Vision Science II
Notetaker: Iwona Miltko
Date: 04/16/2013, 1st hour
Lecture 28: Demonstration 1 experiment explanations, continued
Peripheral Drift Illusion
http://www.ucl.ac.uk/~ucbpmor/docs/case_study3_mor_web.pdf
● the pattern of light and dark
spokes (the luminance gradient)
triggers the illusory perception of
motion
● latency differences generated by
differences in luminance is what
is at the heart of this illusion
● the stimulus would be a RAMP
and would look like a panel with
bars of decreasing intensity.
Plotting the luminance profile
would result in this function.
● 4th RGC is responsible for
signaling the lightest part of the
pattern which is a very bright
stimulus and so quickly
hyperpolarizes the cones,
depolarizes the bipolar cells,
depolarizes the RGCs. The length
of the arrow indicates that there
is a strong and fast response by
the RGC. The latency for
response is quick (so short
latency). The signal released by
the RGC quickly synapses at V1
neuron.
● 5th RGC is responsible for
signaling the darkest part of the pattern which is a very dim stimulus and so slowly hyperpolarizes the
cones, depolarizes the bipolar cells, depolarizes the RGCs. The length of the arrow indicates that there is a
weak and slow response by the RGC. The latency for response is slow (so long latency). The signal
released by the RGC take longer to synapse at V1 neuron.
● Therefore, just as though real target were moving, the parts of the V1 receptive field are not stimulated
simultaneously. As the receptive field is stimulated in left to right direction in time, a left to right
movement is perceived.
● Right to left (leftward) motion is perceived by RGCs 1-3.
● Some neurons are indicating leftward movement and others are indicating rightward movement. The
arrows below V1 neurons indicate strength of direction of movement. The overall direction in rightward
direction is stronger because latency difference between RGC 4-5 is so much greater than latency
differences between RGC 1-3 which indicate leftward movement.
● Post V1 integrating cell adds together all signals from the V1 neurons. The resulting signal is perceived
direction of motion of the object.
● The dark/ light gradient is necessary for this illusory effect.
● This is not the only explanation of this illusion but it is well accepted and will be the only explanation
explained in this class.
Vision Science II
Notetaker: Iwona Miltko
Date: 04/16/2013, 1st hour
Aperture problem
● Looking at a grating pattern moving (in different directions) through aperture makes these gratings appear
to move in the same direction.
● There is ambiguous information when looking at a moving grating through an aperture.
● Although we don’t walk around holding an aperture around what we are viewing our RGCs, V1 cells,
LGNs all only respond to a portion of receptive field. Every neuron in receptive field is looking at the
visual environment through an aperture.
● How do we deal with the ambiguous information received by these cells?
● In this diagram, the blue & green are representing the moving stimuli
○ blue = stimulus at position 1 (before moving)
○ motion is oblique(down & right)
○ green =stimulus at position 2 (after moving)
● if most of the image is covered and only small area of object is visible, the motion direction is ambiguous.
The direction is determined based on which V1 local-motion detector is detecting at specific receptive
field. Each blue stimulus can be perceived to move in one of three directions. The global-motion detector
receives the output from the multiple V1 local-motion detectors, solves the ambiguity and determines
overall perceived motion.
● if the movement occurs at the right speed, stroboscopic movement would be perceived.
● aka barber pole illusion and breathing square
Motion Induced Blindness
*explained by same idea as Cheshire Cat illusion
● Many explanations are accepted but only 1 is being discussed in our class.
● Movement can erase the perception of stationary object in our visual field.
● When something moves across our retina, it acts like motion streak.
● example: when using camera (which uses film) and keeping aperture open for long time and a long
shutter opening when photographing fireworks: streaks (of motion) would be seen because firework is
moving when aperture was open.
● Motion streaks expose to film because the shutter is open for long time. A quick opening time of shutter
would allow for quick capturing of image as it appears.
● Motion streaks are powerful and are often used in cartoon and images.
● When object we are looking at move, we have long shutter opening. We have temporal integration over
roughly 100 ms. Receptors are stimulated by the motion and when the motion passes, the neurons
continue firing until they go off after a period of time.
● In order to clearly perceive moving stimuli, we have to suppress those neurons that are still responding
after the moving object has passed over them. This is motion streak suppression.
● If yellow stationary dots are at trailing edge of moving streak of the moving blue squares, the yellow dots
disappear because they are in the way of motion streak and are suppressed with the motion streak.
● Anything in the way of the trailing edge of the movement of hand come and go as the moving streak is
suppressed.
Change Blindness
● Change in visual field has occurred during eye blink, cut to different scene in film or the change is
occurring very gradually.
● You are unable to detect motion because we can’t encode everything in our visual field for every fixation.
Without motion, we don’t see changes.
● When looking at something, it is very difficult to encode and remember what you weren’t paying direct
attention to.
● This explains why eyewitness testimony is actually very inaccurate.
Inattentional Blindness
● Being unaware of something in the visual field because it is outside of spotlight of attention.
● When counting passes, the gorilla is not noticed.
● We don’t process everything that is in our fixation. We only process what we put our attention to.
Vision Science II
Notetaker: Iwona Miltko
Date: 04/16/2013, 1st hour
Corollary Discharge
● Be able to explain the table about this model (from after image to real motion, passive movement of the
eye and active movement of the eye etc)
● After-image example
○ When 1 eye is bleached and the eye is actively moved around there is no retinal image
displacement.
■ Bleach cannot generate SMS (sensory movement signal).
○ When eye isn’t bleached and the eye is actively moved around there is retinal image
displacement.
○ When bleach occurs, afferent signal can’t be generated
○ If bleach is generated in one eye, and eye is actively moved around, after image does move.
○ If either after image is generated or eye is actively moved around: object motion is seen. If both
are generated, after image will move.
○ If bleach is generated in one eye, and eye is passively touched, after image does not move.
Turning Point Questions
1 Which component of the smooth pursuit eye movement system evolved along with a differentiated
fovea?
a pursuit system
b reflexive system
c both systems
Answer= a
2 If a human observer is accelerated through space from right to left in darkness, with NO fixation light
then:
a the eyes will remain stationary
b the eyes will also move right to left
c the eyes will move from left to right
Answer= c
3 While a subject is accelerated from right to left in darkness, do the eyes actually move? (select two).
a yes, if there is no fixation light
b not if the subject fixated on the fixation light
c there is always a VOR regardless of fixation attempts
Answer= a and b
4 If the newer, pursuit system opposes the VOR response while the subject is accelerated from right to left,
which direction must the compensatory eye movement be in?
a in direction opposite to acceleration
b in the same direction as the acceleration
c in the same direction as the VOR
Answer= b
5 Which of the following is a smooth eye movement?
a microsaccade
b smooth pursuit
c saccade
d VOR
e convergence of eyes
Answer= b
6 in the induced motion of the self illusion, what do the rotating stripes do?
a they trigger the newer pursuit system of eye-control
b they provide vestibular input to the older, reflexive system
c they provide visual input to the older, reflexive system
Answer = c