er ▁filter ▁or ▁complete ▁occ l usion ▁( e . g . ▁with ▁a ▁hand ). ▁ ▁On set ▁DVD ▁typically ▁becomes ▁apparent ▁between ▁ 1 8 ▁months ▁and ▁three ▁years ▁of ▁age , ▁however , ▁the ▁difficulties ▁of ▁ach ieving ▁the ▁prolong ed ▁occ l usion ▁required ▁for ▁accurate ▁detection ▁in ▁the ▁very ▁young , ▁make ▁it ▁possible ▁that ▁on set ▁is ▁generally ▁earlier ▁than ▁these ▁figures ▁suggest . ▁ ▁Mechan ism ▁▁ ▁Dis soci ation ▁refers ▁to ▁the ▁situation ▁where ▁the ▁inner v ation ▁of ▁one ▁eye ▁causes ▁it ▁to ▁move ▁invol unt arily ▁and ▁independently ▁of ▁the ▁other ▁eye . ▁Usually ▁both ▁eyes ▁work ▁together ▁as ▁described ▁by ▁Her ing ' s ▁and ▁Sh err ington ' s ▁laws ▁of ▁inner v ation . ▁A ▁DVD ▁is ▁a ▁slow ▁up ward ▁and ▁sometimes ▁temporal ▁movement ▁of ▁one ▁eye , ▁with ▁cort ical ▁supp ression ▁of ▁the ▁vision ▁in ▁that ▁eye ▁while ▁it ▁is ▁devi ated . ▁On ▁returning ▁down ward ▁and ▁possibly ▁in ward ▁to ▁take ▁up ▁fix ation , ▁the ▁DVD ▁slow ▁movement ▁will ▁be ▁revers ed . ▁ ▁The ▁dis soci ative ▁movement ▁seen ▁' object ively ' ▁should ▁not ▁be ▁confused ▁with ▁the ▁dis soci ation ▁that ▁occurs ▁' subject ively ' ▁- ▁as ▁when ▁the ▁brain ▁begins ▁to ▁not ▁visual ise ▁both ▁images ▁simultaneously ▁( by ▁ign oring ▁or ▁suppress ing ▁vision ▁in ▁that ▁eye ). ▁ ▁Di agn osis ▁A ▁test ▁called ▁the ▁B iel sch ow sky ▁Dark ening ▁W edge ▁Test ▁can ▁be