Neurobiology of Learning and Memory 100 (2013) 77–87
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Neurobiology of Learning and Memory
journal homepage: www.elsevier.com/locate/ynlme
Pre-learning stress that is temporally removed from acquisition exerts
sex-specific effects on long-term memory
Phillip R. Zoladz a,⇑, Ashlee J. Warnecke a, Sarah A. Woelke a, Hanna M. Burke a, Rachael M. Frigo a,
Julia M. Pisansky a, Sarah M. Lyle a, Jeffery N. Talbot b
a
b
Department of Psychology, Sociology, & Criminal Justice, Ohio Northern University, Ada, OH 45810, USA
Department of Pharmaceutical & Biomedical Sciences, Raabe College of Pharmacy, Ohio Northern University, Ada, OH 45810, USA
a r t i c l e
i n f o
Article history:
Received 8 October 2012
Revised 13 December 2012
Accepted 15 December 2012
Available online 22 December 2012
Keywords:
Amygdala
Cortisol
Hippocampus
Learning
Memory
Stress
a b s t r a c t
We have examined the influence of sex and the perceived emotional nature of learned information on
pre-learning stress-induced alterations of long-term memory. Participants submerged their dominant
hand in ice cold (stress) or warm (no stress) water for 3 min. Thirty minutes later, they studied 30 words,
rated the words for their levels of emotional valence and arousal and were then given an immediate free
recall test. Twenty-four hours later, participants’ memory for the word list was assessed via delayed free
recall and recognition assessments. The resulting memory data were analyzed after categorizing the
studied words (i.e., distributing them to ‘‘positive-arousing’’, ‘‘positive-non-arousing’’, ‘‘negative-arousing’’, etc. categories) according to participants’ valence and arousal ratings of the words. The results
revealed that participants exhibiting a robust cortisol response to stress exhibited significantly impaired
recognition memory for neutral words. More interestingly, however, males displaying a robust cortisol
response to stress demonstrated significantly impaired recall, overall, and a marginally significant
impairment of overall recognition memory, while females exhibiting a blunted cortisol response to stress
demonstrated a marginally significant impairment of overall recognition memory. These findings support
the notion that a brief stressor that is temporally separated from learning can exert deleterious effects on
long-term memory. However, they also suggest that such effects depend on the sex of the organism, the
emotional salience of the learned information and the degree to which stress increases corticosteroid
levels.
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
Stress can enhance, impair or have no effect on learning and
memory. One factor that plays an important role in dictating
whether stress enhances or impairs learning is the stage of learning
and memory that is affected by the stress. Perhaps the most consistent finding in this area of work is that stress administered after
learning enhances consolidation (Beckner, Tucker, Delville, &
Mohr, 2006; Cahill, Gorski, & Le, 2003; Hui, Hui, Roozendaal,
McGaugh, & Weinberger, 2006; Preuss & Wolf, 2009; Smeets, Otgaar, Candel, & Wolf, 2008). The effects of stress on encoding and
retrieval, however, have been less clear. Although a great deal of
work has shown that stress impairs retrieval (Buchanan & Tranel,
2008; Buchanan, Tranel, & Adolphs, 2006; Kuhlmann, Piel, & Wolf,
2005; Park, Zoladz, Conrad, Fleshner, & Diamond, 2008; Smeets
et al., 2008; Tollenaar, Elzinga, Spinhoven, & Everaerd, 2008), there
⇑ Corresponding author. Address: Department of Psychology, Sociology, &
Criminal Justice, Ohio Northern University, 525 S. Main St. Hill 013, Ada, OH
45810, USA. Fax: +1 419 772 2746.
E-mail address: p-zoladz@onu.edu (P.R. Zoladz).
1074-7427/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.nlm.2012.12.012
are discrepancies in the literature, with some studies reporting
stress enhancing or having no effect on such processes (Beckner
et al., 2006; Buchanan & Tranel, 2008; Schwabe et al., 2009; Yang
et al., 2003). Investigations examining pre-learning stress have also
been inconclusive and have revealed enhancements, impairments
or no effects at all (Diamond et al., 2006; Duncko, Johnson, Merikangas, & Grillon, 2009; Elzinga, Bakker, & Bremner, 2005; Jelicic,
Geraerts, Merckelbach, & Guerrieri, 2004; Kim, Koo, Lee, & Han,
2005; Kim, Lee, Han, & Packard, 2001; Nater et al., 2007; Park
et al., 2008; Payne et al., 2006, 2007; Schwabe, Bohringer, Chatterjee, & Schachinger, 2008a; Smeets et al., 2008; Zoladz et al., 2011a).
Researchers have claimed that the timing of stress plays a critical role in the stress-induced modulation of learning and memory
(Diamond, Campbell, Park, Halonen, & Zoladz, 2007; Joels, Fernandez, & Roozendaal, 2011; Joels, Pu, Wiegert, Oitzl, & Krugers, 2006;
Schwabe, Joels, Roozendaal, Wolf, & Oitzl, 2012). Diamond and colleagues suggested that stress exerts a biphasic effect on hippocampal function, which consequentially results in differential effects
on long-term memory depending on when the stress is administered relative to learning (Diamond et al., 2007). According to these
investigators, stress activates the amygdala, which produces a ra-
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P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
pid enhancement of hippocampal synaptic plasticity that initially
facilitates the storage of information; however, as times passes,
the hippocampus descends into a refractory state for producing
new plasticity, and the storage of information is impaired. Part of
the basis for this theory was electrophysiological work indicating
that glucocorticoids, as well as electrical stimulation of the amygdala, could exert immediate excitatory, but delayed inhibitory, effects on hippocampal synaptic plasticity (Akirav & Richter-Levin,
1999; Frey, Bergado-Rosado, Seidenbecher, Pape, & Frey, 2001;
Karst et al., 2005; Wiegert, Joels, & Krugers, 2006). Indeed, a general consensus has begun to emerge suggesting that if the stress
converges in time with the learning experience and activates
neurobiological mechanisms that are in common with the learning
experience, then long-term memory can be enhanced (Joels et al.,
2006; Sandi, 2011). Such stress-induced enhancements are
thought to be associated with the rapid effects of noradrenergic
activity, non-genomic actions of corticosteroids and increased
glutamatergic transmission. Stress-induced impairments of hippocampal function, on the other hand, are thought to be associated
with delayed, gene-dependent activity of corticosteroids and
NMDA receptor desensitization (Akirav & Richter-Levin, 2002; Diamond et al., 2007; Halonen, Zoladz, Park, & Diamond, 2007; Joels,
Velzing, Nair, Verkuyl, & Karst, 2003).
Much of the support for this ‘‘temporal dynamics model’’ has
come from non-human animal work. For instance, Diamond et al.
