Stimulus probability and motor response in young and old adults: an ERP study.

The effects of responding hand and stimulus probability were investigated in young and old subjects in an RT task in which both rare and frequent stimuli required a response. It was found that the effect of stimulus probability was less pronounced in old subjects than in young, and that the latency of P3 was longer in the elderly, although their RTs were not different from young subjects. ERPs for right hand responses were larger than for left hand responses; this difference was discernible already in the P2 and N2 peaks of the ERP. A tentative explanation is offered for these large and unexpected hand differences. An interpretation in terms of an age-related decrease in resources is proposed for the increased P3 latency and the decreased probability effect on P3 amplitude in old subjects.

Learning where to look: electrophysiological and behavioral indices of visual search in young and old subjects.

The present investigation explores the way young and elderly subjects use regularities in target location in a visual display to guide search for targets. Although both young and old subjects show efficient use of search strategies, slight but reliable differences in reaction times suggest decreased ability in the elderly to use complex cues. Event-related potentials were very different for the young and the old. In the young, P3 amplitudes were larger on trials where the rule that governed the location of the target became evident; this was interpreted as an effect of memory updating. Enhanced positive Slow Wave amplitude indicated uncertainty in random search conditions. Elderly subjects' P3 and SW, however, seemed unrelated to behavioral performance, and they showed a large negative Slow Wave at central and parietal sites to randomly located targets. The latter finding was tentatively interpreted as a sign of increased effort in the elderly to allocate attention in visual space. This pattern of behavioral and ERP results suggests that age-related differences in search tasks can be understood in terms of changes in the strategy of allocating visual attention.

Components of the event-related potential following degraded and undegraded visual stimuli.

Event-related potentials were recorded in response to visual stimuli in two different reaction tasks in which subjects were instructed to react immediately to the stimuli, or to delay their response for a 2 sec period, respectively. There were four types of stimuli: frequent-degraded, frequent-undegraded, infrequent-degraded and infrequent undegraded letters. In all conditions the stimuli evoked complex waveforms which comprised a frontal-negative wave (N400) and a late positive wave that reached a maximum amplitude on the parietal scalp site (P500). In addition, a slow positive wave with a central-parietal scalp distribution was found in the waveforms that were associated with the delayed reaction task. A principal components analysis of the waveforms yielded two major components: an early composite component that peaked around 400 msec, and a late component that became maximally active towards the end of the recording epoch. The scores of the earlier component were more negative (or less positive) and the scores of the late component were more positive when infrequent or degraded stimuli were presented, in comparison with frequent or undegraded stimuli.