Department of Experimental Psychology
University Department of Psychiatry, University of Oxford.
Correspondence: Catherine J. Harmer, University Department of Psychiatry, Warneford Hospital, Oxford OX3 7JX, UK. Email: catherine.harmer{at}psych.ox.ac.uk
G.M.G. holds research grants from Sanofi-Aventis and Servier; holds honoraria from AstraZeneca, Bristol-Myers-Squibb, Eisai, Ludbeck, Sanofi-Aventis, Servier; and is a member of the advisory board of AstraZeneca, Bristol-Myers-Squibb, Lilly, Lundbeck, P1vital, Sanofi-Aventis, Servier, Wyeth. C.J.H. has acted as a consultant for Lundbeck, Merck, Sharpe & Dohme and P1vital. Funding detailed in Acknowledgements.
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Depression is associated with neural abnormalities in emotional processing.
Aims
This study explored whether these abnormalities underlie risk for depression.
Method
We compared the neural responses of volunteers who were at high and low-risk for the development of depression (by virtue of high and low neuroticism scores; high-N group and low-N group respectively) during the presentation of fearful and happy faces using functional magnetic resonance imaging (fMRI).
Results
The high-N group demonstrated linear increases in response in the right fusiform gyrus and left middle temporal gyrus to expressions of increasing fear, whereas the low-N group demonstrated the opposite effect. The high-N group also displayed greater responses in the right amygdala, cerebellum, left middle frontal and bilateral parietal gyri to medium levels of fearful v. happy expressions.
Conclusions
Risk for depression is associated with enhanced neural responses to fearful facial expressions similar to those observed in acute depression.
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Neuroticism scores were derived from the 12-item neuroticism scale of the shortened Eysenck Personality Questionnaire (EPQ).21 Twelve participants (9 women) were in the high-N group (N range 8–12), and 13 participants (8 women) in the low-N group (N range 0–3). This range of N scores was consistent with our previous behavioural study.20 The two groups were matched for age (mean=20.00, s.d.=0.60 v. mean=20.15, s.d.=0.99), gender, verbal IQ (mean=119.10, s.d.=3.03 v. mean=118.66, s.d.=4.26) and spatial IQ (mean=2584 ms, s.d.=941 v. mean=1974 ms, s.d.=610) assessed by National Adult Reading Test (NART)22 and Wechsler Adult Intelligence Scale – Revised (WAIS–R)23 respectively. Two participants had a first-degree relative with depression (one from each group).
Mood variables
The Beck Depression Inventory
(BDI)24 and
State–Trait Anxiety Inventory
(STAI)25 were used
to assess self-rated mood.
Stimuli and task
Each volunteer participated in a single 16 min experiment employing rapid
event-related fMRI. Eight faces (four male, four female) displaying
prototypical expressions of fear and happiness were taken from a standardised
series of facial
expressions.26 In
addition to the prototypic or high intensity (100%) facial expressions, medium
(60%) and low (30%) intensity expressions generated using morphing
software27 were
used. Each face was also presented in a neutral facial expression. Thus, there
were eight facial stimuli representing each of the following categories: high
fearful (fear-H), medium fearful (fear-M), low fearful (fear-L), high happy
(happy-H), medium happy (happy-M), low happy (happy-L) and neutral. Each of
these faces was presented three times and 24 presentations of a fixation cross
were included as baseline, giving a total of 192 trials. Stimuli were
presented in a random order for 500 ms each and the intertrial interval varied
according to Poisson distribution with a mean intertrial interval of 5000 ms.
Participants were asked to indicate the gender of each face by pressing one of
two keys on an MRI-compatible keypad. No motor response was required for
baseline trials of fixation cross. Stimuli were presented on a personal
computer using E-Prime (version 1.0; Psychology Software Tools Inc,
Pittsburgh, Pennsylvania) and projected onto an opaque screen at the foot of
the scanner bore, which participants viewed using angled mirrors. Behavioural
responses were recorded using a MRI-compatible keypad. Accuracy and reaction
times were recorded by E-Prime.
Functional MRI data acquisition
Imaging data were collected by a 1.5 T Siemens Sonata scanner located at
the Oxford Centre for Clinical Magnetic Resonance Research. Functional imaging
consisted of 30 contiguous T2*-weighted
echo-planar image (EPI) slices (repetition time (TR)=3000 ms, echo time
(TE)=50 ms, matrix=64x64, field of view (FOV)=192x192, slice
thickness=4 mm). A Turbo FLASH sequence (TR=12 ms, TE=5.65 ms, voxel size=1
mm3) was also acquired to facilitate later coregistration of the
fMRI data into standard space. The first two EPI volumes in each run were
discarded to ensure T1 equilibration.
