• Refers to a region of retina which will excite one ganglion, producing a signal to the brain, depending on the pattern of the light falling on it.

Receptive field(center-surround)

Which one gives you the maximum response?

Which one gives you the maximum response?In each case, is the ganglion response greater than, less than, or the same as a background response?

Processing Edges

• The brain relies on edges for information about the the uniform regions between them. 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.

• Cornsweet illusion

To agree with the usage in our book, we would label the y-axis in part das “relative lightness”. In part e the large gray and “white” surfaces away from the joining line actually have the same gray lightness.

Successive Lightness Contrast

• Sensitivity of a given region of the retina decreases after being exposed to a bright light for a period of time

• Prolonged stimulation adapts (desensitizes) part of retina

• This is the principle behind negative afterimages

Negative Afterimages

• Look at a bright image for a while (white filament on a black background)

• Look away at a blank area and see a dark filament on a white background!

• Negative afterimages can last up to 30 sec.

• A negative afterimage does not transfer to the opposite eye. If you look at the light bulb with the right eye and then the white paper with the left eye, no negative afterimage is apparent.

• This implies that the processing happens before information gathers in the brain (retinal processing).

Eye Movements

• Drifts - slow, smooth, ~1/60 degree per sec

• Tremors - small, rapid, jittery (~50 Hz), ~1/240 degree

• Saccades - sharp, abrupt, up to 4 Hz, ~5/60 to 10/60 degree

• Produces sharp image by focusing on the fovea

• Prevents “retinal stabilization”

Retinal Stabilization

• Without eye movements, an image fades to uniform gray within a few seconds and then becomes invisible (even if it has edges)

• Change is required for vision

Positive Afterimage(persistence of vision)

Persistence of the response implies positive afterimage. – Lasts 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 vision’s 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---Movies!

• However, if one detects a discontinuous change, the movie flickers. In fact, the movies used to be called “the flicks”!

• TV: one frame every 1/30 sec– Each frame is scanned twice. First, horizontal

lines 1, 3, 5 …., then 2, 4, 6…(“interlaced”)– Effectively there are 60 frames per sec

• Movie: one frame every 1/24 sec.– Each frame is projected three times. – Effectively there are 72 frames per sec.

Standard Broadcast TV = 480i (480 horizontal lines, interlaced, odd numbered line scanned first 1/60 s, even numbered lines scanned second 1/60 s, complete frame after 1/30 s)

High Definition TV (HDTV) = Three scanning possibilities720p (720 horizontal lines, progressive, all linesscanned in 1/60 s, complete frame after 1/60 s)

1080i (1080 horizontal lines, interlaced, 540 linesscanned each 1/60 s, complete frame after 1/30 s)

1080p (1080 horizontal lines, progressive, all linesscanned in 1/60 s, complete frame after 1/60 s)

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!