The representation of ordinal information: Domain specific or domain general?

Ordinal processing allows for the representation of the sequential relations between stimuli, and is a fundamental aspect of different cognitive domains such as verbal working memory (WM), language and numerical cognition. Several behavioural and neuroimaging studies suggest the possibility of common ordinal coding mechanisms across these different domains but direct between-domain comparisons of ordinal coding are rare and have led to contradictory evidence. This fMRI study assessed the assumption of common ordinal representational mechanisms across the WM, the number and the letter domains. We administered three ordinal judgment tasks (for alphabetical, numerical, and verbal WM judgment) with further manipulation of ordinal distance, and a luminance judgment control task. Neural patterns in fronto-parietal cortices distinguished ordinal distance in all three domains. Critically, between-task predictions of ordinal distance in fronto-parietal cortices were robust between serial order WM and alphabetical order judgment tasks, as well as a luminance judgment control task, but not when involving the numerical order judgment tasks. These results suggest that common neural substrates characterize processing of ordinal information in WM and alphabetical but not numerical domains. This commonality may however reflect attentional control processes involved in judging ordinal distances rather than the intervention of domain-general ordinal codes.