section_path: Mechanism/Neurocircuitry in mammals/Humans
section_name: Humans
paragraph_text: Humans
  
  Evidence of chemosensory alarm signals in humans has emerged slowly: Although alarm pheromones have not been physically isolated and their chemical structures have not been identified in humans so far, there is evidence for their presence. Androstadienone, for example, a steroidal, endogenous odorant, is a pheromone candidate found in human sweat, axillary hair and plasma. The closely related compound androstenone is involved in communicating dominance, aggression or competition; sex hormone influences on androstenone perception in humans showed a high testosterone level related to heightened androstenone sensitivity in men, a high testosterone level related to unhappiness in response to androstenone in men, and a high estradiol level related to disliking of androstenone in women.
  
  A German study from 2006 showed when anxiety-induced versus exercise-induced human sweat from a dozen people was pooled and offered to seven study participants, of five able to olfactorily distinguish exercise-induced sweat from room air, three could also distinguish exercise-induced sweat from anxiety induced sweat. The acoustic startle reflex response to a sound when sensing anxiety sweat was larger than when sensing exercise-induced sweat, as measured by electromyography analysis of the orbital muscle, which is responsible for the eyeblink component. This showed for the first time that fear chemosignals can modulate the startle reflex in humans without emotional mediation; fear chemosignals primed the recipient's "defensive behavior" prior to the subjects' conscious attention on the acoustic startle reflex level.
  
  In analogy to the social buffering of rats and honeybees in response to chemosignals, induction of empathy by "smelling anxiety" of another person has been found in humans.
  
  A study from 2013 provided brain imaging evidence that human responses to fear chemosignals may be gender-specific. Researchers collected alarm-induced sweat and exercise-induced sweat from donors extracted it, pooled it and presented it to 16 unrelated people undergoing functional brain MRI. While stress-induced sweat from males produced a comparably strong emotional response in both females and males, stress-induced sweat from females produced markedly stronger arousal in women than in men. Statistical tests pinpointed this gender-specificity to the right amygdala and strongest in the superficial nuclei. Since no significant differences were found in the olfactory bulb, the response to female fear-induced signals is likely based on processing the meaning, i.e. on the emotional level, rather than the strength of chemosensory cues from each gender, i.e. the perceptual level.
  
  An approach-avoidance task was set up where volunteers seeing either an angry or a happy cartoon face on a computer screen pushed away or pulled toward them a joystick as fast as possible. Volunteers smelling androstadienone, masked with clove oil scent responded faster, especially to angry faces than those smelling clove oil only, which was interpreted as androstadienone-related activation of the fear system. A potential mechanism of action is, that androstadienone alters the "emotional face processing". Androstadienone is known to influence the activity of the fusiform gyrus which is relevant for face recognition.