although habituation has been regarded as a learning process by some as early as 1887, its learning status remained controversial up until the 1920s - 1930s. while conceding that reflexes may "relax" or otherwise decrease with repeated stimulation, the "invariance doctrine" stipulated that reflexes should not remain constant and that variable reflexes were a pathological manifestation. indeed, air pilots who showed habituation of post-rotational nystagmus reflex were sometimes ejected from or not recruited for service for world war i: on the grounds that a variable reflex response indicated either a defective vestibular apparatus or a lack of vigilance. eventually, however, more research from the medical and scientific communities concluded that stimulus-dependent variability reflexes is clinically normal. the opposition to the considering habituation a form of learning was also based on the assumption that learning processes must produce novel behavioral responses and must occur in the cerebral cortex. non-associative forms of learning such as habituation (and sensitization) do not produce novel (conditioned) responses but rather diminish a pre-existing (innate) responses and often are shown to depend on peripheral (non-cerebral) synaptic changes in the sensory-motor pathway. most modern learning theorists, however, consider "any" behavioral change that occurs as a result of experience to be learning, so long as it cannot be accounted for by motor fatigue, sensory adaption, developmental changes or damage.