Several observations suggest that overlearned ordinal categories (e.g., letters, numbers, weekdays, months) are processed differently than non-ordinal categories in the brain. In synesthesia, for example, anomalous perceptual experiences are most often triggered by members of ordinal categories (Rich et al., 2005; Eagleman, 2009). In semantic dementia (SD), the processing of ordinal stimuli appears to be preserved relative to non-ordinal ones (Cappelletti et al., 2001). Moreover, ordinal stimuli often map onto unconscious spatial representations, as observed in the SNARC effect (Dehaene et al., 1993; Fias, 1996). At present, little is known about the neural representation of ordinal categories. Using functional neuroimaging, we show that words in ordinal categories are processed in a fronto-temporo-parietal network biased toward the right hemisphere. This differs from words in non-ordinal categories (such as names of furniture, animals, cars, and fruit), which show an expected bias toward the left hemisphere. Further, we find that increased predictability of stimulus order correlates with smaller regions of BOLD activation, a phenomenon we term prediction suppression. Our results provide new insights into the processing of ordinal stimuli, and suggest a new anatomical framework for understanding the patterns seen in synesthesia, unconscious spatial representation, and SD.
Keywords: overlearned sequence, synesthesia, fMRI, semantic dementia, language, right hemisphere, predictability
Citation: Pariyadath V, Plitt MH, Churchill SJ and Eagleman DM (2012) Why overlearned sequences are special: distinct neural networks for ordinal sequences. Front. Hum. Neurosci. 6:328. doi: 10.3389/fnhum.2012.00328
Received: 02 September 2012; Accepted: 25 November 2012;
Published online: 20 December 2012.
Edited by:Sven Bestmann, University College London, UK
Reviewed by:Frederic Dick, University of California, San Diego, USA
Copyright © 2012 Pariyadath, Plitt, Churchill and Eagleman. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.
*Correspondence: David M. Eagleman, Baylor College of Medicine, 1 Baylor Plaza, Houston, TX 77030, USA. e-mail: firstname.lastname@example.org
†Present address: Vani Pariyadath, National Institute on Drug Abuse, National Institutes of Health, Baltimore, MD, USA.