Humans Can Discriminate More than 1 Trillion Olfactory Stimuli

Humans Can Discriminate More than 1 Trillion Olfactory Stimuli
Science 21 March 2014. 343(6177):1370-1372
C. Bushdid

Humans can discriminate several million different colors and almost half a million different tones, but the number of discriminable olfactory stimuli remains unknown.
The lay and scientific literature typically claims that humans can discriminate 10,000 odors, but this number has never been empirically validated.
We determined the resolution of the human sense of smell by testing the capacity of humans to discriminate odor mixtures with varying numbers of shared components. On the basis of the results of psychophysical testing, we calculated that humans can discriminate at least 1 trillion olfactory stimuli. This is far more than previous estimates of distinguishable olfactory stimuli.
It demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far outperforms the other senses in the number of physically different stimuli it can discriminate.

journalistic version:


Eau de Elderly

Scents and Senescence: “Old Person Smell” Is Real, but Not Necessarily Offensive
Scientific American. May 30, 2012

A new study confirms that people, like many animals, easily recognize a unique—but not unpleasant—eau de elderly

the Japanese even have a word for it, kareishuu

sebaceous glands

apocrine sweat glands

people recognize a characteristic “old people smell” not because of the aroma’s intensity or offensiveness, but because of its uniqueness compared to the body odors of younger people

people over age 40 have higher levels of a fragrant organic compound known as 2-nonenal in their sweat and on their skin. The chemical, which the researchers described as having an “unpleasant grassy and greasy odor,” … But the compound has also been linked to the scent of cucumbers and aged beer, which are not distasteful to most people

or possibly pheromones … subtle chemical communication, much of it subconscious,
helps people recognize family,
changes women’s reproductive cycles and
makes it easier to empathize with others

see also:

Chemosignals Communicate Human Emotions

Chemosignals Communicate Human Emotions
Psychological Science November 2012, 23  (11):  1417-1424

Can humans communicate emotional states via chemical signals?
In the experiment reported here, we addressed this question by examinin g the function of chemosignals in a framework furnished by embodied social communication theory.
Following this theory, we hypothesized that the processes a sender experiences during distinctive emotional states are transmitted to receivers by means of the chemicals that the sender produces, thus establishing a multilevel correspondence between sender and receiver.

In a double-blind experiment, we examined facial reactions, sensory-regulation processes, and visual search in response to chemosignals.

We demonstrated that fear chemosignals generated a fearful facial expression and sensory acquisition (increased sniff magnitude and eye scanning); in contrast, disgust chemosignals evoked a disgusted facial expression and sensory rejection (decreased sniff magnitude, target-detection sensitivity, and eye scanning).
These findings underline the neglected social relevance of chemosignals in regulating communicative correspondence outside of conscious access.

We Are What We Smell
Scientific American. Feb. 3, 2013
A recent study finds that we can instill our emotions in others through chemical signals delivered by scent.

narratives about acceptable emotionality

Hatred Derangement Syndrome

Living without shame

Greater Good in Action


Taste and pheromone perception in mammals and flies

Taste and pheromone perception in mammals and flies
Matsunami H, Amrein H – Genome Biol. (2003)

The olfactory systems of insects and mammals have analogous anatomical features and use similar molecular logic for olfactory coding.
The molecular underpinnings of the chemosensory systems that detect taste and pheromone cues have only recently been characterized. Comparison of these systems in Drosophila and mouse uncovers clear differences and a few surprising similarities.


The chemical bases of human sociality
Trends in Cognitive Sciences, 24 June 2013
Gün R. Semin,   and Jasper H. B. de Groot
Faculty of Social and Behavioral Sciences, Utrecht University, The Netherlands

Are mammal olfactory signals hiding right under our noses?
Naturwissenschaften June 2013, Volume 100, Issue 6, pp 487-506
Peter James Apps
Chemical communication via olfactory semiochemicals plays a central role in the social behaviour and reproduction of mammals, but even after four decades of research, only a few mammal semiochemicals have been chemically characterized. Expectations that mammal chemical signals are coded by quantitative relationships among multiple components have persisted since the earliest studies of mammal semiochemistry, and continue to direct research strategies. Nonetheless, the chemistry of mammal excretions and secretions and the characteristics of those semiochemicals that have been identified show that mammal semiochemicals are as likely to be single compounds as to be mixtures, and are as likely to be coded by the presence and absence of chemical compounds as by their quantities.

Chemosignals, hormones and mammalian reproduction
Hormones and Behavior, 63(5), May 2013, Pages 723–741
Aras Petrulis
Georgia State University, Neuroscience Institute, 100 Piedmont Ave SE, Atlanta, GA

• All aspects of chemical communication are influenced by sex and hormone status.
• Very few chemosignals qualify as “pheromones”.
• Most responses to chemosignals are dependent on context and learning.
• Chemosignal processing is mediated by the medial amygdala and its connections.
• Species-differences in all aspects of chemical communication are evident.

see also: Citing & related articles

Women’s tears contain chemical cues
Female weeping dampens …
6 January 2011 | Nature

Human Tears Contain a Chemosignal
Science 14 January 2011: 331 (6014):226-230
Shani Gelstein, Yaara Yeshurun, Liron Rozenkrantz, Sagit Shushan, Idan Frumin, Yehudah Roth, Noam Sobel
Emotional tearing is a poorly understood behavior that is considered uniquely human. In mice, tears serve as a chemosignal. We therefore hypothesized that human tears may similarly serve a chemosignaling function. We found that merely sniffing negative-emotion–related odorless tears obtained from women donors induced reductions in …

