Discussion
depressed
The facial feedback effect states that when we contract or flex the relevant muscles to create an emotional expression (e.g., happy or angry), it can assist in identifying and experiencing the emotion reflected, even in the absence of an emotional face as the stimulus. There is evidence that signaling between the emotional centers of the brain and facial muscles is bidirectionalNeuroanatomical circuit involved in the processing of emotional faces. Orange lines represent the sensory trigeminal innervating the brain stem and synapsing on the trigeminal nucleus. The blue lines highlight the flow of information along key regions in the circuit. Sensory neurons in the trigeminal nucleus caudalis have reciprocal connections with sensory and limbic structures and are often monosynapticIn addition to the well-established modulation of amygdala activity by emotion, we found evidence consistent with that of prior studies by Kim and colleaguesIn addition to the amygdala, we observed a modulation of activity in the fusiform gyrus following onabotA injections. The fusiform gyrus (also known as Broadmann area 37) contains a region often labeled the fusiform face area because of its importance for processing faces and facial expressionsWe designed this study to parallel that of Kim and colleaguesThe reason(s) for the hemispheric discrepancies are unclear but may be due to experimental design. The baseline contrast condition was different between the current study and that of Kim and colleaguesEach of these studies is relatively underpowered—though that is not unusual in functional imaging studies. Our results are based on a group of 10 participants, while Kim and colleaguesThis study has several strengths and limitations. The sample size of the present study was small; larger studies will help increase the signal-to-noise ratio and improve generalizability. Additionally, the emotional faces task was limited to happy and angry faces. Use of a wider range of emotions (e.g., sad, surprised, scared) would address the generality of the amygdala response to other emotions. Finally, the present study enrolled healthy individuals. Studies of emotional modulation in clinically depressed individuals, already experiencing alterations in emotional processing, may reveal stronger effects of onabotA injections on emotion-related neural activity. Strengths of the present study include use of a controlled, non-emotional baseline that enabled us to assess overall shifts in emotion-related activity in the amygdala. We also conducted a whole-brain exploratory analysis to identify regions other than the amygdala that were modulated by onabotA, which led to the observation of significant effects in the fusiform gyrus.