Scientific Trend Essays Vol 7: Chemical Senses

Chemical Senses

Simay Selek

The studies of the three chemical senses -olfactory, gustatory and trigeminal- regarding their neural processing are quite young when compared to the other senses’ studies. Those systems, especially taste and smell senses are working as “gatekeepers” by helping humans distinguish what to consume or what to avoid (Goldstein, 2010). According to the overview written 8 years ago, our knowledge of these senses have become more advanced in the last 25 years with the effect of increasing interest and the invention of modern olfactometer and gustometer which enhanced the EEG, PET and fMRI studies (Lundström, Boesveldt, & Albrecht, 2011); so now we can say in the last 33 years. Even the experience of the chemical senses have the power to impress us, like sometimes taking us to a specific time period -reminding us people or memories- or maybe changing our mood; when people asked to rate senses that would make them upset if they lose it, the chemical senses took the last places of the ranking (Lundström et al., 2011). In this paper, I’m planning to investigate the developments in the studies of the three chemical senses in humans separately as well as mentioning the interactions between them.

Olfaction

The first neural mapping of the olfactory system of humans was developed by Zatorre and his colleagues by using PET, showed the primary olfactory cortex as the piriform cortex and secondary olfactory cortex as the orbitofrontal cortex (Lundström et al., 2011; Zatorre, Jones-Gotman, Evans, & Meyer, 1992). The latter studies suggested that the primary computations may be occurring in the olfactory bulb (Cleland & Linster, 2005). As the research continued, the olfactory bulb accepted as the place where the central processing occurred so signals that first processed in the olfactory bulb later transferred to the piriform cortex as the place that processes the odorants by merging the chemical information and send it to amygdala (regarding smell and emotion interaction) and then to the orbitofrontal cortex (Goldstein, 2010; Gottfried, 2010).

At first, studies conducted with PET were claiming that amygdala and OFC together were responsible for processing unpleasant smells (Royet et al., 2001; Zald, And, & Pardo, 1997); but later it was shown that there are no differences between pleasant and unpleasant odors while being processed in amygdala; instead, there is a difference between the intensity of the odors during processing in amygdala (Anderson et al., 2003; Lundström et al., 2011). Around 2007, it was observed in the fMRI study which used jasmine and indole smells for pleasant and unpleasant odor stimuli, that the medial parts of the orbitofrontal cortex process the pleasant odor and lateral parts of the orbitofrontal cortex process unpleasant odor (F. Grabenhorst, Rolls, Margot, da Silva, & Velazco, 2007).

Along with the importance of chemical senses’ role in detecting danger -as being “gatekeepers”-; the olfactory system has a huge place in the research about identifying threats. To give an interesting example; the studies used sweat of people during fearful experience resulted in participants’ startle reflexes or fearful face expressions (Wisman & Shrira, 2015).

The interaction of odor and vision senses are quite impressing since it is investigated that the processing of an odor stimulus can be affected when it is manipulated by visual stimuli. People’s memories or expectations are also manipulating the process of odor perception, which is a result of higher- order processing (Goldstein, 2010). For example in a study, white wine was perceived as red wine by the wine tasters because of the red color which was created artificially by dying (Morrot, Brochet, & Dubourdieu, 2001). Steveson’s study which was conducted by pairing the same odor stimulus with 2 different odors separately (like AB / AC), resulted in when the subjects smelled the same stimulus alone, they were affected by the previously paired smell (for example B was sweet stimulus and C was smoky and they experienced A stimulus as sweet if it was paired with B before), that shows the effect of learning on odor perception (Stevenson, 2001)

The decrease in olfactory sensation is also another subject that the researchers are interested in. More than 50% of the olfactory function of the individuals between 65-80 years and more than between 62- 80% function among the individuals older than 80 years, decreases; which affects them in terms of their quality of life and physical well-being as well as in other aspects (Attems, Walker, & Jellinger, 2015). Research of Parkinson’s and Alzheimer’s are investigating the potential connection of the decrease in olfactory sensation and those diseases, since it can be a warning of neurogenerative disorder (Attems et al., 2015). Also, regardless of aging, it is observed that olfactory sensation decreases in the cases of depression (Kohli, Soler, Nguyen, Muus, & Schlosser, 2016).

