Understanding Motivation in Eating and Drinking Behavior

1. CHAPTER 5 MOTIVATION

2. Chapter plan • INTRODUCTION • HUNGER AND THE CONTROL OF FOOD INTAKE • Peripheral factors • Control signals • How the brain controls eating • Taste + smell = flavour • Obesity – possible factors • THIRST AND THE CONTROL OF DRINKING • Cellular dehydration • Extracellular thirst stimuli • Control of normal drinking • SEXUAL BEHAVIOUR • Sociobiology and sexual behaviour • How the brain controls sexual behaviour • SUMMARY

3. What motivates us to work for food when we are hungry, or water when we are thirsty? • How do these motivational control systems ensure that we eat approximately the right amount of food to maintain our body weight, or drink enough to quench our thirst? • And how do we explain overeating and obesity?

4. Reward something for which an animal will work. • Punishment something an animal will work to escape or avoid. • Operant response (or instrumental response) an arbitrary response or behaviour performed in order to obtain a reward or escape from or avoid a punishment.

5. Motivated behaviour is when an animal (either human or non-human) performs an operant response to obtain a reward or avoid a punishment. • Motivation has close links with emotions, since these can be regarded as states elicited (in at least some species) by rewards and punishments.

6. The pleasant smell and taste of food give us immediate reward, a separate process from satiety – or the feeling of fullness. The brain brings the two processes together to control the amount of food we eat. (Fig. 5.1)

7. Sham feeding preparation. When food is drained from a rat’s stomach it will often continue to eat for over an hour. (Fig. 5.2)

8. Hunger and the control of food intake Peripheral factors • Sham feeding preparation • It is the taste and smell of food that provide the immediate reward for food-motivated behaviour. • A second important aspect of sham feeding is that satiety(reduction of appetite) does not occur.

9. Control signals Appetite controlled by: • Sensory-specific satiety • Gastric distension • Duodenal chemosensors • Glucostatic hypothesis • Body fat regulation and the role of leptin • Conditioned appetite and satiety

10. The fall in glucose concentration in the plasma typically seen in rats before a meal is initiated. (Fig. 5.3)

11. How the brain controls eating • Since the early twentieth century, we have known that damage to the base of the brain can influence food intake and body weight. • One critical region is the ventromedial hypothalamus. • Bilateral lesions of this area (i.e. two-sided, damaging both the left and right) in animals leads to hyperphagia(overeating) and obesity (see Rolls, 1999).

12. By contrast, Anand and Brobeck (1951) discovered that bilateral lesions, (that is, damage) of the lateral hypothalamus can lead to a reduction in feeding and body weight. • Evidence of this type led, in the 1950s and ’60s, to the view that food intake is controlled by two interacting ‘centres’ – a feeding centre in the lateral hypothalamus and a satiety centre in the ventromedial hypothalamus. • But problems arose with this dual centre hypothesis.

13. Effects of lesions and stimulation of the lateral and ventromedial hypothalamus on eating. A coronal (transverse or vertical) section through the rat brain is shown. (Fig. 5.4)

14. The ventromedial nucleus of the hypothalamus is now thought of as a region that can influence the secretion of insulin and, indirectly, affect body weight, but not as a satiety centre per se. • On the other hand, the hypothesis that damage to the lateral hypothalamus produces a lasting decrease in food intake and body weight has been corroborated.

15. A matter of taste • During the first few stages of taste processing (from the rostralpart of the nucleus of the solitary tract, through the thalamus, to the primary taste cortex), representations of sweet, salty, sour, bitter and protein tastes are developed (protein represents a fifth taste, also referred to as umami). • In contrast, in the secondary cortical taste area (the orbitofrontal cortex), the responses of taste neurons to a food with which the monkey is fed to satiety decrease to zero (Rolls, Sienkiewicz & Yaxley, 1989, 1990)

16. Schematic diagram showing some of the gustatory, olfactory and visual pathways involved in processing sensory stimuli involved in the control of food intake. Areas of processing where hunger affects the neuronal responses to the sight, smell or taste of food are indicated by the gating or modulatory function of hunger. (Fig. 5.5)

17. Taste + smell = flavour • Flavour refers to a combination of taste and smell. • The connections of the taste and olfactory (smell) pathways in primates suggest that the necessary convergence may also occur in the orbitofrontal cortex.

