Transcript Ch5slides - Blackwell Publishing

CHAPTER 5
MOTIVATION
Chapter plan
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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
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What motivates us to work for food when we are
hungry, or water when we are thirsty?
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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?
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And how do we explain overeating and obesity?
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Reward something for which an animal will work.
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Punishment something an animal will work to
escape or avoid.
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Operant response (or instrumental response) an
arbitrary response or behaviour performed in order
to obtain a reward or escape from or avoid a
punishment.
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Motivated behaviour is when an animal (either
human or non-human) performs an operant
response to obtain a reward or avoid a punishment.
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Motivation has close links with emotions, since
these can be regarded as states elicited (in at least
some species) by rewards and punishments.
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)
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)
Hunger and the control of food
intake
Peripheral factors
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Sham feeding preparation
i)
It is the taste and smell of food that provide
the immediate reward for food-motivated
behaviour.
ii)
A second important aspect of sham feeding is
that satiety (reduction of appetite) does not
occur.
Control signals
Appetite controlled by:
i)
ii)
iii)
iv)
v)
vi)
Sensory-specific satiety
Gastric distension
Duodenal chemosensors
Glucostatic hypothesis
Body fat regulation and the role of leptin
Conditioned appetite and satiety
The fall in glucose concentration in the plasma typically
seen in rats before a meal is initiated. (Fig. 5.3)
How the brain controls eating
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Since the early twentieth century, we have known
that damage to the base of the brain can influence
food intake and body weight.
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One critical region is the ventromedial
hypothalamus.
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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).
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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.
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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.
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But problems arose with this dual centre hypothesis.
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)
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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.
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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.
A matter of taste
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During the first few stages of taste processing (from
the rostral part 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).
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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)
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)
Taste + smell = flavour
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Flavour refers to a combination of taste and smell.
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The connections of the taste and olfactory (smell)
pathways in primates suggest that the necessary
convergence may also occur in the orbitofrontal cortex.
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)
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)
Functions of the brain and nervous
system involved in eating
The orbitofrontal cortex
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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.
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)
Amygdala
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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.
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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.
The striatum and other parts of
the basal ganglia
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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).
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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.
Obesity – possible factors
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With all of these brain function promoting food
regulation, why, then, is there such a high incidence
of obesity in the world today?
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Many different factors can contribute to obesity, and
there is only rarely a single cause.
Obesity has many possible contributing factors and is
rarely the result of a single cause. (Fig. 5.9)
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Possible factors:
 Hyperinsulinemia
 External factors
 Variety of modern foods
 Less exercise
 Fixed meal times
 Eating later in the day
 High stress levels
Thirst and the control of drinking
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But what of water intake, and drinking?
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The human body can survive without food for very
much longer than it can survive without water.
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How does our physiological make-up help direct this
vital function?
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When we are deprived of water, both the cellular and
extracellular fluid compartments are significantly
depleted.
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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).
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Body water is contained within two main
compartments, one inside the cells (intracellular) and
the other outside (extracellular).
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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.
Body water compartments. Arrows represent
fluid movement. (Fig. 5.10)
A summary of the factors that may lead to drinking
after water deprivation. (Fig. 5.11)
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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.
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It is important to note that we continue to drink fluids
every day, even when our bodies aren’t deprived of
water.
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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!).
Sexual behaviour
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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.
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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 childrearing practices of particular animal species?
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How does the human brain control sexual behaviour?
Sociobiology and sexual behaviour
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Monogamous: having only one sexual partner.
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Polygamous: having many sexual partners (sperm
warfare?).
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But what about humans???
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Humans are intermediate in testis size (and penis
size) – bigger than might be expected for a
monogamous species.
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Although humans usually do pair, and are apparently
monogamous, we also live in groups, or colonies.
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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.
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)
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Sexual selection: organism may choose an ‘attractive’
male by responding to indicators of health, strength
and fitness.
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Are humans like swallows?
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)
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)
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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.
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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).
Summary
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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.
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The reward for eating is provided by the taste, smell
and sight of food.
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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.
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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.
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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.
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The orbitofrontal cortex, and the amygdala, are
involved in learning which environmental stimuli are
foods (for example, in learning which visual stimuli
taste good).
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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.
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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.