Expectancy effects in a psychophysiological experiment

Abstract

The effect of the experimenter’s expectations about experimental outcomes on two physiological variables (EMG and EEG) were investigated in an experiment involving a tendon tap stimulus used to elicit a phasic stretch reflex. Experimenter’s and subject’s perceived locus of control scores were incorporated into the design to test their relevance as a mediating variable. Immediately after the presentation of the tendon tap stimulus, subject’s EEG alpha measures were found to be more biased in the direction of the experimenter’s expectations when experimenters and subjects were similar in their scores on the perceived locus of control scale. Biasing effects on EEG alpha measures in the direction of experimenter’s expectations were found early in the experimental session where subjects or their experimenters scored as internally controlled. The EMG measures increased or decreased in the direction of the experimenter’s expectations when subjects were internally controlled but there was a reversal of the direction of the experimenter’s expectations with subjects who scored as externally controlled so that no significant experimenter expectancy effect was observed.