Lectures 3 & 4 - PSYU3352: Appetite The Psychology of Eating and Drinking PDF

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Lectures 3 & 4 - PSYU3352: Appetite The Psychology of Eating and Drinking...

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Sensory aspects of eating and drinking Reading, Logue Ch.4 Appetite: The psychology of eating and drinking 1

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Aim • This and the next lecture aim to familiarise you with the basic way in which we perceive food and drink • We will start with an overview, then examine each sensory system in more depth • Finally, we will return to consider the ‘big picture’ - that is how the brain integrates information from these different sensory systems into what we consciously perceive as ‘flavour’ and the consequences this has for perception 2

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What systems? • So what sensory systems are involved when we eat and drink? – Smell – many qualities (but it is the ‘hidden sense’ - case of JM [TLE & parosmia] illustrates its importance via dysfunction – vomiting & weight loss)

– Taste – few qualities, but motivationally significant – Skin senses (touch) • Common chemical sense – very few qualities (whole body, especially mucosa)

• Somatosensation/Proprioception – few qualities (static & dynamic)

• But be aware now that what we perceive is an integrated sensation - flavour 3

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The sense of taste • The sense of taste (that results in sensations we call ‘tastes’) is located primarily on the surface of the tongue • We appear able to perceive several qualitatively different sensations (note hedonics-function) • Sweet (e.g. sucrose, saccharine) – Energy Pleasant • Sour (e.g. acids) – Ripeness/Vitamin C, fermentation (bacteria) – Un/Pleasant • Bitter (e.g. plant alkaloids) – Toxicity (LD50 correlation) - Unpleasant • Salty (e.g. mineral salts) – Depletion & Preference (Miners) – Un/Pleasant • Umami (e.g. MSG) – Allergy quackery (in toms, cheese, breast milk) - Pleasant • Fat (e.g. fatty acids) – Energy – BUT may have no conscious correlate

• To determine the role of taste (independent of smell) pinch your nose whilst eating

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The human tongue – Receptors are located in structures called taste buds – Taste buds are grouped into structures called papillae • Vallate papillae (fried eggs) - 9 in adults, 250 buds/papillae (function: swallow reflex – last chance to check?)

• Foliate papillae (ridges) - 10 in adults, 120 buds/papillae • Fungiform papillae (dots) - 30/cm2 [tip] - 8/cm2 [mid], 3 buds/pap (function: most sensitive – immediate detection of tastants?)

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The taste bud • Each bud (to cells with mi • These cells l • The bud is fi (effect of tongue scrubbing) • Each bud may have more than one type of receptor located on the microvilli • Taste myths (bottom right) – this diagram reproduced in many text books is wrong 6

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Receptors • There are two basic types of receptor present upon the taste bud’s microvilli – Ion gated channels • Salt detectors (Na+ [sodium ions]) • Acid detectors (H+ [hydrogen ions or protons])

– Protein gated channels • Sweet, bitter, umami, fat • It appears that each of these may occur in several forms – For bitter – may be 14 different receptors perhaps driven by selection pressure to avoid poison? – For sweet – just one receptor

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And to where in the brain? • After the cell depolarises an action potential passes along onto the chorda tympani nerve • The first major processing point is the Nucleus of the solitary tract in the brain stem • Information is then routed along two discrete pathways – To the brain stem (ingestive/protective reflexes) – To the insula and orbitofrontal cortices (perception of taste quality, intensity & hedonics)

• The insula is primary taste cortex, and the orbitofrontal cortex, can be thought of as secondary taste cortex • Patients with discrete insula lesions are able to tell a taste is present, but have some trouble with its quality • The insula also supports taste-related functions – notably the emotion of disgust 8

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Taste and disgust • Animals including humans respond with disgust to bitter tastes (see right) • In humans disgust seems to occur to a much broader range of stimuli than just bitter tastes (in contrast to animals)

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Disease cues (body products, body envelope violations, death, spoiled food, signs of ill-health etc)

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Incest Perhaps even to some moral violations

Disgust responding involves • • • • • •

A characteristic facial expression (right) A particular qualia - revulsion Nausea An intense desire to withdraw If the elicitor is touched - contamination A preparatory immune response

