Examination of the limbic system's role in emotional experience using a human lesion model

The current prevailing notion is that the limbic system is inextricably linked to emotion, and indeed, most textbooks, research articles, and scientific lectures tout the limbic system as being the predominant purveyor of emotional processing in the brain. Yet, more than a half-century of research has produced surprisingly little evidence in support of such a notion, suggesting that the concept of an emotional limbic system is overly simplistic. The primary objective of this thesis is to determine whether the limbic system is necessary for one aspect of emotion, namely, its conscious experience. Neurological patients with focal damage to different regions of the limbic system - including the hippocampus, amygdala, insular cortex, anterior cingulate cortex (ACC), and ventromedial prefrontal cortex (vmPFC) - underwent multiple emotion induction procedures using affectively-laden film clips. For comparison, two other groups of participants were tested: patients with brain damage outside of the limbic system and healthy participants without brain damage. Two emotion inductions aimed at eliciting diffuse emotional states of positive or negative affect, and five emotion inductions aimed at eliciting specific emotional states of happiness, sadness, anger, disgust, or fear. Immediately following the end of each film clip, the return of emotion back to its baseline state was tracked over a three minute "emotion recovery" time period. The results of the experiment revealed three main findings. First, limbic system damage did not disrupt the experience of emotion during the film clips, with patients reporting high levels of the induced target emotion at a magnitude comparable to both comparison groups. Second, patients with bilateral damage circumscribed to either the hippocampus or the vmPFC demonstrated an abnormally slow rate of emotion recovery, indicating that these limbic regions are important for the successful downregulation of emotion. Third, patients with large bilateral lesions affecting multiple limbic structures (including the medial temporal lobes and insular cortices) showed an abnormally rapid rate of emotion recovery, with the induced emotion returning to baseline levels within 60 seconds following the end of each film. Based on these findings, it is concluded that the limbic system is not necessary for the experience of emotion, but is necessary for sustaining and regulating that experience after the emotion-inducing stimulus is no longer directly accessible to consciousness.