x the troxler effect: the fading of a large stimulus with blurred edges presented in the peripheral...
TRANSCRIPT
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Chapter 7
Temporal Factors in Vision
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Spatial vision is not possible
unless the retinal image changes
with time
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Spatial vision is not possible unless the retinal image changes over time
X
The Troxler effect: the fading of a large stimulus with blurred edges presented in the peripheral visual field
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Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)
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Part One
Temporal Acuity:
The critical flicker frequency (CFF)
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The critical flicker frequency (CFF) is a measure of the minimum temporal interval
that can be resolved by the visual system.
CFF is analogous to grating acuity as a measure of spatial resolution acuity
Measure CFF using an episcotister
(a rotating sectored disk used to produce square-wave flickering stimuli)
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the period is the length of time for one complete cycle of light and dark, and the flicker rate,or flicker frequency is the number of cycles per second (Hz)
Duty cycle – the ratio of the time a temporal square-wave pattern is at Lmax to the time it is at Lmin
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Talbot brightness = Lmin + ([Lmax – Lmin] x f) Eq. 7.1
The time-averaged luminance of a flickering light determines its brightness at
flicker rates above the CFF (Talbot-Plateau Law)
where f is the fraction of time that Lmax is present during the total period
How bright does a fused flickering light appear?
To convert duty cycle to f, divide the first number by the sum of the two numbers: 1:1 means f=0.5
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If a square-wave flickering light has a duty cycle of 4:1, what is f?
.1 .2 .4 .8 1.
0% 0% 0%0%0%
1. 0.1
2. 0.2
3. 0.4
4. 0.8
5. 1.0
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To convert duty cycle to f, divide the first number by the sum of the two numbers: 4:1 would be 4/(4+1) so f=0.8
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How bright does a flickering light appear?
At flicker rates slightly below the CFF, brightness is enhanced beyond the mean luminance of the flicker (the Brücke-Bartley phenomenon)
This is related to the Broca-Sulzer effect described later in the chapter
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Time
Stimulus luminance
Neural response
Stimulus luminance
Neural response
Stimulus luminance
Neural response
A
B
C
The neural basis of the CFF is the modulation of firing rates of retinal neurons (ganglion cells)
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Courtesy of Dr. Tim Kraft
Cone flicker response (pig). Contrast 0.49; mean light level 48,300 photon/ square micron
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Rat ganglion cell responses showing CFF
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In order to see a light as flickering
The
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1. The flicker rate must be above the CFF
2. The Troxler effect must occur
3. Retinal neurons must be able to respond with gaps in their firing pattern
4. All of the other answers are correct
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How does this measure of temporal acuity (CFF) change under different conditions (changes in the stimulus dimensions listed in Chapter 1)?
First: stimulus luminance (intensity)
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Important Stimulus Dimensions
intensity
wavelength
size
exposure duration
frequency
shape
relative locations of elements of the stimulus
cognitive meaning
In addition,(NOT stimulus Dimensions!)
location on the subject’s retina
light adaptation of the subject’s visual system
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CFF is directly proportional to the log of stimulus luminance (Ferry -Porter Law)
CFF = k log L + b where k is the slope of the function , b is a constant, and L is the luminance of the flickeringstimulus
Log Retinal IIluminance (Td)
-1 0 1 2 3 4
Critical FlickerFrequency (Hz)
0
10
20
30
40
50642 nm at the fovea
Note: if the luminanceof the stimulus increasesby one log unit, so does the retinal illuminance
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demo
1) Find CFF
2) Raise intensity (luminance) by 1 log unit. The more intense stimulus is below CFF (flicker is seen).
3) Have to increase the flicker rate to again find CFF.
Log Retinal IIluminance (Td)
-1 0 1 2 3 4
Critical FlickerFrequency (Hz)
0
10
20
30
40
50642 nm at the fovea
1
2
3
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The Ferry-Porter law holds at all eccentricities. The slope is steeper
in the periphery. At high luminance, CFF is higher in the periphery
than at the fovea.
Log Retinal Illuminance for 0oand 3
o (td)
-1 0 1 2 3 4 5 6 7
Critical Flicker Frequency (Hz)
0
20
40
60
80
100
Log Retinal Illuminance for 10o-85
o (td)
-4 -3 -2 -1 0 1 2 3 4
0o
3o
10o
35o
65o
85o
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Retinal Eccentricity (deg)
0 20 40 60 80 100
Critical Flicker Frequency (Hz)
0
20
40
60
80
100
0.25 2.5
25 250
2500
Retinal Illuminance (td)
The CFF is highest in the midperipheral retina at high luminance,
but nearly constant across the retina at low luminance.