• Disgust responding/perception is impaired in people with damage to their insular cortex (e.g., in Huntington’s chorea)

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From a neural signal to a‘taste’ percept • Crucially, for taste, we understand the‘stimulus problem’ – That is what particular physical stimulus is associated with what particular psychological state

• If you drip sucrose on to the tongue you will perceive a sweet taste, quinine, a bitter taste, salt….. (etc) – So we can readily address the stimulus problem for taste in a way that we can not for smell (as we will soon see)

• The brain appears to use two approaches to form a representation of what is stimulating the taste receptors – Labeled lines (stimulus A – receptor for A activates only A sensitive neurons – thus the presence of ‘A’ is determined) – Patterns (stimulus A – generates a unique pattern of activity across many neurons – presence of ‘A’ is determined by recognising ‘A’s’ unique neural pattern) 10

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Evidence - labeled line • Labeled line – Certain fibres in the chorda tympani are selectively responsive to different tastes (i.e. fibre X is only active when salt is tasted) – On this basis we might assume that when fibre X is active, this results in a ‘salty taste’ qualia – Such selective fibres have been observed for sweet, salty, sour and bitter

Firing rate

(L) Salt selective

(R) Sweet selective

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Patterns • A pattern based explanation assumes that the brain recognises a pattern of activity across many nerve fibres and that different patterns produce different taste qualities • The following slide shows data from many different nerve fibres in the chorda tympani (taste nerve) of a rat… 12

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Patterns - example Salt (NaCl)

• Based on these patterns of activity which tastes do you think rats have little difficulty in telling apart (NHCL - ammonium chloride, KCL - potassium chloride, NACL - sodium chloride [Salt])?

• Patterns for similar tastes can be learnt presumably based upon these activity patterns Potential to discriminate NHCl from KCl

Generally similar pattern of NHCl and KCl 13

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So how do we ‘taste’? • Basic qualities may be defined by activity within specific nerve fibres (labeled line) – For example, ‘salty’

• But whether it is one sort of saltiness or another may depend upon the pattern of activity – For example, ‘metallic salty’ vs ‘mineral salty’ 14

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Individual differences • Back in the 1940’s it was first noted that some people could taste a very bitter substance called PTC (phenylthiocarbamide) and others could not • This difference was genetically determined • Recent research has focused on a bitter tasting chemical PROP (propylthiouracil), which is not carcinogenic (as is PTC) • There are large and significant individual differences in sensitivity to PROP 15

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Different taste worlds • Recent research suggests three groups of people – Non-tasters (30%), tasters (40%) and supertasters (20%) • Supertasters find PROP disgustingly bitter • Supertasters appears to have more taste buds than, tasters and non-tasters. This has the following effects • Greater sensitivity to sweet & bitter tastes • Dislike for bitter tasting vegetables (especially sprouts and other members of the Brassicae family including cabbage, broccoli and cauliflower) • Greater sensitivity to irritants such as chilli and carbonic acid (responsible for ‘fizz’ in carbonated drinks) • Supertasters are often leaner as well, as they may be more sensitive to fats in food (and so need less fat to get equal ‘pleasure’) 16

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Taste - conclusion • Taste, as you now know, is a relatively simple sensory system, with few qualities • Stimulation of the taste system also stimulates the production of saliva which assists digestion and makes food more palatable • Our ability to taste declines with age, but not until we are into our late 60’s – Reductions in taste sensitivity are associated with lower body weight in the elderly and with reduced appetite 17 17

The common chemical sense • The primary function of the common chemical sense is to allow for the speedy identification and removal of harmful chemical irritants from the skin • We will now examine how it works and then ponder the bizarre question as to why humans (unlike most animals) actively seek to add irritants to their diet 18

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Why is it ‘common’ • It is called the common chemical sense (CCS) as it is located over the whole area of the body but receptors are more densely grouped on the mucosa - mouth, eyes, genitals etc • In the mouth, many CCS receptors are located around the base of taste buds, so if you have more taste buds, you have more of these receptors too • When these receptors are stimulated in sufficient number the body has a reflex response – Tears, salivation, running nose, sweating 19

