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Mechanisms of Motivation
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Motivation and Incentives
Motivation - factors within and outside an organism that cause it to behave a certain way at a certain time
Motivational state or drive - an internal condition, which can change over time, that orients an individual to a specific set of goals (e.g., hunger, thirst, sex, curiosity)
Incentives - goals or reinforcers in the external environment (e.g., good grades, food, a mate)
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Drives as Tissue Needs
Homeostasis - the constancy of internal conditions that the body must actively maintain
Drives may be an upset in homeostasis, inducing behavior to correct the imbalance
Animals do behave in accordance with their tissue needs (e.g., increasing or decreasing caloric intake, drive for salt)
However, homeostasis cannot explain all drives
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Types of Drives
Regulatory drives - helps preserve homeostasis (e.g., hunger, thirst, oxygen)
Nonregulatory drives - serve other purposes (e.g., sex, achievement)
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Drives as States of the Brain
Central state theory of drives - different drives correspond to neural activity in different sets of neurons in the brain
Central drive system - set of neurons in which activity constitutes a drive
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Drives as States of the Brain
Techniques for studying central drive systems include lesions and stimulation
Connecting Socket
Electrode
Brain
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Drives as States of the Brain
The hub of many central drive systems lies in the hypothalamus
Cerebral cortex
Portion of
limbic system
Hypotahlamus
Pituitary
gland
Brainstem
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Hunger Drive
Two areas of the hypothalamus, the lateral and ventromedial areas, play a central role in the hunger drive
Hypothalamus
Hypothalamus
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Lateral Area
Electrical lesions to tract of axons connecting brainstem, hypothalamus and basal ganglia cause a loss of all goal-directed behavior
Stimulation causes drives in response to available incentives
Hypothalamus
Hypothalamus
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Lateral Area
However, chemical lesions to specific cell bodies reduce hunger drive, but do not abolish it - most other drives appear normal
Hypothalamus
Hypothalamus
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Ventromedial Area Lesions alter
digestive and metabolic processes
Food is converted into fat rather than energy molecules, causing animal to eat much more than normal and gain weight
Hypothalamus
Hypothalamus
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Hunger Drive
Other stimuli that act on the brain to increase or decrease hunger include satiety signals from the stomach signals indicating the amount of food
molecules in the blood leptin, a hormone indicating the amount of fat
in the body the appetizer effect
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Research on Weight Regulation and Dieting
No consistent personality trait differences found between obese and non-obese people (e.g., willpower, anxiety)
Dieters and obese are more likely to eat in response to stress than non-dieters
Family environment of little importance in determining body weight - genetics plays a large role
Number of fat-storage cells is a major determinant of body weight
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Research on Weight Regulation and Dieting
Fat cells are determined by genetics and food intake They increase with weight gain, but merely shrink with
weight loss - may stimulate hunger Weight loss causes a decline in basal metabolism
Fat cells
Normal
diet
High-fat
diet
Return to
normal diet
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Effects of Culture and Habits on Body Weight
Settling point - cluster of genetic and environmental factors that cause a person’s weight to settle within a given range
Weight can be affected by factors like diet, exercise, and daily habits (e.g., stairs instead of elevator)
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Sex Drive
Increased production of testosterone and estrogen at puberty is responsible for physical differentiation
Increased secretion of DHEA, primary adrenal sex hormone, is responsible for sexual feelings
0
100
200
300
400
500
600
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Age (in years)Females Males
Final maturation
of ovaries in females
Final maturation
of testes in males
First sexual
attraction in
both sexes
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Male Sex Drive
Testosterone maintains sex drive in adult males castration decreases
drive testosterone injections
or implantation to medial preoptic area restores drive
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Female Sex Drive
Estrous cycle controls drive in nonhuman mammals removal of ovaries
abolishes drive, while hormone injections restore it
Also, lesions to ventromedial area abolish drive, while injection or implantation restores drive
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Female Sex Drive
Female monkeys and apes depend less on hormones for sexual behavior
Human female sex drive may not be consistently affected by hormone cycle at all ovarian hormones play small role adrenal hormones like DHEA and testosterone
play larger role
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Early Effects of Testosterone
Presence of testosterone during critical period will cause rudimentary genitals of fetus to develop into male structures
Testosterone acts in brain to promote development of neural systems for male sex drive and inhibit systems for female drive
Absence causes development of female structures
Stressful events experienced by pregnant rats reduce level of prenatal testosterone
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Human Sexual Orientation
Orientation is an early-emerging, ingrained aspect of the self that probably does not change
No consistent relationship between orientation and childhood experiences (e.g., parenting, abuse, sexual experience)
Controversial findings suggest a possible relationship among prenatal stress, androgens, and the development of brain systems that play a role in sexual attraction
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Reward Pathways in the BrainMedial forebrain bundle runs from the
midbrain through the lateral area of the hypothalamus into the nucleus accumbens in the basal ganglia neurons in this tract secrete dopamine animals will self-stimulate this pathway euphoria-producing drugs affect the level of
dopamine in this tract evolved to promote survival and reproduction