This is why you can see flicker on some PC monitors if you look slightly to the side
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CFF increases
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1. In direct proportion to the log of the stimulus luminance
2. In the periphery at all luminance levels
3. In response to the Brücke-Bartley phenomenon
4. None of the above
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CFF is directly proportional to the logarithm of the area of the flickering stimulus (the
Granit-Harper Law)
Second: area (size)
Demo, since I haven’t found a good figure showing this relationship
CFF = k logA + b
Where k and b are constants and A is the area of the flickering stimulus
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Demo – Granit-Harper
1) Find CFF
2) Increase stimulus area by 1 log unit. The more intense stimulus is below CFF (flicker is seen).
3) Have to increase the flicker rate to again find CFF.
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Chapter 7 – Temporal Factors in Vision
Main points so far:
1) CFF is a measure of temporal acuity – analogous to VA (how small a temporal interval can you detect – in time)?
2) CFF increases linearly with log stimulus luminance (Ferry-Porter Law)
3) CFF increases linearly with log stimulus area (Granit-Harper)
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You will not be responsible for the material starting on page 188, “flicker sensitivity increases….”
and including all of page 189 and 190 (Figs. 7.7 and 7.8).
You will be responsible for material starting again on page 191, “Temporal Contrast Sensitivity”
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Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)
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Contrast, modulation and amplitude
The contrast of a temporal sine wave is defined the same way as the contrast of a spatial sine wave
grating:
Contrast = (Lmax Lmin)/( Lmax + Lmin)
In Figure 7-1, Lmax is 300, Lmin is 100, so contrast = (200)/(400) = 0.5 Another term, modulation
(abbreviated as m),is sometimes used for sine-wave flicker, and may be used interchangeably with
contrast. As illustrated in Figure 7-1, Lmax is the maximum luminance of the flicker, and Lmin is the
minimum luminance. Lmax and Lmin are symmetrically arranged around the mean or average luminance,
defined as:Mean Luminance = L
m = ( Lmax + Lmin)/2
Hence, contrast or modulation can also be expressed as:
Contrast = m = (Lmax Lm)/ L
m
In addition, Lmax - Lm is also called the amplitude of the wave, and, therefore,
Contrast = modulation = amplitude /Lm
Referring again to the sine wave at the bottom of Figure 7-1, the
mean luminance is 200 units, the amplitude is 100, and the contrast (modulation) therefore is 0.5. As
was the case for spatial sine-wave gratings, contrast sensitivity is defined as the inverse of the threshold
contrast.
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Temporal CSFs have several features in common with spatial CSFs:
band pass shape, cutoff high frequency indicating the acuity limit, and a low frequency
rolloff.
This is like Figure 6.9 in the spatial domain
Frequency (Hz)
2 5 10 20 50 100
ContrastSensitivity
1
2
5
10
20
50
100
200
Threshold Contrast
1
0.5
0.2
0.1
0.05
0.02
0.01
0.005
Retinal Illuminance (Td)
9300 850 77 7.1 0.65 0.06
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Temporal CSF Demo
http://psy.ucsd.edu/~sanstis/TMTF.html
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Change in the temporal CSF with luminance:
As luminance decreases,
the peak contrast sensitivity becomes lower
the cutoff high temporal frequency decreases (Ferry-Porter law)
peak contrast sensitivity occurs at lower temporal frequency
the low temporal frequency rolloff disappears
Frequency (Hz)
2 5 10 20 50 100
ContrastSensitivity
1
2
5
10
20
50
100
200
Threshold Contrast
1
0.5
0.2
0.1
0.05
0.02
0.01
0.005
Retinal Illuminance (Td)
9300 850 77 7.1 0.65 0.06
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The temporal contrast sensitivity function
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1. Is the boundary between contrasts you can see and ones you cannot see
2. Has a peak contrast at around 1 Hz at high mean luminance
3. Is a measure of temporal acuity
4. Becomes more bandpass as the mean luminance is decreased
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The center-surround interactions of retinal neurons may account for a low
frequency roll-off in temporal CSF of individual neurons
Actually, there is a mid-temporal frequency enhancement of sensitivity
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The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at some temporal frequencies
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The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at mid-range temporal frequencies
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Temporal CSFs have several features in common with spatial CSFs:
band pass shape, cutoff high frequency indicating the acuity limit, and a low frequency
rolloff.