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What do we perceive? • The receptors responsible for the CCS are called ‘free nerve endings’ and appear to – Detect temperature (hot/cold) - confusion studies with capsaicin and menthol – In snakes the same receptor that detects warmth (and chilli) is used to detect [visualise like a thermal camera] prey at night

– Damage from excessive temperature – Chemical stimulation

• Many researchers believe that we can only experience the following sensory dimensions – Intensity (weak to strong) – Hot/Cold (quality; could be more - Anosmic studies) – Hedonics (pleasure to pain) 20

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What do we like and why? • Many foods, drinks or additives are CCS irritants, all of which have different temporal profiles – Pepper (piperine) – short acting, sharp – Ginger (zingerone) – short acting, sharp – – – – –

Chilli (capsaicin) – longer lasting, burning Fizzy drinks (carbonic acid) Alcohol (ethanol) Mustard (allyl-isothiocyanate) and horseradish, onion, menthol, vinegar, salt etc 21

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Spice it up? • People over the last 1000 years have gone to great lengths to secure irritants – Pepper shortage & price were significant financial motivators for the discovery of the America’s by Europeans (notably Columbus) – They did not find black pepper, but the chilli instead and its use rapidly spread to Europe and then to India and Asia

• So why do people like the burn of chilli for example? 22

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Liking the burn • Why? – Bland diets, Rice (Asia), Corn (Mexico) – salivation – Medicine effect - Vitamin C – Release of endogenous opioids – Naloxone study

• How? – In Mexico exposure starts around 7 years – Chilli sauce is always available, but children are never forced into using it – Concentration is gradually increased – It appears that people learn to love it (i.e. they come to know that it does not harm them and this then allows them to enjoy the ‘burn’) 23

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Conclusion • We have now completed our examination of taste and the common chemical sense • In the next lecture we will turn our attention briefly to the other skin senses and then to our sense of smell • Then we will look at how the brain integrates this information to produce the sensation of ‘flavour’ 24

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Sensory aspects of eating and drinking II Reading, Logue Ch.4 Appetite: The psychology of eating and drinking

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Smell • Our smell receptors are located behind the bridge of the nose and can be accessed by two separate pathways – Sniffing (orthonasal) – Via the back of the throat (retronasal)

• Each of these pathways is associated with its own perceived location • Sniffing makes us feel that an odour is located in the environment, while when the odour is in our mouth, it is perceived as part of that food – how does this happen? – This type of question is called a‘binding problem’ and is a major issue for cognitive neuroscience – Odour location binding may be caused by nasal airflow direction and by inhibition of olfactory attention by the presence of a taste in the mouth

• Much of the sensation that we term ‘taste’ or ‘flavour’ when we eat and drink is in fact smell • The sensations that we can experience in this modality appear to exceed the other flavour senses by many orders of magnitude 26

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Gross anatomy I • Features to notice – Frontal (anterior) nasal passages – Receptors – olfactory epithelium – Cribiform plate and olfactory bulb (and proneness to injury) – Turbinate bones (richly vascularised to warm air and create a turbulent air flow) – Rear (posterior) nasal passages – Soft palat (velopharyngeal flap – and ability to open and close)

Olfactory epithelium

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Gross anatomy II • During certain phases of eating and drinking volatiles ascend via the nasopharynx and bind to the same receptors that are stimulated during sniffing – Volatiles in food are pumped into the nasopharynx during chewing and on exhalation, when the soft palate (velopharyngeal flap) opens – This flap is normally shut during eating and drinking to stop food and drink getting into the nose – The mechanics of this process are poorly understood as it is hard to study 28

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Receptor surface • We have about 4-6cm2 of receptor tissue - the olfactory mucosa • The tissue is bathed in mucus and the ORN’s extend microvilli into this medium • The mucosa has a variety of functions – Clearing ‘old’ smells away – Transport – Protection 29

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The receptors • • •

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There are between 300-500 different olfactory receptors in humans and maybe 800+ in rodents Contrast this with the visual system and its 4 receptor types! Each olfactory receptor neuron on the epithelium (see right for an actual photo of the rat epithelium) expresses just one type of receptor All belong to a group called GProteins Chemicals bind to the G-Protein and result in depolarisation of the cell and an action potential

