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Later experiments located other “pleasure centers” (Olds, 1958). (What the rats actually
experience only they know, and they aren’t telling. Rather than attribute human feelings to rats,
today’s scientists refer to reward centers.) Just how rewarding are these reward centers? Enough
to cause rats to self-stimulate these brain regions more than 1000 times per hour. In other spe-
cies, including dolphins and monkeys, researchers later discovered other limbic system reward
centers, such as the nucleus accumbens in front of the hypothalamus (Hamid et al., 2016).
Animal research has also revealed both a general dopamine-related reward system and
specific centers associated with the pleasures of eating, drinking, and sex. Animals, it seems,
come equipped with built-in systems that reward activities essential to survival. As neuro-
scientist Candice Pert (1986) observed, “If you were designing a robot vehicle to walk into
the future and survive, . . . you’d wire it up so that behavior that ensured the survival of the
self or the species — like sex and eating — would be naturally reinforcing.”

®
AP Science Practice Data Interpretation

Number of Presses

Rats receiving reward center activation Rats not receiving reward center activation
Trial 1 204 202

Trial 2 813 250
Trial 3 857 300
156
Trial 4 900 Copyright © Bedford, Freeman & Worth Publishers.
158
1001
Trial 5 Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.
This module describes research on rats that led to the identification of the hypothalamus as a reward center in the brain.
Consider the data set above.

• Describe the general difference in number of presses • Calculate the mean for each group.
between the groups represented in this table.
• Is the variable “number of presses” qualitative or
• Describe the trends in the data within each group. quantitative? Explain.
Remember, you can always revisit Unit 0 to review information related to psychological research.

Do humans have limbic centers for pleasure? Some evidence indicates we do. When we
meet likable people or read affirming messages from friends, our brain bursts with reward
center activity (Inagaki et al., 2019; Zerubavel et al., 2018). But when one neurosurgeon
implanted electrodes in violent patients’ reward center areas, the patients reported only mild
pleasure. Unlike Olds and Milner’s rats, the patients were not driven to a frenzy (Deutsch,
1972; Hooper & Teresi, 1986). Stimulating the brain’s “hedonic hot spots” (its reward cir-
cuits) produces more desire than pure enjoyment (Kringelbach & Berridge, 2012).
Experiments have also revealed the effects of a dopamine-related reward system in
people. For example, experimentally boosting dopamine levels increases the pleasurable
“chills” response to a favorite piece of music, whereas reducing dopamine levels decreases
hippocampus a neural center
in the limbic system that helps musical-related pleasure (Ferreri et al., 2019). Some researchers believe that many disor-
process explicit (conscious) dered behaviors may stem from malfunctions in the natural brain systems for pleasure and
memories — of facts and well-being. People genetically predisposed to this reward deficiency syndrome may crave
events — for storage. whatever provides that missing pleasure or relieves negative feelings, such as aggression,
rich food, or drugs and alcohol (Blum et al., 1996, 2014; Chester et al., 2016).

68 Unit 1 Biological Bases of Behavior

03_myersAPpsychology4e_28116_ch01_002_163.indd 68 15/12/23 9:23 AM