(2007) found that brief stress administered immediately prior to
learning enhanced long-term spatial memory in rats; however, if
the same stress was administered 30 min before learning, no
enhancement was observed. The stress-induced enhancement of
long-term spatial memory was mimicked by epinephrine and
blocked by propranolol, a b-adrenergic receptor antagonist (Halonen et al., 2007). In contrast, the pre-learning stress-induced
impairment of long-term spatial memory, which was induced by
prolonged (i.e., 30 min) stress prior to training, was not prevented
by propranolol. The findings of human work in this area have been
mixed. Some studies examining pre-learning stress effects on longterm memory have been consistent with the temporal dynamics
model (e.g., Schwabe et al., 2008a), while others have observed
stress-induced enhancements or impairments of memory in situations that do not coincide with predictions that would arise from
this theory (e.g., Smeets et al., 2009). Recently, we found support
for the temporal dynamics model by showing that brief stress
administered immediately before learning enhanced long-term
memory in healthy young adults, while the same brief stressor
administered 30 min before learning impaired long-term memory
(Zoladz et al., 2011a). Importantly, the stress-induced enhancement of memory was associated with participants’ heart rate response to the stress, an indication of autonomic nervous system
and noradrenergic activity, while the stress-induced impairment
of memory was associated with participants’ corticosteroid and
blood pressure responses to the stress. Combined with the above
evidence from rodent research, these findings supported the possibility that noradrenergic mechanisms are responsible for prelearning stress-induced enhancements of long-term memory, but
they are eventually suppressed by the delayed increase in corticosteroids, which results in memory impairment (Schwabe et al.,
2012).
Most, but not all, of the research examining stress effects on
learning and memory has reported greater effects for information
that is emotional in nature (Schwabe, Wolf, & Oitzl, 2010; Wolf,
2009). This finding makes sense considering the amygdala, an area
of the brain that is highly involved in emotional memory formation
(McGaugh, 2004), is significantly activated by emotional material
(Dolcos, LaBar, & Cabeza, 2004; Strange & Dolan, 2004). One limitation of studies of this type, however, is that investigators have
frequently confounded the emotional valence (e.g., positive, nega-
tive, and neutral) of the words with the arousal level (arousing,
non-arousing) of the words. That is to say, few, if any, studies have
systematically manipulated both the emotional valence and arousal level of the learned information in order to examine how such
manipulations influence stress-induced alterations of learning and
memory (see Schwabe et al., 2008a for a consideration of this
point). In the present experiment, we examined the influence of
pre-learning stress on memory for information that varied in both
emotional valence and arousal by systematically varying the levels
of each based on subjective ratings provided by participants.
Sex differences also abound in the stress–memory literature,
and a clear agreement on how sex mediates such effects has not
been reached. Investigators have contended that sex differences
depend on the type of learning that occurs (Conrad et al., 2004),
the stage of learning that is affected by the stress (Andreano & Cahill, 2009) and the hormonal response of the participant to the
stress (Andreano & Cahill, 2006; Andreano & Cahill, 2009). Perhaps
one relatively consistent finding in this area is that corticosteroidmediated modulation of learning and memory has been observed
more in males than in females. Several studies have reported cortisol- or stress-induced alterations of learning and memory or significant relationships between cortisol and memory in males, but
not females (Andreano & Cahill, 2006; Jackson, Payne, Nadel, & Jacobs, 2006; Wolf, Schommer, Hellhammer, McEwen, & Kirschbaum, 2001; Wood, Beylin, & Shors, 2001; Zorawski, Blanding,
Kuhn, & LaBar, 2006). Ultimately, additional work is needed to assess the influence of sex on stress–memory interactions, especially
in humans.
The purpose of the present study was to examine the influence
of sex and the perceived emotional nature of the learned information on pre-learning stress effects on long-term memory. Given the
aforementioned findings, we predicted that a brief stressor administered 30 min before learning would impair long-term memory,
particularly for emotional information (Zoladz et al., 2011a). We
also expected that, since pre-learning stress-induced impairments
of learning may be more dependent on corticosteroid mechanisms,
males might be more adversely affected by the stress than females.
2. Method
2.1. Participants
Ninety-seven healthy men and women (38 men, 59 women;
age: M = 19.18, SD = 1.15) from Ohio Northern University volunteered to participate in the present experiment. Individuals were
excluded from participating if they met any of the following conditions: diagnosis of Raynaud’s disease or peripheral vascular disease; presence of skin diseases, such as severe psoriasis, eczema
or scleroderma; history of severe head injury; current treatment
with psychotropic medications, narcotics, beta-blockers, steroids
or any other medication that was deemed to significantly affect
central nervous or endocrine system function; mental or substance
use disorder; regular nightshift work. Individuals who smoked
were allowed to participate in the study; information regarding
individuals’ smoking habits was collected prior to the experiments
via a short demographic survey. There was only one participant
who reported smoking on a regular basis, and inclusion of the data
from this participant in the statistical analyses did not alter the results. Females who took birth control on a regular basis were also
allowed to participate in the study; prior to participation, we asked
female participants if they took birth control via a short demographic survey. Females who reportedly took birth control were
not significantly different from naturally cycling females on any
physiological or behavioral measure, nor did stress significantly
interact with birth control in these analyses. Therefore, we treated
P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
all females as a single group in the statistical analyses for this
study. Participants were asked to refrain from using recreational
drugs (e.g., marijuana) for 3 days prior to the experimental sessions; to refrain from drinking alcohol or exercising extensively
for 24 h prior to the experimental sessions; and, to refrain from
eating or drinking anything but water for 2 h prior to the experimental sessions. Participants were awarded class credit upon completion of the study. All of the methods for the experiment were
approved by the Institutional Review Board at Ohio Northern
University.
2.2. Experimental procedures
The experimental timeline for the present experiment is presented in Fig. 1. To control for diurnal variations in cortisol levels,
all testing was carried out between 1200 and 1800 h.
2.2.1. Socially Evaluated Cold Pressor Test (SECPT)
Participants were asked to submerge their dominant hand, up
to and including the wrist, in a bath of water for 3 min. Those participants who had been randomly assigned to the stress condition
(N = 49; 20 males and 29 females) placed their hand in a bath of ice
cold (0–2 °C) water, while participants who had been randomly assigned to the control condition (N = 48; 18 males and 30 females)
placed their hand in a bath of warm (35–37 °C) water. The water
was maintained at the appropriate temperature by a Lauda Brinkmann RMT6 circulating water bath. To maximize the stress response, participants in each experiment were encouraged to keep
their hand in the water bath for the entire 3-min period. However,
if a participant found the water bath too painful, he or she was allowed to remove his or her hand from the water and continue with
the experiment. Only five participants from the stress condition removed their hand from the water prior to 3 min elapsing (M water
time = 166.37 s, SD = 40.99), and all participants from the no stress
condition kept their hand in the water for the entire 3-min period.
