| According to two contemporary models of facial emotion recognition, understanding emotions conveyed by the face of one's social partner involves activation of emotional processes in oneself, encompassing somatomotor, internal physiological and experiential components of emotion. In one of these models, mimicking the facial actions of another individual triggers global emotional convergence (i.e., corresponding emotional states), which in turn activates knowledge of emotional states (Hatfield et al., 1994). In the other model, neural systems simulate emotional processes that sufficiently reconstruct relevant knowledge without intermediary changes in other physiological systems (Adolphs, 2002). Embodied cognition theory, which describes cognition as being grounded in and constrained by sensorimotor systems of the brain and body, offers these models a unifying framework. Within this framework, somatosensory regions of the brain are viewed as an integral part of a neural system mediating emotional processes and thus involved in recognizing facial emotional expressions. In two experiments I tested the hypothesis that the human somatosensory cortex is a critical structure involved in recognizing facial emotion by reversibly disrupting somatosensory cortical processes with dual-pulse transcranial magnetic stimulation (TMS) while healthy female participants made judgments of angry, fearful and happy facial expressions. Morphed facial expressions varying by emotional intensity were presented briefly to participants who were instructed to make quick yes-no responses (i.e., did it express the target emotion) and analog rating responses of subjective intensity (i.e., how intense was the emotion in the face?). Based on evidence from lesion studies (e.g., Adolphs et al., 2000), Experiment 1 was conducted specifically to determine whether disruption of right somatosensory cortical processes would impair judgments of negatively valenced facial expressions more than judgments of positive ones and gender from the face. Results showed that participants were impaired at judging angry and fearful expressions, but not happy ones, when right somatosensory cortices were stimulated relative to other stimulation conditions: primarily, participants rated angry and fearful target faces as less intense during right somatosensory stimulation than during sham and vertex stimulation. There was also some evidence that right somatosensory stimulation increased reaction times to make gender judgments, suggesting that judging gender may also recruit somatosensory cortices. To extend these findings, Experiment 2 was conducted to replicate the effects of right somatosensory stimulation on judging negatively valenced facial expressions and to determine whether stimulation of left somatosensory regions causes similar effects or alternatively, whether left somatosensory stimulation causes deficits in judging positive facial emotion. Results showed that right, but not left, somatosensory cortical disruption caused impaired judgments of angry faces. Conversely, left, but not right, somatosensory cortical disruption showed some evidence of causing impaired judgments of happy faces, which was revealed by slower reaction times. These results present tentative evidence for a possible role of left somatosensory cortices in mediating positive emotional inferences. Consistent with an embodied cognition framework, these findings collectively point to the functional necessity of right somatosensory cortical processes in judging facial expressions of negative emotion. |
