Regression of lateral asymmetry scores toward the mean

On most, if not all, laterality tasks, some subjects initially display a left sensory field advantage and others a right sensory field advantage. During the course of testing, the mean asymmetry score of those subjects who start with a left sensory field advantage decreases, whereas the mean asymmetry score of those subjects who start with a right sensory field advantage increases (asymmetry scores computed as L-R). Some previous studies (e.g., Ross-Kossak and Turkewitz, 1986) have suggested that these differential, temporal shifts in sensory field advantage reflect systematic changes in subjects' sensory field advantage (and associated hemispheric advantage) during familiarization and individual differences in those changes. Alternatively, the differential, temporal shifts may reflect unsystematic sources of changes because they are not distinguishable from statistical regression toward the mean. Consistent with this hypothesis, "time-reversed control" analyses of laterality data show that the differential trends do not reflect a factor associated with the specific direction of time passage as would be predicted in the case of increasing stimulus familiarization. Regression of asymmetry scores toward the mean, or its equivalent, imperfect reliability of asymmetry scores, may be significantly attributable to short-term fluctuations in subjects' hemispheric arousal asymmetries.