We report thermal-convection experiments using helium I where the vertical temperature difference has an externally imposed time dependence. Both positive and negative threshold shifts are observed depending on the strength and rate of modulation, as well as a depression of the initial slope of the heat transfer above threshold, in qualitative agreement with predictions of Ahlers, Hohenberg, and L\"ucke and of Rosenblat and Herbert.
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Niemela et al. (1987) studied this question.
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