Chapter 6 Spatial Vision. There is a rational order to the material we cover! Ch. 1 – how to...

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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Log V

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1.0

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B

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G H I

A'

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A''

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Dark Adapted-4.2-2.2

D'

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:


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)


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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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.

http://www.sumanasinc.com/webcontent/animations/content/receptivefields.html

http://www.sumanasinc.com/webcontent/animations/content/receptivefields.html

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Neural responses


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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Neural responses


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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Neural responses


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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Neural responses


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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Neural responses


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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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

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Stimuli are invisiblein gray area

Stimuli are visiblein white area

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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

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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)


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Relative ContrastSensitivity

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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



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)

OnOffOff

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OnA

B

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OnOffOff





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


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ContrastSensitivity

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Do neurons with different receptive-field sizes produce the human spatial CSF?


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ContrastSensitivity

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RelativeThresholdElevation

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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


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ContrastSensitivity

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1 10 100

RelativeThresholdElevation

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This was a test of contrast adaptation at a particular spatial frequency.

Blakemore and Campbell measured contrast adaptation using other spatial frequencies as well


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ContrastSensitivity

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Parvocellular pathway neurons

Magnocellular pathway neurons





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”)


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ContrastSensitivity

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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


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ContrastSensitivity

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1.5o

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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

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M

3F

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litu

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of

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bje

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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

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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

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11 10 100


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!

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Relative Contrast

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Horizontal Position (deg)0.0 0.2 0.4 0.6 0.8 1.0

Relative Luminance

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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?

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.

Chapter 6 Spatial Vision. There is a rational order to the material we cover! Ch. 1 – how to...

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