Short-term memory in the mentally retarded: an application of the dichotic listening technique

Abstract

A series of experiments was conducted to investigate short-term memory in mental retardates with the dichotic listening technique as initiated by Broadbent (1958). The primary purpose of these experiments was to discern whether or not short-term memory capacity and/or strategy of encoding information could account for some of the differences between retardates and normals. Four groups of 15 Ss each were used for the three major experiments. The groups included: two groups of retardates, one organic (group O) and one cultural-familial (group F) in nature, matched in mental age and digit-span with a group of normal controls (group NMA). The fourth group, matched in chronological age with the two mentally retarded groups, served as a second normal control (group NCA). In the first experiment dichotic series of 2, 3, 4 and 5 pairs of numbers were presented to the Ss at the rate of one pair every half-second. This experiment demonstrated that the effective' short-term memory capacity of both retarded groups is much less than that of a comparable chronological- age control, but does not differ greatly from group NMA. The evidence also indicated that the retardates were Subject to a faster rate of information decay in that part of immediate memory which has been termed S-system by Broadbent. The second experiment held the length of dichotic series constant at 3 pairs of numbers, but varied the rate of presentation as follows: 1 pair per quarter-second, 1 pair per half-second, 1 pair per second, and 1 pair per 2 seconds. This experiment demonstrated a marked degree of flexibility by the normals (both NMA and NCA) in their adaptation of different strategies of recall to the various rates of informational input. Such flexibility was not found in the retardates. Experiment III similarly tested the immediate recall of series 3 pairs in length, but held the rate of presentation constant at 1 pair per half-second. In this experiment, However, each pair of items presented together consisted of a letter of the alphabet and a digit, and the side on which the letter was presented varied haphazardly from pair to pair. For the retarded Ss (both groups O and F) recall was more successful when S was instructed to recall the items of one type and then the items of the other type than when instructed to report the items heard on one side and then those heard on the other. Normal Ss (NCA and NMA) recalled equally well in both conditions. In conclusion, the evidence indicated that short-term memory capacity was indeed an important difference between retardates and group NCA. This deficit in apparent capacity, however, was probably enhanced by the retardates' lack of flexibility in the search for and use of appropriate recall strategies, and their manifestation of difficulty with ambiguous types of strategies. Though capacity. was essentially the same for groups O, F and NMA, the two retarded groups also fell below NMA Ss in their ability to adopt a flexible mode of behavior, and to utilize more ambiguous strategies. The differences between groups NMA and NCA, on the other hand, were indicative of the degree to which both memoric capacity and ability to make use of useful strategies develops in normal individuals over time.