Patterns of electromyographic (EMG) activity can give an insight into muscle activity associated with a given behavioral state. The masseter muscle is positioned closely to the temporomandibular joint and controls the position and movement of the jaw. The hypothalamus is the region of the brain associated with emotional behavior. In an effort to further understand the muscle activity underlying emotional display, the hypothalamus in two cats was stimulated to evoke a stereotyped emotional response, known as the "rage response." Unsheathing of the claws, retraction of the ears, significant pupillary dilation and vocalization (hissing) characterize this behavior. EMG data obtained at the masseter muscle during this emotional state were compared to EMG activity recorded during mastication (eating), the simulated voluntary behavior for this study.

The results of this study indicate that the emotional state significantly influences the EMG activity in the masseter muscle. This is evidenced statistically by a larger high frequency component in the EMG data. It is also evidenced by the ratio of stimulation to mastication power levels at different frequencies, which increases as frequency increases.

The frequency range between 5-30 Hz has been utilized in the past in studies assessing fatigue. However, the results of this research indicate that the interpretation of the data in this frequency band must be different in studies of emotionally elicited muscle response. Recordings obtained during voluntary muscular activity reflected the typical fatigue response, and appropriate elevations in the power in the 5-30 Hz frequency range occurred, in agreement with previous findings. Recordings obtained during stimulation indicate that the highest power in this frequency band is achieved at the onset of hypothalamic stimulation, rather than at the point in time when fatigue typically occurs, in contrast to previous findings.