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Chapter 6
Spatial Vision
There is a rational order to the material we cover!
Ch. 1 – how to measure vision
Ch. 2 – absolute threshold (see anything?)
Ch. 3 – intensity discrimination (discriminate different intensities)
Ch. 4 – how do these thresholds change with adaptation to light and dark?
Ch. 5 – thresholds for seeing simple objects spatially arranged on the retina (lines, offsets, pairs of lines, etc.)
Now – Chapter 6, expand to measuring thresholds for detecting more complex objects (edges) and testing contrast sensitivity.
The visual system recognizes objects from patterns of light and dark.
We will focus on the psychophysical measurement of the mechanisms the visual system uses to process complex patterns of light and dark
A fundamental requirement for detecting objects is to determine the location of the edges (boundaries) of the objects
Absolute luminance is less important in spatial vision than relative luminance levels
…because object contrast is relatively constant
The reflectance from real objects does not vary (at least not very much), so
contrast = (LT – LB)/LB
remains fairly constant under different lighting conditions
The visual system responds to luminance differences (e.g., contrast) more than to luminance
(the visual system is a very poor light meter),
so
the brightness of an object is not always predicted by its luminance
We see this in three examples: brightness constancy, simultaneous contrast, & assimilation
Brightness Constancy
the brightness of objects is relatively invariant even though the absolute luminance varies widely.
Brightness is determined largely by relative local contrast,
Log Test Flash Intensity
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Dark Adapted-4.2-2.2
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Neural basis: against different backgrounds, the same increase in luminance produces the same response (V)
Luminance
A.
B.
Simultaneous contrast : The brightness of an object is not always predicted by its luminance
The brightness of an object is affected by the local contrast with surrounding objects.
(brightness is increased when viewed against a background with which the object has positive contrast and reduced when the stimulus has negative contrast relative to the background)
Luminance
A.
B.
Assimilation
the brightness of a stimulus co-varies with the brightness of a surrounding stimulus
unaltered
darkened
lightened
Gradient
blurred
On the right:
Filtered to equalize luminance while accenting the contrast boundaries.
The visual system acts similarly to base brightness on local luminance changes
On the left:
In review of what we’ve just covered:
A fundamental requirement for detecting objects is to determine the location of the edges (boundaries) of the objects
Absolute luminance is less important in spatial vision than relative luminance levels
…because object contrast is relatively constant
The reflectance from real objects does not vary (at least not very much), so
contrast remains fairly constant under different lighting conditions
Several examples: if you can’t see the boundaries (edges), you can’t see the object
“Good camouflage”
The visual system is so “wired” to see edges, it detect them when there is very little information to specify the edge.
(A) (B)
A fundamental requirement for detecting objects is to determine the location of the edges (boundaries) of the objects
To repeat: the visual system recognizes objects from patterns of light and dark
Discovered by Ernst Mach (1865)
Mach also measured the speed of sound; “Mach One” is named in his honor
Mach Bands are a brightness enhancement created by the visual system and demonstrate the accentuation of luminance changes at surface boundaries
Mach bands - regions of increased or decreased brightness caused by the response of the visual system to luminance boundaries
Luminance
Brightness
A.
B.
C.
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Receptive field positions relative to light and dark panels
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3
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Neural responses
The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands
• http://www.sumanasinc.com/webcontent/animations/content/receptivefields.html
This is a really useful website for understanding how center-surround ganglion cells work. I will show it in class but you would benefit from viewing it before (or after) class.
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Receptive field positions relative to light and dark panels
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Neural responses
The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands
Center-surround organization
a more detailed look at how the surround interacts with the center
When light increases in both the center and surround, the surround subtracts from the center.
