Associations of Hemispheric Laterality and Frontal–Temporal Atrophy with Emotional Processing and Regulation in Frontotemporal Degeneration: A Review

It is generally accepted that emotions are represented more in the right than in the left hemisphere and that temporal and frontal brain structures play a prominent role in functions of representation and control of emotions. It is, however, unclear if the right hemisphere prevalence concerns both the generation and the control of emotions. To clarify this problem, studies dealing with the emotional and behavioural disorders of patients with a right/left and a temporal/frontal variant of frontotemporal degeneration were included in a structured narrative review. A strong prevalence of right-sided atrophy was found in patients with emotion comprehension disorders, but different categories of emotions were affected in patients with temporal and frontal lesions. In the first case, comprehension disorders mainly affected ‘basic emotions’, whereas in the second case they preferentially concerned social and cognitive ‘complex emotions’. A weaker prevalence of right-sided atrophy was found in patients with emotional generation disorders (apathy and empathy) and in those with defects of emotional control (disinhibition). This difference between results obtained on tasks of comprehension and of control of emotions can be due to the fact that ‘apathy’, ‘empathy’ and ‘disinhibition’ can be considered as ‘umbrella terms’, covering behavioural domains larger than those allowed by a strict definition of emotions. The methodological heterogeneity and the observational nature of the evidence proposed suggest to consider the observed temporal–frontal hierarchy as a hypothesis supported by converging but heterogeneous clinical and neuroimaging observations.