Data analyses
Functional MRI data analysis was carried out using FSL version
3.2β.28
Preprocessing included slice acquisition time correction, within-participant
image
realignment,29
non-brain
removal,30 spatial
normalisation (to Montreal Neurological Institute (MNI) 152 stereotactic
template), spatial smoothing, and high-pass temporal filtering (to a maximum
of 0.025 Hz).
In the first level analysis, individual activation maps were computed using the general linear model with local autocorrelation correction.31 Eight explanatory variables were modelled, including each intensity (low, medium, high) of fear and happy as well as neutral and fixation. The main contrasts of interest were fear v. happy expressions (and vice versa) for each intensity level, i.e. fear-H v. happy-H; fear-M v. happy-M, fear-L v. happy-L. In addition, each individual activation map was analysed by fitting linear trends at each voxel at the three intensity levels of fear and happy, separately, with orthogonal polynomial trend analysis. Positive linear trends modelled responses for increasing emotional intensity, whereas negative linear trends modelled responses for decreasing emotional intensity. All variables were modelled by convolving the onset of each stimulus with a haemodynamic response function, using a variant of a gamma function (i.e. a normalisation of the probability density function of the gamma function) with a standard deviation of 3 s and a mean lag of 6 s.
In the second level analysis, individual data were combined at the group level (high-N v. low-N scores) using a mixed effects analysis.32 This mixed effects approach accounts for intra-individual variability and allows population inferences to be drawn. We aimed to establish, first, the effect of neuroticism on the responses to fear v. happy facial expressions at each intensity level; and second, the effect of neuroticism on the linear trend across increasing or decreasing intensity of fear and happy expressions. Significant activations were identified using a cluster-based threshold of statistical images (height threshold of z=2.0 and a (corrected) spatial extent threshold of P<0.05).33 Significant interactions were further explored by extracting per cent blood oxygen level dependent (BOLD) signal change within the areas of significant difference, which were then analysed using repeated measures ANOVA (between-participants variable=group; within-participants variable=intensity or valence) followed by appropriate post hoc t-tests (SPSS version 14.0 for Windows). Corresponding Brodmann areas (BA) were identified by transforming MNI coordinates into Talairach space.34
As a result of the strong a priori evidence implicating the amygdala in the processing of facial expressions,5–9 we also performed a region of interest analysis. Amygdala masks (left and right) were segmented for each individual using a robust fully automated Integrated Registration and Segmentation Tool (`FIRST').35 Per cent BOLD signal change for each emotional stimulus (fear and happy) was extracted from each individual amygdala. These data were entered into 2x2x3 repeated measures ANOVA (between-participants variable=group; within-participant variables=valence or intensity). Significant three-way interaction was clarified by two-way ANOVA and subsequent t-tests.
For the behavioural data, independent sample t-tests were used to examine group differences for subjective mood ratings, overall accuracy and reaction time of the gender discrimination responses. As a result of technical difficulties, reaction time and accuracy data (measured during fMRI) from four participants with low N were not recorded. These individuals were included in the analysis of fMRI data because the behavioural response of gender discrimination is incidental to the main outcome measure of neural response to emotional valence.
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Functional imaging results
Neural responses for fearful v. happy expressions: between-group differences
Our primary hypothesis was that fearful and happy faces would be
differentially processed by the participant groups. Indeed, the high-N
v. low-N groups exhibited greater activity for fear v. happy
expressions with medium intensity (i.e. fear-M v. happy-M) in the
following areas: cerebellum (MNI: 0, –64, –26; z=3.91),
left middle frontal gyrus (BA10, MNI: –30, 58, 2; z=3.46), left
superior parietal (BA7, MNI: –18, –66, 60; z=3.25) and
right superior parietal cortex (BA7, MNI: 4, –48, 68; z=3.25).
Analysis of per cent BOLD signal change for fear-M and happy-M stimuli
revealed increased responses in the high-N group during presentation of
fearful facial expressions, which in some areas was accompanied by relatively
reduced responses during the presentation of happy facial expressions
(Fig. 1 details simple main
effect analyses). These effects remained significant after including BDI or
STAI scores as covariates (all P<0.01).
