Brain potentials in a memory-scanning task. II. Effects of aging on potentials to the probes.

Brain potentials accompanying the classification of probe items as being members of a previously presented list were recorded from subjects ranging in age from 18 to 86 years old. A group of older subjects (average age = 66 years) was compared to a younger group (average age = 29 years). The items tested were verbal (digits) and non-verbal (musical notes). Digits were presented in the auditory and visual modalities, and notes were presented acoustically. Reaction times (RTs) and performance accuracy were computed. Potentials are described in terms of scalp distribution, latency and amplitude as a function of the type of stimulus (verbal/non-verbal, auditory/visual) and age group (younger/older). Evoked potentials to target notes in an auditory target-detection ('odd-ball') task were also recorded for comparison with the memory tasks. Potentials evoked by probes consisted of a sequence of sensory components in the first 250 msec followed by a cognitive component that was positive in polarity and sustained in duration (approximately 700 msec labeled P3), consisting of an earlier frontal component, P3a (mean latency: younger = 385 msec, older = 406 msec), and a large (15 microV) and later parietal constituent, P3b (mean latency: younger = 574 msec, older = 630 msec). The frontal derivation of the younger subjects showed a sustained negative bias of the wave forms in the latency range of 200-500 msec (P2 to P3) compared to the older subjects. Reaction times were longer in older subjects than in younger subjects for all stimulus types and set sizes. For the potentials evoked by the probes the younger group had consistently larger late parietal components (P3b) than the older group, whereas the late frontal potentials (P3a) were larger for the older than younger subjects. Except for visual stimuli, the latencies of the parietal sustained potentials were not influenced by subject age in contrast to the uniform changes in RT for all stimulus types. Significant amplitude and latency effects on the parietal sustained potentials accompanied the different stimulus types and memorized-set sizes which were similar for the two age groups. These results suggest that the effects of aging on short-term memory are primarily on response selection, as evidenced by RT slowing with aging, and not on memory-scanning processes as evidenced by the similarity of the latency measures of the accompanying brain potentials between the two age groups.