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Other olfactory receptors? • There may be other classes of receptor that are sensitive to reproductive related chemicals - Scent of symmetry - Faces vary in symmetry - More symmetrical faces are liked more - The smell from people with more symmetrical faces is liked more too

- MHC (Major histocompatability complex) type – immune genes - Needs to be different between sexual partners to maximise off-spring fitness - Partners with dissimilar MHC have more kids - Partners with similar MHC have more miscarriages - Even female perfume choice seems to be selected to complement MHC type - Smell seems to be our main mode for detecting MHC type

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Receptors to glomeruli • As you know each ORN expresses one type of receptor • The olfactory receptor types are randomly distributed across the olfactory epithelium • Each receptor type is sensitive to different chemicals but there is considerable overlap in sensitivity • Information from each receptor type converges on a structure called a glomeruli in the olfactory bulb • There are about the same number of glomeruli (300500) as there are receptor types (300-500) 32

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Schematic diagram of receptor to glomeruli relationship Three receptor types (A, B, C) on the olfactory epithelium (remember there are really 300+ types, not just 3 as here!)

Each receptor type then converges on to the same glomeruli in the olfactory bulb

When we sniff something there is a spatial (and temporal) pattern of activation across all of the 300-500 glomeruli – as we will see this is crucial to how we manage to perceive odours

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Information flow to/in the brain • Information from the glomeruli in the olfactory bulb (OB) travels then to the olfactory cortex (PC paleocortex), orbitofrontal cortex (OFC; neocortex), amygdala (AC; fear), mediodorsal thalamus (MD; attention role) and the hypothalamus (Hy) • The neural architecture of olfaction is unique amongst the senses – Direct access to neocortex without obligatory thalamic processing – Initial paleocortical processing – Direct access to hippocampus & amygdala

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How do we smell? • In essence our sense of smell is a pattern recognition system • Most odours are complex mixtures of chemicals - coffee contains 600 or so volatile (i.e. smelly) chemicals, but we just perceive ‘coffee’ • The olfactory system has to recognise these complex combinations of chemicals how?

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Pattern recognition • As you know each olfactory receptor type is sensitive to many different chemicals • This effectively rules out ‘labeled lines’ just as the complex nature of the stimulus does too (i.e. we don’t have a ‘coffee’ receptor) • Rather the brain uses the pattern of activity across the 300-500 glomeruli to recognise the odour • It appears to do this by matching the glomerular pattern to patterns that have already been experienced before (and encoded in to odour memory) • Crucially, it is this pattern matching process that generates our conscious perception of odour quality (that’s coffee!) 36

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Implications • What if the odour memory store is lost? – The case of Henry Molaison (HM)

• What if we have not smelled that odour before and so have no odour memory of its pattern? • A more bizarre prediction is that we all have different smell worlds that are dependent upon our history of smelling • There is strong and robust evidence for this claim as we shall see 37

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Different smell worlds • Children are poorer at telling odours apart than adults, even though they have normal acuity (they can detect whether an odour is present or absent) • Different cultures perceive different culturally specific odours in different ways • Japanese vs Tibetans (fish) • Japanese vs Germans (soya beans & marzipan)

• Experience based effects can also be readily demonstrated in the laboratory… 38

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Lab demonstration • If participants, smell an odour mixture (cherry-smoky) and then later smell each component alone… – The cherry odour alone now smells somewhat smoky – The smoky odour alone now smells somewhat cherry-like – Both odours are judged to smell more alike – Both odours are less discriminable from each other • The brain has encoded “Cherry-Smoky”, and so smelling either odour alone recovers the memory of the mixture • Interestingly this whole process occurs without explicit knowledge – learning without awareness

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Experts • So in sum, smelling is based upon experience – memory that is • If you loose your smell memories, like HM, a rose smells no different to coffee or petrol • Before turning to our next topic, I want to briefly examine the issue of perceptual expertise in olfaction as this directly relates to our discussion of ‘experiential’ effects and it is also a big deal in the culinary world 40

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Wine tasters • Most of us probably believe that expert wine tasters have the ability to detect ‘notes’ (components) in wine that the rest of us with uneducated palates can not detect – Des...