We briefly review the concept of “vertical splitting” in its relation to dissociation and neurobiology. We then report the case of a patient with dissociative identity disorder for whom the neural correlates of switching between personalities have been studied using functional MRI. Activations during switching were found in the primary sensorimotor cortex, the frontal regions, and, importantly, the nucleus accumbens. The activations in the nucleus accumbens may represent an identifiable neural correlate of this specific kind of defense mechanism, one type of dissociation or “vertical split.” These findings may lead to a better understanding of dissociative processes in general and may help to distinguish between the neural correlates of repression and the neural correlates of dissociation.