Motivation and emotion/Book/2020/Disgust and disease avoidance motivation

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Disgust and disease avoidance motivation:
What role does disgust play in disease avoidance motivation?
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Overview[edit | edit source]

Focus questions:

  • What is disgust?
  • What are the main types of disgust?
  • How does disgust motivate behaviour?
  • What learning mechanisms can influence disgust responses?

Emotion and Disgust[edit | edit source]

In psychology, emotion is often defined as a complex state of feeling that results in changes, physical and psychological, that influence thought and behaviour. Emotionality is associated with a range of psychological phenomena such as; temperament, personality, mood and motivation (Kazdin, Alan E., 2000). Emotions are also a biological state that is associated with the nervous system brought on by neuropsychologial changes associating with thoughts, feelings, behavioral response and various degrees of pleasure or displeasure (Kazdin, Alan E., 2000).

Disgust is one of the seven universal emotions and arises as a feeling of aversion towards something offensive, it produces a strong and visceral emotion that can arouse powerful effective and behavioral responses ("Disgust", 2020). Typically, disgust is experienced as a feeling of revolution, sometimes accompanied by nausea, along with the strong desire to withdraw from the eliciting stimulus ("Disgust", 2020). While disgust arose to defend against infections and disease, it can also cause manipulative behaviour, interfering with the ability to lead a normal life. Disgust is a fundamental asset of human nature. Darwin was the first to propose that disgust is expressed universally, with available data suggesting that there is a universal set of disgust cues ("Disgust", 2020).

Recognising Disgust[edit | edit source]

According to Goleman (1995), the expression of disgust feelings and emotion looks the same throughout the world. Disgust serves to send the message that something is offensive in taste and/or smell. The emotion of disgust may also have a cognitive element of judging someone or something as to be avoided. From an evolutionary perspective, physiological manifestations of disgust may originate from guarding against noxious odors or spitting out poisonous food.

To recognise the feeling of disgust within one's own physiology, Paul Ekman (2003) suggests to:

1. First pay attention to feelings in your throat, notice any slight gagging.

2. Upper lip and nose sensations increase in sensitivity and start to 'turn up'.

3. Focus attention on nose, upper lip and throat.

4. Think about another's actions.

5. Notice if there are any judgements about feeling superior, many times the feelings of disgust may be mistaken for contempt or anger. It is important to feel within the body and make finer distinctions within one's own body. Emotions may shift extremely rapidly.

To recognise the feeling of disgust within the physiology of others, notice the 'upper lip raise'. See if there are any wrinkles above the nose between the eyes which raise the upper lip, forming an 'Inverted-U' shape. The overall appearance of the nostril and upper lip should be raised. The raising of the cheeks and lowering of the eyebrows display the feeling and emotion of disgust to an extreme degree (Ekman, 2003, 2007).

Take this quiz to test your knowledge:

1 What is disgust?

Feeling of aversion towards something offensive
Behavioural immune response
Expressed universally
All of the above

2 How to recognise disgust?

Closed eyes
Upper lip is raised in an inverted "U"
Raised eyebrows


Three Domains of Disgust[edit | edit source]

Tyber et al, outlines that there are three domains in which forms of disgust can be categorized in; pathogen disgust, sexual disgust and moral disgust ("Disgust", 2020).

Pathogen Disgust[edit | edit source]

Pathogen disgust motivates the avoidance of infectious microorganisms and arises from the desire to survive and a fear of death. It is known as the 'behavioural immune system' that is the first line of defense against diseases ("Disgust", 2020).

Sexual Disgust[edit | edit source]

Sexual disgust motivates the avoidance of dangerous sexual partners and behaviours. It develops as a desire to avoid biologically costly mates and a consideration of the consequences of certain reproductive choices. The two primary considerations are intrinsic qualities and genetic compatibility ("Disgust", 2020).

Moral Disgust[edit | edit source]

Moral disgust motivates people to avoid breaking social norms, which may have an important role in certain forms of morality. Moral disgust pertains to social transgressions and may include behaviours such as lying, theft, murder and rape ("Disgust", 2020). Unlike pathogen and sexual disgust, this domain motivates the avoidance of social relationships with norm-violating individuals because those threaten group cohesion.

Take this quiz to test your knowledge :

What are the three domains of disgust?

