Processing Edges We rely on edges for information about the the uniform regions between them. When we see something, we first look for edges and then fill in the regions, taking the clue from the behavior noticed at the edges. Craik-O’Brien illusion.

Craik-O’Brien-Cornsweet illusion

Negative Afterimages The sensitivity of a given region of retina decreases after it is exposed to a bright light for a period of time—successive lightness contrast. Negative afterimage –Look at a white cat with a dark background for a while, you will see a dark cat on a white background! (last for ~30 sec)

Applications in personal defense weapons! Negative afterimage does not transfer to the opposite eye. This means the processing happens before information gathers in the brain. Another illustration that demonstrates that the constant simulation is ignored, but changes are not.

Positive Afterimage Persistence of the response  positive afterimage. –Last as long as 1/20 sec at low ambient light levels –As short as 1/50 sec at high light levels. If two images are presented in rapid succession, the image will appear as one because your response is too slow to separate and distinguish them.

When a rapid sequence of images is shown, each of which differs only slightly from the ones coming directly before or after it, you see a continuous motion! Illusion of a continuous motion from images differing slightly--- Moves! However, if one detects a discontinuous change, the movie flickers. In fact, the movies are used to be called flickers!

TV: one frame every 1/30 sec – Each frame is scanned twice. First, 1, 3, 5 …., then 2, 4, 6… – Effectively there are 60 frame per sec. Movie: one frame every 1/24 sec. – Each frame is projected three times. – Effectively there are 72 frames per sec.

Stroboscope Persistence of vision can make repetitive motion appear “stopped” or frozen. Stroboscope: a source of a regular sequence of bright flashes of light. When a bicycle wheel rotates, it is usually difficult to see the spokes. However, if the strobe flashes once for every revolution, the wheel will appear stopped.

What happens if the flashes are not at the exact frequency that stops the motion? – If the rotation is clockwise and the frequency is too high, the wheel appears to rotate slowly counter- clockwise. – If the frequency is too slow, the wheel appears to rotate slowly clockwise. Problem in cinematography: Use spokes that are spaced irregularly around the wheel!