KNOWLEDGE AND THE KNOWER—PERSPECTIVEs
ACTIVE SENSE PERCEPTION
Rotating Snakes Illusion:
Akiyoshi Kitaoka designs static works that generate “anomalous motion.” On his Illusion Pages website he warns that his creations “might make sensitive observers dizzy or sick.” You can stop local movements of the concentric circles temporarily by staring into the dark centers. Are the concentric circles really moving? What on earth is going on here?
“Bear in mind that the filling in is not just some odd quirk of the visual system that had evolved for the sole purpose of dealing with the blind spot. Rather, it appears to be a manifestation of a very general ability to construct surfaces and bridge gaps that might be otherwise distracting in an image – the same ability, in fact, that allows you to see a rabbit behind a picket fence as a complete rabbit, not a sliced-up one.”
CLASS ACTIVITY I: BLINDSPOT
This blindspot activity was designed by Paul Grobstein. The full version can be found online at the extensive Serendip site.
Begin by ensuring that students are familiar with the anatomical basis for the blind spot. For the second activity they should know about the fovea too.
For the hands-on activity: students should work in pairs. The instructions appear next to the images on a set of cards; which must be printed out in color.
BLIND SPOT CARD #2
BLIND SPOT CARD #2
Close or cover your left eye. Stare at the cross with your right eye. Without actually looking at it notice in your peripheral vision the black spot.
Slowly move the card back and forth until the spot seems to disappear. What is going on?
BLINDSPOT #3
Try again. This time with a colored background. What do you notice? A spot? A hole? Something else?
BLINDSPOT #4
What other tricks are in store? Try again. What do you notice this time?
BLINDSPOT #5
One last variation… Again, what do you notice?
Is the mind oblivious to information it does not receive? Does it somehow fill in the missing piece? If so how? What might this tell us in general about the nature of sense perception?
Printable color Pdf of the Blindspot Activity slides.
CLASS ACTIVITY II: Eye Tracking
The intention of this activity is to instill in students the notion that the eyes are constantly on the move actively searching; hungry for points of interest.
Begin by showing this video of saccadic eye movements in slow motion. Saccadic eye movements occur around three times per second, and are mostly below the level of conscious control. The video is well worth its full mesmerizing three minutes and thirteen seconds!
Next ask a student to read out the quote from Findlay and Gilchrist; and then ask the class to read in silence the It's All About the Fovea notes that follow. After that obtain three student volunteers to read aloud the three paragraphs. Allow clarification questions and some discussion without revealing the details of the Yarbus data.
“A Martian ethologist observing humans using their visual systems would almost certainly include in their report back: ‘they move these small globes around a lot and that’s how they see’... We believe that movements of the eyeballs are a fundamental feature of vision. This viewpoint is not widely current. Many texts on vision do not even mention that the eye can move. ”
IT'S ALL ABOUT THE FOVEA
Our synchronized eyeballs shift in their sockets relentlessly. Stimuli of interest are captured by peripheral vision and processed at lightning speed to determine the precise trajectory of saccadic shifts. Saccadic movements place selected points of interest precisely in line with the fovea, the tiny portion of each retina which provide visual acuity.
The spherical shape of the eyeballs provides maximum maneuverability. The protruding corneas provide the correct focal lengths without compromising the spherical structure of the embedded portions of the eyes. (The corneas are responsible for about 70% of focusing.) Again, mostly in service of optimizing fixation at the foveae, the antagonistic muscles of the irises and the ciliary muscles of the lenses also make constant fine adjustments.
The irises optimize the amount of incoming light by adjusting the pupil apertures. The ciliary muscles stretch the lenses (which in their relaxed states resemble transparent jelly-filled bags) to refract incoming light patterns to the foveae. Focusing by adjusting the shape of the lenses is known as accommodation.
ENCOUNTER WITH YARBUS
Russian psychologist Alfred L. Yarbus investigated the nature of eye movements during the fifties and sixties. His famous eye tracking experiments revealed that subjects who viewed complex scenes, scanned them differently when asked to perform specific tasks. Students should be reminded that the saccadic jumps, of the eye are happening about three times a second; too rapid to be under direct conscious control.
Yarbus used Repin's painting They did not expect him in his most influential study. Repin is revered for his realist painting in Russia, as Tolstoy is for literature.
Repin, Il'ia Efimovich (1884-88) They Did Not Expect Him. Oil on canvas. Tretiakov Gallery, Moscow.
(a) Free examination
(b) Estimate material circumstances of the family?
(c) Give the ages of the people?
(d) What had the family been doing before the arrival of the unexpected visitor?
(e) Remember the clothes worn by the people
(f) Remember the positions of the people and objects in the room
(g) Estimate how long the visitor had been away from the family
From Yarbus (1967)
Students should work in pairs. Provide a copy of the Yarbus gaze traces and the specific tasks a to g. Printable pdf.
Students should respond to the following questions.
For each task scenario encapsulate the result in a single sentence
What is the "control" in this experiment?
What does this tell us about eye scanning in general?
What seem to be some of our default points of interest?
What are the big picture implications for sense perception in general are revealed by the Yarbus data?
More iconic Yarbus results
TOUGHER GROUP QUESTIONS
Students are divided into three groups. A facilitator for each group is given one of the following knowledge questions printed out on a folded paper. After 10 full minutes of discussion, spokespersons publicly read their question, report their findings and field whole class comments and questions. Printable pdf.
GROUP ONE:
If we can be fooled by optical illusions is it reasonable be skeptical about all sense data? Provide reasons and examples.
GROUP TWO:
Do people who think differently also see differently? Why are opinion and belief often used synonymously with perception?
GROUP THREE:
There are no “naked perceptions”―why? Think deeply about this next question: is all seeing, seeing as?
Nielsen lab measures brainwaves and tracks eye movements to provide a real-time picture of consumers’ subconscious reactions to brand messages. Photo: Thinkstock
“A fool sees not the same tree that a wise man sees”