See this interesting article on how “focussed” humans become when they have a specific task at hand.
Excerpt:
Picture yourself watching a one-minute video of two teams of three players each. One team wears white shirts and the other black shirts, and the members move around one another in a small room tossing two basketballs. Your task is to count the number of passes made by the white team – not easy given the weaving movement of the players. Unexpectedly, after 35 seconds a gorilla enteres the room, walks directly through the farrago of bodies, thumps his chest and, nine seconds later, exits. Would you see the gorilla?
Fifty percent of all observers do not see the gorilla.