The Callous Cerebellum: A cerebellar role in aggressive and impulsive behaviour investigated with MRI

The work in this thesis set out to investigate the structural and functional correlates of the cerebellum in aggressive behaviour. This was approached through the study of the cerebellar topography in association with aggressive behaviour and functional connectivity patterns between cerebellar subregions and the cortico-limbic circuit associated with aggressive behaviour. The topographic regions in the cerebellum involved in aggressive behaviour were assessed in Chapters 2-4. In Chapter 2, lobular volumes were investigated in a healthy cohort to provide correlational evidence for a relation between aggressive symptoms and cerebellar regions. Results showed that higher self-reported physical aggression traits were associated with smaller grey matter volumes in the right posterior lobe. In addition, higher self-reported impulsivity scores showed a negative association with right posterior lobe volume. Furthermore, higher impulsivity scores were correlated with higher grey matter volumes of the posterior vermis. In Chapter 3, empirical data in the scientific literature were examined to investigate whether our findings in Chapter 2 extended to pathological forms of aggression. Results indicated associations between cerebellar structure and aggressive symptoms or violent psychopathic tendencies. These studies pointed towards a pattern of grey matter decreases in the right posterior cerebellum and lobule V, and larger grey matter volumes in (para)vermal lobules I-VI in psychopathy. In addition to the structural findings, in Chapter 4 a task-based fMRI aggression paradigm showed activation of the left posterior cerebellar when provocations occurred and the right posterior cerebellar activation when participants engaged in aggressive behaviour. These s- and fMRI findings add to the growing recognition of the posterolateral cerebellar regions in impulsive and aggressive behaviour. In Chapters 5 and 6, functional connectivity patterns between the cerebellum and the cortico-limbic circuit were explored using resting-state fMRI. In Chapter 5, we compared resting-state functional connectivity patterns between veterans with and without reactive aggression symptoms. An exploratory whole-brain analysis demonstrated decreased connectivity between the DCN and the orbitofrontal cortex in veterans with reactive aggression symptoms as compared to control veterans. In Chapter 6, resting-state functional connectivity of several cerebellar regions from Chapters 2-5 was explored with the cortico-limbic aggression circuit in healthy volunteers. Significant resting-state functional connectivity patterns between the posterior vermis and hypothalamus, centromedial amygdala (CMA) and subgenual anterior cingulate cortex (sgACC) were found. In line with Chapter 5, the fastigial nuclei of the DCN showed resting-state functional connectivity to the hypothalamus and CMA. Furthermore, cerebellar regions in left Crus I and right Crus II (Chapter 4) showed functional connections to the left sgACC. These results provide evidence for cerebellar functional connections with cortico-limbic regions linked to emotion regulation, impulsivity and aggressive behaviour. In sum, the series of studies presented in this thesis provides empirical evidence for the involvement of the cerebellum in aggressive and impulsive behaviour. Across structural and functional measures, cerebellar correlates of impulsive and aggressive behaviour were found in the vermis and posterolateral lobules. These cerebellar regions also showed evidence for the existence of a cerebello-cortico-limbic network in the human brain associated with impulsive and aggressive behaviour.