Integrating emotional cues from different senses is critical for adaptive behavior. Much of the evidence on cross-modal perception of emotions has come from studies of vision and audition. This research has shown that an emotion signaled by one sense modulates how the same emotion is perceived in another sense, especially when the input to the latter sense is ambiguous. We tested whether olfaction causes similar sensory modulation of emotion perception. In two experiments, the chemosignal of fearful sweat biased women toward interpreting ambiguous expressions as more fearful, but had no effect when the facial emotion was more discernible. Our findings provide direct behavioral evidence that social chemosignals can communicate emotions and demonstrate that fear-related chemosignals modulate humans' visual emotion perception in an emotion-specific way--an effect that has been hitherto unsuspected.

Summary Vision is widely accepted as the dominant sense in larger primates including humans, whereas olfaction is often considered a vestigial sense yielding only obscure object representations [1]. It is well documented that vision drives olfactory perception [2-3], but the converse is hardly known. Here we introduce smells to a well-established visual phenomenon termed binocular rivalry, perceptual alternations that occur when distinctively different images are separately presented to the two eyes [4]. We show that an odorant congruent to one of the competing images prolongs the time that image is visible and shortens its suppression time in a manner that is automatic, essentially independent of cognitive control, and partly subconscious. Our findings provide the first direct evidence that an olfactory cue biases the dynamic process of binocular rivalry, thereby demonstrating olfactory modulation of visual perception - an effect that has been hitherto unsuspected.

Nonhuman animals communicate their emotional states through changes in body odor. The study reported here suggests that this may be the same for humans. We collected underarm odors on gauze pads from 25 young women and men on two different occasions. On one occasion the donors were induced to feel happy by viewing an excerpt from a funny movie whereas on the other, separated by a day, they were induced to feel afraid by watching an excerpt from a frightening movie. One week later, 40 women and 37 men were asked to smell several different bottles, some of which contained underarm odor pads collected during the happy movie, some contained underarm odor pads collected during the frightening movie, whereas others contained unused pads (control odor). Each odor was identified on two separate tasks that involved identifying the odor from among three odors and identifying it again from among six odors. Data were the number of women and men who identified an odor correctly on both tasks. When asked to select which bottles contained "the odor of people when they are happy," women chose the correct bottles for both tasks significantly more often than chance. Men chose the bottle which contained the body odors collected when women (but not men) viewed the happy movie more often than would be expected by chance. When asked to select which bottles contained "the odor of people when they are afraid," women and men both chose the bottle that contained the body odors collected when men (but not women) viewed the frightening movie more often than would be expected by chance. The finding suggests that there is information in human body odors indicative of emotional state. This finding introduces new complexity in how humans perceive and interact.

It is well documented across phyla that animals experiencing stress and fear produce chemical warning signals that can lead to behavioral, endocrinological, and immunological changes in the recipient animals of the same species. Humans distinguish between fear and other emotional chemosignals based on olfactory cues. Here, we study the effect of human fear chemosignals on the speed and accuracy of cognitive performance. In a double-blind experiment, female participants performed a word-association task while smelling one of the three types of olfactory stimuli: fear sweat, neutral sweat, and control odor carrier. We found that the participants exposed to the fear condition performed more accurately and yet with no sacrifice for speed on meaningful word conditions than those under either the neutral or the control condition. At the same time, they performed slower on tasks that contained ambiguous content. Possible factors that could introduce bias, such as individual differences due to anxiety, verbal skills, and perceived qualities of the smells, were ruled out. Our results demonstrate that human fear chemosignals enhance cognitive performances in the recipient. We suggest that this effect originates from learned associations, including greater cautiousness and concomitant changes in cognitive strategies.

It is well established that both the emotional tone of sensory stimuli and the personality characteristics of an individual can bias sensory perception. What has largely been unexplored is whether the current emotional state of an individual has a similar effect, and how it works together with other factors. Here we carry out a comprehensive study to examine how olfactory perception is affected by the emotional tone of the stimuli, and the personality and current emotional state of the individual. Subjects reported experiencing happiness, sadness, negativity/hostility and neutrality when exposed to corresponding emotionally themed video clips, and in each case, smelled a suprathreshold pleasant, an unpleasant and a neutral odorant. The time taken for the subject to detect each odorant and the olfactory intensity were recorded. We found that women detected the pleasant odorant faster than the neutral one. In addition, personality modulated reaction time and olfactory intensity, such that neurotic and anxious individuals were selectively biased toward affective rather than neutral odorants. Finally, current emotional state augmented intensity in men but not in women, and differentially influenced the response time. These findings provided new insights into the effects of emotion and personality on olfactory perception.