1 INTRODUCTION

It is commonly believed that at times of great stress our behaviour changes. Typically, this is considered to be changes in our ability to make decisions, but acute stress might also affect our ability to retrieve known information or learn new information (Lupien & Lepage, 2001). The direction of the effect of acute stress on behaviour is thought to depend on when it occurs: if stress occurs close to the time of learning it enhances memory, whereas stress immediately before retrieval impairs memory (Shields et al., 2017; Wolf, 2017). This can be argued to be evolutionarily advantageous, as post-learning stress is thought to improve the processing and consolidation of information through rapid acting noradrenergic and non-genomic glucocorticoid responses, in order to avoid similar situations in future (Quaedflieg & Schwabe, 2018; Sandi & Pinelo-Nava, 2007). On the other hand, this effect is suggested to occur at the cost of impaired retrieval of previously learned material, due to the prioritisation of cognitive resources towards the current stressor (Quaedflieg & Schwabe, 2018; Wolf, 2019).

Yet this understanding has been based mainly on animal models, or on findings that do not discriminate between physical and psychosocial stressors. It is not clear to what extent the same conclusion characterises the effect of stressors in everyday life, where a substantial proportion of the stress humans experience is psychosocial. Within the psychosocial stress literature, there is considerable variability of findings about the effect of stress as a function of its induction time relative to the learning episode (Beckner et al., 2006; Corbett et al., 2017; Sheldon et al., 2018; Tollenaar et al., 2009). In part, this may be due to inter-individual variability in stress response, but also to the heterogeneous nature of memory itself, as this construct has been operationalised in many different ways in that literature. In an attempt to better understand how psychosocial stress impacts episodic memory, it may first be crucial to identify which elements of memory are affected. In the present experiments, we use a careful, stepped experimental design, to explore the impact of psychosocial stress on varying aspects of human episodic memory.

Item recognition memory is typically assessed using old/new identification tasks. Exploration of the processes that underlie item recognition can also be carried out using remember-know tasks: these assess levels of recollection and familiarity (e.g., Migo et al., 2012). In remember-know tasks, participants are asked if they have a sense that test stimuli have been encountered before (responding ‘know’ to represent familiarity) or if they have a vivid memory of some other specific details from the previous encounter with the stimuli (responding ‘remember’ to represent recollection). Item memory may also be assessed using free recall tasks that are completed by searching and actively recalling previously learned stimuli. It is also possible to test recognition and recall jointly, in paired-associates tasks. For example, participants could initially be asked to identify if a stimulus has been encountered before (item recognition), and then asked to recall what other stimulus it was paired with (cued recall; Old & Naveh-Benjamin, 2008).

Familiarity and recollection may differentially recruit the neural circuitry implicated in stress responses, which could potentially lead to variable effects of stress on memory (Dedovic et al., 2009; McEwen, 2007). Previously, it has been proposed that only hippocampally dependent memory is influenced by stress (McEwen, 2007). This amounts to the suggestion that recollection, which is often believed to be uniquely based on the hippocampus (Montaldi & Mayes, 2010), will be particularly strongly affected by stress. However, more recent evidence suggests that stress may also influence other, less hippocampally dependent memory processes, including familiarity (Li et al., 2013; McCullough & Yonelinas, 2013; Wolf, 2017). It is not clear however whether psychosocial stress preferentially affects recollection or familiarity.

