Transcript No Slide Title

Chapter 7
Temporal Factors in Vision
Spatial vision is not possible
unless the retinal image changes
with time
Spatial vision is not possible unless the retinal image changes over time
The Troxler effect:
the fading of a large
stimulus with blurred
edges presented in
the peripheral visual
field
X
Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)
Part One
Temporal Acuity:
The critical flicker frequency (CFF)
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 squarewave flickering stimuli)
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
How bright does a fused flickering light appear?
The time-averaged luminance of a flickering light determines its brightness at
flicker rates above the CFF (Talbot-Plateau Law)
Talbot brightness = Lmin + ([Lmax – Lmin] x f)
where f is the fraction of time that Lmax is present during the total period
To convert duty cycle to f, divide the first number
by the sum of the two numbers: 1:1 means f=0.5
Eq. 7.1
If a square-wave flickering light has
a duty cycle of 4:1, what is f?
0%
0%
1.
0%
.4
0%
.2
0%
.8
0.1
0.2
0.4
0.8
1.0
.1
1.
2.
3.
4.
5.
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
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
The neural basis of the CFF is the modulation of firing rates
of retinal neurons (ganglion cells)
A
Neural response
Stimulus luminance
B
Neural response
Stimulus luminance
C
Neural response
Stimulus luminance
Time
Cone flicker response (pig). Contrast 0.49; mean light level 48,300 photon/ square micron
Courtesy of Dr. Tim Kraft
Rat ganglion cell responses showing CFF
In order to see a light as flickering
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
A
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t.
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tm
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.
0% 0% 0% 0%
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)
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
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 flickering
stimulus
Critical Flicker
Frequency (Hz)
50
642 nm at the fovea
40
30
Note: if the luminance
of the stimulus increases
by one log unit, so does
the retinal illuminance
20
10
0
-1
0
1
2
3
Log Retinal IIluminance (Td)
4
Critical Flicker
Frequency (Hz)
50
642 nm at the fovea
40
30
3
1
20
2
10
demo
0
-1
0
1
2
3
4
Log Retinal IIluminance (Td)
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.
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.
Critical Flicker
Frequency (Hz)
100
80
60
o
0
o
3
40
o
10
o
35
o
65
o
85
20
0
-4
-3
-2
-1
0
1
2
o
3
4
o
Log Retinal Illuminance for 10 -85 (td)
-1
0
1
2
3
4
5
o
6
o
Log Retinal Illuminance for 0 and 3 (td)
7
The CFF is highest in the midperipheral retina at high luminance,
but nearly constant across the retina at low luminance.
Critical Flicker
Frequency (Hz)
Retinal
Illuminance (td)
2500
250
100
25
2.5
0.25
80
This is why you
can see flicker on
some PC monitors
if you look slightly
to the side
60
40
20
0
0
20
40
60
Retinal Eccentricity (deg)
80
100
CFF increases
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
th
e
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of
to
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In
ab
ov
B
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at
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ry
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e
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to
...
or
tio
n
pr
op
di
re
ct
In
e
0% 0% 0% 0%
Second: area (size)
CFF is directly proportional to the logarithm of the area of the flickering stimulus (the
Granit-Harper Law)
CFF = k logA + b
Where k and b are constants and A
is the area of the flickering stimulus
Demo, since I haven’t found a good figure
showing this relationship
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.
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)
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”
Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)
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 asm),is sometimes used for sine-wave flicker, and may be used interchangeably with
contrast. As illustrated in Figure 7-1, Lmaxis the maximum luminance of the flicker, and L
minis the
minimum luminance. Lmax and L min are symmetrically arranged around the mean or average luminanc
defined as:
Mean Luminance = Lm
= ( Lmax + Lmin)/2
Hence, contrast or modulation can also be expressed as:
Contrast = m = (Lmax - L m )/ L m
In addition, Lmax - L m 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.
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.
