endogenous

Meaning: Endogenous describes a substance, signal, or cause that originates inside the organism. Its opposite is exogenous, which originates outside it. Three uses matter in psychology. Endogenous opioids (endorphins) are made by the body and reduce pain perception and contribute to reward, unlike exogenous opioids such as morphine. Circadian rhythms are generated by an internal clock (the suprachiasmatic nucleus of the hypothalamus) and continue at roughly 24 hours without outside cues. In attention, endogenous control is voluntary, goal-driven, top-down orienting.

On the MCAT: Expect a passage on attention cueing. A central arrow that participants choose to follow is an endogenous cue, and it works relatively slowly. A sudden flash or loud sound in the periphery is an exogenous cue, which captures attention automatically and quickly (bottom-up). Another setup describes people kept in constant light or a cave. Their sleep-wake cycle persists, which shows an endogenous rhythm. External zeitgebers such as light then entrain (reset) it to the 24-hour day.

Common confusion: Endogenous does not mean innate or genetic. It means generated internally, and the thing can still be changed by experience. A zeitgeber is exogenous even though it acts on an endogenous clock. Do not link endogenous with bottom-up processing; internally driven attention is top-down.