Left Hemisphere Lesions Differentially Impact Conditional Reasoning with Familiar and Emotional Content

Conditional reasoning has been widely studied in the cognitive literature, and in the past decade, neuroimaging studies have started to investigate brain networks recruited to solve these logical conditionals. A meta-analysis of these neuroimaging studies of healthy adults has shown that conditional arguments are primarily associated with left-lateralized activation in the parietal and frontal lobes. Beyond logical form, content factors such as belief- logic congruency, familiarity, and emotion have been shown to recruit networks different from the main effect of reasoning. To date, conditional connectives have not been investigated using traumatic brain injury patients, therefore, the goal of this thesis was to study the effect of brain lesions on conditional reasoning.

 A whole brain analysis using voxel-based lesion symptom mapping (VLSM) was conducted on 72 neurological patients with unilateral lesions in order to explore the impact of brain lesions on reasoning accuracy scores. Results indicated that conditional reasoning with familiar content is highly dependent on left hemisphere intactness, whereas right hemisphere volume loss does not inhibit performance and in some conditions may even lead to improved performance. In particular, we found that familiar believable content failed to benefit patients with left hemisphere lesions. Additionally, VLSM analysis isolated a region in the left medial prefrontal cortex deemed necessary for reasoning with emotional content, the 10 patients with lesions in this cluster performed significantly worse than all other patients and controls on emotional conditionals. 	

 Our findings provide additional evidence that reasoning processes involving familiar content are largely left lateralized and that the ventromedial prefrontal cortex is specifically engaged in reasoning about emotional content. This is the first study to use a lesion analysis to investigate conditionals, and thus contributes important new information to the existing neuroimaging literature.