Fifty years of pheromones
Nature 457, 262-263 (15 January 2009)
Tristram D. Wyatt1
Department of Zoology of the University of Oxford, UK, and is the author of Pheromones and Animal Behaviour.
Powerful chemical signals have been identified in moths, elephants and fish, recounts Tristram D. Wyatt. But, contrary to stories in the popular press, the race is still on to capture human scents.

cited by 50:,d.eWU

Beauty is in the nose of the beholder
Gene found that determines if putative human pheromone smells naughty or nice.
16 September 2007

Neuroscience:  The sweet smell of success
Nature 428, 362-364 (25 March 2004)
Carina Dennis
Nature’s Australasian correspondent.
Smell is arguably the most evocative and mysterious of our senses. But thanks to advances in our understanding of the cells that detect odour, its secrets should now start to be revealed. Carina Dennis sniffs around.

[Web Focus] Sensory transduction

Male sweat relaxes women
Men’s underarms may hold clue to new fertility drug.
28 May 2003 | Nature

Male axillary extracts contain pheromones that affect pulsatile secretion of luteinizing hormone and mood in women recipients.
Biology of Reproduction, 68, 2107 – 2103, (2003).
Preti, G. et al. [ny]

Nosing into pheromone detectors
Nature Neuroscience 6, 438 – 440 (2003)

Mice lacking a functional main olfactory system are shown to be able to detect some odorants via their vomeronasal organ, suggesting this system is not restricted to sensing pheromones.


Molecular detection of pheromone signals in mammals: from genes to behaviour

Nature Reviews Neuroscience 4, 551-562 (July 2003)

Catherine Dulac & A. Thomas Torello

The instinctive and species-specific behavioural response of animals to pheromones has intrigued biologists for a long time. Recent molecular and electrophysiological approaches have provided new insights into the mechanisms of pheromone detection in rodents and into the sensory coding of pheromone signals that lead to gender discrimination and aggressive behaviour. [on disk]

Reproductive biology: Mammary messages

Nature 424, 25-26 (3 July 2003)

Elliott M. Blass

Identification of a pheromone that induces suckling in newborn rabbits sets a standard for studies on other mammals, and should prime investigations of the neurobiological basis of this behaviour.

One of the unique features of mammals is the parental nursing of young and suckling of milk from the mother1. On page 68 of this issue2, Schaal et al.

Chemical and behavioural characterization of the rabbit mammary pheromone
Nature 424, 68-72 (3 July 2003)
Benoist Schaal1,2, Gérard Coureaud1,2, Dominique Langlois2,3, Christian Giniès3, Etienne Sémon3 & Guy Perrier
Centre Européen des Sciences du Goût, CNRS, France

Neuropharmacology: Odorants may arouse instinctive behaviours 
Nature 412, 142 (12 July 2001)
Mehran Sam, et al.

The prevailing view of the mammalian olfactory system is that odorants are detected only in the nasal olfactory epithelium, whereas pheromones are generally detected in the vomeronasal organ. Here we show that vomeronasal neurons can actually detect both odorants and pheromones. This suggests that in mammals, as in insects, odorous compounds released from plants or other animal species may act as ‘semiochemicals’ — signalling molecules that elicit stereotyped behaviours that are advantageous to the emitter or to the receiver.


Human pheromones:  Communication through body odour
Nature 392, 126-127 (12 March 1998)

Human communication is dominated by auditory and visual information. In contrast, many animals use smell to communicate — both immediate and long-term effects of chemical signals have been documented within many species, from yeasts to mammals.


Regulation of ovulation by human pheromones
Nature 392, 177-179 (12 March 1998)
Kathleen Stern & Martha K. McClintock
Department of Psychology, The University of Chicago



‘Quantum smell’ idea gains ground

‘Quantum smell’ idea gains ground
BBC News. 27 January 2013

A controversial theory that the way we smell involves a quantum physics effect has received a boost, following experiments with human subjects.

It challenges the notion that our sense of smell depends only on the shapes of molecules we sniff in the air. Instead, it suggests that the molecules’ vibrations are responsible.

A way to test it is with two molecules of the same shape, but with different vibrations. A report in PLOS ONE shows that humans can distinguish the two.

tantalizingly, the idea hints at quantum effects occurring in biological systems – an idea that is itself driving a new field of science, as the BBC feature article Are birds hijacking quantum physics? points out.

But the theory – first put forward by Luca Turin, now of the Fleming Biomedical Research Sciences Centre in Greece – remains contested and divisive.

Of horses and unicorns
“I like to think of the vibration theory of olfaction and its proponents as unicorns. The rest of us studying olfaction are horses,”


Quantum Physics: What is really real?
May 20, 2015


Can You Smell Yourself?

Functional and anatomical segregation of the two mammalian olfactory systems. MOE: main olfactory epithelium; VNO: vomeronasal organ

Can You Smell Yourself?
by Sarah C. P. Williams
22 January 2013

in 1995, researchers conducted the now famous “sweaty T-shirt study,” which concluded that women prefer the smell of men who have different MHC genes than themselves

Researchers don’t know which receptors in the nose actually sense MHC proteins, because humans don’t have the vomeronasal organ that animals use to sniff out the molecules. “We would really like to continue this research to identify the receptors that recognize these peptides in humans,” Boehm says.

“I cannot wait to hold him and smell him,” said Yanet Leal Concepcion, Jojo’s mother.