In addition to the memory test, a key factor that may influence the impact of stress on memory is emotion. Whether external to or intrinsic within stimuli, emotion is thought to enhance memory relative to neutral materials (Ponzio & Mather, 2014; Sharot & Yonelinas, 2008). This suggestion can also extend to stress in general as stress after learning is thought to enhance memory for emotional stimuli relative to neutral stimuli (LaBar & Cabeza, 2006). This finding supports the modulation model, in which emotional memories recruit additional activity in the amygdala, following secretion of noradrenaline, cortisol or other related hormones. This amygdala activity, in turn, aids consolidation, rendering emotional memories less vulnerable to forgetting (McGaugh, 2004). Amygdala activation is also thought to increase in response to stress-related hormones, such as noradrenaline and cortisol. The combination of both emotional stimuli and increased levels of stress could have additive influence on amygdala activation and ensuing memory enhancement. In practice however, both enhanced and impaired memory for positive and negative emotional materials have been reported after stress (Cornelisse et al., 2011; Elzinga et al., 2005; Kamp et al., 2019; Kuhlmann et al., 2005; Preuß & Wolf, 2009; Wolf, 2012), alongside null findings (Schoofs & Wolf, 2009; Tollenaar et al., 2009). This heterogeneous picture may reflect differences in the population studied, or in the memory tests used.

Within the stress and memory literature, there are a large variety of stress induction procedures and memory assessments, with few very similar or direct replications of these studies. The diversity of methods used may obscure exactly which factor results in stress-related memory alterations. By carefully exploring which aspects of memory may be affected by psychosocial stress, we can gain greater insight into the cognitive effects of psychosocial stress.

In the current study, we attempt to overcome some of these issues by conducting four similar experiments, each manipulating one crucial factor only, to allow for a controlled stepped approach. Through this, we hope to gain a clearer understanding of how and when psychosocial stress influences episodic memory. Each experiment uses the same sample size, the same experimental stress test and the same active control condition.

The first three experiments all induced psychosocial stress immediately after learning and allow 24 hr before delayed retrieval. This series of experiments began by first exploring the effects of stress on a relatively simple measure of memory: the ability to recognise whether neutral items were previously seen. We then further asked whether this item memory was supported by recollection or familiarity using a remember/know task. The second experiment used a cued-pair recall task, allowing further exploration of the impact of psychosocial stress upon specific recollection processes. This built from the first experiment by still examining simple recognition of neutral stimuli, only here also assessing whether stress aided search and retrieval of an item given a cue. The third experiment used the same memory task as the first experiment but assessed memory for different types of stimuli (random-neutral, categorised-neutral and emotionally negative word stimuli). By including a second category of neutral stimuli that were semantically related (categorised-neutral), we aimed to determine if stress specifically affects memory for emotional stimuli, or more broadly for stimuli with high semantic relatedness (inherent to emotionally negative stimuli). The fourth experiment used an identical memory procedure to the first, but induced stress immediately before retrieval instead of after encoding. Comparing studies one and four allowed us to explore the impact of timing of psychosocial stress on item recognition. The overall stepped design of these experiments aimed to reduce issues of heterogeneity where possible and allow for clearer insights into the effects of psychosocial stress on more specific aspects of episodic memory.

Salivary cortisol, a measure of the physiological response to stress, and self-reports of state anxiety were measured at multiple timepoints while participants underwent the Trier Social Stress Test (TSST; Kirschbaum et al., 1993), a commonly used laboratory-based experimental psychosocial stress paradigm. We predicted significantly higher scores for the stress group relative to controls on both these measures. Personality factors, specifically trait stress reactivity scores, were also measured to better identify potential stress responders and non-responders within the stress and control groups.

In line with previous meta-analyses (Shields et al., 2017; Wolf, 2009), and other studies exploring the effects of post-encoding psychosocial stress (Beckner et al., 2006; Preuß & Wolf, 2009), we expected to see memory enhancements in the stress group relative to controls in the three experiments using a post-encoding stress design. We also predicted specific enhancements in recollection across all these experiments, given proposed links between recollection and hippocampal function, and the sensitivity of the hippocampus to stress-related hormones. For the fourth study, we predicted that stress immediately before retrieval would lead to worse memory in the stress group relative to controls. Again, we predicted greatest effects on recollection. Last, beyond group effects of stress, we expected that those showing the greatest response to stress would show the greatest stress-related memory change. We therefore predicted significant correlations between levels of stress, as measured both by self-reports and salivary cortisol, and memory scores, within the stress group. In experiments 1, 2 and 3, we predicted these correlations would be positive (i.e., increased stress after encoding leads to better memory), whereas in experiment 4, we predicted this correlation to be negative (i.e., stress at retrieval impairs memory).

