Adequate stimuli are the kinds of stimuli for which a sense's threshold is lowest. Each sense has a different adequate stimulus. For example, the photoreceptors in the retina of our eye are each most sensitive to a particular piece of the visual field. The hair cells in our ear respond to sound waves and each has a particular frequency of sound waves to which they respond best.
Author: Caroline Benjamins (translated by Anneke Terneusen)