Sensation and Perception of Flavor
Background
Olfaction is the sense of smell. It is the primary window to the environment, especially for animals. Animals are actually macrosmatic which means they have a keen sense of smell that is important for their survival; whereas, humans are microsmatic which means we have a less keen sense of smell that is not crucial to our survival. Olfaction can also be important for sexual reproduction because it triggers mating behaviors in many species. One of the most important reasons for having the human olfaction sense comes from people who suffer from anosmia. Anosmia is the loss of the ability to smell as a result of injury or infection. People who suffer from anosmia express experiencing a great void, because of their inability to taste foods which is due to the close connection between smell and flavor. These people also lose the olfaction warning system which alerts us to gas, smoke, spoiled foods, etc. that could actually be essential for our survival. Even though olfaction is not considered to be the most important aspect to human survival it still enhances the quality of life (Goldstein 2010).
Detecting Odors
Our sense of smell allows us to identify extremely low concentrations of some odorants. Humans have a detection threshold for smell that is the lowest concentration at which an odorant can be detected.
Although humans can detect very low concentrations, we are much less sensitive to odors than animals. For example, rats are 8 to 50 times more sensitive to odors than humans, and dogs are 300 to 10,000 times more sensitive.
Odor detection also contains what is known as the difference threshold. The difference threshold is the smallest difference in the concentration of two odors that can be detected. Measuring the difference threshold highlights a big problem in olfactory research which is the control of concentrations in stimulus presentations. William Cain measured the difference threshold using cotton balls, click here to see his research study.
Identifying Odors
When an odorant approaches the threshold, a person can detect the presence of an odor but cannot detect the quality of the odor, so whether it is cinnamon, floral, or pepperminty. In order for a person to recognize an odor’s quality the odor concentration must be increased by at least 3 times that above the threshold concentration. This is called the recognition threshold.
One fact about odors is that even though humans can discriminate between as many as 100,000 different odors we often find it difficult to accurately identify specific odors (meaning it is hard for us to name an odor). Knowing the correct label for the odor actually seems to transform our perception into that odor. William Cain demonstrates this by using an example of an odor described as a fishy-goaty-oily smell that actually comes from leather, so now that odor is called leather (Goldstein 2010).This trouble we have identifying odors results from an inability to retrieve the odor’s name from our memory, not from an olfactory system deficiency.
Olfaction Parts
Olfaction Mucosa: This is about a dime-sized region located high in the nasal cavity (which contains the receptors for olfaction) and just below the olfactory bulb. Odorant molecules are carried into the nose in an air stream which brings them into contact with the mucosa.
Olfaction Receptor Neurons: These are located in the mucosa and are dotted with molecules called olfactory receptors that are sensitive to chemical odorants. ORN's are proteins that cross the membrane of the receptor neurons seven times, and they are sensitive to a specific range of stimuli and a narrow range of odorants. Linda Buck and Richard Axel were the researchers who discovered that there are 350 different types of olfactory receptors in humans, each sensitive to a certain group of odorants (Goldstein 2010).
Pathway
The Olfactory Receptor Neurons send signals to structures called glomeruli in the olfactory bulb. All 10,000 ORNs, of a certain kind, send their signals to only one or two glomeruli. Each glomerulus then collects the information about the firing of a certain kind of ORN. The signals are transmitted from the olfactory bulb to the piriform cortex (primary olfactory cortex) and the amygdala, and then to the orbitofrontal cortex (secondary olfactory cortex). Amygdala is the area of the brain that is associated with emotions, so it plays a role in the emotional reactions elicited by some odors.
OLFACTION
Functions of Olfaction
Detecting and Identifying Odors
Olfaction
Parts and Pathway
Olfaction System Structure
Neural Pathway
Olfaction Sense
Animals
Leather
