Identifying conceptual neural responses to symbolic numerals

The goal of measuring conceptual processing in numerical cognition is
distanced by the possibility that neural responses to symbolic numerals
are influenced by physical stimulus confounds. Here, we targeted
conceptual responses to parity (even vs. odd), using
electroencephalographic (EEG) frequency-tagging with a symmetry/asymmetry
design. Arabic numerals (2–9) were presented at 7.5 Hz in 50-s sequences;
odd and even numbers were alternated to target differential, “asymmetry”
responses to parity at 3.75 Hz (7.5 Hz/2). Parity responses were probed
with four different stimulus sets, increasing in intra-numeral stimulus
variability, and with two control conditions comprised of non-conceptual
numeral alternations. Significant asymmetry responses were found over the
occipitotemporal cortex to all conditions, even for the arbitrary
controls. The large physical-differences 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 drawn, colored exemplars/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.