Olfactory memory
Olfactory memory refers to the recollection of odors. Studies have found various characteristics of common memories of odor memory including persistence and high resistance to interference. Explicit memory is typically the form focused on in the studies of olfactory memory, though implicit forms of memory certainly supply distinct contributions to the understanding of odors and memories of them. Research has demonstrated that the changes to the olfactory bulb and main olfactory system following birth are extremely important and influential for maternal behavior. Mammalian olfactory cues play an important role in the coordination of the mother infant bond, and the following normal development of the offspring. Maternal breast odors are individually distinctive, and provide a basis for recognition of the mother by her offspring.
Throughout evolutionary history, olfaction has served various purposes related to the survival of the species, such as the development of communication. Even in humans and other animals today, these survival and communication aspects are still functioning. There is also evidence suggesting that there are deficits in olfactory memory in individuals with brain degenerative diseases such as Alzheimer's disease and dementia. These individuals lose the ability to distinguish smells as their disease worsens. There is also research showing that deficits in olfactory memory can act as a base in assessing certain types of mental disorders such as depression as each mental disorder has its own distinct pattern of olfactory deficits.
Mechanism
Physiology
Odorant
An odorant is a physiochemical molecule that binds to a specific receptor protein.
Neuromodulators
Implicit odor memory
Implicit memories of
Habituation
Habituation involves decreased levels of attention and responsiveness to a stimulus that is no longer perceived as being novel.
Explicit memory
Explicit, unlike implicit memory for odors, is thought by some to be a phenomenon that is exclusive to humans.
Odor recognition
Odor recognition is the most common and direct means used to measure odor memory.[8] In an odor recognition test participants are asked whether or not they recognize an odor. More specifically, a participant is subjected to a certain olfactory-related stimulus, and after a delay period is asked to decide if a probe (a stimulus that could or could not be the same as the initial stimulus) is the same as the one he/she initially encountered.[8] Memory accuracy is assessed by the amount of correct recognition decisions that are made.[8] A potential problem with this measure involves the generation of verbal labels that may enhance memory for olfactory stimuli. There are various ways of measuring the effect of verbal labeling, which include comparison of odors and odor names, as well as the speed and accuracy with which lexical decisions are made regarding odor names.[9] It has been suggested that odor recognition testing should be considered as a measure that involves both memory for perceptual information as well as potentially confounding memory due to the generation of verbal labels.[9]
Odor identification
Odor identification requires the specific labeling of presented olfactory stimuli, unlike odor recognition.[8] Neural coding refers to the way that the identity, concentration, and pleasurable value of olfactory stimuli are represented in the pattern of action potentials relayed to the brain from the olfactory bulb.[13] Identification begins with an odorant binding to specific odorant receptor proteins. Olfactory receptor molecules are very similar to G-protein-linked receptors and belong to the odorant receptor gene family.[13] The specificity of odor recognition is the result of the molecular variety of odorant receptor proteins and their interaction with the odorant molecules. However, the specific mechanism of certain receptors binding with certain odorant molecules is not well understood.[13] Odorant receptor genes also play a major role in odor identification. Expression in olfactory receptor neurons has been confirmed for a limited subset of the huge number of odorant receptor genes.[13] Genetic analysis shows that odorant receptor neurons express only one type of odorant receptor gene. It is hypothesized that different odors activate different receptors, and genetic regulation of odorant receptors results in the diversity for olfactory receptor neurons and this allows the capacity of olfactory systems to detect and encode a wide range of complex and novel odors in the environment.[13]
Hemispheric differences
Although bilateral activation of the brain has been seen with unilateral stimulation (accomplished by placing a stimulus under one nostril only), the activation seen is not exactly equal in both hemispheres.
Role of the amygdala
The
Behavioural effects
Odors can evoke positive
Neurological and structural development
Studies demonstrate that the changes to the olfactory bulb and main olfactory system following birth are extremely important and influential for maternal behavior.[18] Pregnancy and childbirth result in a high state of plasticity of the olfactory system that may facilitate olfactory learning within the mother.[19] Neurogenesis likely facilitates the formation of olfactory memory in the mother, as well as the infant.[19] A significant change takes place in the regulation of olfaction just after birth so that odors related with the offspring are no longer aversive, allowing the female to positively respond to her babies.[19] Research with a variety of animals suggest the role of norepinephrine in olfactory learning, in which norepinephrine neurons in the locus coeruleus send projections to neurons in the main and accessory olfactory bulbs.[20] This is significant in the formation of olfactory memory and learning.
The main olfactory bulb is one of the neural structures that experiences profound change when exposed to offspring odors at the time of childbirth.
Olfactory cues
Mammalian studies
Mammalian olfactory cues play an important role in the coordination of the mother infant bond, and the following normal development of the offspring.
