▁the ▁long ▁term . ▁They ▁represent ▁a ▁very ▁small ▁subset ▁( ~ 1 %) ▁of ▁the ▁ret inal ▁gang l ion ▁cells . ▁Their ▁functional ▁roles ▁are ▁non - image - form ing ▁and ▁fund ament ally ▁different ▁from ▁those ▁of ▁pattern ▁vision ; ▁they ▁provide ▁a ▁stable ▁representation ▁of ▁ambient ▁light ▁intensity . ▁They ▁have ▁at ▁least ▁three ▁primary ▁functions : ▁ ▁They ▁play ▁a ▁major ▁role ▁in ▁synchron izing ▁circ ad ian ▁rh yth ms ▁to ▁the ▁ 2 4 - hour ▁light / dark ▁cycle , ▁providing ▁primarily ▁length - of - day ▁and ▁length - of - night ▁information . ▁They ▁send ▁light ▁information ▁via ▁the ▁ret in oh yp oth al am ic ▁tract ▁( R HT ) ▁directly ▁to ▁the ▁circ ad ian ▁pac em aker ▁of ▁the ▁brain , ▁the ▁sup rach ias m atic ▁nucle us ▁of ▁the ▁hypoth al am us . ▁The ▁phys i ological ▁properties ▁of ▁these ▁gang l ion ▁cells ▁match ▁known ▁properties ▁of ▁the ▁daily ▁light ▁ent rain ment ▁( s ynchron ization ) ▁mechanism ▁reg ulating ▁circ ad ian ▁rh yth ms . ▁ ▁Ph otos ens itive ▁gang l ion ▁cells ▁inner v ate ▁other ▁brain ▁targets , ▁such ▁as ▁the ▁center ▁of ▁pup ill ary ▁control , ▁the ▁ol iv ary ▁pre te ct al ▁nucle us ▁of ▁the ▁mid bra in . ▁They ▁contribute ▁to ▁the ▁reg ulation ▁of ▁pup il ▁size ▁and ▁other ▁behavior al ▁responses ▁to ▁ambient ▁light ing ▁conditions . ▁ ▁They ▁contribute