Fear, Social and Moral
Sexual, Moral and Fear
Pathogen, Diseases, Avoidance
Pathogen, Sexual and Moral


Disgust as a Disease-Avoidance Mechanism[edit | edit source]

Disgust is an evolved psychological system for protecting organisms from infection through disease avoidant behaviour. This behavioural immune system is present in a vast array of species and exhibits universal features that orchestrate hygienic behaviour in response to cues of risk of contact with pathogens.

Disgust Cues[edit | edit source]

A fundamental assumption of any disease-avoidance system is that the presence of possible pathogens can be detected by the sense organs in the form of cues. This cue system can be divided into two parts:

  1. The detection of pathogen related cues in visibly inanimate objects and;
  2. In animate objects

For inanimate objects, a range of cues are potentially available in the form of visible, auditory, olfactory and tactile senses to identify possible diseases. In turn, animate objects that are directly or indirectly infectious may display certain characteristics that indicate their potential to infect (Curtis, de Barra & Aunger, 2011). A defensive system designed to identify and activate avoidant behaviours to such cues offers a significant advantage (Curtis, de Barra & Aunger, 2011). That is, it would allow the possessor to detect likely sources of infection and thus avoid contact with them. For example, disgust cues could include bodily wastes, body contents, sick, dead, or unhygienic people, some sexual behaviour, dirty environments, certain foods, and certain animals. Objects that have come into contact with any of the above, or other disgust cues, can also become disgusting by association.

Contact with disgust elicitors, real or imagined, is associated with:

  1. Characteristic facial expressions that are recognised across cultures
  2. Patterns of behaviour that include withdrawing, distancing, stopping or dropping the object of disgust and shuddering
  3. Physiological changes including; lowered blood pressure and galvanic skin response, recruitment of serotonin pathways, increased immune system and;
  4. Physical negative affects that include nausea (Curtis, de Barra & Aunger, 2011).

Disgust Sensitivity Scale[edit | edit source]

The Disgust Scale is a self-report personality scale that was developed by Jonathon Haidt, Clark McCauley and Paul Rozin as a general tool for the study of disgust ("The Disgust Scale Home Page", 2020). It is used to measure individual differences in sensitivity to disgust, and to examine the relationship among different types of disgust ("The Disgust Scale Home Page", 2020).

To test your disgust sensitivity see: http://www.yourmorals.org/

Disgust Motivates Hygiene Behaviours[edit | edit source]

Good hygiene practices are integral to bringing about lasting change at community and national level. Improved hygiene behaviour reduces stigma, prevents the spread of disease and infection, reduces malnutrition and maintains health ("Department of Health | 7 Personal hygiene", 2020). Disgust plays a crucial role in the development of good hygiene by contributing a motivational factor in practicing hygiene behaviours.

While there is little data on correlations between disgust sensitivity and hygiene behaviour, a recent study conducted by Paul Rozin focused on disgust sensitivity and avoidance behaviour. The study found that individual self-reports of disgust during the task were predictors of the avoidance behaviour of each task. Using more aversive behavioural tasks, such as touching urine or eating a cookie of the floor, found that higher disgust sensitivity was associated with greater behavioural avoidance. Those who rated their subjective experience of disgust on paper as being particularly strong were more likely to avoid aversive disgust cues. This indicates that disgust motivates individuals to avoid diseases by keeping themselves and their surroundings clean, as well as practicing good hygienic behaviours that are repulsive because of disgust.

Take this quiz to test your knowledge:

1 What item would NOT produce a disgust response?

Spoiled apple
Raw chicken
Fresh fruit
Mold

2 Are individuals with a high disgust sensitivity more likely to avoid aversive stimuli?

No
Yes
Not enough information to answer correctly


Sexual Function and Disgust[edit | edit source]

Having heightened levels of sexual disgust may lead to disruptions in the ability to become sexually aroused, which may lead to deleterious effects in sexual functioning over time.

The Sexual Disgust Questionnaire[edit | edit source]

The Sexual Disgust Questionnaire; a Psychometric Study and a First Exploration in Patients with Sexual Dysfunctions, was conducted in 2013 to examine whether sexual dysfunctions lead to a heightened sexual disgust sensitivity rating (Van Overveld et al., 2013). The study aimed to examine whether or not sexual disgust is heightened throughout several sexual dysfunctions including; vaginismus, dyspareunia and erectile dysfunction (Van Overveld et al., 2013). The development of the Sexual Disgust Quiz (SDQ) was to help test whether high sexual disgust is linked to sexual dysfunctions (Van Overveld et al., 2013).

