ion ▁cell ▁phot ore ceptor ▁ ▁cont ributes ▁to ▁conscious ▁sight ▁as ▁well ▁as ▁to ▁non - image - form ing ▁functions ▁like ▁circ ad ian ▁rh yth ms , ▁behaviour ▁and ▁pup ill ary ▁re actions . ▁Since ▁these ▁cells ▁respond ▁mostly ▁to ▁blue ▁light , ▁it ▁has ▁been ▁suggested ▁that ▁they ▁have ▁a ▁role ▁in ▁mes op ic ▁vision ▁and ▁that ▁the ▁old ▁theory ▁of ▁a ▁purely ▁du plex ▁ret ina ▁with ▁rod ▁( dark ) ▁and ▁cone ▁( light ) ▁light ▁vision ▁was ▁simpl istic . ▁Za idi ▁and ▁colle agues ' ▁work ▁with ▁rod less , ▁con eless ▁human ▁subjects ▁hence ▁has ▁also ▁opened ▁the ▁door ▁into ▁image - form ing ▁( visual ) ▁roles ▁for ▁the ▁gang l ion ▁cell ▁phot ore ceptor . ▁ ▁The ▁discovery ▁that ▁there ▁are ▁parallel ▁path ways ▁for ▁vision ▁was ▁made : ▁one ▁classic ▁rod - ▁and ▁cone - based ▁ar ising ▁from ▁the ▁outer ▁ret ina , ▁the ▁other ▁a ▁rud iment ary ▁visual ▁bright ness ▁det ector ▁ar ising ▁from ▁the ▁inner ▁ret ina . ▁The ▁latter ▁seems ▁to ▁be ▁activ ated ▁by ▁light ▁before ▁the ▁former . ▁Classic ▁phot ore cept ors ▁also ▁feed ▁into ▁the ▁novel ▁phot ore ceptor ▁system , ▁and ▁colour ▁const ancy ▁may ▁be ▁an ▁important ▁role ▁as ▁suggested ▁by ▁F oster . ▁ ▁It ▁has ▁been ▁suggested ▁by ▁the ▁authors ▁of ▁the ▁rod less , ▁con eless ▁human ▁model ▁that ▁the ▁re ceptor ▁could ▁be ▁instrument al ▁in ▁understanding ▁many ▁dise ases , ▁including