Light on in the receptive field center produces depolarization in the bipolar cell and increased firing rate (spikes/second) in the on-center ganglion cell
i = sign-inverting (metabotropic) synapses
c = sign-conserving (ionotropic) synapses
Light on in the receptive field surround produces hyperpolarization in the bipolar cell and decreased firing rate (spikes/second) in the on-center ganglion cell
The depolarization caused by light on in center is opposed by the hyperpolarization produced by light on in the surround
Thus, when both the whole center and the whole surround are stimulated with light, surround cancels almost all the depolarization caused by light in the center. The result is a very small depolarization and slight increase in ganglion cell firing (the center is slightly stronger than the surround)
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Receptive field positions relative to light and dark panels
1
2
3
4
Neural responses
The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands
Receptive field center and surround of #1 are entirely in light. Surround subtracts from center, but center is a little stronger so cell responds a little to the high intensity
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Receptive field positions relative to light and dark panels
1
2
3
4
Neural responses
The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands
Receptive field center and surround of #4 are entirely in darker band. Surround subtracts from center, but center is a little stronger so cell responds a little to the low intensity, but less than #1
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Receptive field positions relative to light and dark panels
1
2
3
4
Neural responses
The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands
Receptive field center of #2 is are entirely in light. Only part of the surround is in the light, so the surround subtracts less from the center, so cell responds more strongly than #1. This causes the brightness increase at the right edge of the light bands.
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Receptive field positions relative to light and dark panels
1
2
3
4
Neural responses
The responses of retinal neurons underlie both the dependence of brightness on contrast and the brightness enhancement of Mach Bands
Receptive field center of #3 is entirely in the darker region, so the center is not strongly driving the cell. Some of the surround is in the light, so it subtracts strongly from center, so cell responds less than #4, causing the brightness decrease at the left edge of the dark bands.
Another reason having a receptive-field surround is important!
The sensitivity (of the visual system) to spatial luminance changes is measured with gratings of varying spatial frequency and contrast
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RelativeLuminance
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Horizontal Position (arbitrary units)
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Sensitivity of spatial luminance changes is measured with gratings of varying frequency and contrast
Horizontal Position (arbitrary units)
0 90 180 270 360 450 540 630 720
Relative Luminance
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20
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60
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Width of 1 cycle
B
ALmax of A
Lmax of B
Lmin of B
Lmin of A
Mean Luminance
Sine-wave gratings are measured in terms of their spatial frequency defined as the number of cycles per degree of visual angle
Michelson (grating) CONTRAST
When periodic stimuli like gratings are used, contrast is defined as:
Contrast L L L L ( ) / ( )max min max min
where Lmax is the maximum luminance in the grating, and Lmin is the minimumluminance in the grating.
In a sine-wave grating, Lmax and Lmin are symetrically arranged around the average
luminance, which is defined as:
(Lmax + Lmin)/2
where Lmax is the maximum luminance in the grating, and Lmin is the minimum
luminance in the grating.
The spatial CSF is the inverse of the contrast threshold to detect various spatial frequencies
Spatial Frequency (cycles/deg)
0.5 1 2 5 10 20 50 100
ContrastSensitivity
1
2
5
10
20
50
100
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500
1000
Stimuli are invisiblein gray area
Stimuli are visiblein white area
High
Low
Large Small
AcuityLimit
Contrast Sensitivity Function(CSF)
Contrast
Grating Bar Width
Snellen 6/6Equivalent
The spatial CSF is the inverse of the contrast threshold to detect various spatial frequencies
1000
100
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10.1 1 10 100
Spatial Frequency (cycles/deg)
Co
ntr
ast
Sen
siti
vity
Snellen 6/6 Equivalent
Low spatial frequency rolloff
Cutoff highspatial frequency
Peak contrast sensitivity
The spatial CSF is “band pass.” We are more sensitive to a “band” of mid-range spatial frequencies (3 – 10 cycles/deg)
As shown in Chapter 5, the cutoff high spatial frequency (the highest spatial frequency that can be resolved at a contrast of 1.0) is very close (nearly identical) to the spatial resolution acuity (MAR)
How to measure the spatial CSF
1. Pick a series of spatial frequencies (cycles per degree)
2. At each spatial frequency, determine the threshold contrast
3. Contrast sensitivity is the inverse (reciprocal) of the threshold contrast
4. Plot the contrast sensitivity for that all the spatial frequencies that were measured
5. Connect the data points with straight lines
Spatial Contrast Sensitivity Function
(Spatial CSF)
Spatial Frequency (cycles/deg)
0.1 0.3 0.5 1 3 5 10 20 50 100
Relative ContrastSensitivity
0.1
0.3
0.5
1FalconMacaqueOwl
MonkeyCatGoldfish
The band-pass shape of the spatial CSF is the same in many species (but peak at different spatial frequencies)
Optical defocus (blur – uncorrected refractive error) affects high spatial frequencies more than low
The cutoff high spatial frequency matches the optical limit
So far in this chapter:
1) Detecting the boundaries of objects is essential for seeing objects
2) The center-surround receptive fields of retinal ganglion cells cause the visual system to emphasize luminance changes (contrast) that typically occur at the edges of objects (this produces Mach bands)
3) The sensitivity of the visual system to spatial changes is measured by the spatial contrast sensitivity function (CSF)
Now: the individual neurons (retina, LGN, visual cortex) have a band pass spatial CSF with more narrow tuning than the whole CSF due to receptive field size
Relative Amplitude ofNeural Response
Spatial Frequency (cyles/deg)
A. Retinal Ganglion Cells
B. Lateral Geniculate Cells
C. Cortical Simple Cells
Textbook figure Original figure
Underlying the spatial CSF are individual neurons, each with a spatial CSF
What is plotted is relative amplitude (strength, firing rate, # of action potentials/sec) of neural response to a constant contrast.
