Dalton is doing science. Male squirrel monkeys don’t see color well; they have a kind of red-green color blindness. Dalton's eyes really only see medium and short wavelengths of light—blues and greens, and their overlap color, yellow. He's what vision scientists call a protanope. With no receptor for reddish hues, he sees reds as dark yellows and yellow-browns, and greens as mostly yellow—to the extent that human color words mean anything to a monkey.
He isn’t really bonking his head; Dalton is trained to indicate when he can see a color on the screen. “He’s actually fairly carefully touching his tongue to the screen,” says Jay Neitz, a color vision researcher at the University of Washington. Dalton sticks his tongue out, Jay says, because he knows that when he recognizes a color, a drop of grape juice will appear in the basin. Dalton really likes grape juice. And a little click will sound in the background, another bit of reinforcement. When he sees a color, he gives it a little kiss.
Please select this link to read the complete article from WIRED.