Based on previous work (Schwabe, Haddad, & Schachinger,
2008b), a social evaluative component was added to the cold pressor manipulation. Participants in the stress condition were misleadingly informed that they were being videotaped during the
procedure for subsequent evaluation of their facial expressions,
and throughout the water bath manipulation, they were asked to
keep their eyes on a camera that was located on the wall of the
laboratory.
79
2.2.2. Subjective pain and stress ratings
All participants were asked to rate the painfulness and stressfulness of the water bath manipulation at 1-min intervals on 11point scales ranging from 0 to 10, with 0 indicating a complete lack
of pain or stress and 10 indicating unbearable pain or stress. If a
participant removed his or her hand from the water before 3 min
had elapsed, the remaining data points were automatically scored
as 10 s for each measure.
2.2.3. Word presentation
Thirty minutes following exposure to the water bath manipulation, participants were presented with a list of 30 words, which
were selected from the Affective Norms for English Words (Bradley
& Lang, 1999). Based on standardized valence and arousal ratings,
we chose 10 neutral (5 arousing and 5 non-arousing), 10 positive
(5 arousing and 5 non-arousing) and 10 negative (5 arousing and
5 non-arousing) words, which, across emotional valence and arousal categories, were balanced for word length and word frequency.
As per the methods employed by Schwabe et al. (2008a), participants were instructed to read each word aloud and rate its emotional valence on a scale from 3 (very negative) to +3 (very
positive) and its arousal level on a scale of 1 (not arousing) to 7
(very highly arousing) on a sheet of paper containing the list of
words. These manipulations were performed to promote encoding
of the words, and they allowed us to analyze the final memory data
based on participants’ own ratings of the words (see Section 2.3).
Two versions of the word list were used in the experiments.
According to the Affective Norms for English Words (Bradley & Lang,
1999), the mean (±SEM) standardized valence and arousal ratings
for the words that made up these lists were as follows: word list
1 (positive arousing words (e.g., erotic): valence = 7.77 ± 0.16, arousal = 6.84 ± 0.29; positive non-arousing words (e.g., butterfly): valence = 7.41 ± 0.13, arousal = 3.23 ± 0.08; negative arousing words
(e.g., poison): valence = 2.32 ± 0.20, arousal = 6.71 ± 0.37; negative
non-arousing words (e.g., bored): valence = 2.50 ± 0.20, arousal = 3.75 ± 0.24; neutral arousing words (e.g., lightning): valence = 4.61 ± 0.25, arousal = 6.53 ± 0.12; neutral non-arousing
words (e.g., hairpin): valence = 5.00 ± 0.18, arousal = 3.54 ± 0.13)
and word list 2 (positive arousing words (e.g., lust): valence = 7.69 ± 0.26, arousal = 6.86 ± 0.22; positive non-arousing
words (e.g., secure): valence = 7.40 ± 0.09, arousal = 3.46 ± 0.21;
negative arousing words (e.g., bloody): valence = 2.30 ± 0.20, arousal = 6.74 ± 0.17; negative non-arousing words (e.g., trash): va-
Fig. 1. Timeline for the methodology employed in the present study. Participants were exposed to the water bath manipulation at time point 0. Participants in the stress
condition placed their dominant hand in ice cold (0–2 °C) water while believing that they were being videotaped [Socially Evaluated Cold Pressor Test (SECPT)]; participants
in the no stress condition placed their dominant hand in warm (35–37 °C) water. Thirty minutes following the initiation of the water bath manipulation, participants were
presented with a list of 30 words. They were asked to read each word aloud and to rate each word’s emotional valence and arousal level. Immediately following word list
encoding, participants were given a free recall assessment. To verify the induction of a stress response, saliva samples (S in the figure) and cardiovascular measurements (C in
the figure) were obtained from participants throughout the experimental session. Twenty-four hours later, participants returned to the laboratory to complete free recall and
recognition tests regarding the word list that was studied on the previous day. Recognition memory was assessed 15 min following the free recall assessment.
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P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
lence = 2.39 ± 0.25, arousal = 4.35 ± 0.24; neutral arousing words
(e.g., volcano): valence = 4.71 ± 0.14, arousal = 6.58 ± 0.33; neutral
non-arousing
words
(e.g.,
bland):
valence = 5.01 ± 0.24,
arousal = 3.48 ± 0.13).
2.2.4. Memory testing
Immediately following word list encoding, participants were given 5 min to write down as many words as they could remember
from the list of words they just studied (i.e., immediate free recall).
One day following the first experimental session, participants returned to the laboratory to have their memory for the list of words
assessed. Participants were again given 5 min to write down as
many words as they could remember from the list of words that
they studied on the previous day (i.e., delayed free recall). Then,
participants sat quietly and completed school work that they had
brought to the laboratory. After 15 min had elapsed, participants
were given a recognition test. They were presented with a list of
words containing 30 ‘‘old’’ words (i.e., words that were presented
on the previous day) and 30 ‘‘new’’ words (i.e., words that were
not presented on the previous day) and were instructed to label
each word as ‘‘old’’ or ‘‘new’’. The ‘‘new’’ words were matched to
the ‘‘old’’ words on emotional valence, arousal level, word length
and word frequency, according to the ratings obtained from the
Affective Norms for English Words (Bradley & Lang, 1999). To assess
participants’ ability to discriminate between ‘‘old’’ and ‘‘new’’
words, we calculated a sensitivity index (d0 = z[p(hit) p(false
alarm)]) for each category of word (i.e., positive-arousing, positive-non-arousing, negative-arousing, etc.) to be used for statistical
analysis (Wickens, 2002).
2.2.5. Cardiovascular analysis
Heart rate (HR) and blood pressure (BP) measurements were taken 2 min before (baseline), halfway through and 10 min after the
water bath manipulation. Cardiovascular activity was measured
with a vital signs monitor (Mark of Fitness WS-820 Automatic
Wrist Blood Pressure Monitor) placed on the wrist of each participant’s non-dominant hand.
2.2.6. Cortisol analysis
Saliva samples were collected from participants 2 min before
(baseline) and 28 min following exposure to the water bath manipulation to analyze salivary cortisol concentrations. The samples
were collected in a Salivette saliva collection device (Sarstedt,
Inc., Newton, NC). Participants were asked to place a swab of cotton in their mouths and chew on it so that it would easily absorb
their saliva. Following 1 min of chewing, the swab was collected
and placed in the Salivette conical tube and kept at room temperature until the experimental session was completed. The samples
were subsequently stored at 20 °C until assayed for cortisol.
Saliva samples were thawed and extracted by low-speed centrifugation. Salivary cortisol levels were determined by enzyme immuno assay (Caymen Chemical Co., Ann Arbor, MI) according to
the manufacturer’s protocol. The minimum detectable concentration of cortisol was approximately 8 pg/ml, and the average interand intra-assay percent coefficients of variation were less than
6.9% and 6.8%, respectively.