Threshold
Contrast
Contrast
Sensitivity
200
0.005
100
0.01
50
0.02
20
0.05
10
0.1
5
0.2
2
0.5
1
1
2
5
10
20
Frequency (Hz)
This is like Figure 6.9 in the spatial domain
50
100
Retinal
Illuminance (Td)
9300
850
77
7.1
0.65
0.06
Temporal CSF Demo
http://psy.ucsd.edu/~sanstis/TMTF.html
Change in the temporal CSF with luminance:
As luminance decreases,
Threshold
Contrast
Contrast
Sensitivity
200
0.005
100
0.01
50
0.02
20
0.05
10
0.1
5
0.2
2
0.5
1
1
2
5
10
20
Frequency (Hz)
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
50
100
Retinal
Illuminance (Td)
9300
850
77
7.1
0.65
0.06
The temporal contrast sensitivity
function
B
a
Is
ec
m
ea
ba
n
m
or
e
om
es
su
re
of
t
em
nt
ra
s
co
k
pe
a
a
as
d.
..
po
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...
ta
.
be
t..
nd
ar
y
H
4.
0% 0% 0% 0%
bo
u
3.
th
e
2.
Is the boundary between
contrasts you can see and
ones you cannot see
Has a peak contrast at
around 1 Hz at high mean
luminance
Is a measure of temporal
acuity
Becomes more bandpass
as the mean luminance is
decreased
Is
1.
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
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
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
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.
Threshold
Contrast
Contrast
Sensitivity
200
0.005
100
0.01
50
0.02
20
0.05
10
0.1
5
0.2
2
0.5
1
1
2
5
10
20
Frequency (Hz)
This is like Figure 6.9 in the spatial domain
50
100
Retinal
Illuminance (Td)
9300
850
77
7.1
0.65
0.06
The temporal CSF is a useful measure for diagnosing
retinal disorders
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 low temporal frequency rolloff
of the temporal CSF
Is
re
l
cr
at
ed
ea
t
to
f..
.
to
e
th
e
M
ac
h
cu
b.
..
...
pr
om
m
or
e
el
ps
ec
B
H
a
om
es
“m
id
-te
m
po
r ..
0% 0% 0% 0%
lly
3.
4.
re
a
2.
Is really a “mid-temporal
frequency enhancement
produced by the longer
latency of the receptive
field surround
Becomes more prominent
at low mean luminance
levels
Helps create Mach bands
Is related to the cutoff high
temporal frequency
Is
1.
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)
Log Threshold
Luminance
(quanta/s/deg2)
Fig. 2.5
Log Background Intensity
7.83
5.94
4.96
3.65
No Background
9
8
7
6
5
4
Stimulus area = 0.011 deg2
0.001
0.01
0.1
1
Flash Duration (s)
10
100
Bloch’s Law holds for durations shorter than the critical duration
Lxt=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/txA
quanta
x duration  C
duration x area
Eq. 2.6
There is a constant # of quanta in a threshold flash as L decreases
Part A – threshold measures
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”
A
B
Number
of
Quanta
Critical Duration
Time
Critical Duration
Time
Two ways to show Bloch’s Law: L x t = C
Log Threshold
Luminance
Bloch’s Law holds
Log Threshold
Luminance x Time
Bloch’s Law holds
1
10
100
1
10
100
Flash Duration (msec)
“Holds” means that Bloch’s Law accounts for the threshold values
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
Bloch's law is a consequence of the temporal filtering properties of vision.
Fourier Synthesis: can construct complex
waveforms by adding together simple ones
A
Luminance
0.0 0.2 0.4 0.6 0.8 1.0
Horizontal Position (deg)
11F
F+3F+5F+7F+9F+11F
10 00
B
Relative 1 00
Contrast
10
1
9F
7F
0.1
F+3F+5F+7F+9F
F+3F+5F+7F
1
3
5 7 11 17 25
Spatial Frequency (cycles/deg)
10
C
Relative
Sensitivity 1
0.1
0.01
5F
3F
F+3F+5F
F+3F
1
3
5 7 11 17 25
Spatial Frequency (cycles/deg)
D
Relative
Contrast
10 00
1 00
10
1
F
0.1
F
1
3
5 7 11 17 25
Spatial Frequency (cycles/deg)
E
Brightness
0.0 0.2 0.4 0.6 0.8 1.0
Horizontal Position (deg)
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.