This is based on the fact that neurons respond more strongly to a stimulus that is farther above threshold
High contrast sensitivity
High contrast sensitivity
High contrast sensitivity
Low (0) contrast sensitivity
Low (0) contrast sensitivity
Low (0) contrast sensitivity
A retinal cell’s center-surround receptive-field organization produces a spatial CSF for that cell
In previous slides, we saw how the surround subtracts from the center (when light increases in both the center and surround)
Now, we see how the surround adds to the center (when light increases in the center and decreases in the surround)
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B
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Good fit; strong response;Therefore, lowest threshold
Light in center & light in Surround; center slightlyStronger. Will respond but contrast threshold is high(low sensitivity)
Light & dark in center cancel each other; light and dark in surroundCancel each other;infinite threshold (cut-off)
Sensitivity profile of the Receptive-field center and surround
The surround adds to the center (when light increases in the center and decreases in the surround)
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So far in this chapter:
1) Detecting the boundaries of objects is essential for seeing objects
2) The center-surround receptive fields of retinal ganglion cells cause the visual system to emphasize luminance changes (contrast) that typically occur at the edges of objects (this produces Mach bands)
3) The sensitivity of the visual system to spatial changes is measured by the spatial contrast sensitivity function (CSF)
4) the individual neurons have a band pass spatial CSF with more narrow tuning than the whole CSF due to receptive field size
NOW:
Cells with a variety of receptive-field sizes provide the basis for the behaviorally measured spatial CSF
Test the whole creature’s threshold, rather than the threshold for an individual neuron
Spatial Frequency (cycles/deg)
0.3 1 3 10 30
ContrastSensitivity
1
3
10
30
100
Do neurons with different receptive-field sizes produce the human spatial CSF?
Spatial Frequency (cycles/deg)
1 10 100
ContrastSensitivity
1
10
100
1 10 100
RelativeThresholdElevation
0.01
0.1
1
A B
Contrast adaptation demonstrates that there are multiple spatial frequency channels in the human visual system
Blakemore and Campbell (1969); contrast adaptation with 7.1 cycles/deg
A
Spatial Frequency (cycles/deg)
1 10 100
ContrastSensitivity
1
10
100
1 10 100
RelativeThresholdElevation
0.01
0.1
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A B
This was a test of contrast adaptation at a particular spatial frequency.
