. ▁They ▁noted ▁that ▁in ▁humans , ▁cogn itive ▁bi ases ▁vary ▁according ▁to ▁emot ional ▁state ▁and ▁suggested ▁this ▁as ▁a ▁possible ▁starting ▁point ▁to ▁examine ▁animal ▁em otion . ▁They ▁propose ▁that ▁research ers ▁may ▁be ▁able ▁to ▁use ▁controlled ▁stim uli ▁which ▁have ▁a ▁particular ▁meaning ▁to ▁trained ▁animals ▁to ▁indu ce ▁particular ▁emot ions ▁in ▁these ▁animals ▁and ▁assess ▁which ▁types ▁of ▁basic ▁emot ions ▁animals ▁can ▁experience . ▁ ▁C ogn itive ▁bias ▁test ▁ ▁A ▁cogn itive ▁bias ▁is ▁a ▁pattern ▁of ▁deviation ▁in ▁judgment , ▁where by ▁infer ences ▁about ▁other ▁animals ▁and ▁situations ▁may ▁be ▁drawn ▁in ▁an ▁il log ical ▁fashion . ▁Ind ividual s ▁create ▁their ▁own ▁" subject ive ▁social ▁reality " ▁from ▁their ▁per ception ▁of ▁the ▁input . ▁It ▁refers ▁to ▁the ▁question ▁" Is ▁the ▁glass ▁half ▁empty ▁or ▁half ▁full ? ", ▁used ▁as ▁an ▁indicator ▁of ▁optim ism ▁or ▁p ess im ism . ▁To ▁test ▁this ▁in ▁animals , ▁an ▁individual ▁is ▁trained ▁to ▁anticip ate ▁that ▁stim ulus ▁A , ▁e . g . ▁a ▁ 2 0   Hz ▁tone , ▁preced es ▁a ▁positive ▁event , ▁e . g . ▁highly ▁desired ▁food ▁is ▁delivered ▁when ▁a ▁lever ▁is ▁pressed ▁by ▁the ▁animal . ▁ ▁The ▁same ▁individual ▁is ▁trained ▁to ▁anticip ate ▁that ▁stim ulus ▁B , ▁e . g . ▁a ▁ 1 0   Hz ▁tone , ▁preced es ▁a ▁negative ▁event , ▁e . g . ▁bland ▁food ▁is