Source memory retrieval is affected by aging and prefrontal lesions: behavioral and ERP evidence

Abstract

Age-related deficits in source memory have been attributed to alterations in prefrontal cortex (PFC) function, but little is known about the neural basis of such changes. The present study examined the time course of item and source memory retrieval by recording event-related potentials (ERPs) in patients with focal lesions in lateral PFC and in healthy older and young controls. Both normal aging and PFC lesions were associated with decrements in item and source memory. However, older controls showed a decrease in item hit rate with no change in false alarms, whereas patients showed the opposite pattern. Furthermore, ERPs revealed notable differences between the groups. The early positive-going old/new effect was prominent in the young but reduced in patients and older adults, who did not differ from each other. In contrast, older adults displayed a prominent left frontal negativity (600-1200 ms) not observed in the young. This left frontal effect was substantially smaller and delayed in the patients. The current results provide novel insights into the effects of aging on source memory and the role of the lateral PFC in these processes. Older controls appeared to adopt alternate memory strategies and to recruit compensatory mechanisms in left PFC to support task performance. In contrast, the lateral frontal patients were unable to use these mechanisms, thus exhibiting difficulties with strategic memory and monitoring processes.

Fig. 1

Lesion reconstructions for the lateral frontal patients (7 with left, 2 with right hemisphere damage). Lesions were estimated from MRI or CT scans and transcribed onto sequential axial templates. Left hemisphere lesions are shown from ventral to dorsal (left to right), and right hemisphere lesions from dorsal to ventral (left to right).

Fig. 2

Item accuracy (hits and correct rejections divided by the total number of trials, chance performance 50%) compared to source accuracy (correct source decisions divided by the total number of correct “old” decisions, chance performance 50%).

Fig. 3

Grand average ERPs from young control subjects for correctly rejected new words (NEW) and correctly recognized old words with accurate source judgments (H/H). Unlike the older controls, note the absence of the anterior negativity and the widespread scalp distribution of the positivity to correctly remembered old words (400–800 ms).

Fig. 4

Grand average ERPs from age-matched controls for correctly rejected new words (NEW) and correctly recognized old words with accurate source judgments (H/H). The most prominent feature is a left lateralized frontal negativity (600–1200 ms) to old words that obscures the typical ERP old/new positivity (400–800 ms) at left temporal and parietal electrodes (compare T4 to T3).

Fig. 5

Grand average ERPs from patients with damage to lateral PFC. The lateral ERPs from the 2 right frontal patients were flipped so that, for all patients, the left side of the head is the ipsilesional side. The frontal negativity is significantly reduced, particularly at left posterior frontal scalp sites (e.g., F7, FC5) over lesioned cortex. The item ERP old/new effect is actually larger over left centro-parietal and temporal electrodes (e.g., at CP5, T5) than in older controls, due to reduction of the overlapping anterior negative component.

Fig. 6

Difference waveforms (hit/hit minus correct rejection) for the three groups.

Fig. 7

Topographic maps showing the spatial distribution of the difference between correct old (hit/hit) and correct new trials.