18. The effect of feeding to satiety with glucose solution on the responses of a neuron in the secondary taste cortex to the taste of glucose and of blackcurrant juice (BJ). The spontaneous firing rate is also indicated (SA). Below the neuronal response data for each experiment, the behavioural measure of the acceptance or rejection of the solution on a scale from +2 to -2 (see text) is shown. The solution used to feed to satiety was 20 per cent glucose. The monkey was fed 50 ml of the solution at each stage of the experiment as indicated along the abscissa (x-axis) until he was satiated, as shown by whether he accepted or rejected the solution. Pre – the firing rate of the neuron before the satiety experiment started; OFC – orbitofrontal cortex; CC167 and CC170 – two different neurons. (Fig. 5.6)

19. The responses of a bimodal neuron recorded in the caudolateral orbitofrontal cortex. The neuron responded best to the tastes of NaCl and monosodium glutamate and to the odours of onion and salmon. G – 1M glucose; N – 0.1M NaCl; H – 0.01M HCl; Q – 0.001M Quinine HCl; M – 0.1M monosodium glutamate; Bj – 20 per cent blackcurrant juice; Tom – tomato juice; B – banana odour; Cl – clove oil odour; On – onion odour; Or – orange odour; S – salmon odour; C – control no-odour presentation. The mean responses + se (standard error of the mean) are shown. (Fig. 5.7)

20. Functions of the brain and nervous system involved in eating The orbitofrontal cortex • The orbitofrontal cortex is important not only in representing whether a taste is rewarding, and so whether eating should occur, but also in learning about which visual and olfactory stimuli are actually foods.

21. There are neurons in the orbitofrontal cortex that respond to the texture of chocolate. Add its distinctive flavour (taste + smell) and you have an appealing combination. (Fig. 5.8)

22. Amygdala • Many of the amygdala’s connections are similar to those of the orbitofrontal cortex, and indeed it has many connections to the orbitofrontal cortex itself. • In primates, the orbitofrontal cortex may be performing some of the functions of the amygdala but doing it better, or in a more ‘advanced’ way, since as a cortical region it is better adapted for learning.

23. The striatum and other parts ofthe basal ganglia • This route is important as a behavioural output/feeding system, because disruption of striatal function results in aphagia (lack of eating) and adipsia(lack of drinking) in the context of a general akinesia(lack of voluntary movement). • Neurons in the ventral striatum also respond to visual stimuli of emotional or motivational significance (i.e. associated with rewards or punishments), and to types of reward other than food, including drugs such as amphetamine.

24. Obesity – possible factors • With all of these brain function promoting food regulation, why, then, is there such a high incidence of obesity in the world today? • Many different factors can contribute to obesity, and there is only rarely a single cause.

25. Obesity has many possible contributing factors and is rarely the result of a single cause. (Fig. 5.9)

26. Possible factors: • Hyperinsulinemia • External factors • Variety of modern foods • Less exercise • Fixed meal times • Eating later in the day • High stress levels

27. Thirst and the control of drinking • But what of water intake, and drinking? • The human body can survive without food for very much longer than it can survive without water. • How does our physiological make-up help direct this vital function?

28. When we are deprived of water, both the cellular and extracellular fluid compartments are significantly depleted. • The depletion of the intracellular compartment is shown in figure 5.11 as cellular dehydration, and the depletion of the extracellular compartment is known as hypovolaemia (meaning that the volume of the extracellular compartment has decreased).

29. Body water is contained within two main compartments, one inside the cells (intracellular) and the other outside (extracellular). • Intracellular water accounts for approximately 40 per cent of total body weight, and extracellular water (blood plasma and interstitial fluid) for about 20 per cent.

30. Body water compartments. Arrows represent fluid movement. (Fig. 5.10)

31. A summary of the factors that may lead to drinking after water deprivation. (Fig. 5.11)

32. Experiments show that, in many species, it is the depletion of the cellular, rather than the extracellular, thirst system that accounts for the greater part of the drinking, typically around 75 per cent. • It is important to note that we continue to drink fluids every day, even when our bodies aren’t deprived of water. • The changes in this type of thirst signal are smaller, partly because drinking has become conditioned to events such as eating foods that deplete body fluids, and also because humans have a wide range of palatable drinks, which stimulate the desire to drink even when we are not thirsty (e.g. students in the college bar!).