This is like Figure 6.9 in the spatial domain
Frequency (Hz)
2 5 10 20 50 100
ContrastSensitivity
1
2
5
10
20
50
100
200
Threshold Contrast
1
0.5
0.2
0.1
0.05
0.02
0.01
0.005
Retinal Illuminance (Td)
9300 850 77 7.1 0.65 0.06
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1. Artificially increased IOP produces reduced temporal CSF (but no effect on CFF)
2. Temporal CSF is reduced with glaucoma and ocular hypertension
• Glaucoma - Frequency-doubling perimeter measures contrast threshold for 0.25 c/deg grating flickering at 25 Hz (mediated by MY [nonlinear magno] cells?)
3. Eyes at risk for exudative (wet) AMD show reduced sensitivity at 5 - 40 Hz (5 Hz & 10 Hz alone discriminate from healthy eyes)
Importance? Early diagnosis can lead to earlier treatments
The temporal CSF is a useful measure for diagnosing retinal disorders
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The low temporal frequency rolloff of the temporal CSF
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1. Is really a “mid-temporal frequency enhancement produced by the longer latency of the receptive field surround
2. Becomes more prominent at low mean luminance levels
3. Helps create Mach bands4. Is related to the cutoff high
temporal frequency
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Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation (Bloch’s Law & Broca-Sulzer)
Masking
Motion Detection (Real and Apparent)
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Flash Duration (s)
0.001 0.01 0.1 1 10 100
4
5
6
7
8
9
Log Threshold Luminance(quanta/s/deg2)
Stimulus area = 0.011 deg2
Log Background Intensity
7.83 5.94 4.96 3.65 No Background
Fig. 2.5
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Bloch’s Law holds for durations shorter than the critical duration
L x t = C Eq. 7.7
where L is the threshold luminance of the flash, t is its duration, and C is a constant
Remember: luminance (L) is directly proportional to the number of quanta (Q) in a flash
and inversely proportional to the duration (t) and area (A) of the flash, or
L = Q / t x A
C x duration area duration x
quanta Eq. 2.6
There is a constant # of quanta in a threshold flash as L decreases
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Temporal Summation and Bloch's Law
When a brief flash is used to determine the threshold intensity, the visual system does not distinguish the “temporal shape” of the flash if the flash duration is less than the “critical duration”
Part A – threshold measures
Numberof
Quanta
TimeCritical Duration
TimeCritical Duration
BA
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Flash Duration (msec)
1 10 100
Log ThresholdLuminance
1 10 100
Log Threshold Luminance x Time
Bloch’s Law holds
Bloch’s Law holds
“Holds” means that Bloch’s Law accounts for the threshold values
Two ways to show Bloch’s Law: L x t = C
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Bloch's law is a consequence of the temporal filtering properties of vision.
But I will not hold you responsible for this section
Bottom of 198 & top of 199
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Bloch's law is a consequence of the temporal filtering properties of vision.
F
F+3F
F+3F+5F
F+3F+5F+7F
F+3F+5F+7F+9F
F+3F+5F+7F+9F+11F
F
3F
5F
7F
9F
11F
Fourier Synthesis: can construct complexwaveforms by adding together simple ones
Horizontal Position (deg)
0.0 0.2 0.4 0.6 0.8 1.0
ALuminance
Spatial Frequency (cycles/deg)1 3 5 7 11 17 25
BRelativeContrast
0.1
1
10
100
1000
Spatial Frequency (cycles/deg)1 3 5 7 11 17 25
CRelativeSensitivity
0.01
0.1
1
10
Spatial Frequency (cycles/deg)1 3 5 7 11 17 25
DRelativeContrast
0.1
1
10
100
1000
Horizontal Position (deg)0.0 0.2 0.4 0.6 0.8 1.0
EBrightness
Flashes of various durations shorter than the critical duration all have the same temporal frequency spectrum. Flashes longer than the critical duration contain less contrast at intermediate temporal frequencies, after filtering through the temporal CSF and are therefore less visible. Thus, more quanta are need to be added to bring them up to threshold.