2 GENERAL METHODS APPLICABLE TO ALL STUDIES

2.3 General procedure

The study took place across two sessions, separated by 1 day. Stress occurred on day 1 for experiments 1, 2 and 3, but on day 2 for experiment 4. The TSST stress procedure used followed original details provided by Kirschbaum et al. (1993). For the stress group, participants were asked to prepare and present a 5-min presentation for a job interview in front of a panel of two socially neutral judges. After the speech task, participants then had to complete a verbal mental arithmetic challenge, also in front of the panel. Participants believed that both tasks were being filmed and voice recorded, and that one panel member was making notes regarding the manner and content of their speech. We used a control task similar to Kuhlmann et al. (2005) with matched cognitive conditions, but removing elements of social stress. Participants were told they would be having a 5-min conversation about a topic of their choosing that they enjoy (i.e., sports or a movie) with two friendly panel members. After the conversation, participants were asked to complete as many very simple maths questions as they could on paper (i.e., without peer evaluation). The study took place in two rooms, A and B. Participants were led between rooms A and B during the session by the researcher. Two panel members remained in room B on the day the stress task occurred and were not present on the other day. Timelines for post-encoding and retrieval stress are shown in Figure 1 and further procedural details are given as part of the relevant experiments.

Five times throughout the testing session, including the stress task, participants were asked to add a mark to a 10 cm line that best described their current feelings of anxiety, from “not at all anxious (0)” to “extremely anxious (10)”. The five time points were: (1) immediately after baseline rest period (2) after preparation for the Trier (3) after the Trier speech task (4) after the Trier maths task (5) immediately before study debrief. At time points 1, 4 and 5 passive drool saliva samples were also collected from participants. Stress levels were not measured on the other day of testing (day 2 in Experiments 1, 2 and 3, and day 1 in experiment 4). Session timelines are shown in Figure 1 (with post-encoding stress at the top, and stress at retrieval displayed at the bottom).

3 RESULTS COMMON TO ALL FOUR EXPERIMENTS

3.1 Stress results

We begin by reporting results for the stress manipulation, which was the same for all four studies. Time course plots for self-reported anxiety and salivary cortisol can been seen in Figure 2. The stress groups reported significantly greater anxiety (t(142) = 6.10, p < 0.001) and had significantly higher salivary cortisol concentrations (t(85.489) = 3.33, p = 0.001) than control groups. Descriptive statistics for measures of stress between groups are shown in Table 3.

TABLE 3. Mean and SD values for stress and control groups for measures of stress collapsed across all four experiments

	Stress		Control
	M	SD	M	SD
Cortisol (μg/dl, AUC)	0.87	0.69	0.55	0.30
Self-reported Anxiety (AUC)	20.02	6.97	12.95	6.94

• Abbreviation: AUC, area-under-the-curve.

We found a significant positive correlation between self-reported anxiety and salivary cortisol (r = 0.2, p = 0.025). Significant correlations between self-reported anxiety and trait stress were also seen in both the stress (r = 0.44, p < 0.001) and control (r = 0.26, p = 0.03) groups. Cortisol reactivity was not associated with trait stress in either stress (r = −0.05, p = 0.71) or control (r = −0.11, p = 0.38) groups.

4 EXPERIMENT 1: POST-ENCODING STRESS AND NEUTRAL ITEM RECOGNITION

4.2 Results

4.2.1 Measures of stress

The stress group reported higher levels of anxiety and showed higher cortisol reactivity than the control group (anxiety: t(34) = 3.76, p = 0.001; Cortisol: t(20.14) = 2.87, p = 0.01). Means and SD are shown in Table 4. Time course plots for self-reported anxiety and salivary cortisol can been seen below in Figure 3.