The study concluded that sexual disgust and willingness to handle sexually contaminated stimuli were associated with sexual functioning in women, but not in men. Specifically, women with vaginismus displayed heightened sexual disgust compared to without sexual problems, while men who suffered from erectile disorders demonstrated a lower willingness to handle sexually contaminated objects compared to men without sexual problems (Van Overveld et al., 2013).

In the context of this study, sexual disgust has become a motivator for women with vaginismus to avoid sexual acts, as it will bring pain to the individual. Women are also more likely to have a heightened sexual disgust than men due to this pain factor.

Take this quiz to test your knowledge:

1 What does SDQ stand for?

Serious Disgust Quiz
Sensitivity Disgust Quiz
Sexual Disgust Questionnaire

2 Men are more likely than women to rate higher sensitivity on the SDQ than women for sexual dysfunctions?

No
Yes


Disgust Images[edit | edit source]

There is an abundance of media platforms that can influence millions of people and disgust images are a crucial factor in the role of disease-avoidant behaviours. Through the the use of ads, news and social media disgust images about health related topics can be spread quickly and affect behaviours of individuals, one such health topic is smoking. Smoking has been an ongoing health problem for years, one of the tactics employed to encourage smokers to quit, or people to never start smoking, is to include images of how smoking can effect your health (Clayton, Leshner, Tomko, Trull & Piasecki, 2017). Using repulsive images showing how parts of your body can become rotten, or up close images of cancer inside a smokers body. Two studies focus on the effects of disgust images in smoking ads.

Figure 3: No Smoking sign that is indictive of where it is socially acceptable to smoke.

Motivated Processing of Fear Appeal and Disgust Images[edit | edit source]

A study titled 'Motivated Processing of Fear Appeal and Disgust Images in Televised Anti-Tobacco Ads' conducted in 2011, examined the effects of two types of content commonly found in anti-tobacco advertisements;

  1. Content focused on communicating a health threat about tobacco use (fear)
  2. Content containing disgust related images (how viewers process these messages)

The results of the study suggests that both fear and disgust content in anti-tobacco ads have significant effects on resources allocated to encoding the messages on recognition memory and on emotional responses (Leshner, Bolls & Wise, 2011). Although messages high in fear and disgust content were rated the most unpleasant, the same messages reduced corrugator (a muscle that contracts the skin into wrinkles especially) responses, accelerated heart rate and made recognition memory worse (Leshner, Bolls & Wise, 2011). The implications of this study suggest that while fear and disgust are unpleasant to viewers, the physical responses detract from the message being received in a memorable way.

Countering Craving with Disgust Images[edit | edit source]

'Countering Craving with Disgust Images: Examining Nicotine Withdrawn Smokers’ Motivated Message Processing of Anti-Tobacco Public Service Announcements' was a study conducted in 2017 (Clayton, Leshner, Tomko, Trull & Piasecki, 2017). Whereas the first study examined the effect of disgust as a motivator to change smoking behaviour, the current study focuses on whether the smoking cues in anti-tobacco ads elicit cravings, or if this effect is counteracted with disgust images (Clayton, Leshner, Tomko, Trull & Piasecki, 2017). The participants were 50 nicotine-deprived adults who were tested for cognitive memory and recognition memory after each of the 12 anti-tobacco ads (Clayton, Leshner, Tomko, Trull & Piasecki, 2017). The ads conveyed varying depictions of smoking cues and disgust content, with participants self-reporting smoking urges and intentions to quit after each message.

The results from this study indicated that smoking cue messages activated approach motivation, resulting in enhanced attention and memory, but increased craving and reduced quit intentions (Clayton, Leshner, Tomko, Trull & Piasecki, 2017). Disgust messages also enhanced attention and memory, but activated avoidance motivation resulting in reduced craving and increased quit intentions (Clayton, Leshner, Tomko, Trull & Piasecki, 2017). The combination of smoking cues and disgust content resulted in moderate amounts of craving and quit intentions, but also led to heart acceleration (defensive response) and poorer recognition of message (Clayton, Leshner, Tomko, Trull & Piasecki, 2017). However, including smoking cues in anti-tobacco advertisements is counterproductive to nicotine deprived smokers craving and prolonged abstinence. Therefore, anti-tobacco messages should omit smoking cues whilst including disgust images.