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The critical duration for a brief flash against a background decreases as the luminance
of a background light or area of the flash increases
Log Flash Duration (msec)
0 1 2 3
Log Threshold RetinalIlluminance (Td)
-2
-1
0
1
2
3
4
2500 3400 456 115 21 9.5 1.9 0 0.43
Background Luminance (Td)
fovea 1o
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Critical duration also depends on stimulus area. As
the area of the flash is increased, the critical duration
decreases.
When the stimulus diameter is small (1.5 - 2 min
arc), Bloch's Law holds for flash durations up to
around 0.10sec (100 msec).
If the test flash diameter increases to approximately
5 deg., Bloch's Law only holds for flashes up to about
30 msec in duration.
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For flash durations less than the critical duration, Bloch’s Law holds and
1. The flash cannot be seen when it is above threshold
2. The number of quanta in a threshold flash is the same for different flash durations
3. L x C = t4. None of the above
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Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation (Bloch’s Law & Broca-Sulzer)
Masking
Motion Detection (Real and Apparent)
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Supra-threshold flashes of a certain brief duration appear brighter than longer and
shorter flashes of the same physical intensity (Broca-Sulzer effect)
170
16.2 lux
32.4 lux
64.5 lux
126 lux
170 lux
126
64.5
32.416.2
0.0
1
0.2
0.5
0.2
5
0.1
25
0.1
0.0
62
0.0
46
0.0
37
Flash Duration (sec)
Co
mp
ara
tive
Brig
htn
ess
700
600
500
400
300
200
100
0
Part B – above-threshold brightness
Broca
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Neural Explanation
•Intense stimuli produce photoreceptor overshoot
•This produces (via the bipolar cells) an initial burst of action potentials in the ganglion cells
•Brightness is related to the firing rate of the cells (spikes/second)
•For long flashes, the firing rate after the initial burst signals the brightness
•For brief flashes, only the initial burst occurs, so the only information the neurons in central structures can use is a high firing rate, which makes the flash appear brighter than when it is long.
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Neural explanation of the Broca-Sulzer effect
Note: the photoreceptor membrane potentials are upside down (negative is up on the graph) to demonstrate the similarity in shape to the Broca-Sulzer effect.
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Neural Explanation
•Intense stimuli produce photoreceptor overshoot
•This produces (via the bipolar cells) an initial burst of action potentials in the ganglion cells
•Brightness is related to the firing rate of the cells (spikes/second)
•For long flashes, the firing rate after the initial burst signals the brightness
•For brief flashes, only the initial burst occurs, so the only information the neurons in central structures can use is a high firing rate, which makes the flash appear brighter than when it is long.
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Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking (Temporal interactions between visual stimuli)
Motion Detection (Real and Apparent)
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Temporal Interactions between Visual Stimuli
Masking is any situation in which the detection of a visual stimulus is reduced by
another stimulus presented before, during, or after the target stimulus.
The effects of a masking stimulus may continue forward, after its
cessation, and backwards, before its onset
1) masking of light by light, 2) masking of a pattern by light, and 3) masking of a pattern
by a pattern.
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Test Field Onset Time (msec)
0 250 500 750 0
Log Threshold Test Field Energy
Test Off
Test On
Mask Off
Mask On
-4
-3
-2
-1
0Masking Stimulus (1.38o)
Test Stimulus (0.36o)
MaskingForwardmasking
Backwardmasking
Backward Masking might remind you of Early Dark Adaptation
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Test flash
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Masking flash
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Test Field Onset Time (msec)
0 250 500 750 0
Log Threshold Test Field Energy
Test Off
Test On
Mask Off
Mask On
-4
-3
-2
-1
0Masking Stimulus (1.38o)
Test Stimulus (0.36o)
MaskingForwardmasking
Backwardmasking
Backward Masking might remind you of Early Dark Adaptation
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Simultaneous and forward masking are signal detection problems
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Target pulse aloneS
pik
es p
er s
eco
nd 60
Mask follows target by 300 milliseconds
Sp
ikes
per
sec
on
d 60
0
Mask pulse alone
Sp
ikes
per
sec
on
d 60
0
0
Mask follows target by 100 milliseconds
Spi
kes
per
seco
nd 60
0
Mask follows target by 50 milliseconds
Spi
kes
per
seco
nd 60
0
Mask follows target by 20 milliseconds
Spi
kes
per
seco
nd 60
0
Mask follows target by 10 milliseconds
Spi
kes
per
seco
nd
Time (milliseconds)
60
00 1000 2000
Time (milliseconds) 0 1000 2000
Backward masking may be explained by the response latency and duration of the test flash
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Masking effects do not require spatial coincidence of test and masking stimuli; they
may occur when the test and mask are spatially separated by as much as 3 degrees
Masking effects may occur when the test and mask are spatiallyseparatedmetacontrast (backwards) and paracontrast (forwards)are masking in which the test flash and masking flash do not overlap spatially on the retina
This suggests that the same cells must be stimulated by the edges of both stimuli to obtain metacontrast
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When the gap between the stimuli becomes large enough, different populations of retinal neurons are stimulated by the test and masking flashes. Any masking has to occur upstream in the visual pathway, where receptive fields get larger
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Masking
• Masking is any situation in which the detection of a visual stimulus is reduced by another stimulus presented before, during, or after the target stimulus.