Amniotic fluid is one of the primary olfactory cues that the ewe is exposed to after birth, allowing her to be attracted to any newborn lamb associated with that amniotic fluid.[19] The amniotic fluid produces olfactory cues, and a response from the ewe that cause her to be attracted to the newborn lamb.[19] When newborn lambs were washed with soap (or even water) it greatly reduced the degree of licking behavior by the maternal ewe, and consequently prevented her from displaying acceptance behavior towards the newborn.[19] The main olfactory system in sheep is quite significant in the developing appropriate maternal behaviors in sheep.[18]
Human studies
Research studies provide evidence that the fetus becomes familiar with chemical cues in the intrauterine environment.[22] Intrauterine olfactory learning may be demonstrated by behavioral evidence that newborn infants respond positively to the smell of their own amniotic fluid.[22] Infants are responsive to the olfactory cues associated with maternal breast odors.[20] They are able to recognize and react favorably to scents emitted from their own mother's breasts, despite the fact that they also may be attracted to breast odors from unfamiliar nursing females in a different context.[20] The unique scent of the mother (to the infant) is referred to as her olfactory signature.[20] While breasts are a source of the unique olfactory cue of the mother, infants are also able to recognize and respond with familiarity and preference to their mother's underarm scent.[20]
Olfactory cues are widespread within parental care to assist in the dynamic of the mother-infant relationship, and later development of the offspring.[19] In support of fetal olfactory learning, newborn infants display behavioral attraction to the odor of amniotic fluid.[20] For example, babies would more often suck from a breast treated with an amount of their own amniotic fluid, rather than the alternative untreated breast.[20] Newborns are initially attracted to their own amniotic fluid because that odor is familiar. Although exposure to amniotic fluid is eliminated after birth, breast fed babies have continued contact with cues from the mother's nipple and areola area. This causes breast odors to become more familiar and attractive, while amniotic fluid loses its positive value.[20] Maternal breast odors are individually distinctive, and provide a basis for recognition of the mother by her offspring.[20]
Role of olfaction in maternal bonding and subsequent development
As demonstrated by animals in the wild (the great apes, for example), the offspring is held by the mother immediately after birth without cleaning and is continually exposed to the familiar odor of the amniotic fluid (making the transition from the intrauterine to extrauterine environment less overwhelming).
While infants are generally attracted to the odors produced by
Evolution
Search for food
Studies of the mammalian brain have discovered that the excess of cerebral neurons is a phenomenon of mainly animals which had to seek and capture food. These neurons have become a large part of the olfactory system throughout evolution to allow higher mammals such as primates to have a better chance for survival through more advanced methods of hunting and finding food.[23] For example, the vulture has a large part of its brain committed to olfactory senses. This allows for it to be able to detect food at long ranges without being able to see it.[24] Having memory for various types of food aids in survival by allowing the animals to remember which scent is edible and which is not.
Communication and identification
Olfactory memory has also been developed throughout evolution to help animals recognize other animals.
Sexual reproduction
Olfaction is a very important aspect in
Warning stimulus
The development of a sense of smell is also thought to have arisen to function as an arousal system. Once an odor enters into conscious memory, it can signal the presence of a threat, like the smell of gas or smoke. However, odor memory can also be an implicit or unconscious process. This ability to respond automatically to a warning stimulus is much like pre-attentive processes in other sensory systems which involve the use of automatic forms of memory. These response patterns have evolved over time and involve a wide variety of motor and autonomic responses which are integrated into the behaviour pattern of reacting to a warning stimulus. odor-induced anxiety can be caused when an animal senses a predator. A study conducted on rats showed that when a rat was exposed to cat odors, there was increased anxiety-related behaviour in the rat. The cat odor induced an inhibition of the endocannabinoid system in the amygdala which has been suggested to induce anxiety-related responses.
Deficits
Olfactory deficits in the brain
Olfactory memory deficits can be significant indicators of brain damage and pathology.
Olfactory deficits and testing
Many tests have been developed to test olfactory memory in patients with mental disorders. The 40-item University of Pennsylvania Smell Identification Test (UPSIT)[44] and the 12-item Brief Smell Identification Test,[45] that was developed from the UPSIT, both test olfactory identification using a scratch and sniff booklet. The Sniffin' Sticks olfactory test consists of several pens that hold different scents and different dilutions, and this test provides scores for three olfactory domains: identification, threshold and discrimination.[46]
Olfactory deficits and prediction of mental illness or disease
Olfactory deficits have been found in patients with mental disorders and there is evidence suggesting that olfactory deficits can be a predictor of
See also
- Olfaction
- Olfactory system
- Memory
- University of Pennsylvania Smell Identification Test
- Insect olfaction
- Olfactory heritage
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