▁photos ens itive ▁ret inal ▁gang l ion ▁cells ▁( ip R GC s ), ▁also ▁called ▁photos ens itive ▁ret inal ▁gang l ion ▁cells ▁( p R GC ), ▁or ▁mel an ops in - cont aining ▁ret inal ▁gang l ion ▁cells ▁( m R GC s ), ▁are ▁a ▁type ▁of ▁neur on ▁in ▁the ▁ret ina ▁of ▁the ▁m amm al ian ▁eye . ▁The ▁presence ▁of ▁ip R GC s ▁were ▁first ▁noted ▁in ▁ 1 9 2 3 ▁when ▁rod less , ▁con eless ▁m ice ▁still ▁respond ed ▁to ▁a ▁light ▁stim ulus ▁through ▁pup il ▁con str iction , ▁suggesting ▁that ▁ro ds ▁and ▁con es ▁are ▁not ▁the ▁only ▁light - sens itive ▁neur ons ▁in ▁the ▁ret ina . ▁It ▁wasn ' t ▁until ▁the ▁ 1 9 8 0 s ▁that ▁adv anc ements ▁in ▁research ▁on ▁these ▁cells ▁began . ▁Rec ent ▁research ▁has ▁shown ▁that ▁these ▁ret inal ▁gang l ion ▁cells , ▁unlike ▁other ▁ret inal ▁gang l ion ▁cells , ▁are ▁intr ins ically ▁photos ens itive ▁due ▁to ▁the ▁presence ▁of ▁mel an ops in , ▁a ▁light - sens itive ▁protein . ▁Therefore ▁they ▁const itute ▁a ▁third ▁class ▁of ▁phot ore cept ors , ▁in ▁addition ▁to ▁rod ▁and ▁cone ▁cells . ▁ ▁Over view ▁ ▁Comp ared ▁to ▁the ▁ro ds ▁and ▁con es , ▁the ▁ip R GC s ▁respond ▁more ▁sl ugg ish ly ▁and ▁signal ▁the ▁presence ▁of ▁light ▁over