Numbers and time doubly dissociate.

The magnitude dimensions of number, time and space have been suggested to share some common magnitude processing, which may imply symmetric interaction among dimensions. Here we challenge these suggestions by presenting a double dissociation between two neuropsychological patients with left (JT) and right (CB) parietal lesions and selective impairment of number and time processing respectively. Both patients showed an influence of task-irrelevant number stimuli on time but not space processing. In JT otherwise preserved time processing was severely impaired in the mere presence of task-irrelevant numbers, which themselves could not be processed accurately. In CB, impaired temporal estimation was influenced by preserved number processing: small numbers made (already grossly underestimated) time intervals appear even shorter relative to large numbers. However, numerical estimation was not influenced by time in healthy controls and in both patients. This new double dissociation between number and time processing and the asymmetric interaction of number on time: (1) provides further support to the hypothesis of a partly shared magnitude system among dimensions, instead of the proposal of a single, fully shared system or of independent magnitude systems which would not explain dissociations or interactions among dimensions; (2) may be explained in terms of a stable hierarchy of dimensions, with numbers being the strongest.