We all have our 'good days' and 'bad days'. But what's actually going on inside our heads to explain the difference between these sorts of personal and persistent emotional experiences?
Part of the reason behind long-lasting negative moods appears to have now been identified in a new study, with scientists finding that activity within the amygdala plays a key role in whether or not people are able to shake off the feeling of negative emotions.
The amygdala is part of the brain's limbic system, and is involved with a number of aspects of emotional processing, memory, and decision-making.
In the new study, a team led by researchers from the University of Miami examined data from 'Midlife in the US', a landmark longitudinal study that began in the 1990s, and which has since collected physical and mental health data from thousands of midlife Americans, including measurements of their psychological wellbeing (PWB).
As part of the program, individuals answered PWB surveys and filled out daily diaries, and a sample of the participants completed a number of interviews with researchers over the phone, who asked questions measuring the individuals' daily affect: their overall feeling and mood from day to day.
Part of the group also had their brain activity measured by fRMI scans in an experiment, during which the participants were shown positive, negative, or neutral images from a database commonly used in psychological research, with each affective image being followed by a neutral image of a face, before the next positive, negative, or neutral image was displayed.
In a subset of 52 adults who completed all the tests, the fRMI data indicated that negative outcomes in people's daily affect and PWB were tied to continued and persistent activation in the left amygdala after seeing the negative affective images.
In other words, the effect of seeing the negative images (which included pictures of violence and mutilated bodies) stayed with some people, coloring their emotional processing even after the unpleasant image was removed.
By contrast, in the people whose amygdala activity did not persist after the aversive images were seen, their emotional response seemed to normalize more quickly, with the other data collected in the study suggesting they enjoyed more frequent positive and less frequent negative affect in their daily life.
The amygdala plays an important role in keeping us safe by helping us to detect threats, but that ability seems to come with a kind of 'spill over' effect, as sometimes the negative emotions we feel when we appraise something that looks dangerous don't quickly fade, and can persist after, affecting how we view subsequent things.
According to the new findings, that 'spill over' effect may in fact spill over into our lives to a much greater extent than perhaps we realized.
"It may be that for individuals with greater amygdala persistence, negative moments may become amplified or prolonged by imbuing unrelated moments that follow with a negative appraisal," the researchers explain in their paper.
"Ultimately, this persistence could result in more negative affect and less positive affect, on average, in daily life. This brain-behavior link between left amygdala persistence and daily affect can inform our understanding of more enduring, long-term evaluations of well-being."
There's a lot more that needs to be unpacked here, and the subgroup of patients involved in this aspect of the Midlife in the US study was not a large sample.
Beyond replicating the experiment with larger numbers of participants, the team says we should experiment with alternative forms of neutral images, as well as further explore with higher-resolution scans what other kinds of brain regions might be demonstrating persistent effects from affective imagery.
"As psychological well-being is a complex, cognitive self-evaluation requiring one to integrate across a lifetime of experiences, such an evaluative process is likely supported by distributed brain networks rather than any one individual region," the researchers write.
The findings are reported in The Journal of Neuroscience.