2.3. Statistical analyses
Prior to performing any statistical analyses, we conducted correlations to verify that participants’ valence and arousal ratings
of the studied words were not significantly correlated; our analysis
confirmed this prediction. Therefore, mixed-model ANOVAs were
used to analyze all data; the between-subjects factors utilized in
these analyses were stress and sex, and the within-subjects factors
were word valence and arousal (for recall and recognition analy-
ses) or time (for physiological and pain/stress ratings analyses).
Participants in the stress condition were divided into ‘‘Responders’’
and ‘‘Non-Responders’’ based on their cortisol responses to the
SECPT. Those participants exhibiting a cortisol increase of at least
1.5 nmol/l following the SECPT were considered Responders; all
other participants were considered Non-Responders. The cutoff
for dividing participants into Responder and Non-Responder
groups was based on previous work using a similar criterion
(Schwabe et al., 2008a; Zoladz et al., 2011a).
The analyses of participants’ valence and arousal ratings and
memory for the words (i.e., immediate free recall, delayed free recall, and recognition) were performed based on categorizing the
words (i.e., distributing the words to ‘‘positive-arousing’’, ‘‘positive-non-arousing’’, ‘‘negative-arousing’’, etc. groups) according to
participants’ subjective valence and arousal ratings that were obtained during the study (see Section 2.2.3).
For each analysis of memory data, we conducted a mixed-model
ANOVA analyzing the effects of stress, overall (i.e., stress vs. no
stress), on memory performance, and then we conducted a subsequent mixed-model ANOVA to assess the influence of cortisol response to stress (i.e., Responders vs. Non-Responders) on
memory performance. For memory measures affected by stress,
we conducted additional ANCOVAs to control for the influence of
participants’ valence and arousal ratings of the studied words on
such effects. We also conducted bivariate correlations (Pearson’s
r) to explore possible relationships between participants’ physiological responses to stress and cognitive performance. To limit
the inflation of Type I error rates in these analyses, the correlations
were performed only for memory measures affected by stress.
Alpha was set at 0.05 for all analyses, and Bonferroni-corrected
post hoc tests were employed when necessary. Outlier data points
that were at least 3 standard deviation units beyond the exclusive
group means were eliminated from the analyses; less than 1% of all
data were outliers. SPSS (version 18.0; SPSS, Inc.) was used to perform all statistical analyses.
3. Results
3.1. Physiological responses
3.1.1. Heart rate (see Table 1)
The stress manipulation had no effect on HR (no significant effect of condition: F(2,88) = 1.43, p > 0.05, g2 = 0.03). However, females exhibited an overall greater HR than males (significant
effect of sex: F(1,88) = 4.53, p < 0.05, g2 = 0.05), and HR decreased
over time across all participants (significant effect of time:
F(2,176) = 6.92, p < 0.001, g2 = 0.07). No other significant effects
were observed.
3.1.2. Systolic blood pressure (see Table 1)
Stressed participants, independent of cortisol response to the
stressor, exhibited greater systolic BP than non-stressed participants during the water bath manipulation (significant effect of
condition: F(2,88) = 8.36, p < 0.001, g2 = 0.16; significant effect of
time: F(2,176) = 145.68, p < 0.001, g2 = 0.62; significant Condition Time interaction: F(4,176) = 8.54, p < 0.001, g2 = 0.16). Male
participants also exhibited greater systolic BP than female participants, particularly in response to the water bath manipulation (significant effect of sex: F(1,88) = 31.16, p < 0.001, g2 = 0.26;
significant Sex Time interaction: F(2,176) = 4.43, p < 0.05,
g2 = 0.05). No other significant effects were observed.
3.1.3. Diastolic blood pressure (see Table 1)
Stressed participants, independent of cortisol response to the
stressor, exhibited greater diastolic BP than non-stressed partici-
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P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
sample sizes for each group: 20 Responders (8 male, 12 female)
and 29 Non-Responders (12 male, 17 female). As expected,
Responders exhibited greater salivary cortisol levels than nonstressed participants after the water bath manipulation (significant
effect of condition: F(2,91) = 4.71, p < 0.05, g2 = 0.09; significant effect of time: F(1,91) = 41.07, p < 0.001, g2 = 0.31; significant Condition Time interaction: F(2,91) = 26.09, p < 0.001, g2 = 0.36). No
other significant effects were observed.
Table 1
Cardiovascular activity before, during and after the water bath manipulation.
DV/condition
Heart rate (bpm)
Responders
Male
Female
Non-Responders
Male
Female
No stress
Male
Female
Pre
During
Post
75.38 (5.14)
74.75 (4.29)
71.88 (3.86)
73.67 (4.94)
64.50 (3.04)
70.42 (2.90)
74.58 (5.80)
77.94 (3.52)
72.45 (4.40)
76.53 (2.70)
66.45 (5.44)
76.53 (3.18)
72.56 (1.86)
81.10 (2.50)
75.28 (2.65)
78.38 (1.96)
71.94 (2.20)
80.37 (1.80)
166.13 (8.54)*
148.33 (4.40)*
132.50 (5.48)
116.42 (3.38)
161.25 (5.90)*
137.71 (4.47)*
129.08 (2.50)
118.12 (2.57)
140.94 (3.11)
128.62 (1.80)
123.59 (3.51)
115.30 (1.93)
112.38 (3.04)*
99.33 (3.44)*
78.50 (3.44)
74.08 (2.41)
107.75 (5.19)*
95.35 (4.09)*
80.00 (2.01)
76.41 (1.95)
88.47 (2.55)
83.60 (1.40)
73.82 (2.69)
73.97 (1.17)
Systolic blood pressure (mm Hg)
Responders
Male
134.38 (4.33)
Female
123.50 (2.63)
Non-Responders
Male
135.75 (3.08)
Female
127.47 (4.16)
No stress
Male
129.83 (2.86)
Female
119.21 (2.06)
Diastolic blood pressure (mm Hg)
Responders
Male
83.25 (2.93)
Female
78.33 (1.90)
Non-Responders
Male
82.75 (2.53)
Female
80.63 (2.26)
No stress
Male
82.41 (2.31)
Female
77.59 (1.09)
3.2. Subjective ratings of water bath manipulation (see Table 2)
3.2.1. Pain ratings
Stressed participants, independent of cortisol response to the
stressor, reported greater pain ratings than non-stressed participants throughout the water bath manipulation, and these ratings
increased
over
time (significant
effect
of condition:
F(2,88) = 232.05, p < 0.001, g2 = 0.84; significant effect of time:
F(2,176) = 8.00, p < 0.001, g2 = 0.08; Condition Time interaction
trending toward significance: F(4,176) = 2.26, p = 0.06, g2 = 0.05).
