Odor and Memory
The improvement through experience of the ability to detect and or/discriminate sensory stimuli.
It is widely believed that the sense of smell has minimal impact on the critical aspects of existence. However, research has shown that olfaction is deeply involved in every aspect of our lives. Of all the forms of odor memory, odor recognition is the most common and direct measure of odor memory (Crowder et al. 1995). Odor-evoked memory is responsible for providing us a sense of self, who we are, where we are in time and place, present and past (Herz 2011). The anatomy of olfaction seems to be tied to our emotions, motivations, and associations in a unique way which provides us with our personal and interpersonal experiences.
What Is Odor-Evoked Memory?
Odor memory is used to define both memory of odors and memory evoked by odors (Herz and Engen 1996). Research in this field begun early in the twentieth century when early investigators compared verbal associations using images and odors as cues and found that odors were weaker reminders than images, described the emotions and associations odors evoked, and also described the characteristics of odor-evoked memories (Bolger and Titchener 1907; Kenneth 1927; Laird 1935). Over the past decades, modern experimental methods have been used to investigate olfactory memory using cognitive experimentation such as the role of verbal mediation in olfactory processing, the duration of memory, olfactory recall and imagery, and neurological techniques that focused on impaired versus spared dissociation in clinical populations (Pelosi 1994).
Most odors experienced in nature are a combination of many different odorant molecules. The ability to discriminate between these odors is one of the main functions of olfaction. This process involves initial analysis of the inhaled stimulus into its component molecular features which are then combined to provide a unitary odor object such as “chocolate” (Wilson et al. 2009). The more familiar the odors become the more enhanced the encoding of the feature and their synthesis into objects becomes as well. This process leads to improvements in sensory discrimination in other words to determine whether two stimuli are different or the same. However, the ability to remember odors and the experience of memory evoked by an odor are two distinct cognitive-perceptual processes. Remembering odors refers to the ability to identify that you have smelled a specific smell before, and it does not bring back any particular memory event (Herz 2011). On the contrary, memory evoked by an odor refers to the ability to recollect a specific event from one’s life, but it does not have much to do with the odor itself. There is a possibility for the two processes to overlap in certain situations.
In the last 20 years, research concerning the special attributes of odor-evoked memory has been widely conducted. Herz et al. (2004) found left hemisphere dominance during episodic odor-evoked recall. Rubin et al. (1984) compared odor memories of participants with memories evoked by photographs or names and found that memories evoked by odors were reported as never having talked about prior to the experiments. They also suggested that odors might evoke more pleasant and emotional memories compare to other stimuli (Rubin et al. 1984). Hinton and Henley (1993) conducted a study to compare responses to stimuli that were resented in visual, lexical, and olfactory modalities. Their results indicated that the olfactory mode is significantly more emotionally arousing than verbal or visual stimuli and that odors can be more personally involving than those elicited by other sensory stimuli (Hinton and Henley 1993). However, odor-evoked memories do not elicit more accurate memories than other sensory cues (Hertz and Cupchik 1992). Research that followed suggested that memories triggered by olfactory information differ compared to memories evoked by verbal and visual information (Herz and Schooler 2002; Willander and Larsson 2006) in that they are more emotional. The results of Willander and Larson (2007) in their study of 72 older adults indicated that semantic knowledge of an odor’s name significantly affects the age distribution of memories such that the memory peak in childhood observed for odors only was attenuated.
Characteristics of Odor-Evoked Memory
In order for an odor to be remembered, it needs to be encoded and stored in the olfactory system. This process is regulated by neuromodulators which store information in a way that maintains the olfactory experience significant (Wilson and Stevenson 2006). The neurotransmitters norepinephrine and acetylcholine that affect both the implicit and explicit memory regulate these systems. Implicit odor memories do not require the deliberate recollection of an odor experience in order to form in the brain (Rouby et al. 2002). Previous exposure to a familiar stimulus seems to help the formation of implicit memories (de Wijk et al. 1995) through the process of habituation due to prolonged exposure to a stimulus (Wilson and Stevenson 2006). Jonesgotman and Zatorre (1993) studied odor memory in 121 patients with unilateral cerebral excision from temporal, frontal, frontotemporal, or centroparietal areas, and in 20 control subjects and found that participants showed impairment only after excision from the right temporal or right orbitofrontal cortex. All groups showed significant forgetting over time, and verbalized odors were recognized more efficiently than unlabeled ones. Rasch et al. (2007) in their study cued new memories in humans during sleep. They presented an odor that had been presented as context during prior learning. Their results showed that reactivation indeed causes memory consolidation during sleep and that reexposure to the odor during slow-wave sleep (SWS) improved the retention of hippocampus-dependent declarative memories but not of hippocampus-independent procedural memories. On the contrary explicit odor memories refer to memories that are consciously remembered, which in terms of olfaction refers to attribution of associative meaning to a specific odor (Radvansky 2006; Wilson and Stevenson 2006). By associating meaning to odors information is stored in a way that will aid in processing and comparing to other odors (Wilson and Stevenson 2006). Odor recognition and odor identification are probably the most commonly used tests to measure odor memory (de Wijk et al. 1995).
