▁the ▁expected ▁accuracy . ▁In ▁meas uring ▁sens ory ▁threshold , ▁noise ▁must ▁be ▁account ed ▁for . ▁Sign al ▁noise ▁is ▁defined ▁as ▁the ▁presence ▁of ▁extra , ▁unw anted ▁energy ▁in ▁the ▁observ ational ▁system ▁which ▁obsc ures ▁the ▁information ▁of ▁interest . ▁As ▁the ▁measurements ▁come ▁closer ▁to ▁the ▁absolute ▁threshold , ▁the ▁vari ability ▁of ▁the ▁noise ▁increases ▁causing ▁the ▁threshold ▁to ▁be ▁obsc ured . ▁D ifferent ▁types ▁of ▁internal ▁and ▁external ▁noise ▁include ▁excess ▁stim uli , ▁nerv ous ▁system ▁over - ▁or ▁under - st im ulation , ▁and ▁conditions ▁that ▁f als ely ▁stim ulate ▁n erves ▁in ▁the ▁absence ▁of ▁external ▁stim uli . ▁ ▁A ▁universal ▁absolute ▁threshold ▁is ▁difficult ▁to ▁define ▁a ▁standard ▁because ▁of ▁the ▁vari ability ▁of ▁the ▁measurements . ▁While ▁sens ation ▁occurs ▁at ▁the ▁physical ▁n erves , ▁there ▁can ▁be ▁reasons ▁why ▁it ▁is ▁not ▁consistent . ▁Age ▁or ▁n erve ▁damage ▁can ▁affect ▁sens ation . ▁Similarly , ▁psych ological ▁factors ▁can ▁affect ▁per ception ▁of ▁physical ▁sens ation . ▁M ental ▁state , ▁memory , ▁mental ▁ill ness , ▁fat igue , ▁and ▁other ▁factors ▁can ▁alter ▁per ception . ▁ ▁A viation ▁use . ▁ ▁When ▁related ▁to ▁motion ▁in ▁any ▁of ▁the ▁possible ▁six ▁degrees ▁of ▁freedom ▁( 6 - Do F ), ▁the ▁fact ▁that ▁sens ory ▁th resh olds ▁exist ▁is ▁why ▁it ▁is ▁essential ▁that ▁aircraft ▁have ▁blind - f lying ▁instruments . ▁ ▁S ust ained ▁flight ▁in ▁cloud