The odor’s possible mood-lifting effect is widely used in aromatherapy. One of the studies investigating this issue, especially focusing on citrus (which is claimed to be a mood lifting substance) resulted with no mood change according to a specific odor (which is d-(+)-limonene in this study) but they observed that pleasantness of the smell affected the mood positively since vanillin also made the participants feel good (Hoenen, Müller, Pause, & Lübke, 2016).

Another field studying olfaction is its interaction with memory. To some extent, we all experience the remembering of someone, somewhere, something because of a smell’s trigger. Research shows that the memories triggered by olfaction are more emotional than the ones evoked verbally (Willander & Larsson, 2007).

Lastly, I’d like to mention some of the studies investigating the odors’ effect on socio-emotional communication. The research reveals that the chemosignals, especially created by smell stimuli have an important place in sexual attraction (Doty & Cameron, 2009). If we look at one of the studies investigating the odor’s effect among men and women, Herz and Inzlicht observed that (Herz & Inzlicht, 2002)women are more sensitive to odor perception in terms of pleasantness than men in mate selection (Herz & Inzlicht, 2002). It is also observed that men are able to understand whether a woman is in ovulating period, by smell perception (Kuukasjärvi et al., 2004). Olfactory signs are also affecting the judgments of personality; especially fragrance industry is aware of this situation and providing products that help people to modify their self-perception and self- confidence which results in increased attractiveness (Sorokowska, Sorokowski, & Havlícek, 2016).

Gustatory Senses

Even though gustatory system is mostly thought to be the place where the flavor is perceived — because of the tactile process occurring while eating-, studies show that, the flavor is the combination of olfactory and gustatory sense (Goldstein, 2010). According to Lundströn (2011), Claudius Galenus was the first person who successfully described the tongue’s innervations and the conveyance of the taste signals to the brain, in the 2nd century. The first neuroimaging study that explored the neural network that involves in the gustatory sense in humans (which is also confirmed by proceeding studies) was conducted in 1994 by using PET (Lundström et al., 2011). Before that, there were single recording studies on non-human primates which were used to make hypothesis about humans’ gustatory sense system (Haase, Cerf-Ducastel, & Murphy, 2009) According to the first PET study in 1994, the taste processing occurs in parahippocampal gyrus, thalamus, anterior cingulate gyrus, insular cortex, lingual gyrus, temporal gyri, and caudate nucleus (Kinomura et al., 1994). After the main regions that are responsible for gustatory senses were mostly identified; the questions whether the different tastes are processed in different parts of the brain and the physiological states’ -like hunger and satiety- effect on reaction to the taste arisen . For the first question; just like in olfaction, amygdala was first suggested to respond to the aversive stimuli; which was observed during the PET study which saline solution, pure water and chocolate’s gustatory stimuli were compared (Zald, Lee, Fluegel, & Pardo, 1998); but later the fMRI study conducted by using sweet taste of glucose as pleasant stimulus and NaCl’s salt taste as unpleasant stimulus resulted in activation of amygdala for both tastes (O’Doherty, Rolls, Francis, Bowtell, & McGlone, 2001). The studies about the hunger and satiety’s effects on brain activation started around 1999, in spite of inconsistencies in the literature, as the research continued it is observed that hunger and satiety “produce divergent activation in multiple brain areas” (Haase et al., 2009). The study can be summarized as; when the subjects were hungry, the primary gustatory regions -inferior insula- and secondary regions — OFC 11 and 47- activated higher for all kinds of used stimuli when compared to the satiety condition (Haase et al., 2009).

Around 2007, researchers started to investigate selective attention’s effect on taste processing. Veldhuizen and her colleagues carried out an fMRI study by using sweet, salty, sour and tasteless solutions and discovered that the activity in insula and overlying operculum increases by the tasteless stimulus, and the OFC responded only when the subject encountered unpredicted stimulus. They interpreted those results as, the taste detection process occurs in the insula and overlying operculum and process of unpredicted gustatory stimulus occur in OFC (Veldhuizen, Bender, Constable, & Small, 2007).