Take this quiz to test your knowledge:

1 What should anti-tobacco advertisements do to help people successfully quit smoking?

Not use disgust images
Display smoking cues
Don't display smoking cues

2 What did both studies results indicate?

Individuals were not affected by the anti-tobacco ads
People are more likely to quit smoking through the use of disgust images
Participants did not display any physical affects while watching the advertisements


Learning Mechanisms That May Modify Disgust Response[edit | edit source]

Disgust is also a dynamic adaptive system within which individuals show variation in pathogen avoidance associated with psychological traits like having a neurotic personality, as well as being in certain physiological states such as pregnancy or infancy. There are three specialized learning mechanisms that can influence the disgust response; the Garcia effect, Evaluative Conditioning and the Law of Contagion.

The Garcia Effect[edit | edit source]

Conditioned Taste Aversion, also known as the Garcia Effect, occurs when an animal associates the taste of a certain food with symptoms caused by a toxic, spoiled, or poisonous substance ("Conditioned taste aversion", 2020). The ability to develop a taste aversion is considered an adaptive trait, or survival mechanism, that trains the body to avoid poisonous substances before they cause harm ("Conditioned taste aversion", 2020). The association reduces the probability of consuming the same substance in the future and avoid further poisoning. However, it can be become more of a maladaptive behaviour when taste aversion occurs when sickness is merely coincidental to, and not caused by, the substance consumed ("Conditioned taste aversion", 2020).

There is another similar process that is considered interchangeable to Taste Aversion, termed Taste Avoidance. Taste Avoidance may be motivated by fear rather than conditioned nausea (Parker, 2003). Both processes however, demonstrate learning mechanisms that can modify an individuals disgust response. In the context of food, avoiding substances that have or can caused an illness is a survival mechanism produced by disgust cues, motivating a behaviour that becomes better suited to avoid diseases (Parker, 2003).

Evaluate Conditioning[edit | edit source]

Evaluate Conditioning is defined as a change in the valence of a stimulus that is due to the pairing of that stimulus with another positive or negative stimulus. The first stimulus is often referred to the conditioned stimulus, while the second is the unconditioned stimulus. A study conducted in 2009, titled 'Evaluate conditioning of fear and disgust in blood-inject-injury (BII) phobia: specificity and impact of individual differences in disgust sensitivity, examined whether pairing neutral facial expression stimuli with BII pictures had an impact in lowering disgust sensitivity in individuals with phobia of BII (Olatunji, Lohr, Smits, Sawchuk & Patten, 2009).

The results indicated that Evaluate Conditioning did have a positive correlation with reducing disgust sensitivity in individuals with BII phobia (Olatunji, Lohr, Smits, Sawchuk & Patten, 2009). The study found that individuals who had a phobia of BII and a high disgust sensitivity, had decreased disgust post test. Heightened fear and disgust were also subsequently reduced by the extinction procedure (Olatunji, Lohr, Smits, Sawchuk & Patten, 2009). These findings indicate that Evaluate conditioning can have an impact on the disgust response, this learning mechanism is important in disgust-avoidance motivation mainly in the areas of maladaptive behaviour that can form from extreme responses.

The Law of Contagion[edit | edit source]

The Law of Contagion is one of two laws within sympathetic magic, developed by Sir James Frazer (1959), in "The Golden Bough", and Marcel Mauss (1972) in "A General Theory of Magic" (Rozin, Millman & Nemeroff, 1986). The principles of sympathetic magic are thought to be a more primitives approach to disgust response belief systems and rituals, however, there are some aspects that could still apply in modern culture (Rozin, Millman & Nemeroff, 1986). According to the Law of Contagion, things that have once been in contact with each other may influence each other through transfer of some of their properties via an 'essence'. That essence can remain a permanent feature on the item it has come into contact with (Rozin, Millman & Nemeroff, 1986).

The phenomenon of disgust offers an opportunity to re-evaluate the laws of sympathetic magic, because disgusting stimuli produce strong effects that are in accord with these laws. A feature of disgusting substances is that it can render perfectly good food inedible after brief contact, even if there is no detectable trace. The history of contact is sufficient to warrant a disgust response (Rozin, Millman & Nemeroff, 1986). It can also apply directly in other aspects of daily life, for example, individuals tend to avoid contact with objects a sick person may have touched (like tissues), or find the idea of wearing a shirt worn by a murderer repulsive, therefore applying the Law of Contagion in the present day.

