Examining the role of stimulus complexity in item and associative memory.

Episodic memory comprises memory for individual information units (item memory) and for the connections among them (associative memory). In two experiments using an object pair learning task, we examined the effect of visual stimulus complexity on memory encoding and retrieval mechanisms and on item and associative memory performance. Subjects encoded pairs of black monochrome object images (low complexity, LC condition) or color photographs of objects (high complexity, HC condition) via interactive imagery, and subsequently item and associative recognition were tested. In Experiment 1, event-related potentials (ERPs) revealed an enhanced frontal N2 during encoding and an enhanced late posterior negativity (LPN) during item recognition in the HC condition, suggesting that memory traces containing visually more complex objects elicited a stronger effort in reconstructing the past episode. Item memory was consistently superior in the HC compared to the LC condition. Associative memory was either statistically unaffected by complexity (Experiment 1) or improved (Experiment 2) in the HC condition, speaking against a tradeoff between resources allocated to item versus associative memory, and hence contradicting results of some prior studies. In Experiment 2, in both young and older adults, both item and associative memory benefitted from stimulus complexity, such that the magnitude of the age-related associative deficit was not influenced by stimulus complexity. Together, these results suggest that if familiar objects are presented in a form that exhibits a higher visual complexity, which may support semantic processing, complexity can benefit both item and associative memory. Stimulus properties that enhance item memory can scaffold associative memory in this situation.

Effects of acute psychosocial stress on the neural correlates of episodic encoding: Item versus associative memory.

Acute stress is known to modulate episodic memory, but little is known about the extent to, and the circumstances under, which stress affects encoding of item vs. inter-item associative information for words of different valences. Furthermore, the precise neuro-cognitive mechanisms underlying stress effects on episodic encoding in humans are largely unknown. To address these questions, in the present study we recorded EEG activity while male participants encoded neutral, negative and positive words, each paired with another word that was always neutral. Immediately before encoding, half of the participants experienced a psychosocial stressor, the Trier Social Stress Test, while the other half underwent a control procedure. Twenty-four hours later, participants completed separate item and associative recognition tests. Pre-learning stress enhanced item recognition accuracy for the positive, but not for the negative words. By contrast, there was no evidence for stress effects on associative recognition. The increase in item recognition was accompanied by a higher familiarity-, but not recollection-, based item retrieval of positive and neutral, but not negative words. Crucially, in the event-related potential (ERP) stress affected the amplitude of the frontal slow wave in general, and the frontal slow wave subsequent memory effect for positive words in specific, and the subsequent memory effect was correlated with cortisol levels after the stress manipulation. Our results suggest that positive words are encoded more elaboratively under stress, leading to a higher likelihood of subsequent item retrieval. An interaction of cortisol with frontal-lobe dependent control processes as well as a shift in attentional biases may contribute to this stress-induced modulation of episodic encoding.

Subjective cognitive decline in healthy older adults is associated with altered processing of negative versus positive feedback in a probabilistic learning task.

Older adults who worry about their own cognitive capabilities declining, but who do not show evidence of actual cognitive decline in neuropsychological tests, are at an increased risk of being diagnosed with dementia at a later time. Since neural markers may be more sensitive to early stages of cognitive decline, in the present study we examined whether event-related potential responses of feedback processing, elicited in a probabilistic learning task, differ between healthy older adults recruited from the community, who either did (subjective cognitive decline/SCD-group) or did not report (No-SCD group) worry about their own cognition declining beyond the normal age-related development. In the absence of group differences in learning from emotionally charged feedback in the probabilistic learning task, the amplitude of the feedback-related negativity (FRN) varied with feedback valence differently in the two groups: In the No-SCD group, the FRN was larger for positive than negative feedback, while in the SCD group, FRN amplitude did not differ between positive and negative feedback. The P3b was enhanced for negative feedback in both groups, and group differences in P3b amplitude were not significant. Altered sensitivity in neural processing of negative versus positive feedback may be a marker of SCD.

An examination of task factors that influence the associative memory deficit in aging.

Aging is accompanied by a decline in associative memory, whereas item memory remains relatively stable compared to young adults. This age-related associative deficit is well replicated, but its mechanisms and influencing factors during learning are still largely unclear. In the present study, we examined mediators of the age-related associative deficit, including encoding intentionality, strategy instructions, the timing of the memory test (immediate vs. 24 h delayed) and the material being learned (words vs. pictures) in a within-subject design. Older and younger adults performed seven encoding tasks on word pairs and picture pairs on two consecutive days, followed by item and associative recognition tests. The associative deficit was evident after all encoding tasks. We found no evidence for a difference in the magnitude of the associative deficit between incidental vs. intentional learning conditions. However, there was some evidence for a larger associative memory deficit with pictures versus words when the encoding task was held equal. Sentence generation and interactive imagery instructions in which participants generated their own mediators reduced the magnitude of the associative deficit. However, increased encoding guidance through the provision of mediators did not lead to an alleviation of the deficit, potentially because the specified mediators were implausible or difficult for the older adults to reconcile with prior knowledge. Finally, we found some evidence for a reduced age-related associative deficit with a test delay of 24 h. These results contribute to a better understanding of the factors affecting the relative difficulty of older adults with encoding and retrieving novel associations.