3 , ▁C ly de ▁E . ▁Ke eler ▁observed ▁that ▁the ▁pup ils ▁in ▁the ▁eyes ▁of ▁blind ▁m ice ▁he ▁had ▁accident ally ▁b red ▁still ▁respond ed ▁to ▁light . ▁The ▁ability ▁of ▁the ▁rod less , ▁con eless ▁m ice ▁to ▁retain ▁a ▁pup ill ary ▁light ▁ref lex ▁was ▁suggest ive ▁of ▁an ▁additional ▁phot ore ceptor ▁cell . ▁ ▁In ▁the ▁ 1 9 8 0 s , ▁research ▁in ▁rod - ▁and ▁cone - def ic ient ▁r ats ▁showed ▁reg ulation ▁of ▁dop am ine ▁in ▁the ▁ret ina , ▁a ▁known ▁neur om od ulator ▁for ▁light ▁adaptation ▁and ▁photo ent rain ment . ▁ ▁Research ▁continued ▁in ▁ 1 9 9 1 , ▁when ▁Russell ▁G . ▁F oster ▁and ▁colle agues , ▁including ▁Ig na cio ▁Pro ven cio , ▁showed ▁that ▁ro ds ▁and ▁con es ▁were ▁not ▁necessary ▁for ▁photo ent rain ment , ▁the ▁visual ▁drive ▁of ▁the ▁circ ad ian ▁rh ythm , ▁nor ▁for ▁the ▁reg ulation ▁of ▁mel aton in ▁secret ion ▁from ▁the ▁p ine al ▁g land , ▁via ▁rod - ▁and ▁cone - k no ck out ▁m ice . ▁Later ▁work ▁by ▁Pro ven cio ▁and ▁colle agues ▁showed ▁that ▁this ▁phot ores ponse ▁was ▁medi ated ▁by ▁the ▁phot op ig ment ▁mel an ops in , ▁present ▁in ▁the ▁gang l ion ▁cell ▁layer ▁of ▁the ▁ret ina . ▁ ▁The ▁phot ore cept ors ▁were ▁identified ▁in ▁ 2