Previous research has shown that loudness perception is modified in adult listeners following 2 weeks of continuous sensory deprivation or stimulation [Formby, C. et al. (2003). J. Acoust. Soc. Am. 114, 55-58]. However, it is not known if the auditory system undergoes physiological changes or if the listeners simply recalibrate their behavioral criteria such that they become more, or less, conservative following sensory deprivation and stimulation, respectively. The results of this study, comparing threshold of the middle ear acoustic reflex in the two ears of adult listeners after use of a unilateral earplug, are consistent with adaptive plasticity. Acoustic reflexes were measured at a lower sound pressure level in the ear that had been plugged for 7 days. Thus, the effect is consistent with a central gain mechanism mediated by a process within the brainstem.