TABLE 4. Experiment 1 group mean and SD’s for stress and memory measures

	Stress		Control
	M	SD	M	SD
Cortisol (μg/dl, AUC)	1.4	0.9	0.7	0.4
Self-reported anxiety (AUC)	20.6	4.8	12.7	7.6
Recognition	0.13	0.71	−0.11	0.64
Recollection	0.35	0.75	0.38	0.61
Familiarity	0.43	0.65	0.14	0.59

• Abbreviation: AUC, area-under-the-curve.

No group differences were seen for scores of trait stress reactivity (p > 0.5) confirming that no prior differences exist between the groups for susceptibility to stress.

4.2.2 Stress and memory

For overall recognition accuracy, no significant difference was shown between stress and control groups (t(34) = 1.07, p = 0.294). Similarly, no group differences were observed between stress and control groups for recollection (t(34) = −0.15, p = 0.879). Familiarity scores also did not differ between groups (t(34) = 1.30, p = 0.2). Results are shown in Figure 4 and descriptive statistics in Table 4.

No significant correlations were seen between persistence scores for recognition accuracy or familiarity with either self-reported anxiety or cortisol. Recollection also did not significantly correlate with self-reported anxiety but showed a strong significant positive correlation with cortisol response (r = 0.66, p = 0.006).

5 EXPERIMENT 2: POST-ENCODING STRESS AND CUED-PAIR RECALL

5.2 Results

5.2.1 Measures of stress

Elevated levels of self-reported-anxiety were seen for participants in the stress group compared to the control group (t(1, 34) = 2.5, p = 0.018; Figure 5). Salivary cortisol concentrations did not differ between stress and control groups during the stress task (t(1, 26) = 1.236, p = 0.227). Descriptive statistics is shown in Table 5.

TABLE 5. Experiment 2 group mean and SD’s for stress and memory measures

	Stress		Control
	M	SD	M	SD
Cortisol (μg/dl, AUC)	0.84	0.70	0.59	0.28
Self-reported anxiety (AUC)	19.97	7.87	13.74	7.11
Recognition	0.66	0.62	0.47	0.34
Cued pair recall	0.2	0.36	0.46	0.54

• Abbreviation: AUC, area-under-the-curve.

5.2.2 Stress and memory

Two participants’ recognition persistence scores were classified as outliers and removed. Additionally, one participant's cued pair recall persistence score was identified as an outlier and removed. Removing these outliers did not affect the results of this experiment. Group means and SD shown in Table 5. Stress and control groups did not differ in recognition accuracy scores (t(27.045) = 1.151, p = 0.26) or in cued pair recall persistence scores (t(29.887) = −1.68, p = 0.1; Figure 6). No significant correlations were seen between either memory or stress metric during this study.

6 EXPERIMENT 3: POST-ENCODING STRESS AND EMOTIONAL MEMORY

6.2 Results

6.2.1 Measures of stress

The stress (self-reported anxiety: M = 19.80, SD = 6.98, cortisol: M = 0.72, SD = 0.48) group showed greater acute stress response than controls (self-reported anxiety: M = 11.13, SD = 6.35, cortisol: M = 0.41, SD = 0.16) both in terms of self-reported anxiety (t(34) = 3.9, p < 0.001) and cortisol, (t(28) = 2.342, p = 0.027), as shown in Figure 7. Groups did not differ for trait stress reactivity (p > 0.45) confirming no prior group differences in self-reported susceptibility to stress.