The first study about how the cognitive states affect appetite and the neural pathways regarding the gustatory senses was conducted in 2008 by using fMRI; showed that activations in the orbitofrontal cortex related to the umami and flavor can be tuned by language which is interpreted as the top- down cognition created by language can affect the earliest cortical regions that are involved in appetite, so it can be useful for appetite regulation (Fabian Grabenhorst, Rolls, & Bilderbeck, 2008). In 2009, Genevieve and her colleagues wanted to see if the task which is given to the subject affects the neural response to the gustatory stimulus. They used sweet, sour, salty and tasteless stimuli and used fMRI and observed that selective attention relating to the task effects the encoding of gustatory stimulus (Bender, Veldhuizen, Meltzer, Gitelman, & Small, 2009).

Along with olfaction, the gustatory system has an important place in reward mechanism of humans. Researchers studying obesity are especially focusing on this domain of neural signaling in chemical senses. Because of the olfactory and gustatory stimulus’ effect of creating pleasurable responses, people sometimes have the desire to eat in order to experience the feeling created by the reward mechanism (Islam et al., 2015). The possibility of obese people’s impairments in neuronal pathways that regulate connections with reward system has also been a subject of the obesity research (Volkow, Wang, & Baler, 2011). Another kind of study that has been conducted in the obesity research is whether obese people have different taste perception than normal weighted people. One of the studies resulted in difference in taste perception between obese and normal weighted participants, by showing less accurate identification of taste qualities by obese participants (Overberg, Hummel, Krude, & Wiegand, 2012).

Individual differences in taste perception is also an interesting subject of gustatory sensation. The reason for people’s experiencing different taste perception from the same stimuli is mostly claimed to be genetic differences (Goldstein, 2010). According to (Bachmanov & Beauchamp, 2007) the taste sensitivity cannot be fully explained by physiological or cognitively but it is clear that the case is affecting the eating behavior of the individuals (Puputti, Aisala, Hoppu, & Sandell, 2018).

Tregeminal Sense

Our sensation of freshness, burning feeling of chili pepper or an odorant stimulus’ tickling are all results of our trigeminal system (Frasnelli, Albrecht, Bryant, & Lundström, 2011). Neuroimaging studies of the trigeminal senses are not as rich as the other two chemical senses. The reason for this situation is said to be the difficulty in separating related olfactory stimulation while triggering trigeminal system (Lundström et al., 2011). Although researchers were able to investigate the intranasal trigeminal systems’ characteristics, basic anatomy and physiology earlier (Hummel & Livermore, 2002); Boyle and her colleagues were the first to create the neural map of intranasal trigeminal processing, which demonstrated the brain areas regarding nociceptive processing which are activated by trigeminal stimuli (Boyle, Heinke, Gerber, Frasnelli, & Hummel, 2007). Nowadays researchers are seeming to have found a way to discriminate odor and trigeminal perception to some extent. It is claimed that, if the odor stimulus activates trigeminal sensation, the researchers would able to localize which nostril is stimulated (when the odorant only presented to one of the nostrils) (Tremblay & Frasnelli, 2018). In order to create a trigeminal perception, there must be a specific interaction between trigeminal chemoreceptors and relevant chemicals (Friedland & Harteneck, 2017). Recently the place of interaction between trigeminal and olfactory systems are found at mucosal (peripheral) levels (Tremblay & Frasnelli, 2018).

As the distribution of the domains in this paper shows, the study of chemical senses mostly focused on olfactory and gustatory senses (olfactory may have received a little bit higher attention). This situation can be observed in journals too. For example, if you check the Oxford Academic’s Chemical Senses Journal’s “Top 10 Articles Making Impact in 2016 and 2017” list, which is sorted according to an attention score, created by evaluating the articles’ impact on platforms like Mendeley, Twitter, Facebook, Google+, Wikipedia as well as the uploaded videos, blogs and news sources; 4 of 10 journals are about gustatory perception, 6 of 10 are about olfactory perception (1 is investigating both so counted in both categories), and the last one is about neuromodulation of the chemical senses. The subject is still is a work in progress and there are various kinds of domains that have been and will be investigated under the same topic, which is so exciting that there are so many things to be discovered.

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This article was submitted by Simay Selek as a course assignment for COGS 507: Cognitive Neuroscience course (Spring 2019) at Cognitive Science Master’s Program, Yeditepe University, Istanbul, Turkey.