![]() View larger version (98K): [in a new window] [as a PowerPoint slide] |
Fig. 1 The image and blood oxygen level dependent (BOLD) per cent signal change of
the brain regions where the high-N group (blue) showed greater activation for
fearful v. happy faces at medium intensity than the low-N group
(white). Colour bar represents z-score between 2.0 and 3.9. Asterisks
represent significant group comparisons of P<0.05. Fear-M, medium
fearful; Fear-H,medium happy.
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![]() View larger version (50K): [in a new window] [as a PowerPoint slide] |
Fig. 2 The image and blood oxygen level dependent (BOLD) per cent signal change of
right fusiform gyrus (MNI: 26, –66, –14), in which high-N group
(blue) showed increased signals for increasing intensity of fearful
expressions whereas the low-N group (white) showed the reverse pattern. Colour
bar represents z-score between 2.0 and 3.5. Asterisks represent
significant group comparison of P<0.05. Fear-L, low fearful;
Fear-M, medium fearful; Fear-H, high fearful.
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Fig. 3 The image and blood oxygen level dependent (BOLD) per cent signal change of
left middle temporal gyrus (MNI: –56, –32, 0), in which high-N
group (black) showed increased signals for increasing intensity of fearful
expressions whereas the low-N group (white) showed the reverse pattern. Colour
bar represents z-score between 2.0 and 3.5. Asterisks represent
significant group comparison of P<0.05. Fear-L, low fearful;
Fear-M, medium fearful; Fear-H, high fearful.
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Region of interest analysis of amygdala responses
Amygdala volumes were not significantly affected by group (main effect of
group: F(1,23)=0.563, P=0.461; group hemisphere:
F(1,23)=0.261, P=0.614), allowing functional responses to be
examined in the absence of potentially confounding structural differences. In
the right amygdala there was a non-significant trend for an
emotionxintensityxgroup interaction (P=0.092). Due to the
strong a priori hypothesis regarding the effects on ambiguous facial
expressions, two-way ANOVAs were run for each intensity level. These revealed
a significant group valence interaction for medium intensity (i.e. fear-M
v. happy-M; P=0.024). This interaction was driven by the
high-N group having greater amygdala activation for medium fearful expressions
relative to the low-N group (P=0.029)
(Fig. 4). By contrast, there
was no significant effect in the left amygdala.
![]() View larger version (8K): [in a new window] [as a PowerPoint slide] |
Fig. 4 Per cent blood oxygen level dependent (BOLD) signal change in right
amygdala for fearful and happy expressions at medium intensity by group.
Asterisks represent group comparison P<0.05. Fear-M, medium
fearful; Happy-M, medium happy.
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A key role for the amygdala and the fusiform gyrus in facial expression recognition and depression has been proposed previously from studies of individuals who are currently depressed5,7,8,17,18 and a similar pattern of effect was seen here as a function of neuroticism. Thus, the increased responses shown by our high-N group for increasing intensity of fear in the right fusiform gyrus and heightened amygdala responses to ambiguous fearful facial expressions are similar to those observed in individuals with depression.5,8 Vulnerability to depression has also been associated with aberrant amygdala responses to negative facial expressions in people with familial risk for depression (e.g. van der Veen et al, Monk et al).36,37 These results are also consistent with recent evidence which suggests that amygdala responses to emotional information correlates with neuroticism scores in unselected populations.38,39 Together, these findings suggest that increased amygdala responses to negative affective stimuli may be involved in risk for depressive disorders.
It is notable that while the high-N group showed the expected increase in fusiform response as a function of increasing fear value, the low-N group showed the opposite pattern. This implies decreased visual processing with increasing fear in volunteers at low risk of developing depression. Perception of fearful faces is believed to convey social signals of threat or danger.13,40 Such a pattern of effect could be explained by differential evaluation of threat value in the high- v. low-N groups, according to the curvilinear response function of Mogg & Bradley's (1998) cognitive motivational account of anxiety.41 This theory suggests that low-threat stimuli may be avoided in order to reduce distraction, whereas high-threat stimuli are monitored for potential importance. The observed pattern of results would be expected, if the low-N group estimated the face stimuli as having a lower threat value, leading to the high-intensity fearful faces being perceived as low threat and thus `avoided'. In other words, the current observation of reducing neural responses towards fearful expressions with higher intensity suggest that low risk for depression may be manifest as a reduced estimation of threat value in the environment.
