Why Do Humans Have Weak Sense of Smell

Compared to nearly all other animals, humans have the weakest sense of smell. You would think that humans and our closest evolutionary cousins (chimpanzees, apes, etc) would have similar sense of smell, but humans still have a much weaker olfactory sense.

Why might that be?

Well, research from Karolinska Institutet have discovered that humans are the ONLY mammals whereby no new neurons are formed in the olfactory bulb after birth. Or if there is any, it is VERY little.

In all other mammals including apes, new neurons are constantly being formed in the olfactory bulb which might explain why all mammals with the exception of humans have a superb sense of smell.

How did scientists figure out the absence of neuron growth in humans?

They looked at Carbon-14 levels within the DNA of the olfactory bulb. Carbon-14 levels in the atmosphere increased significantly due to nuclear bomb testing during the Cold War.

As such, IF new neurons were being formed, one would expect Carbon-14 levels which gets incorporated into developing DNA, to be similar to current atmospheric levels. Or at least find a certain turn-over percentage of both "old" and "new" neurons reflective of the Carbon-14 atmospheric levels at time of neuron development.

However, what researchers have found was Carbon 14 concentrations that corresponded to the atmospheric levels at the time of birth of the individuals establishing that there is very limited, if any, postnatal neuron development in the human olfactory bulb.

Reference:
The age of olfactory bulb neurons in humans. Neuron. May 24, 2012, 10.1016/j.neuron.2012.03.030

"Mouthwatering" Foods May Be a Misnomer?

In the spirit of Thanskgiving where thoughts of food predominate, British researchers have determined that humans are NOT able to produce more saliva by merely thinking of foods whether hungry or not.

This includes looking at pictures of sumptuous foods.

What researchers did determine is that SMELLING foods increased salivary production significantly. Tasting and chewing was even more influential in salivary production.

So at least for humans, unless the food is smelled, tasted, or chewed, mouthwatering just does not happen.

Of note, salivary production was determined by placing cotton pledgets inside the cheeks (saliva produced by the parotid glands) and a suction device under the tongue (saliva produced by the submandibular glands) on healthy volunteers.

Read MSNBC report here.

Reference:
IS THE MOUTHWATERING SENSATION A TRUE SALIVARY REFLEX? Journal of Texture Studies Special Issue: Food Oral Processing - Physics, Physiology, and Psychology University of Leeds, U.K. Volume 42, Issue 3, pages 212–216, June 2011

Diagnosing Smell and Taste Disorders

This article is reprinted with kind permission from Dr. Robert Henkin of the Smell and Taste Clinic located in Washington DC.

Answers to the Four Questions Required to Determine Cause of Smell and Taste Disorders: 

Question 1. DETECTION THRESHOLD – Can you detect the least concentrated stimulus from among three presented stimuli – two of which are either water (tastants) or neutral (odorants) and one of which is different?

This answer tells us of the presence and character of functioning taste and/or smell receptors. If this threshold number is higher (less sensitive) than normal then the patient requires more tastant or odorant than normal to stimulate taste and/or smell receptors. If the number is higher than normal then the patient has lost sensory receptor function due to some pathology.

Question 2. RECOGNITION THRESHOLD – Can you recognize correctly the different stimulus you just detected?

This answer tells us of the interaction between taste and/or smell receptors and the brain which processes this information. If the threshold number is higher (less sensitive) than normal then the patient requires more tastant or odorant than normal both to stimulate receptors and for the brain to process the sensory information correctly. This means that the patient has either lost sensory receptor sensitivity and/or has brain impairment which inhibits processing of the sensory stimulus due to some pathology.

Question 3. MAGNITUDE ESTIMATION – Can you judge the intensity of the correct recognition you just made using a scale from 0-100?

This answer tells us of the number of functioning taste and/or smell receptors. Taste and smell receptor activation involves a “field effect”; the larger the field, the more functioning receptors, the higher the magnitude estimation number. Thus, if the magnitude estimation number is lower than normal then the receptor number (taste and/or smell) is diminished and indicates pathology of the sensory system related to decreased receptor number.

Question 4. HEDONIC ESTIMATION – Can you judge the unpleasantness on a scale of -100 – 0, or pleasantness on a scale from 0 – +100, of the stimulus you just correctly recognized?

This answer tells us of the interaction between sensory receptor and the brain with respect to intensity of distortions of the sensory stimulus. This test uses a scale from 0 - −100 to indicate unpleasantness and a scale from 0 - +100 to indicate pleasantness. If a sweet tastant stimulus is considered unpleasant (e.g., -50% at a moderate concentration of sucrose, 300mM) then this response indicates a sensory distortion and thereby pathology of the taste system. Similarly, if the fruity odorant stimulus of amyl acetate is considered unpleasant (e.g., -50% at a moderate concentration of 10-1M) then the response indicates a sensory distortion and thereby pathology of the smell system.


By use of answers to these four questions it is possible to determine the nature and character of the pathology affecting taste and smell dysfunction. Answers to each of these questions are essential to determine this pathology.

These tests are used at The Taste and Smell Clinic in Washington, DC to determine the nature and character of sensory abnormalities of patients with taste and smell dysfunction. Only by use of this entire test battery can the nature and character of sensory dysfunction be determined. Simpler tests cannot fulfill this requirement.

Dr. Robert Henken
The Taste and Smell Clinic
Phone: (202) 364-4180

Reference:
An open label controlled trial of theophylline for treatment of patients with hyposmia. American Journal of the Medical Sciences. Vol. 337, pp. 396-406, 2009.