No other significant effects were observed.
3.2.2. Stress ratings
Stressed participants, independent of cortisol response to the
stressor, reported greater stress ratings than non-stressed participants throughout the water bath manipulation (significant effect
of condition: F(2,88) = 89.49, p < 0.001, g2 = 0.67; significant effect
of time: F(2,176) = 3.12, p < 0.05, g2 = 0.03). No other significant effects were observed.
Data are presented as means ± SEM.
p < 0.05 relative to the no stress group.
3.3. Word list ratings
*
pants during the water bath manipulation (significant effect of
condition: F(2,87) = 10.49, p < 0.001, g2 = 0.19; significant effect
of time: F(2,174) = 219.76, p < 0.001, g2 = 0.72; significant Condition Time interaction: F(4,174) = 18.43, p < 0.001, g2 = 0.30).
Male participants also exhibited greater diastolic BP than female
participants, particularly in response to the water bath manipulation (significant effect of sex: F(1,87) = 9.21, p < 0.01, g2 = 0.10; significant Sex Time interaction: F(2,174) = 7.38, p < 0.001,
g2 = 0.08). No other significant effects were observed.
3.3.1. Valence ratings
Positive words were given more positive ratings than neutral
words, which were given more positive ratings than negative
words (significant effect of valence: F(2,180) = 847.38, p < 0.001,
g2 = 0.90). Non-arousing words were also given more positive ratings than arousing words (significant effect of arousal:
F(1,90) = 5.61, p < 0.05, g2 = 0.06). The latter effect seemed to be
driven by neutral words being rated more negatively if they were
arousing
(significant
Valence Arousal
interaction:
F(2,180) = 14.80, p < 0.001, g2 = 0.14). No other significant effects
were observed.
3.1.4. Salivary cortisol (see Fig. 2)
Based on the criteria employed to divide participants into
Responders and Non-Responders, we ended up with the following
3.3.2. Arousal ratings
Arousing words were rated as more arousing than non-arousing
words (significant effect of arousal: F(1,90) = 190.49, p < 0.001,
Males
Females
12
12
Responders
Non-Responders
No Stress
*
8
6
4
-10
0
10
Time (min)
20
30
6
4
2
Learning
0
8
Water
Bath
Learning
Water
Bath
2
*
Responders
Non-Responders
No Stress
10
Cortisol (nmol/l)
Cortisol (nmol/l)
10
0
-10
0
10
20
30
Time (min)
Fig. 2. Salivary cortisol concentrations before and after the water bath manipulation in males (left) and females (right). In both sexes, Responders were the only participants
to exhibit significantly increased cortisol concentrations following the water bath manipulation. Data are presented as means ± SEM; = p < 0.05 relative to the no stress
group.
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P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
Table 2
Pain and stress ratings of the water bath manipulation.
DV/Condition
Minute 1
Painfulness (scale of 0–10)
Responders
Male
6.88
Female
5.75
Non-Responders
Male
6.42
Female
6.18
No stress
Male
0.28
Female
0.10
Minute 2
Minute 3
(0.64)*
(0.60)*
6.75 (0.88)*
6.25 (0.66)*
7.25 (0.88)*
6.75 (0.88)*
(0.65)*
(0.60)*
6.67 (0.62)*
6.65 (0.49)*
6.92 (0.56)*
7.18 (0.46)*
(0.14)
(0.08)
0.17 (0.09)
0.11 (0.06)
0.18 (0.10)
0.10 (0.06)
5.75 (1.01)*
5.33 (0.70)*
5.75 (1.03)*
5.67 (0.71)*
5.42 (0.92)*
5.76 (0.63)*
5.67 (0.85)*
6.06 (0.58)*
0.53 (0.19)
0.27 (0.10)
0.41 (0.15)
0.30 (0.10)
Stressfulness (scale of 0–10)
Responders
Male
6.00 (0.94)*
Female
5.25 (0.72)*
Non-Responders
Male
5.00 (0.84)*
Female
5.35 (0.70)*
No stress
Male
0.24 (0.11)
Female
0.10 (0.06)
Data are presented as means ± SEM.
p < 0.05 relative to the no stress group.
*
g2 = 0.68). Interestingly, however, males rated the words, overall,
as less arousing than females (significant effect of sex:
F(1,90) = 4.82, p < 0.05, g2 = 0.05). In addition, positive words were
rated as more arousing than negative words, which were rated as
more arousing than neutral words (significant effect of valence:
F(2,180) = 103.16, p < 0.001, g2 = 0.53). This effect was dependent
on both the sex of the participant and the arousal level of the
words, as males, in general, rated negative, arousing words as less
arousing than females did (significant Valence Arousal interaction: F(2,180) = 7.63, p < 0.01, g2 = 0.08; significant Sex Valence Arousal interaction: F(2,180) = 3.52, p < 0.05, g2 = 0.04). No
other significant effects were observed.
3.4. Memory testing
3.4.1. Immediate free recall (see Fig. 3)
The analysis of stress effects, overall, revealed that the stress
manipulation had no effect on short-term memory (no significant
effect of stress: F(1,92) = 0.01, p > 0.05, g2 = 0.00). We then analyzed for the involvement of cortisol response in the effects of
stress on immediate free recall; this analysis again revealed that
the stress manipulation had no effect on short-term memory (no
significant effect of condition: F(2,90) = 1.36, p > 0.05, g2 = 0.03).
However, participants recalled more arousing words than nonarousing words (significant effect of arousal: F(1,90) = 50.93,
p < 0.001, g2 = 0.36). This effect was dependent on word valence,
as participants recalled more arousing words than non-arousing
words only when the words were neutral or positive in valence
(significant
Valence Arousal
interaction:
F(2,180) = 9.61,
p < 0.01, g2 = 0.10). No other significant effects were observed.
3.4.2. Delayed free recall (see Fig. 4)
The analysis of the effects of stress, overall, revealed that the
stress manipulation had no effect on delayed free recall (no significant effect of stress: F(1,90) = 0.05, p > 0.05, g2 = 0.00). We then
analyzed for the involvement of cortisol response in the effects of
stress on delayed free recall; this analysis revealed that male
Responders recalled fewer words than all other groups (significant
Condition Sex interaction: F(2,88) = 3.30, p < 0.05, g2 = 0.07).
Also, arousing words were better recalled than non-arousing
words, particularly when they were neutral or positive words (sig-
nificant effect of arousal: F(1,88) = 75.06, p < 0.001, g2 = 0.46; significant Valence Arousal interaction: F(2,176) = 5.25, p < 0.01,
g2 = 0.06). No other significant effects were observed.