In addition to the effects on the fusiform gyrus and amygdala, our results implicate a network of brain areas that are involved in facial processing and vulnerability to depression. First, the middle temporal gyrus revealed differential responses for fearful expressions in the high- and low-N groups similar to that observed in the fusiform gyrus. The temporal gyrus is within the core system of face perception13,42 and the increased responses seen here in the high-N group appear to be consistent with greater processing of threat-relevant facial stimuli in this group.
The medium intensity of fear v. happy expressions revealed group differences in the left middle frontal gyrus and bilateral parietal cortex. Indeed, such a frontoparietal network plays a central role in the concept of self, perception of social relationships and attention (e.g. Uddin et al, Feinberg et al).43,44 Thus, the specific activations for fearful expressions in these regions by the high-N group could be explained by their greater tendency to view negative expressions as self-relevant or self-threatening and thereby requiring activation of attentional systems. In line with this, the reduced activation for happy expressions may reflect their inclination to disregard positive social information as self-referent and deserving of further attention. In other words, these individuals are more likely to interpret negative social signals to be personally relevant or threatening, but at the same time unable to translate positive social signals for positive self-regard. This interpretation is consistent with the self-referent and facial expression processing biases observed in a similar high-risk sample.20
The same analysis also revealed increased cerebellum responses in the high-N group towards fearful v. happy facial expressions. The cerebellum is well known to play a key role in fear conditioning, anticipation of pain and coordination of motor action.45–48 Its role in processing fearful facial expressions is therefore not unexpected and the greater response in the high-risk group may represent an increase in threat anticipation and/or readiness to respond to these threat cues.49
The current study demonstrated differential responses to emotional cues in the high-N group in the absence of current or past Axis I psychiatric disorders from DSM–IV, thereby indicating that these biases exist prior to mood or anxiety disorder. Analyses including mood scores as covariates confirmed that the current effects were a function of neuroticism per se independent of mood state. The absence of family history of depression in the high-N group further suggested that the aberrant signals found in the high-risk group were a function of high neuroticism per se independent of familial risk for depression. As noted in the introduction, neuroticism has been identified as a robust predictor for depression. For example, Kendler et al15 found that a 1 standard deviation difference in neuroticism translates into a 100% difference in the rate of first onsets of depression over 12 months. Similarly, in a recent report based on a large Swedish twin sample (>20 000 individuals)16 neuroticism strongly predicted the risks for lifetime and first-onset depression assessed in 25-years follow-up. Thus, the differences in neural response to positive and negative affective stimuli seen here may be involved in predisposition to depression, consistent with cognitive theories of depression.
The differential responses for positive v. negative expressions shown here were seen largely with the medium intensity level of facial expression. This probably represents maximal ambiguity as behavioural data suggest that low-intensity levels are usually perceived as neutral and high-intensity levels usually elicit ceiling levels of performance, with the longest reaction time to identify facial expressions being seen around mid-intensity level.50,51 Such ambiguous social signals may be particularly relevant for problematic social interaction and, experimentally, for differentiating group differences. The current findings were also obtained from direct contrast between positive and negative emotional stimuli, which avoided potential confounds linked to the interpretation of neutral stimuli.
Limitations
A number of limitations also need to be considered in the evaluation of
these results. First, the generalisation of the current finding is limited by
the relative small sample and these results need to be replicated. The current
study also specifically investigated the emotional processing of fearful and
happy expressions, which have been previously shown to be affected by
depression and its
treatment.5,52
However, future studies may also wish to examine responses to other negative
facial expressions including sadness that have also been used in studies of
individuals with
depression.3,4,7
Finally, although high neuroticism is a robust risk factor for depression, the
relatively low prevalence rates of depression imply that only a small
proportion of the high-N group will go on to develop depression, thereby
potentially diluting any effects that we may have seen. Longitudinal studies
are required to assess the predictive power of negative biases for subsequent
depression in a sample adequately powered for the detection of infrequent
events.
In conclusion, our results illuminate the role for a distributed neural network, including the fusiform gyrus and amygdala, in facial expression processing biases in individuals at high risk for developing major depression. These areas overlap with those thought to be important in depression and those targeted by antidepressant drug administration.5,7,12 Longitudinal studies are underway to estimate whether, and to what extent, this aberrant neural behaviour predicts onset of depression.
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