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C82COG - Perception > Taste and smell > Flashcards

Taste and smell Flashcards

1
Q

What is the technical term for taste?

A

Gustation.

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2
Q

What is the technical term for smell?

A

Olfaction.

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3
Q

What are taste and smell often referred to as, and why?

A

The chemical senses, because they’re mediated by chemoreceptors - receptors stimulated by chemical substances.

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4
Q

How do we taste?

A

Chemicals dissolve in our mouth (they must be water soluble) and stimulate the taste buds in the oral cavity (tongue, soft palate, cheek, etc.)

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5
Q

How do we smell?

A

Volatile (gaseous) chemicals are inhaled into the nasal passages (or enter via the mouth), where olfactory receptors line the membranes.

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6
Q

How are taste and smell closely linked?

A

They are both involved in activities such as food seeking and sampling (flavour includes both).

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7
Q

What information does smell convey?

A

Important non-nutritive information such as the presence of prey, predators and, in some species, mates (pheremones regulate sexual activity).

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8
Q

What is taste useful for?

A

The regulation of nutrients and enabling organisms to ‘test’ substances prior to ingestion (important for identifying nutritious and harmful substances).

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9
Q

What are the four primary taste qualities/sensations (Henning, 1916)?

A

Salty, sour, sweet and bitter.

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10
Q

What general rules can be used for the relationship between a substance’s taste and chemical composition?

A
  • salty = organic salts e.g. NaCl
  • sour = acidic substances e.g. vinegar
  • bitter = alkaloids, often poisonous, e.g. quinine, strychnine and cocaine
  • sweet = carbs and amino acids, e.g. glucose
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11
Q

How are primary taste sensations evolutionary mechanisms?

A

Nutritious substances taste sweet and poisonous ones bitter, so the ability to distinguish between them has survival value.

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12
Q

Why is specifying the adequate stimulus for evoking a primary taste sensation difficult in practice?

A

Because taste quality depends on factors such as substance concentration, e.g. lithium chloride changes from sweet to sour as concentration increases (Dzendolet and Meiselman, 1967).

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13
Q

About how many taste buds does the average human have?

A

10,000.

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14
Q

Where are taste buds found?

A

They’re found in three types of papillae on the tongue, each containing 1-hundreds of taste buds (Bradley, 1979).

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15
Q

What is the lifespan of a tastebud?

A

About ten days (Beidler and Smallman, 1965).

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16
Q

What are chemicals dissolved in saliva in direct contact with?

A

Microvilli of receptor cells on the edges of taste buds.

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17
Q

What do taste stimuli interact with on the microvilli?

A

Receptor sites and ion channels.

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18
Q

How is chemical stimulation converted into neural responses?

A

Through several types of transduction mechanisms (see Smith, 1997).

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19
Q

What are the three sets of afferent nerve fibres that carry taste information derived from taste buds?

A
  • chorda tympani (front part of the tongue)
  • glossopharyngeal (back part of the tongue)
  • vagus - throat, pharynx and larynx.
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20
Q

Where do afferent taste fibres project to?

A

To nuclei in the brainstem, then via the thalamus to the primary taste area in the parietal lobe (near somatosensory cortex).

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21
Q

What did Pritchard (1991) find?

A

Brain damage to the primary taste area impairs taste.

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22
Q

Aside from the primary taste area, where else do afferent taste fibres project?

A

To the orbitofrontal cortex, where they are involved in the behavioural significance/reward value of food and perhaps the degree of ‘pleasantness’ of sensory stimuli in general (e.g. Francis et al., 1999).

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23
Q

What did Arvidson and Friberg (1980) show about taste receptor cells?

A

Most of them respond to some extent, although with different sensitivity, to the four primary tastes.

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24
Q

What do taste responsive cells in the thalamus respond to?

A

All kinds of tastes (Doetsch et al., 1969).

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25
Q

Given that both taste receptors and responsive cells respond to all tastes, how dies the brain differentiate between different substances?

A

Cross-fibre theory (Pfaffman, 1955; Erickson, 1968, 1984).

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26
Q

What is the main premise of cross-fibre theory?

A

That although most neurons respond to several taste stimuli, each is tuned to a particular substance. The pattern of firing across a group of neurons is thus different for each stimulus - this activity pattern encodes information about taste quality or identity.

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27
Q

What support is there for cross-fibre theory?

A

Electrophysiological recordings from individual taste sensitive cells in hamsters (Frank, 1973), rats (Scott and Chang, 1984) and primates (Pfaffman et al., 1984).

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28
Q

What do detection thresholds for taste (minimum substance concentration) depend on?

A

Substance tested, temperature, mouth region, viscosity, and presence of other substances.

29
Q

Between what temperatures is taste sensitivity greatest?

A

22-32 degrees C.

30
Q

How does the sensitivity of the tongue to specific substances vary?

A

Front is more sensitive to sweet and bitter, the back sides to sour and the front sides to salt.

31
Q

What taste is the soft palate sensitive to?

A

Bitter.

32
Q

What did Borg et al. (1967) do?

A

Made electrophysiological (AP) recordings from taste fibres innervating the front of the tongue (chorda tympani) during (inner ear) surgery. Had patients make magnitude (intensity) estimates of taste sensations applied in various concentrations to the tongue.

33
Q

What did Borg et al. (1967) find?

A

Very close correspondence between magnitude estimation and neural response - both increased with substance concentration - neural code for taste intensity seems to be the overall activity evoked by a stimulus.

34
Q

What taste intensity differences can humans discriminate for sucrose?

A

Minimum molar concentrations of 15-25% (McBurney, 1978).

