Using parity cross-format adaptation to probe abstract number representation in the human brain.

It is debated whether there is an abstract, format-independent representation of number in the human brain, eg whether "four" shares a neural representation with "4." Most previous studies have used magnitude to investigate this question, despite potential confounds with relative quantity processing. This study used the numerical property of parity. Electroencephalogram recordings were collected from participants performing a fixation-cross task, while viewing 20-s sequences of alternating even and odd Arabic numerals presented at 7.5 Hz: responses to parity were selectively tagged at the asymmetry frequency of 3.75 Hz. Parity asymmetry responses emerged significantly over the occipito-temporal (OT) cortex, and were larger than control asymmetry responses to isolated physical stimulus differences, replicating a previous study. Following 20-s adaptation to cross-font even numerals, larger parity responses were recorded over the right OT cortex, further supporting distinct representations of even/odd numbers; there was no corresponding control adaptation effect. Interestingly, adaptation to even canonical dot stimuli also produced significantly larger parity asymmetry responses; adaptation to even number words trended non-significantly. These results are in line with parity being processed automatically, even across formats. More generally, they suggest that parity is a useful means for probing abstract representation of number in the human brain.