Age differences in short-term memory: organization or internal noise?

Older and young adults' letter search performances were examined on a short-term memory (STM) task where subjects compared a five-letter target sequence stored in short-term memory to a subsequently presented five-letter probe sequence. The same five letters were always presented on target and probe portions of a given trial, but on half of the trials, two letters were transposed in the first chunk, second chunk, or between the first and second chunks of the probe sequence ("No" trials). On the remaining half of the trials, the target and probe sequences were identical ("Yes" trials). Both young and older adults showed increases in reaction time (RT) when the chunk boundary for "yes" trials was different for the target and probe sequences of a given trial. This finding indicated that both age groups were organizing the sequences in STM in the same qualitative manner. However, older adults showed a relatively greater increase in RT and errors than young adults for second-chunk transpositions than for first-chunk or between-chunk transpositions, and this finding suggested that an age difference in task complexity could not account for this effect. We propose, though, that these data are consistent with an internal noise model.