Take this quiz to test your knowledge:

What are the three learning mechanisms that could modify the disgust response?

The Garcia Effect, Evaluate Conditioning and the Law of Contagion
Taste Aversion, Pavlovian Conditioning and the Garcia Effect
The Law of Contagion, Evaluate Conditioning and Disgust cues


Conclusion[edit | edit source]

To sum up this chapter in a sentence, the role disgust plays in disease avoidance is absolutely crucial, it is biologically imbedded in various species to ensure their survival. Within humans, disgust is universally recognised, different countries and nations having similar responses to avoid disease and identity markers of disgust. It unconsciously guides peoples behaviour to best suit the needs of the individual in order to survive.

The three domains of disgust identified; pathogen, sexual, and moral, form broad categories with which we can mark and explore disgust. Pathogen disgust is the first barrier between infectious diseases and the motivation used to help a potentially harmful stimuli. It plays a valuable role in being the motivator behind actions of avoidance, crucial to the survival of many species. Imagine that pathogen disgust did not exist, humans would not have any motivating factor behind learning to avoid toxic substances or infectious diseases. The need to survive would be hindered by the fact that there would be nothing to guide behaviour a different way to avoid harmful stimuli.

Sexual disgust plays a less dominant role than pathogen disgust, but still an important factor to avoiding diseases. Through sexual disgust, an individual is motivated to find partners that they find appealing, while avoiding potential partners that could cause harm and sexual acts that they deem uncomfortable. It relies on individual desires and different qualities one would choose for a partner, other factors being involved heavily (such as personality, looks, social acceptability), but essentially it is an important motivator in avoiding sexual acts that could be uncomfortable and partners that are unsuitable.

Moral disgust is a motivating addition in social norm choices, in order to make a group work cohesively, individuals need to avoid norm-violating individuals, to be able to function in society. Both of the two studies examining anti-tobacco ads, indicated that including disgust image content captures the viewers attention, but implicated that it draws a defensive response in the form of increased heart rate and poorer recognition memory. Through anti-tobacco advertisements smoking has become a violation of social norm, people are more aware of the health risks connected to smoking and therefore, avoid individuals who do smoke. Since this behaviour is now becoming out of the norm, more individuals avoid smoking and people associated with it. However, morality disgust has been known to influence many things, including views on gay rights, abortion and skin colour. This domain of disgust shifts with what is socially acceptable at the time, but at the core, it helps guide individuals into a group mentality of thinking.

Disgust responses can be influenced by different learning mechanisms, in various ways. The Garcia Effect is very common in the case of food poisoning with individuals avoiding certain foods or places that made them ill. Its a response that affects people on an individual level through their own personal experiences. The Evaluate Condition effect is a way for people to overcome any maladaptive behaviour that has been a product of an extreme disgust response. It gives people a chance to align their behaviour into what is considered socially acceptable on the disgust stimuli. One has the ability for an individual to learn personal objects of disgust while the other give people a chance to overcome a maladaptive disgust response. However, the Law of Contagion is more of a belief system that can apply to how people view transference of disgust on objects. In Western society, people do avoid objects that could have been in contact with contaminated stimuli, such as avoiding places like public toilets, or fresh food that has come into contact with rotten food.

Ultimately, disgust is a key motivating factor driving our everyday actions in avoiding diseases. It influences our behaviour in ways we are still exploring, in countless ways, that are still trying to be understood. The role disgust plays in our life is biological, sexual and social, branching unconsciously into our behaviour and altering it to get the best survival result possible.

See also[edit | edit source]

References[edit | edit source]

Aarøe, L., Osmundsen, M. and Petersen, M., 2016. Distrust As a Disease Avoidance Strategy: Individual Differences in Disgust Sensitivity Regulate Generalized Social Trust. Frontiers in Psychology, [online] 7. Available at: <https://www.frontiersin.org/articles/10.3389/fpsyg.2016.01038/full> [Accessed 30 August 2020].