• Metacontrast (backwards masking with physically-separated stimuli) and paracontrast (forward masking with physically separated stimuli)
• Dichoptic masking – masking where the two stimuli are presented to different eyes
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Dichoptic masking - A masking stimulus presented to one eye affects vision of a test
stimulus at a corresponding retinal location in the other eye
Cannot occur until inputs from the two eyes meet at abinocular cell in V1 or later
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Saccadic suppression
Saccadic suppression is defined as a reduction in sensitivity to visual stimuli that occurs before, during and after a saccade
(Look at your eyes in a mirror and try to see them move when you make a saccade)
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Decreased sensitivity (increased threshold) to visual stimuli
occurring before, during, and after saccadic eye movements
100
-120 -80 -40 0 40 80 120 160
80
60
40
20
0
100
80
60
40
20
0
Visual suppression
Pupil suppression
Pu
pil
Re
sp
on
se
(p
erc
en
t)
Vis
ua
l R
es
po
ns
e (
pe
rce
nt)
Time of Flash (msec)
Eyes moving
But you can see the strobe lights atop Red Mountain if you time your saccade just right
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Masking includes
1. any situation in which the detection of a visual stimulus is reduced by another stimulus presented before, during, or after the target stimulus
2. Paracontrast
3. Dichoptic masking
4. All of the above
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Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)
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Motion is a continuous change in an object’s location as a function of time
Three reasons motion detection is important:
•detect moving objects against a background (see edges)
•detect own motion through the environment
•determine 3-D shape (crudely)
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Demo – shape from motion
(If you can’t see the edges, you can’t see the object)
http://www.biomotionlab.ca/Demos/BMLwalker.html
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Real Motion:Motion involve an image changing its location on the retina
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Contrast with smooth pursuit (moving the eyes smoothly). This prevents the image from changing its location on the retina. We are not studying smooth pursuit.
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There is an upper limit to our ability to see motion – stimuli can be moving “too fast to see”
It turns out that the reason is that rapidly moving images have a temporal frequency that is too high for our visual system to detect (frequency is above the temporal high-frequency cutoff).
To understand this – need to look at movement from the point of view of an individual retinal neuron.
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From the viewpoint of any one cell in the retina, motion is a change in luminance that occurs at a rate that depends on the speed with which the object moves andon the spatial frequency composition of the object
•A high spatial frequency grating moving at constant velocity (degrees per second) has a faster temporal frequency than a lower spatial frequency moving at the same velocity
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This grating moved one full cycle
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Motion involves interactions of both spatial frequency and temporal frequency
•Now, a lower spatial frequency (about half of the first one)moving at the same velocity (degrees per second).
It has lower temporal frequency (cycles per second) at a given spot
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This grating moved about ½ cycle.
Measured at the green dot (symbolizing a receptive field), it has a temporal frequency (flicker rate) about half of the higher spatial frequency
Can determine the temporal frequency of a drifting grating by multiplying its spatial frequency times its velocity in degrees per second
A 3 cycle/deg grating moving 10 deg/sec has a temporal frequency of 30 Hz; 30 cycle/deg =300 Hz
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Temporal Frequency (cycles/s)
0.01 0.1 1 10 25 50
Spatial Frequency (cycles/deg)
0.01 0.1 1 10 25 50
ContrastSensitivity
1
10
100
10000110100800
Velocity (deg/s)A B
As object velocity increases, spatial CSF shifts to lower spatial frequencies; temporal CSF remains constant
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How fast a velocity can you see moving?
