Hemispheric differences between left and right number representations: effects of conscious and unconscious priming.

The contribution of each hemisphere to the generation of number representations was investigated by two lateralized priming experiments in which participants had to compare Arabic digits to a fixed standard of four. In Experiment 1, unmasked primes (Arabic digits or word numerals) were used. In Experiment 2, masked primes were presented consciously or subconsciously. In both experiments similar priming effects were found in the left (LH) and the right hemisphere (RH) when the prime was presented consciously. However, asymmetries emerged when the primes were presented subconsciously: while the priming effects of digits and word numbers were equally large in the right visual half field (RVF-LH), the influence of the word prime on the semantic and the response stage of the left visual half field (LVF-RH) was absent, indicating that a word prime was no longer processed when it was presented subconsciously in LVF-RH. We believe that the origin of the latter effect can be attributed to a failure to transfer word number primes from the RH to the LH when attentional resources are restricted.