Increased bilateral occipitoparietal activity during retention of binary versus unary indexed lists in pair...

Cognitive complexity has been characterized by relations processed, rather than items stored. Separating these factors is difficult, because processing more complex relations often involves holding more items in memory. Previous research, NeuroImage, 17, 1031-1055) identified parietal lobes with more item relationships, but not more items by varying index length-fewest number of positions having a unique combination of items. For example, AB CD EF is a unary (length one) indexed list of three pairs, because all items are unique at the first (or second) position; AB AD CB is a binary indexed list, because only pairs of items are unique. But, these lists also differ in number of associates. In this experiment, index length was varied independently of the numbers of items and associates. Subjects were asked to make a recognition judgment for each three-pair list: Was the test pair in the previous list? Random effects analysis contrasting two binary indexed lists (AB AC CB and AB AD CB) minus two unary indexed lists (AB BC CA and AB BC CD) revealed increased occipital and parietal activity (bilaterally) during the retention period for both binary indexed list types. This result is explained by index length, but not by item load or item fan, because the numbers of items and item associates were the same for the corresponding unary and binary list types. For peak voxels in left and right precuneus, activity during retention for both binary list types was also greater than for a third unary indexed list (AB CD EF). Because binary indexes require more positions (roles) to individuate pairs, we suggest that the increased activity in precuneus relates to spatial rehearsal in that more attention is directed to both positions to maintain the integrity of the memory trace.