The limiting factor in motion detection is the temporal resolution of the visual system.
If you present a very, very low spatial frequency (and high contrast) can see motion of several thousand degrees per second
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The ability to see rapidly-moving (high velocity) objects
1. Is limited by the temporal frequency
2. Occurs only in the visual cortex
3. Is set by the velocity of the objects
4. Cannot be measured
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Apparent Motion:Apparent motion is the perception of real motion that can be produced when a stimulus is presented discontinuously.
Phi phenomenonhttp://www.yorku.ca/eye/balls.htm
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Apparent Motion:Apparent motion is the perception of real motion that can be produced when a stimulus is presented discontinuously.
The “rules” for producing apparent motion are the same as for real motion: the optimal stimulus duration and spacing is the same as would occur if a real object moved.
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Real vs. Apparent Motion Motion sampled stroboscopically
appears like real motion due to the
insensitivity of vision to high
temporal and spatial frequencies
Velocity (deg/sec)
0.2 0.5 1 2 5 10 20 50 100
Optimal Time (msec)
20
50
100
200
0.2 0.5 1 2 5 10 20 50 100
Optimal Distance (arc min)
2
5
10
20
50
100
200
Burr and Ross, 1982Van Deenna and Kimurama, 1982Nakayama and Silverman, 1984Kelly, 1979
To produce optimal apparent motion of 10 degrees per second, need each spot to be about 25’ apart and be on for about 35 msec. A real object, traveling at 10 degrees per second would move 21’ in the same time
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In order to make apparent motion look like real motion
1. You have to “fool” some of the neurons all of the time
2. You need a string of lights
3. You need real motion4. You need to present
the stimuli with the same separation and duration as would occur with real motion
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Detection of motion and sensitivity to direction of motion is achieved in hierarchic
fashion in Areas V1 of the striate and middle temporal region of the cortex
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Newsome and colleagues sampled the activity of neurons in area MT
Each cell has a receptive field that responded to motion in some location in the visual field (some retinal location). Each neuron was direction selective; it had an optimal direction (most spikes per second) and a null direction (fewer spikes per second).
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Stimuli with a range of correlation of the motion of the spots were used to determine threshold amount of correlation for the monkey, and also the threshold for neurons in the monkey’s area MT (in a two-alternative, forced-choice situation).
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Using signal detection theory, a “neurometric” function could be produced for each neuron and compared with the monkey’s psychometric function
Spikes per Trial0 100
20
20
Number ofTrials
20
Non-preferred directionPreferred direction
Correlation - 12.8%
Correlation - 3.2%
Correlation - 0.8%
A
B
Correlation (%)0.1 1 10 100
PercentCorrect
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Psychometric FunctionNeurometric Function
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Frequency ofOccurence
0
1
2
3
4
5
6
7
Mean of Noise
Number of Action Potentials in 50 msec Period
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0
1
2
3
4
5
6
7
Mean of Noise + Signal
Overlap: PossibleConfusion
Maintained Discharge (Noise)Distribution
Maintained Discharge (Noise) +Response to Flash (Signal)
Distribution
A
B
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Srimulus AbsentStimulus Present
d'=1.5d'=1.0d'=0.5
A
B
C
ROC Curve
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Using signal detection theory, a “neurometric” function could be produced for each neuron and compared with the monkey’s psychometric function
Spikes per Trial0 100
20
20
Number ofTrials
20
Non-preferred directionPreferred direction
Correlation - 12.8%
Correlation - 3.2%
Correlation - 0.8%
A
B
Correlation (%)0.1 1 10 100
PercentCorrect
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Psychometric FunctionNeurometric Function
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Real & apparent motion seem to be detected by neurons in the parietal (MT) “stream”
Threshold Ratio (neuron/behavior)
0.1 1 10
Number ofNeurons
0
5
10
15
20
The psychometric function for the monkey was matched well by direction-selective neurons in area MT.
Monkey more sensitive than the neuronNeuron more sensitive than the monkey
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The monkeys’ “neurometric function”
1. Did not match the psychometric function
2. Could not be accurately estimated
3. Closely matched the psychometric function
4. None of the above
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Adapting to one direction of motion can produce a motion aftereffect when the movements stops (the “waterfall illusion”)
May be due to neurons in MT
Waterfall Illusion http://www.yorku.ca/eye/mae.htm
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Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)
Right click here to download.url
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