Effects of perceptual and cognitive difficulty on P3 and RT in young and old adults.

Ten healthy old and 10 healthy young subjects each received a series of trials in a memory retrieval task similar to that devised by Sternberg (1967). On each trial the subject saw a memory set of 2 or 4 digits (set size) followed by a probe. The task was to indicate whether the probe was a positive or negative instance (response type) of the memory set for that trial. On half the trials, the probe digits were degraded by a mask of random dots (stimulus quality). For both young and old subjects, RT was later to probes following the larger set size, later to degraded probes, and later to negative probes. For the young subjects only, P3 latency was delayed by the same variables affecting RT although to a lesser degree. P3 latency in the elderly responded quite differently: it was unaffected by set size or response type. However, P3 was somewhat delayed by the degraded probes suggesting that the failure to reflect set size or ceiling effect in the elderly. The correlation between single-trial P3 latency and RT in the elderly is lower than in the young. The data are discussed in terms of age-related differences in the meaning of P3 latency.