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J Exp Psychol Learn Mem Cogn. 2008 Jan;34(1):237-42. doi: 10.1037/0278-7393.34.1.237.

The intermixed-blocked effect in human perceptual learning is not the consequence of trial spacing.

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  • 1School of Psychology, University of New South Wales, Sydney, New South Wales 2052, Australia.


A robust finding in humans and animals is that intermixed exposure to 2 similar stimuli (AX/BX) results in better discriminability of those stimuli on test than does exposure to 2 equally similar stimuli in 2 separate blocks (CX_DX)--the intermixed-blocked effect. This intermixed-blocked effect may be an example of the superiority of spaced over massed practice; in the intermixed, but not the blocked exposure regime, each presentation of a given stimulus (e.g., AX) is separated from the next by the presentation of its partner (BX). Two experiments with human participants replicated the intermixed-blocked effect and showed that the effect was not due to the spacing of exposure trials. A mechanism for the intermixed-blocked effect is proposed, which combines theories from associative learning and memory.

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