Temporal

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Temporal Properties of Vision
Objects and our eyes move, and the input signal
to individual photoreceptors is constantly
changing and thus full of temporal transients.
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Retinal Images are constantly in motion due to
eye movements. Saccades (velocities up to 100
deg/second), slow drifts/smooth pursuits, and
fixational eye movements.
Example of fixations and scanning of a face
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Example of fixations of a 20/20 letter during a
25 second period.
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Interestingly, most stimuli that modulate in time
appear stable (Movies, TV, Fluorescent lights,
sodium lamps).
T1/60 sec
Shutter occluding projection beam
Example Movie
luminance
time
T1/60 sec
Example TV
Route taken by electron beam across TV screen.
Intensity increase and then decay of phosphors.
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• Demonstration of visual persistence
• Spin light source round in a circle. Notice that
  although the light only exists at one point in
  space at any one time, it appears to exist at all
  points around the circle at the same time. We
  see a complete circle because the neural
  responses persist more than one revolution of the
  light.
• In this example, we see whole words, even though
  the stimulus is only a line of small lights
  (LEDs) at any one time. Because the neural
  response persists, and the LEDs are programmed to
  light up is a special order, we see whole words.

t1
t2
t3
t4
t5
Time and space while moving
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Temporal Resolution (Critical Flicker Fusion
Frequency, CFF) the highest flicker rate that
we can detect (about 60 Hz)
Temporal resolution is limited by Visual
Persistence Neural responses to a very brief
flash persist (continue) after the stimulus has
disappeared. This temporal spread of the neural
response is analogous to spatial point spread
function, and just as the PSF limits spatial
resolution, so visual persistence limits
temporal resolution. Because of visual
persistence, we sum photons over time (Blochs
Law). This is temporal summation or shutter
speed
Example where flicker cycle is shorter than
persistence, neural signals add, no resulting
modulation.
Example where flicker cycle is long relative to
persistence little summation, neural response
modulates with signal.
Summation effect
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Temporal properties of inhibition has a longer
latency than excitation
excitation
inhibition
Resultant transient response
Neural Response
MD
Dt
time
Stimulus flash
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Low temporal Frequency
Dt
In this example, we see the delayed inhibition
for low and medium temporal frequencies. With
low temporal frequencies, the extra latency of
the inhibition is only a small fraction of a
cycle, and thus still cancels the excitation, but
at 8 Hz, the inhibition is 1/2 a cycle out of
phase and thus adds rather than cancels the
excitation resulting in increased and not
decreased modulation of the neural signal.
Medium temporal frequency
excitation
inhibition
Excit Inhib
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Temporal Contrast Sensitivity Function
Persistence (temporal summation)
Inhibition Gain control
Delayed inhibition increases modulation
Contrast Sensitivity
CFF
8 Hz
Temporal Frequency (cycles/second or Hz)
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As explained in the spatial vision section, low
spatial frequency contrast sensitivity is reduced
because of lateral inhibition. However, since
this inhibition is delayed, flickering the low
spatial frequencies make them highly visible.
Flickering grating
Stationary grating Carefully fixated
Answer clearly increased responsivity of Parvo
cells occurs with temporal modulation.
CS
Is the increase in CS at low SF due to
recruitment of magno/transient cells, or due to
the increased responsivity of parvo/sustained
cells to stimulus transients?
Spatial Frequency
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The interplay of space, time and eye movements
This picture contains a low spatial frequency low
contrast target and if we can fixate carefully,
it will disappear (Troxler Fading). However,
with sufficient eye movements introducing
temporal modulation in the receptors the target
becomes readily visible.
.
With careful fixation, and because of the low SF,
small fixational eye movements are insufficient
to modulate the light absorbed by receptors.
Thus, the stimulus is effectively stabilized.
The neural after-image that is generated cancels
the positive optical image exactly, and thus we
see nothing. You can see the negative (and thus
light) after-image of the dark smudge if you
fixate off to one side after the dark smudge has
faded.
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An example of monocular rivalry. As the eyes
move horizontally, the vertical line retinal
images move, but the horizontal images do not,
and the horizontal image fades, and vice versa
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The Ouchi Illusion See your own eye movements!
In this and the following illusions from Japan we
find that the perceptual fading occurs because a
negative neural after image cancels the
positive optical image, but when the eyes move we
see a combination of the after-image and the new
optical image. This combination is typically not
observed, except in the following illusions
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TWO VARIATIONS OF THE OUCHI ILLUSION
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Peripheral motion illusion. Also created by
after images and eye movements.
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Illusions from Japanese guy
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What is the ultimate Low spatial frequency? So
low that even normal eye movements are
insufficient to modulate the response of
individual receptors. This is the ambient light
level the whole retina is modulated up and down
during the day, but we are generally unaware,
and, as we showed in the night and day section,
gain control eliminates this signal. Thus we
can see, that the very same reason that we are
insensitive to low spatial and temporal
frequencies is the same reason we have brightness
constancy and a huge dynamic range inhibition
and gain control.
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Temporal Vision Laws
2. Brucke-Bartley effect Just as temporal CSF
peaks at 8 Hz, the perceived brightness of 8 Hz
flicker is greater than that of same light on
constantly (paradox!).
1. Talbot-Plateau law Above CFF, perceived
brightness of flickering light same as
constant light with same mean luminance.
8Hz
B-B
Brightness
Luminance
T-P
Temporal frequency
time
CFF
3. Ferry-Porter Law CFF is directly proportional
to the log of stimulus luminance.
Log luminance
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Motion Perception
Objects move, and because of eye movements,
retinal images of stationary objects move.
Interestingly, objects that we see moving on TV
or at the movies never actually move.
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Distinction between continuous motion and
stepped motion of a stimulus. When stepped
motion of stimulus results in perceived motion,
it is sometimes referred to as apparent motion.
Three stimuli with same average velocity, but
only one really moves (red), while other two are
always stationary but step from position to
position.
Slope (m/sec) velocity
position