Blakemore and Campbell measured contrast adaptation using other spatial frequencies as well
Spatial Frequency (cycles/deg)
0.3 1 3 10 30
ContrastSensitivity
1
3
10
30
100
Parvocellular pathway neurons
Magnocellular pathway neurons
So far in this chapter:
1) Detecting the boundaries of objects is essential for seeing objects
2) The center-surround receptive fields of retinal ganglion cells cause the visual system to emphasize luminance changes (contrast) that typically occur at the edges of objects (this produces Mach bands)
3) The sensitivity of the visual system to spatial changes is measured by the spatial contrast sensitivity function (CSF)
4) the individual neurons have a band pass spatial CSF with more narrow tuning than the whole CSF due to receptive field size
5) the spatial CSF is produced by neurons with more narrow tuning than the whole CSF (e.g., “channels”)
Spatial Frequency (cycles/deg)
1 10
ContrastSensitivity
1
10
100
900 90 9 0.9 0.09 0.009 0.0009
Mean Retinal Illumiance (Trolands)
The spatial CSF varies with luminance
As mean luminance decreases, peak contrast sensitivity decreases, the spatial frequency of the peak decreases, cutoff (acuity) decreases and low frequency rolloff becomes less
Spatial Frequency (cycles/deg)
1 2 4 8 16 32 64
ContrastSensitivity
1
10
100
1.5o
4.0o
7.5o
14o
30o
Fovea
The spatial CSF shifts toward lower spatial frequencies with increasing retinal eccentricity
Applying the idea of spatial frequency channels to looking at real objects in the world:
We are not always presented with high-contrast images:
Applying the idea of spatial frequency channels to looking at real objects in the world:
1) All spatial luminance patterns on the retina (from real objects in the world) are comprised of sine-wave gratings of particular spatial frequencies and contrasts
2) Different subsets of neurons respond to the pattern of spatial frequencies present in the image
All spatial luminance patterns are comprised of sine-wave gratings of particular spatial frequencies and contrasts
Luminance profile of a single horizontal line in the photo
FF+3FF+3F+5FF+3F+5F+7FF+3F+5F+7F+9FF+3F+5F+7F+9F+11F
F3F5F7F9F11F
Fourier Analysis: can determine the component sine waves & contrasts of complex waveforms
Combined
F
M
3F
M F 3F
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of
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Do a Fourier analysis of this single line to get the component spatial frequencies and contrasts
Spatial Frequency
1 2 4 8 16 32 64 128
RelativeContrast
0.0001
0.001
0.01
0.1
1
This shows the spatial frequencies that are present in the image along that narrow line. There are many different spatial frequencies present, and they occur in differing amounts (relative contrast)
Spatial Frequency
1 2 4 8 16 32 64 128
RelativeContrast
0.0001
0.001
0.01
0.1
1
100
10
11 10 100
Spatial Frequency (cycles/deg)
The high frequencies are not detected by our visual system and the low spatial frequencies are attenuated by our visual system, emphasizing the mid-range spatial frequencies
FF+3FF+3F+5FF+3F+5F+7FF+3F+5F+7F+9FF+3F+5F+7F+9F+11F
F3F5F7F9F11F
Fourier Synthesis: can construct complex waveforms by adding together simple ones
Can use this to predict what we will see!
1 3 5 7 11 17 25
0.0 0.2 0.4 0.6 0.8 1.0
Spatial Frequency (cycles/deg)
1 3 5 7 11 17 25
Relative Contrast
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1000
Horizontal Position (deg)0.0 0.2 0.4 0.6 0.8 1.0
Relative Luminance
A
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Fourier Analysis
Obvious examples of spatial frequencies in the world
O.K. The visual scene contains many spatial frequencies.
Do we have to have high spatial frequencies (near the acuity limit) to “see objects”, or can we “see objects” with just low & intermediate spatial frequencies?
Rev 2
We CAN “see objects” with just low & intermediate spatial frequencies!
Good contrast sensitivity is more important than resolution for some spatial tasks
We do not always need to see in high contrast conditions
VA measures “quantity” of vision
spatial CSF measures “quality” of vision
In the Pelli-Robson chart, letter size remains the same, but contrast decreases
Mobility: Pelli-Robson chart performance is a good predictor of time for patients with age-related macular degeneration to complete an obstacle course & number or errors (collisions).
Low vision patients: reading speed is affected by spatial CSF losses
Possible new treatments: selective contrast enhancement at certain spatial frequencies to boost face recognition
Many forms of ocular pathology affect the spatial CSF
glaucoma, ARMD, diabetic retinopathy, cataract, keratoconus
but many of these affect high spatial frequencies and are detected by loss of VA
no unique patterns of CSF loss have emerged that are of use in differential diagnosis of a particular condition
VA is easier to measure and almost as effective for most conditions.
But, at some time, you will see a patient whose VA is OK but they complain about difficulty seeing, and have a spatial CSF deficit.