Lecture 3 and 4 PDF

Sensory aspects of eating and drinking Reading, Logue Ch.4 Appetite: The psychology of eating and drinking 1 1 Aim This and the next lecture aim to...

Sensory aspects of eating and drinking Reading, Logue Ch.4 Appetite: The psychology of eating and drinking 1 1 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 2  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 3 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 4 4  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?) 5 5 The taste bud Each bud (top right) contains cells with microvilli These cells last 2 days The bud is filled with mucus (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 6  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 7 7 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 8  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) Disease cues (body products, body envelope violations, death, spoiled food, signs of ill-health etc) 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) 9 9 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 10  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 11 11 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 12  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 Generally similar discriminate pattern of NHCl and KCl 13 NHCl from KCl 13 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 14  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 15 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 16  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 18  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 19 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 20  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 21 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 22  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 23 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 24  Sensory aspects of eating and drinking II Reading, Logue Ch.4 Appetite: The psychology of eating and drinking 25 25 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 26  Gross anatomy I Features to notice – Frontal (anterior) nasal passages Olfactory epithelium – 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) 27 27 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 28  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 29 The receptors 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 G- Proteins Chemicals bind to the G-Protein and result in depolarisation of the cell and an action potential 30 30  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 31 31 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 (300- 500) as there are receptor types (300-500) 32 32  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 33 33 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 34 34  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? 35 35 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 36  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 37 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 38  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 39 39 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 40  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 – Description of a Hunter Valley Sauvignon blanc – “It is a great wine of phenomenal length and character. The light-yellow colour and bouquet are strikingly youthful for its age, the latter showing little sign of toasty development, instead, subtle notes of lemon, herbs, beeswax and candle- wax. It’s amazingly full of fruit and richness of palate, filling up the entire mouth with flavour that lasts and lasts. All this is delivered with impeccable harmony.” Wine tasters are somewhat better, but not much – They are no better at discrimination than regular wine drinkers, but both are better than non-wine drinkers – They can match a description they gave to a particular wine about 48% of the time, compared to 28% regular wine drinkers – Their expertise lies in applying language to sensation and knowing feature clusters associated with particular varieties (and some tricks) – The phenomenon of verbal overshadowing 41 41 Smell - in sum Our sense of smell is based upon recognising patterns I have not mentioned this, but there are also likely to be important genetic differences in the number and type of receptors we each have, but the full implications of this are at present unclear Both of the above mean that we probably live in relatively unique, but culturally defined, ‘smell worlds’ 42 42  Somatosensation & Proprioception We have, in the mouth, a range of receptors located in and on the tissue surface, and deep in the muscles and joints of the mouth (as with elsewhere in the body) – Somatosensation is our perception of objects (and their properties) contacting the body – it is both an active and passive sense – Proprioception is our perception of the location of our muscles and joints in ‘space’ – They are intimately related and I’ll deal with them as one‘system’ These are important in feeling – Pressure (i.e. chewing food) – Texture (i.e. crispness & fattiness) – Astringency (i.e. pinched-up & shriveled, such as tannins in wine) This is probably the most poorly explored sensory system in the context of flavour, but it is clearly important in the perception of texture and fat 43 43 Fat perception A significant component of fat perception in food involves texture Descriptive terms for fat are textural – Greasy, oily, creamy, thin, watery Fat content can be accurately gauged by the fingers alone However, this is not the whole story Smell (rat studies - Yes vs human studies – No?) Taste (as noted earlier) - currently contentious… – In rat ‘yes’, they have receptors that detect fat – Humans do appear able to discriminate fats in the absence of textural and olfactory cues, which just leaves taste (sort of) 44 44  Putting it all together So, when we go now to eat or have a coffee, how does the brain put all these pieces together (i.e. a binding problem again) to give us a unitary impression of flavour? Well we will approach this in three ways – Is flavour really a unitary sensation? – If it is, what impact does this have on our perception of food and drink? – Finally, how does the brain do it? 45 45 Flavour No language surveyed has a term that distinguishes the olfactory from the taste component of eating & drinking – Taste and smell in the mouth seem to be treated linguistically as a single entity (contrast with ‘red’ and ‘green’ for example) When people lose their sense of smell, they typically report also having lost their sense of taste, as food now tastes bland People are poor at discriminating the components of flavours, even if trained to do so, and especially odours While children know you need eyes to see and ears to hear, most adults do not know that you need a‘nose to taste’ So we can conclude – broadly – that at least for the two major components of flavour, taste and smell, these seem to be treated as a single entity in the mouth 46 46  What impact does this have One consequence of experiencing taste and smell as a unitary experience is that we seem to encode this flavour information in the same way Notably we appear to always encode flavour information irrespective of whether we wish to do so or not – unconsciously that is The most striking consequence of this is odour- taste synesthesia - sweet smelling odours 47 47 Odour-taste synesthesia I Synesthesia refers to the experience of a sensation normally associated with one sensory system, when another sensory system is stimulated Most sysnesthesias (sound/word-colour; word- flavour) are rare (