Cognitive Science: An Introduction/Olfaction
We can define olfaction as the sensory system that detects chemicals in some kind of fluid substance, such as air or water (however, when we detect chemicals in a substance we are drinking, we consider it gustation.)
Olfaction in PlantsEdit
Some plants have olfaction. Fruit ripens in the presence of ethylene in the air--even very minute amounts of it. This can be interpreted as a primitive sense of smell. Ripening fruits also emit ethylene, resulting in many fruits on the same tree ripening at once. Cuscuta pentagona is a parasitic plant. It’s green, and doesn’t get energy through photosynthesis. Instead, it sucks nutrients from other plants. It uses perception to know which nearby plant to attack. As it grows, it moves in circles. It can smell its favorite hosts, for example tomato plants, and the slow circles tend toward them. When it reaches the host, it goes down to the stem and starts to suck the life out of it.
Video of the plant in action: *https://www.youtube.com/watch?v=NDMXvwa0D9E*
Plants use chemicals to communicate with each other, too. This is described in more detail in the section on language and communication. 
Neuroscience of OlfactionEdit
There are more than five hundred different receptors for smells in the nasal mucosa. These can be activated by chemicals in the air to differing degrees, resulting in an enormous space of possibilities for smells--it numbers in the trillions.
Chemical stimuli are first detected by olfactory receptors in olfactory mucosa of the nasal cavity. Humans have around 350 types of olfactory receptors, most of which recognize multiple odors. Most odors we can detect are actually combinations of many chemicals. The receptors are affected by a set of 1000 genes which was discovered by Linda Buck and Richard Axel, earning them a Nobel Prize in 2004. A series of reactions is triggered causing a signal to be sent to the receptor neurons which send the signal to the brain's olfactory bulb into structures called glomeruli. From there, the signals are transmitted and activate various parts of the cortex associated with olfaction, including the olfactory and orbitofrontal cortices. The level of activation decreases as humans age, and is also lower in men than in women. The signals bypass the thalamus on the way to the cortex, a feature unique to olfactory stimuli. Research has confirmed that olfactory sensitivity has a genetic basis; odorant-receptor genes are one of the largest families of genes in the human genome .
In a large room, a single drop of perfume is the minimum concentration that can be detected. This is assuming one has not had prior exposure to the scent; A stronger prior exposure will increase the threshold of sensitivity and make it more difficult to detect. Within four minutes of exposure, the perceived strength of an odor is cut in half.
Once an odor has been detected, its source must be identified to determine whether or not it is harmful. Experiments more specifically examining the parts of the brain associated with odor detection and memory have had varying results. It was theorized that the right side of the brain was more specialized for encoding sensory memory; while evidence for this was found, it was also observed that the left side of the brain was more specialized at storing memories of aversive odors . As long as an odor's source is unknown, it will get conscious attention and cause apprehensiveness. Any odor that cannot be named but can be remembered is considered to have been identified. A smell may be caused by a single compound or a mixture of many compounds. Humans have difficulty distinguishing whether or not a smell has multiple components, all parts of the mixture are perceived as a whole. The average person is able to discriminate between more than 1 trillion mixtures of olfactory stimuli.
Olfaction is often seen by people as the least important sense, and one that is used purely for indulgence. It is rarely tested by doctors, and when there are problems they often go unnoticed. However, the loss of the olfactory sense, known as anosmia, is an indicator of severe underlying health problems. For patients suffering from Parkinson's disease, Alzheimer's disease and many other neurodegenerative diseases, anosmia is one of the earliest indicators. In addition to this, a reduced sense of smell can put one in danger of environmental hazards and health risks, such as a gas leak or spoiled food. MRI studies done on individuals who have been anosmic since birth have found that their olfactory bulbs are often quite deformed or missing completely; for individuals who developed anosmia after a traumatic head injury, frontal and temporal damage was found, and the size of their olfactory bulbs was reduced.
