Scott Stevenson Instructor
Exam Schedule for Spring 2017:
Test 1: February 9
Test 2: March 9
Test 3: (Finals week)
Perception demos from AB Watson http://visionscience.com/perceptiondemos/
Final will cover primarily material from the last third of the class, with some material from the first two thirds.
Grades will be distributed via Blackboard. At the end of the summer, when the Clinical
Integration course is finished, the grades will be entered into the UH system. The
grades for this class depend only on the three exams, the clinical integration course
does not affect your grade.
--------------------- Movies and other demos -------------
Spinning dancers: Can you ever see the silhouettes turning in opposite directions? Can you force your brain to switch the direction you see?
Pinhole viewing activity: Take a small pinhole from your trial lens set, or make one with an opaque card and a pin. The smaller the better. Cover one eye, and hold the pinhole up in front of the other eye with a bright homogeneous field behind it. The brighter the better.
The pinhole should be at the anterior focal point of the eye, about 17 mm in front of the cornea. You will see a disc of light, and the rim of this disc is an image of your own pupil margin. Any shadows you see in the disc are coming from objects on or inside your eye.
Lower your lid slowly. Where does the shadow appear? Why?
Close your eye halfway and then open it. Do you see horizontal ripples? Why?
Uncover and cover your other eye quickly. What happens to the disc you see? Why?
Move the pinhole around in small circles without moving your eye. Do you see blood vessels? Why?
The movie Floater_WhiteBloodCells.MP4 shows an image of the retina near the fovea. (If that link doesn’t work, try this one.) Halfway through the movie a large shadow drifts across the field, which the subject sees as a floater. Motion in the fine capillaries is due to white blood cells.
White blood cells and floaters appear to us because they cause a change in the retinal image. Stationary objects may cast shadows, but don’t change, so we adapt to them.
Scheerer’s phenomenon is the appearance of small white spots against a bright blue sky, caused by white blood cells moving through capillaries.
This simulation explains the appearance. Fixate on the dot in the center, notice that the “blood vessel” fades out, and the moving gap starts to look bright and white. When white blood cells float through your visual field, they create a gap in the blood vessel shadow. The shadows are stable on your retina, so they fade completely whether you move your eyes or not. In this demo, it only fades if you hold your eye very still.
TroxlerDemo3Hz.mp4: A more elaborate demonstration of how a moving gap becomes an object of its own, due to adaptation. Fixate the center steadily.
Your eye is never steady. How do small eye movements produce change in the retinal image when viewing this pattern? Why do the big blurry spots fade more quickly than the sharper black spots?
A minus spectacle lens minifies the image and requires recalibration of the VOR when the head moves.
Notice that the image seen through the lens moves less than the image seen directly. Tracking a point through
the lens requires a lower VOR gain than tracking it without the lens.
A moving pattern is just a lot of local flickers. The “holes” in the opaque gray band show what an individual cone photoreceptor would see as a grating drifts across the retina.
Seeing motion requires a correlation across space and time among these individual flickers.
Stare at the center of this spiral pattern for a minute or two. Then look away at something else, and you’ll see an aftereffect of motion.
The aftereffect occurs because of adaptation or fatigue of motion detectors tuned for particular directions. Motion is a push-pull, or
“opponent” system, so adaptation of one set of directions causes activity in the opposite set. The perception is that the texture
you look at is moving or melting, even though it isn’t going anywhere.
An important function of visual motion processing is busting camouflage. Objects may have the same texture as their background,
and be unnoticed, but when they move they suddenly pop out and are obvious. Here the texture moves in a way to reveal four letters.