33. Sexual behaviour • Just as we need to eat to keep ourselves alive, working to obtain a reward (life!), so we need to have sex and reproduce in order to keep our genes alive. • How can a socio-biological approach (that is, an approach which seeks to reconcile our biological heritage as a species with our highly social organization) help us to understand the different mating and child-rearing practices of particular animal species? • How does the human brain control sexual behaviour?

34. Sociobiology and sexual behaviour • Monogamous: having only one sexual partner. • Polygamous: having many sexual partners (sperm warfare?). • But what about humans???

35. Humans are intermediate in testis size (and penis size) – bigger than might be expected for a monogamous species. • Although humans usually do pair, and are apparently monogamous, we also live in groups, or colonies. • But for most primates (and indeed most mammals) it is the female who makes the main parental investment – not only by producing the egg and carrying the foetus, but also by feeding the baby until it becomes independent; also, because of its large size, the human brain is not fully developed at birth.

36. The gaudy tail of the male peacock is one indicator of attractiveness in birds. Given that the tail handicaps movement, any male that can survive with such a large tail must be very healthy or fit, which may explain why peahens have evolved to choose males with large tails. (Fig. 5.12)

37. Sexual selection: organism may choose an ‘attractive’ male by responding to indicators of health, strength and fitness. • Are humans like swallows?

38. How the brain controls sexual behaviour A midline view of the rat brain showing some of the brain regions involved in the control of sexual behaviour. (Fig. 5.13)

39. We now know that the pleasantness of touch is represented in the human orbitofrontal cortex. This finding contributes to our understanding of the motivational rewards involved in sexuality. (Fig. 5.14)

40. Much of the research on the sociobiological background of human sexual behaviour is quite new and speculative, and many of the hypotheses have still to be fully tested and accepted. • But this research does have interesting implications for understanding some of the factors that may influence human behaviour (see Baker, 1996; Baker & Bellis, 1995; Buss, 1999; Ridley, 1993).

41. Summary • Motivational states are states that lead animals (including humans) to work for goals. Motivation also has close links with emotions, since these can be regarded as mental states (present in at least some species) that are elicited by rewards and punishments. • Goals can be defined as rewards that animals will work to obtain, while punishments are events or situations that animals will escape from or avoid. • One example of a goal is a sweet taste, which is rewarding when the motivational state of hunger is present. Hunger is signalled by decreases of glucose concentration in the bloodstream.

42. The reward for eating is provided by the taste, smell and sight of food. • Satiety is produced by (a) the sight, taste, smell and texture of food, (b) gastric distension, (c) the activation by food of duodenal chemosensors, (d) rises in glucose concentration in the blood plasma, and (e) high levels of leptin. Satiety signals modulate the reward value of the taste, smell, and sight of food to control appetite and eating.

43. The orbitofrontal cortex contains the secondary taste cortex and the secondary olfactory cortex. In this brain region, neurons respond to the sight, taste and smell of food, but only if hunger is present. The orbitofrontal cortex is the first stage of processing at which the reward or hedonic aspects of food is represented. It is the crucial site in the brain for the integration of the sensory inputs activated by food (taste, smell, sight etc) and satiety signals.

44. The lateral hypothalamus has inputs from the orbitofrontal cortex and it also contains neurons that are necessary for the normal control of food intake. Once again, neurons in the lateral hypothalamus respond to the sight, taste and smell of food, but only if hunger is present. These neurons thus reflect the reward value of food, by reflecting the integration between the sensory inputs that maintain eating and satiety signals. • The orbitofrontal cortex, and the amygdala, are involved in learning which environmental stimuli are foods (for example, in learning which visual stimuli taste good).

45. Sexual behaviour has been influenced in evolution by the advantages to genes of coding for behaviours such as parental attachment, which increase the probability of survival of those genes. • As with other motivational systems, such as hunger, genes achieve this by coding for stimuli and events that animals find rewarding. This is achieved by specifying, in parts of the brain such as the amygdala, orbitofrontal cortex, preoptic area and hypothalamus, which sensory inputs and events should be represented as rewards.