A loss of your sense of smell can also lead to social problems. It turns out that smell is very important for understanding and bonding with others, often eliciting feelings of compassion, affection, and romantic love. Mothers' nurturing instincts are activated by the scent of their new baby. More than 50 percent of people with anosmia feel socially isolated and say their relationship troubles are exacerbated by it. Anosmic men have fewer sexual relationships than men who can smell--a full one-fifth. Women have a drop, but it's not as strong. We can even detect emotions, such as fear or disgust, from sweat smells, and even feel them ourselves, in a kind of emotional contagion. It seems that you really can smell fear. 
Hyperosmia, a heightened olfactory sense, is also possible. It is a symptom of asceptic meningitis and Lyme disease, and is associated with migraines. The enhanced and reduced ability to detect only a certain odorant are described as "specific anosmia" and "specific hyperosmia" respectively.
Phantosmia is the condition under which one has olfactory hallucinations. These can be quite vivid, much more vivid that imagined smells, and can generate experiences of smells that have never been experienced in real life. Many of these are utterly indescribable. As with many hallucinations, phantosmia can be caused by sensory deprivation. Sometimes people who lose their sense of smell will eventually start hallucinating smells, for example.
Interaction with GustationEdit
Olfaction (smell) and gustation are the two chemical senses. They are also known as the "gatekeeper" senses because they determine whether something is harmful or beneficial to the body. This is possible because bad smelling and tasting things are generally bad for the body, while things that taste and smell good are generally safe to be consumed. Gustation and olfaction are fairly unprotected relative to the other senses. They are exposed to bacteria, toxins, dirt and other irritants regularly so they have a renewal mechanism called neurogenesis. Both systems regenerate every 5 to 7 weeks.
When nasal passages are blocked due to illness, difficulty tasting things is common. This effect is easily reproducible by pinching your nose closed and eating or drinking something with a strong flavor; the flavor you perceive should be greatly diminished but not gone completely. This is because some molecules can also reach the olfactory mucosa through the mouth by travelling up the nasopharynx.
- Chamovitz, D. (2012). What a plant knows: A field guide to the senses. Scientific American: New York. Pages 30--45
- Sacks, O. (2012). Hallucinations. New York: Alfred A. Knopf. page 52.
- Lazarini, Francoise. (2011). Is adult neurogenesis essential for olfaction. Trends in Neurosciences, 34, 20-30.
- Chamovitz, D. (2012). What a plant knows: A field guide to the senses. Scientific American: New York. Page 28
- Weiten, Wayne, McCann, D. (2016). Psychology: Themes and Variations. Nelson.
- Goldstein, Bruce (1999). Sensation & Perception. Nelson.
- Doty, R. (2001). Olfaction. Annual Review of Psychology, 2001, 423.
- Spence, C., McGlone, F. P., Kettenmann, B., & Kobal, G. (2001). Attention to olfaction. Experimental Brain Research, 138(4), 432-437.
- Jaeger, S. (2010). A preliminary investigation into a genetic basis for cis-3-hexen-1-ol odour perception: A genome-wide association approach. Food Quality and Preference, 21, 121.
- Engen, Trygg (1991). Odor Sensation and Memory. Praeger.
- Rokni, D. (2014). Analysis and Synthesis in Olfaction ACS Chemical Neuroscience, 5, 870-872.
- Bray, N. (2014). The nose knows one trillion smells. Nature Reviews Neuroscience, 15, 281.
- Engen, Trygg (1982). The Perception of Odors. Academic Press.
- Gaines, A.D. (2010). Anosmia and Hyposmia. Allergy and Asthma Proceedings, 31, 185.
- Paliwal, A. (2016). To find love, you may need your nose. Nautilus, Jan/Feb, p15.
- Basant, P. (2014). Hyperosmia in Lyme disease. Arquivos de Neuro-Psiquiatria, 72, 596.
- Sacks, O. (2012). Hallucinations. New York: Alfred A. Knopf. page 50.