We then conducted an ANCOVA on the delayed free recall data,
using the valence and arousal ratings as covariates. This analysis
revealed that when controlling for such word ratings, the aforementioned stress effects on memory remained significant (significant Condition Sex interaction: F(2,86) = 3.16, p < 0.05,
g2 = 0.07).
3.4.3. Delayed recognition (see Fig. 5)
The analysis of the effects of stress, overall, on recognition revealed that stressed participants recognized fewer words than
non-stressed participants, particularly when they were positive
and non-arousing (effect of condition trending toward significance:
F(1,86) = 3.89,
p = 0.052,
g2 = 0.04;
significant
Stress Valence Arousal interaction: F(2,172) = 3.10, p < 0.05,
g2 = 0.04). We then analyzed for the involvement of cortisol response in the effects of stress on recognition; this analysis revealed
that neutral words were better recognized than negative words
(significant effect of valence: F(2,168) = 3.92, p < 0.05, g2 = 0.05).
This effect may have been more evident in Non-Responders and
non-stressed participants because Responders recognized fewer
neutral words than all other groups (significant Condition Valence interaction: F(4,168) = 2.55, p < 0.05, g2 = 0.06).
Male Responders and female Non-Responders also tended to recognize fewer words, overall, than their respective no stress group,
but this effect was only approaching significance (Condition Sex
interaction trending toward significance: F(2,84) = 2.70, p = 0.073,
g2 = 0.06). Arousing words were also better recognized than nonarousing words, particularly when they were positive in valence
(significant effect of arousal interaction: F(1,84) = 4.65, p < 0.05,
g2 = 0.05;
significant
Valence Arousal
interaction:
F(2,168) = 9.55, p < 0.001, g2 = 0.10). No other significant effects
were observed.
We then conducted an ANCOVA on the recognition data, using
the valence and arousal ratings as covariates. This analysis revealed that when controlling for such word ratings, the aforementioned stress effects on memory were only approaching
significance (Condition Valence interaction trending toward significance: F(4,164) = 2.03, p = 0.093, g2 = 0.05; Condition Sex
interaction trending toward significance: F(2,82) = 2.82, p = 0.065,
g2 = 0.06).
3.5. Associations between physiological stress response and memory
There were significant negative correlations between cortisol
and (1) overall delayed free recall, r(19) = 0.46, and (2) emotional
word (i.e., positive, negative, neutral) delayed free recall,
r(19) = 0.48, which were observed in stressed males (i.e.,
Responders and Non-Responders combined) only (p’s < 0.05).
4. Discussion
We have found that brief stress administered 30 min before
learning exerts deleterious effects on long-term (24-h) memory
in human participants. Importantly, however, the magnitude of
these effects and the type of influence that corticosteroids had on
such effects appeared to differ across the sexes. We did find that
Responders, independent of sex, exhibited impaired recognition
of emotionally neutral information, but this effect seemed to be
visibly more pronounced in males. In addition, males exhibiting a
robust stress-induced increase in cortisol displayed significantly
impaired recall and a marginally significant impairment of recognition memory, while females exhibiting a blunted stress-induced
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P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
Males
Responders
Non-Responders
No Stress
80
Free Recall (% of total)
80
Free Recall (% of total)
Females
Responders
Non-Responders
No Stress
60
40
20
0
60
40
20
0
Arousing
NonArousing NonArousing NonArousing
Arousing
Arousing
Positive
Negative
Neutral
Arousing
Total
NonArousing NonArousing NonArousing
Arousing
Arousing
Positive
Negative
Neutral
Total
Fig. 3. Immediate free recall performance in males (left) and females (right). No statistically significant group differences were observed, suggesting that stress exposure did
not impact short-term memory. Data are presented as means ± SEM.
Males
Females
Responders
Non-Responders
No Stress
60
50
40
30
*
20
10
*
0
Free Recall (% of total)
50
Free Recall (% of total)
Responders
Non-Responders
No Stress
60
40
30
20
10
0
Arousing
NonArousing NonArousing NonArousing
Arousing
Arousing
Positive
Negative
Neutral
Arousing
Total
NonArousing NonArousing NonArousing
Arousing
Arousing
Positive
Negative
Neutral
Total
Fig. 4. Long-term (24-h) free recall performance in males (left) and females (right). Male Responders recalled fewer words, overall, than all other groups. This effect seemed to
be driven by a large reduction in their recall of non-arousing words (verified via planned comparisons; see Section 4); however, the Condition Sex Arousal interaction was
not significant. Data are presented as means ± SEM; = p < 0.05 relative to the Non-Responder and no stress groups.
increase in cortisol exhibited a marginally significant impairment
of recognition memory. In general, our findings support the temporal dynamics model of stress–memory interactions by revealing
that even a brief stressor that is temporally separated from the
learning experience can impair long-term memory, hypothetically
via amygdala-induced inhibition of hippocampal processing. However, the differences observed across male and female participants
draw attention to the fact that sex differences must be integrated
into this model for it to be a more comprehensive approach to
understanding emotional memory processing.
4.1. Neurobiological mechanisms
Extensive work has shown that stress exerts differential effects
on various stages of learning and memory. For instance, stress can
enhance, impair or have no effect on encoding/acquisition (e.g.,
Diamond et al., 2006; Payne, Nadel, Allen, Thomas, & Jacobs,
2002; Schwabe et al., 2008a). On the other hand, stress generally
facilitates consolidation (Beckner et al., 2006; Cahill et al., 2003;
Hui et al., 2006; Preuss & Wolf, 2009; Smeets et al., 2008), while
impairing retrieval (Buchanan & Tranel, 2008; Buchanan et al.,
2006; de Quervain, Roozendaal, & McGaugh, 1998; Diamond
et al., 2006; Kuhlmann et al., 2005; Smeets et al., 2008; Tollenaar
et al., 2008), effects that have both been associated with stress-induced increases in glucocorticoids. In the present study, we observed significant effects of stress on delayed, but not immediate,
memory testing, suggesting that stress largely affected the consolidation of information. Since we observed significant stress-induced impairments of long-term memory in both males and
females, it might appear that our results are in conflict with the
prevailing view that stress enhances consolidation. However, it is
important to emphasize that a significant portion of studies demonstrating stress-induced enhancements of memory consolidation
have administered the stress after learning (Beckner et al., 2006;
Cahill et al., 2003; Hui et al., 2006; Preuss & Wolf, 2009; Smeets
et al., 2008). The effects of pre-learning stress on memory consolidation are less clear and often depend on the timing of the stress
relative to learning (Diamond et al., 2007; Zoladz et al., 2011a). For
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P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
Males
3.0
2.5
Responders
Non-Responders
No Stress
3.5
β
2.0
1.5
1.0
0.5
0.0
Discrimination Index (d' )
3.5
Discrimination Index (d' )
Females
Responders
Non-Responders
No Stress
3.0
β
2.5
2.0
1.5
1.0
0.5
0.0
Arousing
NonArousing NonArousing NonArousing
Arousing
Arousing
Positive
Negative
Neutral
Arousing
Total
NonArousing NonArousing NonArousing
Arousing
Arousing
Positive
Negative
Neutral
Total
Fig. 5. Long-term (24-h) recognition performance in males (left) and females (right). Responders recognized fewer neutral words than all other groups. The Condition Sex
interaction was approaching significance, suggesting that male Responders and female Non-Responders tended to recognize fewer words, overall, than their respective no
stress group. The observed effect in male Responders seemed to be driven by a large reduction in their recognition of arousing words, while the observed effect in female NonResponders seemed to be driven by a large reduction in their recognition of non-arousing words (verified via planned comparisons; see Section 4); however, the
Condition Sex Arousal interaction was not significant. Data are presented as means ± SEM; b = p = 0.073 relative to the no stress group.