Several characteristics of odor-evoked memories contribute in distinguishing them from memories evoked by stimuli perceived through other sensory modalities (Herz 2011; Larsson et al. 2014). The most distinctive characteristic of these memories is their ability to evoke more emotional and evocative recollections than any other stimuli (Herz 2016). This is due to the neuroanatomy of the olfactory cortex which includes the amygdala, the part of the brain that processes emotional experience and emotional memory as well as the hippocampus, which is responsible for associative learning (Aggleton and Mishkin 1986; Cahill et al. 1995; Eichenbaum 2001). The amygdala is a complex set of nuclei involved in the formation of memories of emotional experiences associated with defense, flight, and fear. The direct projections from the amygdala to the hippocampus involve integration of various sensations into memory which is extremely important for the development of olfactory memories (Buchanan et al. 2003). The olfactory system is the only one that makes direct and intense contact with neural substrates of emotion and memory, which may provide a better understanding of why odor-evoked memories are usually emotionally potent. Other sensory modalities are mediated by limbic projections which are connected to associational areas (Herz and Engen 1996).
Another unique characteristic of the olfactory system that is directly associated to odor memory is the fact that incoming information is not processed in the thalamus before it is projected directly to the cerebral cortex, it occurs in the primary olfactory cortex (Carmichael et al. 1994). Neural transduction in the olfactory system seems to be different than any other sensory modalities. The neurons are unmyelinated, thereby assisting the sensation of an odor to remain for a greater period of time than other sensations. These unique interconnections are responsible for the emotionally evoked memories.
Based on the principle that environmental features encoded as part of a memory trace can contribute to memory for stored odors, context-depended memory is formed. In other words, odors facilitate the retrieval of information already learned (Schab 1990; Smith et al. 1992). Ehrlichman and Halpern (1988) found that participants recalled significantly more positive memories when presented with a pleasant odor than when they were presented with an unpleasant odor. Herz and Engen (1996) note that hedonic and emotional responses that maybe elicited from an odor depend on how the odor was first encountered. Hence, they seem to be based on learning principles.
It has been demonstrated in studies that hedonic and emotional responses an odor elicits are dependent on how the odor was first encountered. Jellinek (2004) notes that awareness of the emotions associated with an olfactory experience is prior to awareness of the presence of the sensory stimulus which elicits the experience. Almost all responses of odors have been shown to be based on associative learning principles (Davis and Ludvigson 1995). Therefore, if an odor is perceived as pleasant, it was first experienced in a pleasant context.
Herz and Cupchik (1992) proposed a theory based on the primacy of associative mechanisms in olfactory learning in order to explain autobiographical odor-evoked memories. He stated that if an odor is first experienced in an emotionally salient context, it will become a very effective trigger for the recollection of this memory. This is due to the higher level of activation in the amygdala which is critical for emotional memory (Cahill et al. 1995).
To examine the relationship between odor and emotion studies investigated whether heightened emotional states during the encoding of information relating to a new odor enhance the retrieval of that odor. The results indicated that word recall was higher when a specific odor was present at encoding and retrieval compare to no odor cue being available (Herz and Engen 1996). These findings support the theory that emotion is a vital variable in the formation of autobiographical odors.
Many studies have proved the existence of long-term odor memory and of previous experiences triggered by odors. Odors that are associated with emotion arousing events tend to be remembered for years maybe even throughout the life span. The available data from various studies suggest that memories evoked by odors have several distinct characteristics related to their emotional quality such the fact that memories elicited by odors seem to be more emotionally potent than memories triggered by other sensory stimuli, and it seems this saliency produces the impression that odor memories are more real. Neuroanatomical evidence is consistent with this finding (Herz and Engen 1996).
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