6.2.2 Stress and emotional memory

Persistence scores for recognition accuracy, recollection and familiarity were compared using mixed 2 (group) × 3 (list type) ANOVAs. Descriptive statistics for groups overall and separated by list type are shown in Table 6. For recognition accuracy, no significant effects of list type or group were observed, neither did the two factors interact. No significant main effects or interactions were found between stress and control groups for familiarity scores. For recollection persistence rates however, a significant list type x group interaction was seen (F(2, 62) = 3.7, p = 0.03), in the absence of main effects of either factor. Figure 8 shows stress and control group persistence scores for each memory metric.

TABLE 6. Experiment 3 memory metric group means and SD below in brackets for each memory metric, split by list type

	Negative-emotional		Related-neutral		Random-neutral
	Stress	Control	Stress	Control	Stress	Control
Recognition accuracy	0.48 (0.27)	0.61 (0.27)	0.51 (0.26)	0.56 (0.53)	0.62 (0.33)	0.59 (0.42)
Recollection	0.57 (0.52)	0.72 (0.32)	0.78 (0.46)	0.40 (0.34)	0.57 (0.57)	0.69 (0.49)
Familiarity	2.04 (18.75)	−1.73 (6.33)	−3.83 (23.03)	−0.11 (1.10)	−0.36 (1.85)	−7.28 (24.43)

Post hoc analyses were used to explore the significant interaction between list type and group for recollection persistence rates. Analyses were conducted separately for list type to compare persistence scores for each group. Recollection persistence scores did not differ between stress and control groups for emotionally negative (t(32) = −1.04, p = 0.31) or random neutral stimuli (t(32) = −0.68, p = 0.5). Persistence scores for related-neutral items, however, were significantly reduced for the control group compared to stress group (t(34) = 2.86, p = 0.007).

No significant relationship was shown between any stress, list type or memory measure. There was, however, a significant positive correlation between self-reported anxiety and recollection responses (r = 0.50, p = 0.036).

7 EXPERIMENT 4: RETRIEVAL STRESS AND NEUTRAL-ITEM RECOGNITION

7.2 Results

7.2.1 Measures of stress

Overall, the stress group reported significantly higher levels of anxiety during the task than the control group (t(34) = 2.16, p = 0.038), but did not exhibit a greater cortisol response (t(32) = 0.71, p = 0.48; Figure 9). No group differences were seen for trait stress reactivity (p > 0.25) confirming that no prior differences exist between the groups for self-reported susceptibility to stress. All descriptive statistics are shown in Table 7.

TABLE 7. Experiment 4 group mean and SD’s for stress and memory measures

	Stress		Control
	M	SD	M	SD
Cortisol (μg/dl, AUC)	0.55	0.28	0.48	0.27
Self-reported anxiety (AUC)	19.72	8.27	14.25	6.88
Recognition	0.38	0.2	0.46	0.15
Recollection	0.85	0.56	0.84	0.29
Familiarity	0.16	0.51	0.25	0.4

• Abbreviation: AUC, area-under-the-curve.

7.2.2 Stress and memory

Independent samples t tests were used to compare groups’ persistence scores for recognition accuracy, recollection and familiarity. Two recollection persistence scores and five familiarity persistence scores were identified as outliers and removed. The results of this experiment were not affected by removing outliers. Group means and SD shown in Table 7. Stress and control groups did not differ in recognition accuracy (t(34) = −1.42, p = 0.16), recollection scores (t(32) = 0.08, p = 0.94) or familiarity scores (t(29) = −0.58, p = 0.57; see Figure 10). No significant correlations were seen between either memory or stress metric during this study.

8 DISCUSSION

The current set of experiments cautiously and carefully assessed the effects of stress on episodic memory. By using the same stress induction procedures, we provide consistent evidence that our psychosocial stress test significantly increased perceived anxiety and salivary cortisol levels when considered overall. This stress induction had a limited effect on recognition memory accuracy, recollection, familiarity, and on paired-associate memory, measured through cued recall. In particular, we found no significant effects of psychosocial stress, induced either after learning or before retrieval, on item recognition or on cued-recall. Participants who experienced greater stress did, however, show evidence of greater change in recollection in some experiments. Additionally, the stress group in experiment 3 showed more persistence in their recollection of semantically related-items compared to controls. Together, these findings suggest that the impact of psychosocial stress on memory may be more subtle than expected. Inter-individual differences in stress responsivity may, however, begin to help explain some of the discrepancies in findings, particularly when recollection is assessed.