instance, according to the temporal dynamics model, pre-learning
stress would enhance consolidation only if the stressor was brief
and presented in close temporal proximity to the learning experience. If, on the other hand, the stressor was long-lasting or was a
brief stressor that was temporally separated from the learning
experience, then consolidation would hypothetically be impaired
(see Diamond et al., 2006 for an example). Research has also shown
that pre-learning stress effects on long-term memory depend on
the sex of the organism and the type of information being acquired.
For instance, studies have reported that pre-learning stress often
facilitates memory for emotional information, while impairing
memory for neutral information (Jelicic et al., 2004; Payne et al.,
2006; Payne et al., 2007). Moreover, the stress-induced enhancement of emotional memory has been reported to be stronger in females, while the stress-induced impairment of neutral memory
has been reported to be stronger in males (Payne et al., 2006).
Along similar lines, we found that stress impaired the recognition
of neutral words in Responders, an effect that seemed to be more
pronounced in males, and that the stress-induced impairment of
recall in males and recognition in females was largely driven by reduced memory for non-arousing information (discussed further
below). Thus, how stress affects consolidation depends on several
factors related to the timing of the stressor, the type of information
being learned and the sex of the organism.
Research has accumulated over the past decade to suggest that
corticosteroids act as ‘‘two-stage rockets’’, exerting immediate
excitatory, but delayed inhibitory, effects on cellular function and
synaptic plasticity (Joels et al., 2006; Joels et al., 2011). In relation
to the temporal dynamics model of emotional memory processing,
the initial excitatory phase of hippocampal function that immediately follows stress onset is thought to depend, at least in part, on
noradrenergic activity coupled with non-genomic activity of corticosteroids and an ensuing increase in glutamatergic transmission
(Diamond et al., 2007; Karst et al., 2005). The inhibitory phase that
follows shortly thereafter is thought to depend on NMDA receptor
desensitization and delayed, gene-dependent activity of corticosteroids (Diamond et al., 2007; Joels et al., 2003). The present findings
support this model, at least to a degree, in that Responders, overall,
exhibited impaired recognition memory for neutral words, while
male Responders exhibited impaired recall and a marginally signif-
icant impairment of recognition memory. However, the predictions
based on the temporal dynamics model were less evident in female
participants. That is to say, despite the finding of impaired recognition memory for neutral information in Responders, this effect
seemed to be driven largely by deficits in males. Moreover,
stressed females demonstrated no significant alteration of recall
performance, and the marginally significant impairment that was
observed on recognition memory in females was observed in
Non-Responders, not Responders. There was also a significant negative correlation between cortisol and memory in stressed males,
but no such association was observed in females. In fact, if anything, our findings would suggest that cortisol had some sort of
‘‘protective’’ effect on memory, at least for that of recognition
memory overall, in females. Ultimately, these results support the
temporal dynamics model by showing that brief stress that was
temporally separated from the learning experience impaired
long-term memory in both male and female participants. However,
the sex-dependent influence of corticosteroids on these effects
indicate that there is a need to update the model, which was based
largely on research conducted in males, by considering how sex
differences influence the relationship between timing, stress–
memory interactions and their neurobiological basis.
Another, fairly complementary, approach to the biological basis
of stress–memory interactions takes into account general differences in the effects of stress on amygdala and hippocampus function. Extensive work has shown that an intact amygdala is essential
for enhanced memory of emotional information (Adolphs, Cahill,
Schul, & Babinsky, 1997; Cahill et al., 1996; Canli, Zhao, Brewer,
Gabrieli, & Cahill, 2000), as well as for the stress-induced effects
on hippocampus-dependent learning and synaptic plasticity (Kim
et al., 2001, 2005; Zoladz, Park, & Diamond, 2011b). There is also
considerable evidence for the direct modulation of hippocampal
function via amygdala activation (Akirav & Richter-Levin, 1999,
2002; Richter-Levin & Akirav, 2003; Vouimba, Yaniv, & Richter-Levin, 2007), which provides a potential mechanism to explain
stress-induced effects on information storage. It has been speculated that amygdala activation underlies the stress-induced
enhancement of emotional memory, while hippocampal inhibition
underlies the stress-induced impairment of neutral memory (Diamond et al., 2007; Jelicic et al., 2004; Payne et al., 2007). Theoret-
P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
ically, in the face of stress, the processing of information with survival value (e.g., arousing) would take priority over the storage of
other, less important (e.g., non-arousing) information (Diamond
et al., 2007). In the present study, it is possible that stress enhanced
amygdala function while at the same time inhibited hippocampal
function, which manifested as impaired memory, particularly for
emotionally neutral information. Indeed, a close examination of
the effects of stress on memory in the present study revealed that
the stress-induced impairment of recall in males and the stress-induced marginally significant impairment of recognition in females
seemed to be largely driven by reduced memory for non-arousing
information. These speculations were corroborated by planned
comparison analyses (p’s < 0.05), despite the lack of statistically
significant Condition Sex Arousal interactions. Also, Responders, independent of sex, recognized significantly fewer neutral
words than all other groups, even though this effect also seemed
to be more pronounced in males. On the other hand, the marginally
significant impairment of recognition memory observed in males
seemed to be largely driven by reduced memory for arousing information, which according to the aforementioned perspective, might
be expected to be enhanced, if anything, as a result of a stress-induced increase in amygdala activity. It is possible that stress does
not always enhance one’s memory for arousing information. Indeed, some research has shown that whether or not the information is related to the stressor is important in determining if
memory is affected by the stress. Smeets et al. (2009) found that
stress enhanced participants’ memory for words only if the words
were highly arousing and directly related to the stressor that was
employed in the study. Thus, the traditional stress-induced
enhancement of arousing information may not have taken place
in the present study because the studied words were not related
to the cold pressor stress.
