) ▁which ▁have ▁been ▁post ulated ▁to ▁reflect ▁a ▁positive ▁affect ive ▁state ▁( em otion ) ▁analog ous ▁to ▁primitive ▁human ▁joy ; ▁these ▁calls ▁have ▁been ▁term ed ▁" la ugh ter ". ▁ ▁The ▁ 5 0   k Hz ▁US V s ▁in ▁r ats ▁are ▁uniqu ely ▁elev ated ▁by ▁h ed onic ▁stim uli — such ▁as ▁tick ling , ▁reward ing ▁elect rical ▁brain ▁stim ulation , ▁am ph et am ine ▁in jections , ▁m ating , ▁play , ▁and ▁ag gression — and ▁are ▁suppress ed ▁by ▁a vers ive ▁stim uli . ▁ ▁Of ▁all ▁manip ulations ▁that ▁el ic it ▁ 5 0   k Hz ▁ch ir ps ▁in ▁r ats , ▁tick ling ▁by ▁humans ▁el ic its ▁the ▁highest ▁rate ▁of ▁these ▁calls . ▁ ▁Some ▁vocal izations ▁of ▁domestic ▁c ats , ▁such ▁as ▁p urr ing , ▁are ▁well ▁known ▁to ▁be ▁produced ▁in ▁situations ▁of ▁positive ▁val ence , ▁such ▁as ▁mother ▁k itten ▁interactions , ▁contacts ▁with ▁familiar ▁partner , ▁or ▁during ▁tact ile ▁stim ulation ▁with ▁in animate ▁objects ▁as ▁when ▁rolling ▁and ▁rub bing . ▁Therefore , ▁p urr ing ▁can ▁be ▁generally ▁considered ▁as ▁an ▁indicator ▁of ▁" ple asure " ▁in ▁c ats . ▁ ▁Low ▁pitch ed ▁ble ating ▁in ▁sheep ▁has ▁been ▁associated ▁with ▁some ▁positive - val ence ▁situations , ▁as ▁they ▁are ▁produced ▁by ▁males ▁as ▁an ▁est rus ▁female ▁is ▁approaching ▁or ▁by ▁la ct ating