Both appear to move smoothly
Continuous motion
Frequent small steps
Less frequent large steps, we see motion, but it
is jerky, we notice that it is not smooth
time
TV and movies are same as blue lines
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All motion has SPEED and DIRECTION.
Neural mechanism capable of providing this
information
Excitatory Direction
Null Direction
Because of the time delay on the inhibitory
connection, excitation and inhibition will arrive
simultaneously and thus cancel if object is
moving in the null direction.
receptors
Dt
Dt
Dt
-
-
-




Post-receptor neurons
Response in these post receptoral mechanisms
indicates movement from right to left
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DEMO
Motion Aliasing occurs with stepped motion or
real motion illuminated stroboscopically.
Why do wagon wheels go backwards in old movies?
t0
t3
t1
t2
Notice that, because of radial symmetry of wheel
and spokes, the retinal image of wheel be the
same after each 1/4 rotation (red point just to
disambiguate the image). Now imagine that we
illuminate the moving wheel intermittently
(stroboscopically), or view through a shutter
that opens and closes every 1/60 second (movie
camera). If the wheel has move 1/4 cycle in 1/60
second, it will appear stopped. Notice that if
it goes 1/8 of a rotation (or 1/2 of the
repetition cycle) between frames, direction of
motion will be ambiguous, between 1/8 and 1/4 the
wheel appears to move backwards. So, as wagon
wheel speeds up we see increased speed in correct
direction, then it suddenly reverses, then stops,
and then the cycle repeats.
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Saccadic Suppression We do not see the world
fly by during a saccade? Can you see your eyes
move during a saccade (look in a mirror)? Three
Hypotheses 1. There is no need for saccadic
suppression because we cannot see much anyway
when the retinal image is moving at hundreds of
degrees per second. 2. Specific neural mechanism
that suppresses the neural signal generated
during a saccade. 3. New hypothesis there is
little need to suppress signals generated during
a saccade, but there is need to suppress
lingering neural signals from previous fixation
(look at dyslexia notes). The mechanisms may be
inhibition of parvo by magno systems.
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• Motion Acuity
• (1) What is the slowest movement that appears to
  move?
• What is the fastest motion tat can be seen?
• What is the smallest movement that can be seen?
• What is motion coherence threshold?

Dyslexics experience a selective elevation in
motion coherence thresholds
100 coherent motion (typical object motion)
0 coherent motion (random motion)
27 coherent motion (motion acuity test)
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Examine some web sites showing interesting motion
phenomena
1. Apparent motion e.g. phi motion http//www.
yorku.ca/eye/balls.htm 2. Motion After-effect
(waterfall illusion) http//epunix.biols.susx.ac.
uk/Home/George_Mather/Motion/MAE.HTML http//dogf
eathers.com/java/spirals.html 3. Aperture
Problem and Barber Pole Illusion (motion of
contours determined by terminators or ends of
lines) http//www.psico.univ.trieste.it/labs/perc
lab/integration/english_version/aperture.php3 htt
p//www.psico.univ.trieste.it/labs/perclab/integra
tion/english_version/barberpole.php3 4. The
correspondence problem http//epunix.biols.susx.a
c.uk/Home/George_Mather/Motion/RDK.HTML 5. Shape
from motion 1. Biological motion
http//www.at-bristol.org.uk/Optical/DancingLights
_main.htm