Current literature exploring the memory effects of post-encoding psychosocial stress is somewhat mixed, with some studies reporting no effects (Corbett et al., 2017), whereas others report significant enhancements in memory (Beckner et al., 2006). We found no significant effects of experimentally induced psychosocial stress after learning on recognition accuracy or familiarity. This was seen in experiments one, two and three. In each of these experiments, we see no difference in persistence rates between stress and control groups. This was extended in experiment 4, where we also found no significant effect of stress immediately before retrieval. This finding is in line with some previous psychosocial stress studies suggesting minimal impact of stress immediately before retrieval on episodic memory (Beckner et al., 2006; Schoofs & Wolf, 2009), but differs from others suggesting psychosocial stress immediately before retrieval leads to impairments in episodic memory (Kuhlmann et al., 2005; Merz et al., 2019).

We did, however, observe some significant positive correlations for stress and recollection in experiment 1 for cortisol and experiment 3 for self-reported anxiety. These findings could indicate a relationship between level of stress experienced and recollection ability within the stress group. Given that these associations were observed on different measures of stress (self-perceived vs. cortisol), and that such correlations within each experiment were likely underpowered, we should be careful in how we interpret these findings. These correlations do, however, suggest a potential relationship between high levels of post-encoding stress and enhanced recollection. If replicated in a larger sample, these findings could further highlight that each individual's level of stress reactivity and subsequent experience of the stressor, may play a key role in the effects of stress on episodic memory (Nater et al., 2007; Oei et al., 2006; Takahashi et al., 2004). One way to further explore this effect could be to design appropriate studies to better explore how the level of stress reactivity relates to recollection ability.

The findings from this series of experiments differ from studies using physical stressors, which often report that post-encoding stress enhances recognition memory, in particular familiarity (McCullough & Yonelinas, 2013; Yonelinas et al., 2011). A key question could be whether a study directly comparing the effects of physical and psychosocial stress would reveal differences in their effects on memory, as suggested here.

In experiment three, recollection of related-neutral words showed greater rates of persistence in the stress group than in controls. This finding suggests that post-encoding psychosocial stress specifically enhances recollection persistence for neutral words that are highly semantically related. This could be expanded to explain why stress has previously been thought to increase memory for emotional over neutral stimuli (Buchanan & Lovallo, 2001; Jelici et al., 2004), as emotional stimuli is thought to be more intrinsically semantically related than neutral stimuli (Buchanan et al., 2006; Talmi & Moscovitch, 2004). In our case, related-neutral stimuli were more semantically related than both emotional and random-neutral words, as shown by significant differences in latent semantic analysis scores between list types. It could therefore be suggested that psychosocial stress influences the recollection of words that are highly semantically related, regardless of their emotional valence. Future studies should carefully consider the semantic relatedness of their emotional (as well as their neutral) stimuli.

In addition to the stepped design used for these experiments, a further benefit of this study is that unlike many others, we also examine individuals’ trait stress reactivity and their subjective experience of stress during the TSST in addition to their cortisol reactivity. Self-reported measures of anxiety during the stress task provided an insight into emotional and cognitive responses to stress (Campbell & Ehlert, 2012), offering a quick and more holistic view of the individuals’ experience of the stress task than cortisol measures alone (Lazarus & Folkman, 1984; Schlotz et al., 2011). Although potentially susceptible to bias, self-reports of anxiety, measured using VAS, have been shown to be reliably sensitive to changes in mental state, such as those experienced during a social stress task (Cella & Perry, 1986; Taylor et al., 2018).

