Eyeblink cross-habituation between tactile and acoustic systems in humans

To study cross-habituation of the eyeblink reflex in humans, subjects were repetitively exposed to a blink-eliciting stimulus in one modality and then shifted to a second stimulus in a different modality. One group of subjects began with a tactile stimulus and switched after 40 trials to an acoustic stimulus. The other began with the acoustic and switched to the tactile. No evidence of cross-habituation was found in the shift from tactile to acoustic, suggesting that habituation takes place in the sensory limb of the reflex. Cross-habituation was found, however, in the shift from acoustic to tactile stimuli. This result was expected because the act of blinking to the acoustic stimulus activates trigeminal afferents from the eyelid; as a result, the tactile stimulation from the blink to the acoustic stimuli had already begun habituation in the tactile pathway before the first external tactile stimulus was received.