35
Q

What is the difference between nontasters, tasters and supertasters?

A

PTC and PROP taste quite bitter to tasters (50%), tasteless to nontasters (25%) and extremely bitter to supertasters (25%).

36
Q

How do nontasters and tasters differ genetically?

A

Tasters have the dominant allele for a single pair of genes (Bartoshuk, 1988).

37
Q

How do supertasters and nontasters differ?

A

Supertasters have double the amount of papillae on their tongues.

38
Q

What common substances do nontasters, tasters and supertasters differ in sensitivity to?

A

Caffeine (bitterness, Hall et al., 1975), KCl and sodium benzoate (Bartoshuk et al., 1988), and supertasters generally find taste stimuli more bitter and hot stimuli hotter (Bartoshuk, 1993).

39
Q

How many distinct odours are there?

A

Around ten thousand.

40
Q

What did Henning (1916) propose?

A

6 primary odour sensations - fragrant, putrid, ethereal, burned, resinous and spicy.

41
Q

What is the problem with Henning’s theory?

A

It is impossible to reliably classify odours using just six categories - there is no agreed set of primary odour qualities.

42
Q

Outline the general relationship between a substance’s odour and its chemical properties.

A

Typical chemical stimuli are organic volatile substances, usually composed of complex mixtures of chemical compounds.

43
Q

What are the common sources of odour (i.e. organic volatile substances)?

A

Vegetation, decaying matter, and the scent-producing glands of animals.

44
Q

What is the survival value of identifying different odours?

A

Identification and location of foods, toxic substances, predators and potential mates.

45
Q

What happens when molecules enter the nasal cavity?

A

The air is warmed and humidified by baffles.

46
Q

How many olfactory receptors do humans have?

A

About ten million.

47
Q

Where are olfactory receptors located?

A

On the olfactory epithelium.

48
Q

How often are olfactory receptor cells replaced?

A

Every 4-8 weeks (Constanzo and Graziadei, 1987).

49
Q

What are odourants picked up by?

A

Specialised odourant binding proteins in mucus.

50
Q

After being picked up, what happens to odourants?

A

They are transported to receptor sites on cilia at the end of each olfactory cell.

51
Q

What do odourant molecules interact with?

A

Specific sites on membranes of the cilia.

52
Q

What hypothesis is supported for the creation of smell APs?

A

The lock and key hypothesis (Amoore, 1970).

53
Q

Outline the lock and key hypothesis for olfaction.

A

Specific proteins in the cilia membrane have a unique 3-D structure to which particular odourant molecules may bind, depending on their size. This initiates a number of biochemical processes that result in APs in the cell axon.

54
Q

In the olfactory lock and key hypothesis, what are the locks and keys, respectively?

A

The locks are the proteins in the cilia membrane, the keys are the odourant molecules.

55
Q

Once olfactory perception has reached the olfactory receptor cells and become an AP, what happens?

A

The receptor cells send axons through holes in the critiform plate at the top of the nasal cavity, to form the olfactory nerve, leading to the olfactory bulb of the brain.

56
Q

Where do axons from the olfactory bulb project to?

A

Several regions of the brain, including the olfactory cortex, thalamus, and lower brain centres of the limbic system (involved in emotion).

57
Q

So what pathway does smell perception follow?

A

Nasal cavity (baffles), picked up by odourant binding proteins in mucus, transported to cilia membrane receptor sites, where they bind, resulting in an AP, goes to olfactory receptors on olfactory epithelium, then axon through critiform plate to olfactory nerve, then olfactory bulb. Then sent to olfactory cortex, thalamus and limbic system.

58
Q

What ability is impaired by olfactory cortex damage?

A

The ability to identify and/or detect odours (Richardson and Zucco, 1989).

59
Q

Is there an orderly map of different odours in the brain?

A

Apparently not, neither in the olfactory cortex or other regions.

60
Q

How is odour encoded?

A

Olfactory nerve fibres respond to a wide variety of odours, but with different sensitivity (Kauer, 1991), so each fibre alone is ambiguous.

61
Q

How does the brain differentiate between different odours?

A

The cross-fibre patterns of activity like those proposed for taste (Kauer, 1987, 1991).

62
Q

How is information about odour quality/identity encoded?

A

Cross-fibre patterns of activity in an ensemble of neurons.

63
Q

What evidence is there to support the idea of cross-fibre patterns of activity in olfaction?

A

Skarda and Freeman (1987) - the patterns of neural activation across the olfactory bulb.

64
Q

What do detection thresholds (minimum substance concentrations) for smells depend on?

A

Substance tested, odourant purity, the way it’s delivered to the olfactory epithelium, and time (Stevens et al., 1988).

65
Q

Give an example of how the human olfactory system is remarkably sensitive.

A

Subjects can detect mercaptan, a foul-smelling substance added to natural gas) at a concentration of 1 part per 50 billion parts of air (Geldard, 1972).

66
Q

What factors does odour sensitivity depend on?

A

Gender and age - females are generally more sensitive (Koegla and Koster, 1974) and the elderly are less sensitive than young adults (Cain and Gent, 1991).

67
Q

What is anosmia?

A

Odour blindness - Amoore et al. have reported 76 different anosmias, some of which are common (1 in 3 cannot smell 1,8 cineole) and some of which are rate (1/1000 for n-butyl mercapton).

68
Q

What are the different anosmias consistent with the idea of?

A

A very large number of olfactory receptor types.

69
Q

Does smell interact with taste?

A

Yes - without smell, the ability to identify foods by taste alone is poor (Mozel et al., 1960). Smell also greatly affects food flavour (Murphy and Cain, 1980).

C82COG - Perception (9 decks)

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