerals can strongly influence, and even account for, automatic brain responses. However, carefully designed control conditions and highly variable stimulus sets may be used towards identifying truly conceptual neural responses.