In mammals, the hypothalamus and periaqueductal gray of the midbrain are critical areas, as shown in studies on cats, rats, and monkeys. These brain areas control the expression of both behavioral and autonomic components of aggression in these species, including vocalization. Electrical stimulation of the hypothalamus causes aggressive behavior and the hypothalamus has receptors that help determine aggression levels based on their interactions with serotonin and vasopressin. In rodents, activation of estrogen receptor-expressing neurons in the ventrolateral portion of the ventromedial hypothalamus (VMHvl) was found to be sufficient to initiate aggression in both males and females. Midbrain areas involved in aggression have direct connections with both the brainstem nuclei controlling these functions, and with structures such as the amygdala and prefrontal cortex.