Grouping Effects in Short-Term Memory

Presenting short sequences of items in temporal or spatial groups is known to improve recall of their order. Theories about this effect propose either that groups of items are represented in a hierarchical structure in which the positions of items in a group are nested under codes denoting the groups themselves, or in a matrix structure where each item is directly tagged for its group and position. In a matrix system, position codes are independent of group codes, and the retrieval of a code near the bottom of a hierarchy depends on the successful retrieval of the codes above it. Experiments 1-4 tested these dependence r lationships with a probed recall procedure in which subjects were presented with a grouped sequence of items and were then required to recall the position and group of one of the items. This technique provided information about how well subjects correctly recalled both the group and position, the group only, the position only, and neither the group nor the position of an item. When the items in a group were letters, digits, or musical notes, the data conformed to a hierarchical structure. When the nonalphanumeric char-acters ~, $, %, &,., +, @, T, <, were used, a matrix structure merged. Experiments 5-7 required subjects to judge the dissimilarity oftwo sequences of groupe