4.2. Sex differences
Our finding of greater deleterious effects of pre-learning stress
on long-term memory in males, relative to females, is not uncommon. Others have reported significant effects of stress on memory
in males, while finding no differences (e.g., Andreano & Cahill,
2006) or an enhancement of memory (e.g., Payne et al., 2006) in females. Comparable findings have also been reported in studies
examining stress effects on working memory (e.g., Cornelisse,
van Stegeren, & Joels, 2011). It is possible that participants’ perceived arousal of the studied words influenced the observed sex
differences in the present study. We found that males rated the
studied words as less arousing than females did. Thus, it is possible
that stress exerted greater deleterious effects on memory in males
because the studied information, overall, was not perceived as
emotionally arousing as it was in females. This speculation would
relate to previous work that has shown greater stress-induced
impairments of memory for neutral information in males (Payne
et al., 2006).
The present findings are also markedly similar to those from
investigators who have reported negative or curvilinear relationships between cortisol and memory in males, but not females
(Andreano & Cahill, 2006; Wolf et al., 2001). We found that male
Responders exhibited impaired long-term free recall and a marginally significant impairment of long-term recognition memory. Female Non-Responders, on the other hand, displayed a marginally
significant impairment of long-term recognition memory. Thus,
as indicated above, cortisol was potentially associated with better
memory, at least for that of recognition memory overall, in females, in contrast to males. A closer examination of the female recognition memory data, however, suggests that a strong cortisol
response to the stress might have simply ‘‘preserved’’ their memory (Payne et al., 2006), rather than facilitating it, since female
85
Responders performed better relative to female Non-Responders
only, and female Non-Responders performed more poorly than
the no stress group. Since we did not observe any significant sex
differences in cortisol response to the stressor, these differential effects that we observed on measures of memory are not likely
attributable to variations in hypothalamus–pituitary–adrenal axis
response.
Studies have shown that differences in the menstrual cycle play
an important role in the stress-induced alteration of cognition. For
instance, Andreano, Arjomandi, and Cahill (2008) observed a positive correlation between stress-induced salivary cortisol and memory only when female participants were in the mid-luteal phase of
the menstrual cycle, a time when progesterone levels are significantly elevated. Subsequent work substantiated these findings by
reporting that females exhibiting high levels of progesterone (i.e.,
mid-luteal phase) demonstrate better memory for emotional information, greater stress-induced elevations of salivary cortisol and
stronger stress-induced enhancements of emotional memory than
women with low levels of progesterone (i.e., non-luteal phase)
(Ertman, Andreano, & Cahill, 2011; Felmingham, Fong, & Bryant,
2012). Females also exhibit stronger responses of the amygdalahippocampus neural network to emotional stimuli when they are
in the luteal phase (Andreano & Cahill, 2010). Although we did
not control for the menstrual cycle in the present study, it is possible that the differential effects observed in males vs. females
could be attributable to such a factor. If female Responders and
Non-Responders were in different stages of the menstrual cycle,
they could have exhibited differential sensitivity to stress hormones, which led to different effects on memory. Indeed, research
has shown that the luteal phase of the menstrual cycle is associated with altered glucocorticoid sensitivity (Rohleder, Schommer,
Hellhammer, Engel, & Kirschbaum, 2001; Rohleder, Wolf, Kirschbaum, & Wolf, 2009) and elevated norepinephrine levels (Minson,
Halliwill, Young, & Joyner, 2000). In order to verify this speculation,
however, future work will need to control for this factor when
examining pre-learning stress effects on memory.
It is also possible that the female brain does not respond to
emotion in the same manner that the male brain does. Some research has shown that the brain responses of males and females
to emotional stimuli are hemisphere-specific. For instance, Cahill
and others have reported that, when presented with emotional
stimuli, males tend to exhibit greater activity in the right amygdala, while females tend to exhibit greater activity in the left amygdala (Cahill, Uncapher, Kilpatrick, Alkire, & Turner, 2004; Cahill
et al., 2001; Canli, Desmond, Zhao, & Gabrieli, 2002; Mackiewicz,
Sarinopoulos, Cleven, & Nitschke, 2006). Interestingly, the right
amygdala has been associated with one’s memory for gist, while
the left amygdala has been associated with one’s memory for details (Fink, Marshall, Halligan, & Dolan, 1999; Fink et al., 1996).
Thus, it may not be surprising to note that, under emotionally
arousing conditions, males exhibit greater memory for the central
aspects of a scene, while females exhibit greater memory for the
details (Cahill & van Stegeren, 2003). This differential activation
of brain areas in response to emotional stimuli could play an
important role in sex influences on stress–memory interactions.
It could also be related to stress-induced switches in learning style
that appear to differ across the sexes (Beck & Luine, 2010).
4.3. Limitations and caveats
Although the present findings provide important insight into
the factors that influence stress–memory interactions, there were
some limitations that deserve attention. First, when we divided
the stressed participants into Responders and Non-Responders,
we ended up with a relatively small sample size of male Responders (N = 8). Since a significant portion of our effects were evident in
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P.R. Zoladz et al. / Neurobiology of Learning and Memory 100 (2013) 77–87
this group, caution should be employed when interpreting the
data, and future work should be conducted to replicate the results.
Second, we did allow females who were regularly taking birth control to participate in the study. Even though we detected no statistically significant differences between birth control users and
naturally cycling females, the presence of both groups could have
added noise to the data, influencing the resulting stress effects
on cognition. Third, it is unknown as to how the menstrual cycle
influenced the results for female participants in the present study,
since we did not control for this factor. Fourth, we have interpreted
some of the findings based on marginally significant effects, and in
order to verify our speculations, future work, perhaps with larger
sample sizes and greater statistical power, will need to be conducted. Finally, additional research should address the involvement of neuromodulators other than cortisol in the pre-learning
stress-induced alteration of learning and memory. For instance, it
is well known that noradrenergic mechanisms interact with corticosteroids to influence cognition, and recent work has shown that
endocannabinoids play an important part in this process.
4.4. Conclusions
In sum, we have shown that brief stress administered 30 min
prior to learning exerts differential effects on long-term memory
in males and females, effects that depend largely on the cortisol response of participants and the emotional salience of the learned
information. Responders, overall, exhibited impaired recognition
of neutral information. Additionally, male Responders exhibited
impaired recall and a marginally significant impairment of recognition, while female Non-Responders exhibited a marginally significant impairment of recognition. From these findings, it can be
hypothesized that there is a sex-dependent involvement of corticosteroids in pre-learning stress effects on long-term memory. Thus,
the temporal dynamics model, in its current form, perhaps better
summarizes stress–memory interactions in males, and it is necessary to revise this approach to produce a more comprehensive
understanding of how stress affects cognition. On the other hand,
general stress effects on amygdala vs. hippocampus function and
how they interact with sex provides for an equally appealing, complementary explanation of the results.
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