Clayton, R., Leshner, G., Tomko, R., Trull, T., & Piasecki, T. (2017). Countering Craving with Disgust Images: Examining Nicotine Withdrawn Smokers’ Motivated Message Processing of Anti-Tobacco Public Service Announcements. Journal Of Health Communication, 22(3), 254-261. doi: 10.1080/10810730.2016.1268222

Conditioned taste aversion. (2020). Retrieved 18 October 2020, from https://en.wikipedia.org/wiki/Conditioned_taste_aversion#In_humans

Curtis, V., de Barra, M., & Aunger, R. (2011). Disgust as an adaptive system for disease avoidance behaviour. Philosophical Transactions Of The Royal Society B: Biological Sciences, 366(1563), 389-401. doi: 10.1098/rstb.2010.0117

Department of Health | 7 Personal hygiene. (2020). Retrieved 18 October 2020, from https://www1.health.gov.au/internet/publications/publishing.nsf/Content/ohp-enhealth-manual-atsi-cnt-l~ohp-enhealth-manual-atsi-cnt-l-ch3~ohp-enhealth-manual-atsi-cnt-l-ch3.7

Disgust. (2020). Retrieved 18 October 2020, from https://en.wikipedia.org/wiki/Disgust

Disgust protects people against infection. (2020). Retrieved 18 October 2020, from https://www.healio.com/news/infectious-disease/20180621/disgust-protects-people-against-infection

Ekman, P. (2007). Emotions revealed: Recognizing faces and feelings to improve communication and emotional life, New York, NY: St. Martin’s Griffin. Kazdin, Alan E. (Ed). (2000). Encyclopedia of Psychology, Vol. 3 , (pp. 162-167). Washington, DC, US: American Psychological Association; New York, NY, US: Oxford University Press, , 507 pp.

Kemp, D., Niederdeppe, J. and Byrne, S., 2019. Adolescent Attention to Disgust Visuals in Cigarette Graphic Warning Labels. Journal of Adolescent Health, [online] 65(6), pp.769-775. Available at: <https://www.sciencedirect.com/science/article/pii/S1054139X19303581> [Accessed 30 August 2020].

Leshner, G., Bolls, P., & Wise, K. (2011). Motivated Processing of Fear Appeal and Disgust Images in Televised Anti-Tobacco Ads. Journal Of Media Psychology, 23(2), 77-89. doi: 10.1027/1864-1105/a000037

Olatunji, B., Lohr, J., Smits, J., Sawchuk, C., & Patten, K. (2009). Evaluative conditioning of fear and disgust in blood-injection-injury phobia: Specificity and impact of individual differences in disgust sensitivity. Journal Of Anxiety Disorders, 23(2), 153-159. doi: 10.1016/j.janxdis.2008.06.002

Parker, L. (2003). Taste avoidance and taste aversion: Evidence for two different processes. Animal Learning & Behavior, 31(2), 165-172. doi: 10.3758/bf03195979

Rozin, P., Millman, L., & Nemeroff, C. (1986). Operation of the laws of sympathetic magic in disgust and other domains. Journal Of Personality And Social Psychology, 50(4), 703-712. doi: 10.1037/0022-3514.50.4.703

The Disgust Scale Home Page. (2020). Retrieved 18 October 2020, from http://people.stern.nyu.edu/jhaidt/disgustscale.html

Van Overveld, M., de Jong, P., Peters, M., van Lankveld, J., Melles, R., & ter Kuile, M. (2013). The Sexual Disgust Questionnaire; a Psychometric Study and a First Exploration in Patients with Sexual Dysfunctions. The Journal Of Sexual Medicine, 10(2), 396-407. doi: 10.1111/j.1743-6109.2012.02979.x

External Links[edit | edit source]

  • Centers for Disease Control and Prevention [1]
  • Disgust Sensitivity Scale [1]
  • Neural substrates for conditioned taste aversion in the rat [2]
  1. "YourMorals.Org". YourMorals.Org. Retrieved 2020-10-18.
  2. Yamamoto, Takashi; Shimura, Tsuyoshi; Sako, Noritaka; Yasoshima, Yasunobu; Sakai, Nobuyuki (1994-12). "Neural substrates for conditioned taste aversion in the rat". Behavioural Brain Research 65 (2): 123–137. doi:10.1016/0166-4328(94)90097-3. ISSN 0166-4328. http://dx.doi.org/10.1016/0166-4328(94)90097-3.