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. 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. An odorant is a physiochemical molecule that binds to a specific receptor protein. In mammals, each olfactory receptor protein has one type of molecule that it responds to, known as the one-olfactory-one-neuron rule, and approximately one thousand kinds of which have been identified. Structure and complexity constitute an odorant’s features, with changes resulting in altered odorant quality. An odorant’s features are detected by the olfactory system’s glomeruli and mitral cells which can be found in the olfactory bulb, a cortical structure involved in the perceptual differentiation of odorants. The olfactory bulb itself affects how odors come to be encoded through its temporal structure and firing rate, which in turn influences the likelihood of an odorant being remembered. Neuromodulation exists in the olfactory system and is responsible for neural plasticity and behavioural change in both mammals and insects. In the context of olfactory memory, neuromodulators regulate storage of information in a way that maintains the significance of the olfactory experience. These systems are highly dependent on norepinephrine and acetylcholine, which affect both implicit and explicit memory. Studies involving the noradrenergic system of mice demonstrate elimination of habitual learning when areas involving this system are lesioned, and subsequent restoration of habitual learning abilities when noradrenaline is injected into the olfactory bulb. The importance of cholinergic systems has been demonstrated in studies of rats and the effects of scopolamine, with acetylcholine being involved in initial learning stages and more specifically in the reduction of interference between stored memories. Implicit memories of stimuli do not require conscious recollection of the initial encounter of the stimulus. In regards to olfactory memory, deliberate recollection of an odor experience is not necessary in order for implicit memories of odors to form in the brain. Techniques used to study implicit olfactory memory are considered to be applicable to both humans and animals. In tests of implicit memory, memory of a stimulus is shown to be aided by previous exposure to that same stimulus. Evidence of the formation of implicit memory is found in tests of habituation, sensitization, perceptual learning and classical conditioning. In olfaction there exists a strong tendency for habituation, which is discussed further in the following paragraph. By evaluating memory performance of tasks involving one of these ‘subsets’ of implicit memory, the effect of previous odor stimulus experience not involving conscious recollection can be measured. Further knowledge can be gained about implicit memory of odor through the study of the implications of cognitive deficits. The effects of brain injury on odor memory can be investigated through the use of these implicit memory measures leading to further overall understanding of the brain. Habituation involves decreased levels of attention and responsiveness to a stimulus that is no longer perceived as being novel. In the realm of olfactory memory, habituation refers to a decrease in responsiveness to an odor as a result of prolonged exposure (restricted to a certain repeated stimulus), which involves adaptation of cells in the olfactory system. Receptor neurons and mitral cells located in the olfactory system adapt in response to odors. This includes the involvement of piriform cortical neurons which adapt rapidly, more completely and selectively to novel odors and are also thought to play a very important role in the habituation of odors. Norepinephrine is considered to have an effect on the functioning of the mitral cells by increasing their responsiveness. Acetylcholine is also regarded as an important neurotransmitter involved in the habituation of olfactory stimulus, though the exact means through which it operates are not yet clear. Explicit, unlike implicit memory for odors, is thought by some to be a phenomenon that is exclusive to humans. Explicit memory refers to memories that are remembered with conscious awareness of doing so. In olfaction, explicit memory refers to attributing associative meaning to odors. Through the assignment of associations to odors as well as non-odor stimuli, olfactory stimuli can gain meaning. Explicit memories of odors include information which can be used to process and compare other encountered odors. Attention focused on odors aids in the functioning of everyday life as well as the engagement of proper responses to experienced events. Evidence of explicit olfaction memory is seen through behaviours in tasks involving a working memory component. The two most commonly used tests for explicit odor memory are odor identification and odor recognition, which are discussed in greater detail below. Odor recognition is the most common and direct means used to measure odor memory. 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. Memory accuracy is assessed by the amount of correct recognition decisions that are made. 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. 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. Odor identification requires the specific labeling of presented olfactory stimuli, unlike odor recognition. 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. 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. 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. 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. 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.