Dissociations between memory for temporal order and recognition memory in aging

Event-related brain potential investigations of memory and aging

A review of the literature that examines event-related brain potentials (ERPs) and memory with respect to aging reveals some consistency in the processes that might be spared and those that might be compromised with increasing age. By and large, the ERP repetition effect, recorded during indirect memory paradigms, appears to be relatively intact with aging, suggesting spared repetition priming mechanisms and the brain substrates upon which they depend. Some age-related findings during direct (i.e. explicit) memory testing suggest that a left-sided posterior old/new effect ( approximately 500-800 ms), thought to reflect a relatively automatic retrieval of item information, is equivalent in young and old. A later, long-duration, right-sided, prefrontal old/new effect, allied with the search for and/or the retrieval of contextual information (i.e. source memory), has been found to be smaller or absent in the waveforms of the old in two of three studies, suggesting impaired source memory mechanisms in the elderly. It is argued that the data are relatively consistent with spared item retrieval mechanisms in the elderly presumably supported by medial temporal lobe structures. However, although the data are suggestive, there are too few studies at this time to reach a firm conclusion as to whether the mechanisms that support contextual retrieval, presumably mediated by prefrontal cortical structures, are impaired in the elderly.

An event-related potential study of encoding in young and older adults

Event-related brain potentials (ERPs) were recorded from young (M age=25) and older (M age=71) adults during the study phase of a recognition memory paradigm. Participants studied two temporally distinct lists of sentences (each containing two unassociated nouns). During recognition testing, in response to the nouns, participants made old/new, followed by remember (context)/know (familiarity) and temporal source (i.e., list) judgments. To assess age-related differences in encoding, the ERPs recorded during study were averaged as a function of the correctness of the judgment of old in conjunction with subsequent temporal (list correct/incorrect) and subsequent remember and know judgments, i.e., subsequent memory or Dm effects were computed. Both young and old showed robust Dm activity regardless of whether the subsequent temporal source judgment was correct or incorrect. Although both young and old produced robust Dm effects for study items subsequently associated with a remember judgment, only the older participants showed a reliable Dm effect associated with study trials that were subsequently associated with a know judgment. This pattern of results suggests that older adults did not differentially encode items that would be subsequently retrieved with context from those that would be retrieved without such attributes.

Associative learning impairments in patients with frontal lobe damage

The performance of 18 frontal lobe lesion (FL) and 10 frontal lobe dementia (FLD) patients on an associative memory test was compared with the performance of their matched normal controls. The FL group was severely impaired on cued and free recall and was moderately impaired on a recognition condition. Left FL patients performed the poorest on the cued and free recall conditions. The FLD patients were moderately impaired on the free recall condition only but there was a subgroup of FLD patients with additional left temporal atrophy who appeared severely impaired on both cued and free recall. These findings indicate that both left frontal and temporal lobe damage can impair associative learning and that this impairment is more strikingly seen with free rather than cued recall.

An overview of age-related changes in the scalp distribution of P3b

In this overview of 7 studies, the scalp distribution of the P3b component (i.e. the P3 or P300) of the event-related potential elicited by target events in young and older adults was assessed. The target P3b data were recorded in either auditory oddball paradigms or in visual study tasks in which orienting activity was manipulated (as a within-subjects variable) in investigations of indirect memory. Some of the studies required choice reaction time responses, whereas others required responses only to the target stimuli. Motor response requirements had a profound effect on the P3b scalp distribution of older but not of younger subjects. The presence of a frontally oriented scalp focus in the topographies of the older adults in most of the tasks described here is consistent with older adults continuing to use prefrontal processes for stimuli that should have already been well encoded and/or categorized. However, although older subjects generally had different P3b scalp distributions than younger subjects, their scalp distributions were modulated similarly by task requirements. These data suggest that similar mechanisms modulate the scalp distribution of P3b in older compared to younger adults. However, in the older adult, these scalp distribution changes in response to task demands are superimposed on a frontally oriented scalp focus due to a putative frontal lobe contribution to target P3b topography.

Item and source memory: differential age effects revealed by event-related potentials

The neural substrates of age-related memory differences were evaluated by recording event-related potentials (ERPs) from young and older adults during a recognition memory paradigm. Subjects studied two temporally distinct lists of sentences (each with two nouns) and were tested for their memory of the nouns and of the list (i.e. temporal source) in which they had occurred. Compared with the young, the old showed a greater source than item memory performance decrement. Both age groups showed equivalent posterior-maximal old/new ERP effects. However, only the young produced a frontal-maximal, late onset old/new effect that differed as a function of subsequent source attribution. Age-related explicit memory differences may be due to a deficit in a prefrontal cortical system that underlies source memory.