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 Threshold Retinal
Illuminance (Td)
4
fovea 1o
3
Background
Luminance (Td)
2
2500
3400
456
115
21
9.5
1.9
0
0.43
1
0
-1
-2
0
1
2
Log Flash Duration (msec)
3
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.
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 = t
4. None of the above
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)
Part B – above-threshold brightness
Supra-threshold flashes of a certain brief duration appear brighter than longer and
shorter flashes of the same physical intensity (Broca-Sulzer effect)
600
500
400
300
170 lux
Broca
126 lux
200
170
126
64.5 lux
100
64.5
32.4
16.2
Flash Duration (sec)
0.5
0.25
0.2
0.125
0.1
0
0.037
0.046
0.062
32.4 lux
16.2 lux
0.01
Comparative Brightness
700
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.
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 BrocaSulzer effect.
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.
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)
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.
1) masking of light by light, 2) masking of a pattern by light, and 3) masking of a pattern
by a pattern.
The effects of a masking stimulus may continue forward, after its
cessation, and backwards, before its onset
Log Threshold
Test Field Energy
Masking Stimulus (1.38o)
0
Test Stimulus (0.36o)
Backward
Masking
might
remind you
of Early
Dark
Adaptation
-1
-2
-3
-4
Mask On
Mask Off
Test On
Test Off
Forward
masking
Backward Masking
masking
0
250
500
750
Test Field Onset Time (msec)
0
Test flash
Masking flash
Log Threshold
Test Field Energy
Masking Stimulus (1.38o)
0
Test Stimulus (0.36o)
Backward
Masking
might
remind you
of Early
Dark
Adaptation
-1
-2
-3
-4
Mask On
Mask Off
Test On
Test Off
Forward
masking
Backward Masking
masking
0
250
500
750
Test Field Onset Time (msec)
0
Simultaneous and forward masking are signal detection problems
Mask pulse alone
0
Mask follows target by 300 milliseconds
0
60
0
Mask follows target by 100 milliseconds
0
1000
2000
Time (milliseconds)
Spikes per second
0
60
Spikes per second
60
Target pulse alone
60
Spikes per second
Spikes per second
Spikes per second
60
Spikes per second
60
Spikes per second
Backward masking may be explained by the response latency
and duration of the test flash
60
Mask follows target by 50 milliseconds
0
Mask follows target by 20 milliseconds
0
0
Mask follows target by 10 milliseconds
0
1000
2000
Time (milliseconds)
Masking effects may occur when the test and mask are spatially
separated
metacontrast (backwards) and paracontrast (forwards)
are masking in which the test flash and masking flash
do not overlap spatially on the retina
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
This suggests that the same cells must be stimulated
by the edges of both stimuli to obtain metacontrast
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
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 physicallyseparated stimuli) and paracontrast (forward masking
with physically separated stimuli)
• Dichoptic masking – masking where the two stimuli
are presented to different eyes
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 a
binocular cell in V1 or later
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)
Decreased sensitivity (increased threshold) to visual stimuli
occurring before, during, and after saccadic eye movements
100
100
Eyes moving
80
Visual suppression
60
60
40
40
Pupil suppression
20
20
0
0
-120
-80
-40
0
40
80
120
Pupil Response (percent)
Visual Response (percent)
80
160
Time of Flash (msec)
But you can see the strobe lights atop Red Mountain
if you time your saccade just right
Masking includes
1.
2.
3.
