Identifying the behavioural and neural mechanisms that may explain why late chronotype individuals are at increased risk of developing major depressive disorder

Abstract

Introduction: Major Depressive Disorder is estimated to affect 322 million people worldwide. Increasing evidence suggests that late chronotype individuals (or ‘night owls’) are at increased risk of developing depression. However, the underlying cognitive and neural mechanisms that confer risk are not fully understood.
Methods: Here, healthy individuals without a current or previous diagnosis of depression, family history of depression or sleep disorder were recruited and their chronotype estimated. Participants completed lab-based behavioural tasks, an online facial expression recognition task and underwent magnetic resonance imaging (MRI) in order to examine cognitive and neurophysiological functioning in relation to chronotype.
Results: The results indicated that late chronotypes display negative emotion processing biases compared to early chronotypes, including self-referent memory and recognition of personality trait words and facial expression recognition. In particular, late chronotypes showed biases towards negative stimuli and/or away from positive stimuli and also displayed a blunted response to reward during a risk-taking task. Unexpectedly, late chronotypes recalled fewer words compared to early chronotypes suggesting general memory impairments that may also be relevant to risk for depression. The neural substrates of the negative biases were further illustrated using MRI. Later chronotype was associated with enhanced amygdala reactivity to fearful vs. happy facial expressions and reduced functional connectivity between amygdala and dorsal anterior cingulate cortex. Moreover, later chronotypes displayed reduced resting-state connectivity within key nodes of the default mode network. These findings highlight the putative neural mechanisms underlying altered emotion processing/regulation and self-critical thoughts in this population. In contrast, these neurocognitive differences were not accompanied 4 by negative attentional biases, structural differences in brain regions associated with emotion processing (e.g. hippocampus) or changes in key neurometabolites (GABA, glutamate, NAA).
Discussion: The current findings appear consistent with theories emphasising negative cognitive biases in the aetiology and maintenance of depression and may, in part, explain the increased vulnerability for depression in late chronotype individuals. These neurocognitive biases are thought to represent trait vulnerability markers for depression and have important theoretical and clinical implications for the prevention of the disorder, and for promoting psychological well-being in late chronotypes.
“The early bird catcheth the worm” – John Ray.

Date of Award	23 Nov 2018
Original language	English
Awarding Institution	University of Roehampton
Supervisor	Leigh Gibson (Supervisor) & Ray Norbury (Supervisor)