Neuroscientists have identified four dimensions of an odor including odor concentration (an odor’s pervasiveness), hedonic tone (the odor is ranked from extremely unpleasant to extremely pleasant), character (description of the odor to distinguish it from another), and odor intensity (how strongly an odor is perceived). Odor threshold is the lowest concentration of an odorant that can be first detected by the human nose in either air or a liquid, such as water or oil. Thus, the resulting minimal concentration of the odor is the odor threshold value (OTV), which is generally expressed as a concentration in air or as a concentration in liquid (for example, 0.1 percent in mineral oil).
Testing Odor Threshold in Research
Mice are often used in research as their olfaction abilities can easily be tested with an olfactometer. An olfactometer is a specialized machine that presents a subject (a mouse in this case) with an odorant at different concentrations. The olfactometer can be used in a simple go-no-go behavioral task that the mouse learns and is rewarded with water. In these types of experiments, “go” is the positive stimulus (S+) that rewards with water and “no-go” is the negative stimulus (S—) that provides no reward. Thus, a go-no-go behavioral task requires mice to learn to lick on a water-rewarded stimulus (for example, 1 percent isoamyl acetate—a banana-like smell—in mineral oil; S+) and not to lick on an unrewarded stimulus (mineral oil alone—the control; S-). After completion of three consecutive blocks with greater than 85 percent accuracy, it is determined that a mouse has learned the go-no-go task. The olfactometer can be used for additional behavioral experiments, such as Maximum Likelihood Parameter ESTimation (MLPEST, Clevenger & Restrepo, 2006). This method is reliable and accurately assesses olfactory detection threshold within a single day. As MLPEST is a difficult behavioral task, mice are first trained on the go-no-go behavioral task before being tested in MLPEST to determine their ability to identify an odor’s threshold.
Odor Threshold and Neurological Diseases or Disorders
Changes in a person’s sense of smell can be associated with neurological disease such as Alzheimer’s disease and schizophrenia or neurodegenerative disorders like Parkinson’s disease. Because it is an easy and noninvasive way to quickly determine a person’s olfactory dysfunction, neurologists will test a patient’s sense of smell. Generally, odor threshold (T) is often tested along with odor discrimination (D) and odor identity (I). Patients are presented with a series of 16 “sniffing sticks” using a three-alternative forced-choice task and a staircase paradigm. Each stick has a specific concentration of an odorant, which is usually a rose-like odor (phenylethyl alcohol). Patients are given the lowest concentration stick along with two blank sticks (for controls). They sniff each stick independently and are asked if they can smell anything. If not, they are given the next concentration stick with two blank sticks and repeat the sniffing sequence. Two successive correct identifications or one incorrect identification, respectively, triggers a reversal of the staircase. Odor threshold is then represented by the mean of the last four out of seven staircase reversals. In general, most patients with Parkinson’s disease will need a higher concentration of odorant for them to be able to detect the odor’s threshold compared to control adults.
Jennifer L. Hellier
See also: Anosmia; Odor Intensity Scale; Olfactory Mucosa; Olfactory Reference Syndrome; Olfactory Sensory Neurons; Olfactory System; Primary Odors
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Clevenger, Amy C., & Diego Restrepo. (2006). Evaluation of the validity of a maximum likelihood adaptive staircase procedure for measurement of olfactory detection threshold in mice. Chemical Senses, 31, 9—26.
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