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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)

3

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

15

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