There were, however, some limitations that must be considered. Our studies were carried out in comparatively small numbers of participants. It must be acknowledged that small effects may be missed, and the results may be prone to type II errors due to low sample sizes. As such, replications in larger samples are needed to support the findings of these experiments. There are, however, some reasons to have confidence in our findings. As we used persistence scores to calculate change in memory, this removes an important aspect of inter-individual difference: the underlying memory ability of each participant. By minimising this issue, we get a more accurate view of how memory is affected by psychosocial stress, regardless of individual ability. Our findings are also consistent with the effects observed across the field as shown in our recent (pre-print) systematic review and meta-analysis of studies examining the effects of psychosocial stress on episodic memory (McManus et al., 2020). Overall, this found no significant effect of psychosocial stress on memory, across the field. Our findings are in line with these overall trends. This could suggest that our results may not simply the results of low sample size; however, replications in larger samples are still needed.

The sample used in each of the experiments also does not take into consideration potential effects of sex, hormonal contraceptive use or menstrual cycle phase, which have previously been shown to influence cortisol response (Duchesne & Pruessner, 2013; Kirschbaum, Wüst, et al., 1992; Roche et al., 2013). However, some studies in young adults examining the impact of mild-to-moderate stress on memory have reported no effects of sex, (Beckner et al., 2006; Olver et al., 2015). Future studies could, however, use larger sex-stratified samples that could also consider contraceptive use and menstrual phase to explore small effects of psychosocial stress on memory.

Additionally, although self-reported anxiety was seen to increase significantly in the stress over control groups, the rise in cortisol levels did not always reach significance in individual experiments. Increases in salivary cortisol that do not reach significance have been seen in other studies within the stress and cognition literature, both for psychosocial and physical stressors (Duncko et al., 2009; Jelici et al., 2004). Critically, when pooling the findings across all four experiments we did see significant differences between stress and control groups for both cortisol levels and self-reports of anxiety, suggesting it may have been an issue of power when considering cortisol comparisons in individual experiments alone. We also show an overall significant positive relationship between self-reported anxiety and in cortisol, suggesting that the stress manipulation had the desired effects.

During these studies, our participants were exposed to only moderate levels of stress. More highly stressful situations could have had more dramatic effects on memory. It has been reported that increasing the number of panel members can increase the effects of stress, while in contrast an all-female panel may reduce overall feelings of stress during the task (Goodman et al., 2017). Although our panel always had two members, gender of the panellists was not fixed within or across studies; an all-female panel was not uncommon during these experiments. In future, we could also stipulate a minimum level of stress experienced for each individual participant, this would help to minimise the influence of inter-individual differences in stress-reactivity. This suggestion is supported by the significant relationship we see between recollection and stress experienced in experiments 1 and 3.

In addition to levels of stress experienced, there is also the possibility that varying the timing of post-encoding stress impacts on its effects on memory, for instance, non-adrenergic and non-genomic effect may occur immediately after the onset of the stressor, whereas peak cortisol response is thought to occur after around 15 min (Quaedflieg & Schwabe, 2018). These biological responses could feasibly have different effects on memory. Discovering where boundaries for these effects lie would help us understand exactly how different forms of stress affect episodic memory.

In summary, we suggest that mild to moderate levels of psychosocial stress may have minimal effects on item recognition accuracy and familiarity at the group level, regardless of the timing of stress. It may however be that greater responsivity to stress could influence recollection ability. Finally, we also observed that post-encoding stress enhances recollection persistence rates of semantically related words relative to controls. Replications of these effects are needed in larger samples to support these suggestions further.

CONFLICT OF INTEREST

None declared.

AUTHOR CONTRIBUTIONS

All authors were involved in the formulation of the research ideas and development of methodology. Elizabeth McManus collected and analysed the data and created the original draft. Deborah Talmi, Hamied Haroon and Nils Muhlert provided critical reviews of the drafts and acquired financial support for the project.