The Role of Long-Term Memory in Digit–Symbol Test Performance in Young and Older Adults

Abstract

ABSTRACT The psychological functions assessed by substitution tests, and the age-related performance decline, are not well understood. Here several aspects of long-term memory were manipulated across younger and older adults. A 45-page Digit–Symbol test was employed. Each page contained a 9-item digit symbol code-table and 9 response items. There were 9 study conditions with each condition deployed across 5 pages, or trials, of the test. The conditions were formed by crossing two within-subjects factors, each with 3 levels. The first factor, Digit Order, pertained to having the code table digits in numerical order vs. a pseudo-random order fixed across trials vs. a pseudo-random order that varied across trials. The second factor, Symbol Pairing, pertained to having a fixed digit–symbol pairing across trials vs. having a varying digit–symbol pairing across trials vs. having a novel set of 9 symbols introduced on each of the 5 trials. Including the additional factor, Age, resulted in a 2 × 3 × 3 mixed randomised block design. The older group was slowed, F(1, 22) = 17.267, p < .001, and overall-performance was poorer when the digits were arranged non-numerically, F(1,44) = 55.403, p < .001. An Age by Symbol–Order interaction indicated that use of novel symbols disadvantaged only the older participants, F(1, 44) = 6.577, p = .014. While there was no evidence that incidental paired-associate learning or spatial memory affect digit–symbol performance, symbol familiarity may be important to digit symbol test completion in older adults. The benefit of ordinally arranged digits in the coding table highlights a fundamental process difference between Digit–Symbol and Symbol–Digit test formats.