4.
any situation in which the
detection of a visual stimulus
is reduced by another
stimulus presented before,
during, or after the target
stimulus
Paracontrast
Dichoptic masking
All of the above
Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)
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)
Demo – shape from motion
(If you can’t see the edges, you can’t see the object)
http://www.biomotionlab.ca/Demos/BMLwalker.html
Real Motion:
Motion involve an image changing its location on the retina
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.
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 highfrequency cutoff).
To understand this – need to look at movement from the
point of view of an individual retinal neuron.
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 and
on 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
This grating moved one full cycle
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
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
As object velocity increases, spatial CSF shifts to lower
spatial frequencies; temporal CSF remains constant
Contrast
Sensitivity
A
B
Velocity (deg/s)
0
1
10
100
800
1000
100
10
1
0.01
0.1
1
10
Spatial Frequency (cycles/deg)
25 50
0.01
0.1
1
10
Temporal Frequency (cycles/s)
25 50
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
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
Apparent Motion:
Apparent motion is the perception of real motion that can
be produced when a stimulus is presented
discontinuously.
Phi phenomenon
http://www.yorku.ca/eye/balls.htm
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.
Real vs. Apparent Motion
Motion sampled stroboscopically
appears like real motion due to the
Optimal
Distance (arc min)
insensitivity of vision to high
200
temporal and spatial frequencies
100
50
20
10
5
2
0.2
0.5
1
2
Optimal
Time (msec)
5
10
20
50
100
Burr and Ross, 1982
Van Deenna and Kimurama, 1982
Nakayama and Silverman, 1984
Kelly, 1979
200
100
50
20
0.2
0.5
1
2
5
10
Velocity (deg/sec)
20
50
100
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
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 motion
4. You need to present
the stimuli with the
same separation and
duration as would
occur with real motion
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
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).
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 twoalternative, forced-choice situation).
A
Number of
Trials
20
Correlation - 12.8%
20
Correlation - 3.2%
Correlation - 0.8%
20
Non-preferred direction
Preferred direction
0
B
Spikes per Trial
100
Percent
Correct
1.0
0.9
0.8
0.7
0.6
0.5
Psychometric Function
Neurometric Function
0.4
0.1
1
10
Correlation (%)
100
Using signal detection
theory, a “neurometric”
function could be
produced for each
neuron and compared
with the monkey’s
psychometric function
Frequency of
Occurence
7
Mean of Noise
A
6
5
Maintained Discharge (Noise)
Distribution
4
3
2
1
0
Overlap: Possible
Confusion
Mean of Noise + Signal
7
B
6
5
Maintained Discharge (Noise) +
Response to Flash (Signal)
Distribution
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
10
11
12
Number of Action Potentials in 50 msec Period
13
14
15
d'=1.5
A
d'=1.0
B
d'=0.5
C
Srimulus Absent
Stimulus Present
ROC Curve
A
Number of
Trials
20
Correlation - 12.8%
20
Correlation - 3.2%
Correlation - 0.8%
20
Non-preferred direction
Preferred direction
0
B
Spikes per Trial
100
Percent
Correct
1.0
0.9
0.8
0.7
0.6
0.5
Psychometric Function
Neurometric Function
0.4
0.1
1
10
Correlation (%)
100
Using signal detection
theory, a “neurometric”
function could be
produced for each
neuron and compared
with the monkey’s
psychometric function
The psychometric
function for the
monkey was matched
well by directionselective neurons in
area MT.
Number of
Neurons
20
15
10
5
0
0.1
1
10
Neuron more sensitive than the monkey
Monkey more sensitive than the neuron
Threshold Ratio (neuron/behavior)
Real & apparent motion seem to be detected by
neurons in the parietal (MT) “stream”
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
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
Five Parts to this Chapter
Temporal Acuity (critical flicker frequency [CFF])
The Temporal Contrast Sensitivity Function
Temporal Summation
Masking
Motion Detection (Real and Apparent)