The neural mechanism of fluency-based memory illusions: the role of fluency context

Enhanced memory and hippocampal connectivity in humans 2 days after brief resistance exercise

INTRODUCTION

Exercise has significant health benefits and can enhance learning. A single bout of high-intensity resistance training may be sufficient to improve memory. This study aimed to assess memory enhancement by a single bout of high-intensity resistance training and to examine the neural underpinnings using resting-state functional magnetic resonance imaging (MRI).

METHODS

Sixty young adults (34 men and 26 women), divided into the training and control groups, participated. The first session included verbal memory recall tests (cued- and free-recall), resting-state functional MRI (rs-fMRI), and a single-bout high-intensity resistance training for the training group. Two days later, they underwent post-intervention memory tests and rs-fMRI. The study design was 2 groups × 2 sessions for memory tests, and within training group comparisons for rs-fMRI.

RESULTS

Compared to the control group without resistance training, the training group showed higher cued-recall performance 2 days after the brief resistance training (training: +0.27, control: -0.13, interaction: p = .01), and their free-recall scores were associated with enhanced left posterior hippocampal connectivity (r = .64, p < .001).

CONCLUSIONS

These results suggest that brief high-intensity resistance exercise/strength training could enhance memory without repeated exercising. The quick effect of resistance training on memory and hippocampal connectivity could be revealed. A focused and one-shot exercise may be sufficient to enhance memory performance and neural plasticity in a few days.

Context dissociations of the FN400 and N400 are evidence for recognition based on relative or absolute familiarity

Fluency, the ease of processing, can increase familiarity and the ability to recognize previously encountered information. Mixing or blocking the pre-experimental familiarity of test probes alters the pattern of recognition Event-Related Potentials (ERPs) for picture stimuli (Bruett & Leynes, 2015). The present study tested this idea further by mixing or blocking pre-experimental familiarity of word stimuli. A test with only pre-experimentally familiar words (Experiment 1) elicited the prototypical mid-frontal FN400 old/new ERP difference, which is a correlate of familiarity. However, tests with a mix of pre-experimentally familiar and unfamiliar words (Experiment 2) elicited posterior N400 old/new differences. Based on the Unexpected Fluency Attribution model (Mecklinger & Bader, 2020), this pattern of results suggests that pre-experimental fluency can influence the use of relative (changes from recent exposure) or absolute (baseline levels accrued from experience) familiarity to make a recognition judgment.

Study-test congruence of response levels in item stimulus-response priming

We investigated stimulus-response (S-R) memory links during object priming using a binary associative size judgement paradigm. At study, participants decided which of two objects was bigger in real life and, at test, made the same or the reverse judgement. We examined the effects of response congruence on item S-R priming in the associative paradigm. In Experiment 1, a task reversal manipulation had minimal impact on RT priming when classifications were congruent for both recombined objects between study and test. Experiment 2 found that RT priming was more disrupted by classification incongruence of the selected than of the nonselected item alone, with incongruence of the nonselected object having no effect on RTs. Experiment 3, however, found that classification incongruence of both items eliminated RT priming, indicating that a significant effect of classification incongruence for the nonselected item is only evident if both items are classification-incongruent. Finally, across all experiments, we found that accuracy was more sensitive than RTs to decision/action incongruence. We interpret these findings in light of a two-stream account of S-R priming, and suggest a few extensions to account for interactions between S-R links of recombined items.

Can pupillometry distinguish accurate from inaccurate familiarity?

Pupillometry, the measurement of pupil diameter, has become increasingly popular as a tool to investigate human memory. It has long been accepted that the pupil is able to distinguish familiar from completely novel items, a phenomenon known as "pupil old/new effect". Surprisingly, most pupillometric studies on the pupil old/new effect tend to disregard the possibility that the pupillary response to familiarity memory may not be entirely exclusive. Here, we investigated whether the pupillary response to old items correctly judged familiar (hits; accurate familiarity) can be differentiated from the pupillary response to new items wrongly judged familiar (false alarms; inaccurate familiarity). We found no evidence that the two processes could be isolated, as both accurate and inaccurate familiarity showed nearly identical mean and across-time pupillary responses. However, both familiarity hits and false alarms showed pupillary responses unequivocally distinct from those observed during either recollection or novelty detection, which suggests that the pupil measure of familiarity hits and/or false alarms was sufficiently sensitive. The pupillary response to false alarms may have been partially driven by perceptual fluency, since novel objects incorrectly judged to be old (i.e., false alarms) showed a higher degree of similarity to studied images than items correctly judged as novel (i.e., correct rejections). Thus, our results suggest that pupil dilation may not be able to distinguish accurate from inaccurate familiarity using standard recognition memory paradigms, and they also suggest that the pupillary response during familiarity feelings may also partly reflect perceptual fluency.

Context influences the FN400 recognition event-related potential

Recognition of studied items often elicit more positive event-related potentials (ERPs) than unstudied items at mid-frontal electrodes about 300-500 ms (i.e., the FN400). The debate over the psychological processes associated with the FN400 has led to two competing hypotheses. One hypothesis is that the FN400 reflects familiarity, whereas another hypothesis is that it reflects conceptual implicit memory (i.e., conceptual fluency). The present experiment tested these hypotheses by presenting meaningless images that lack familiarity and conceptual fluency, off-brand products that lack pre-experimental familiarity, and name-brand products that have both pre-experimental familiarity and conceptual fluency. ERPs were recorded during judgments of lifetime and recent recognition. During both forms of recognition, ERPs in the FN400 window were greater for meaningless images than name- or off-brand products. Because this evidence is difficult to reconcile with either the familiarity or conceptual fluency hypotheses, the results are interpreted within a broader theoretical framework that includes top-down psychological (i.e., context) influences on the FN400.

Electrophysiological correlates of the perceptual fluency effect on recognition memory in different fluency contexts

The present study used event-related potentials (ERPs) to investigate the contribution of perceptual fluency to recognition memory in different fluency contexts. In a recognition memory test with a modified remember-know paradigm, we employed conceptually impoverished items (kaleidoscope images) as stimuli and manipulated the perceptual fluency of recognition test cues through masked repetition priming. There were two fluency context conditions. In the random fluency context (RC) condition, primed and unprimed trials were randomly inter-mixed. In the blocked fluency context (BC) condition, primed and unprimed trials were grouped into blocks. Behavioral results showed that priming elevated the incidence of remember hits and the accuracy of remember judgements in the RC condition; no such effects were evident in the BC condition. In addition, priming effects on reaction times were found only for remember hit responses in the RC condition. The ERP results revealed an early100-200 ms effect related to masked repetition priming, which took the form of greater positivity for primed than unprimed trials. This effect was modulated neither by fluency context or response type. The present findings suggest that perceptual fluency induced by masked repetition priming affects recollection-related memory judgments in a specific fluency context and indicate that relative, rather absolute, fluency plays a critical role in influencing recognition memory judgments.

Too Much Familiarity! The Developmental Path of the Fluency Heuristic in Children

We tested whether changes in attribution processes could account for the developmental differences observed in how children's use fluency to guide their memory decisions. Children ranging in age from 4 to 9 years studied a list of familiar or unfamiliar cartoon characters. In Experiment 1 (n = 84), participants completed a recognition test during which the perceptual fluency of some items was enhanced using a prime. In Experiment 2 (n = 96), participants completed a source recollection judgment on their recognition decisions. Primed items were recognized at a higher rate than unprimed items. However, while young children rely on fluency for all items, older children use fluency only for unfamiliar items. This pattern came together with a reduction in familiarity-based-but not recollection-based-memory responses.