▁people . ▁To ▁get ▁a ▁clear ▁view ▁of ▁the ▁world , ▁the ▁brain ▁must ▁turn ▁the ▁eyes ▁so ▁that ▁the ▁image ▁of ▁the ▁object ▁of ▁regard ▁falls ▁on ▁the ▁f ove a . ▁E ye ▁movement ▁is ▁thus ▁very ▁important ▁for ▁visual ▁per ception , ▁and ▁any ▁failure ▁can ▁lead ▁to ▁serious ▁visual ▁dis abilities . ▁To ▁see ▁a ▁quick ▁demonstr ation ▁of ▁this ▁fact , ▁try ▁the ▁following ▁experiment : ▁hold ▁your ▁hand ▁up , ▁about ▁one ▁foot ▁( 3 0   cm ) ▁in ▁front ▁of ▁your ▁nose . ▁Keep ▁your ▁head ▁still , ▁and ▁sh ake ▁your ▁hand ▁from ▁side ▁to ▁side , ▁slowly ▁at ▁first , ▁and ▁then ▁faster ▁and ▁faster . ▁At ▁first ▁you ▁will ▁be ▁able ▁to ▁see ▁your ▁fingers ▁quite ▁clearly . ▁But ▁as ▁the ▁frequency ▁of ▁sh aking ▁passes ▁about ▁ 1 ▁H z , ▁the ▁fingers ▁will ▁become ▁a ▁bl ur . ▁Now , ▁keep ▁your ▁hand ▁still , ▁and ▁sh ake ▁your ▁head ▁( up ▁and ▁down ▁or ▁left ▁and ▁right ). ▁No ▁matter ▁how ▁fast ▁you ▁sh ake ▁your ▁head , ▁the ▁image ▁of ▁your ▁fingers ▁remains ▁clear . ▁This ▁demonstr ates ▁that ▁the ▁brain ▁can ▁move ▁the ▁eyes ▁opposite ▁to ▁head ▁motion ▁much ▁better ▁than ▁it ▁can ▁follow , ▁or ▁purs ue , ▁a ▁hand ▁movement . ▁When ▁your ▁purs uit ▁system ▁fails ▁to ▁keep ▁up ▁with ▁the ▁moving ▁hand , ▁images ▁s lip ▁on ▁the ▁ret ina ▁and ▁you ▁see ▁a ▁bl ur red ▁hand . ▁ ▁The ▁brain ▁must