Andrew Stockman
Achromatic and
chromatic vision,
rods and cones.
Andrew Stockman
Outline
Introduction
Rod and cone vision
Rod vision is achromatic
How do we see colour with cone vision?
Vision and visual pathways
Achromatic and chromatic cone vision
(colour and luminance)
NEUR3045
Visual
Neuroscience
Light
400 - 700 nm is important for vision
ROD AND CONE VISION
Achromatic and chromatic vision, rods and cones.
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Andrew Stockman
The retina is carpeted with lightsensitive rods and cones
An inverted
image is formed
on the retina
Rods and
cones
Webvision
Human photoreceptors
Cones
Rods
 Daytime, achromatic
and chromatic vision
 3 types
Long-wavelengthsensitive (L) or
“red” cone
Achromatic and chromatic vision, rods and cones.
Human photoreceptors
Cones
 Achromatic
night vision
 1 type
Rod
 Daytime, achromatic
and chromatic vision
 3 types
Long-wavelengthsensitive (L) or
“red” cone
Middle-wavelengthsensitive (M) or
“green” cone
Middle-wavelengthsensitive (M) or
“green” cone
Short-wavelengthsensitive (S) or
“blue” cone
Short-wavelengthsensitive (S) or
“blue” cone
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Our vision has to operate over an enormous range of 1012 (1,000,000,000,000) levels
Sunlight
Moonlight
Why do we have rods and
cones?
Typical ambient light levels
Indoor
lighting
Starlight
Visual function
Absolute rod
threshold
Cone
threshold
Rod saturation
begins
Damaging
levels
To cover that range we have two different types of photoreceptor...
Two systems
Rods that are optimized for low light levels
Cones that are optimized for higher light levels
Indoor
lighting
Starlight
Sunlight
Moonlight
Typical ambient light levels
Indoor
lighting
Starlight
Photopic retinal illuminance
(log phot td)
-4.3
-2.4
-0.5
1.1
2.7
Scotopic retinal illuminance
(log scot td)
-3.9
-2.0
-0.1
1.5
3.1
SCOTOPIC
Visual function
Visual function
Absolute rod
threshold
Cone
threshold
Rod saturation
begins
Sensitive
ROD SYSTEM
Lower range
Achromatic and chromatic vision, rods and cones.
Damaging
levels
Less sensitive
CONE SYSTEM
Upper range
Sunlight
Moonlight
Typical ambient light levels
Absolute rod
threshold
Scotopic levels
(below cone threshold)
where rod vision
functions alone.
A range of c. 103.5
MESOPIC
Cone
threshold
Rod saturation
begins
Mesopic levels
where rod and cone
vision function
together.
A range of c. 103
4.5
6.5
8.5
4.9
6.9
8.9
PHOTOPIC
Damage
possible
Photopic levels
(above rod saturation)
where cone vision
functions alone.
A range of > 106
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Rod vision
 Achromatic
 High sensitivity
 Poor detail and no
colour
Cone vision
 Achromatic and chromatic
 Lower sensitivity
 Detail and good colour
Facts and figures
There are about 120 million rods. They are absent in
the central 0.3 mm diameter area of the fovea,
known as the fovea centralis.
There are only about 6 to 7 million cones. They are
much more concentrated in the fovea.
Rod and cone distribution
0.3 mm of eccentricity is
about 1 deg of visual angle
Achromatic and chromatic vision, rods and cones.
Rod density peaks at about
20 deg eccentricity
At night, you have to look
away from things to see
them in more detail
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Cone distribution and photoreceptor mosaics
During the day, you have to look at
things directly to see them in detail
Original photograph
Cones peak at the centre
of vision at 0 deg
The human visual system
is a foveating system
Simulation of what we see when we fixate with cone vision…
Visual acuity gets
much poorer with
eccentricity
Credit: Stuart Anstis, UCSD
Achromatic and chromatic vision, rods and cones.
Credit: Stuart Anstis, UCSD
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The foveal region is magnified in
the cortical (brain) representation
ROD AND CONE
DIFFERENCES
What is visual psychophysics?
Psychophysicists study human vision by measuring an observer’s performance on carefully chosen perceptual tasks.
INPUT
Rod and cone differences can be
demonstrated using several techniques,
including visual psychophysics.
PROCESSING
?
STIMULUS
VISUAL
SYSTEM
OUTPUT
?
PERCEPTION
The idea is to work out what is going on inside the visual system from the relationship between the stimulus at the input and the response of the observer.
Achromatic and chromatic vision, rods and cones.
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Rod and cone
threshold versus
intensity curves
Rod-cone threshold sensitivity
differences
Rod‐cone break
How might we measure them?
Rods are about one thousand times more sensitive than cones. They can be triggered by individual photons. Achromatic and chromatic vision, rods and cones.
Spectral sensitivity differences
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Threshold versus target wavelength
measurements
Threshold versus target wavelength
measurements
Incremental flash
Incremental flash
10-deg eccentric fixation
Intensity
Intensity
10-deg eccentric fixation
Space (x)
Space (x)
Threshold versus target wavelength
measurements
Threshold versus target wavelength
measurements
Incremental flash
Incremental flash
10-deg eccentric fixation
Intensity
Intensity
10-deg eccentric fixation
Space (x)
Achromatic and chromatic vision, rods and cones.
Space (x)
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Relative sensitivity (energy)
-1
Rod and cone
spectral sensitivity
curves
Plotted as the more
conventional spectral
“sensitivity” curve
Plotted as “thresholds” versus wavelength curves
-2
-3
-4
-5
Sensitivity = 1/threshold
or
log (sensitivity) = -log(threshold)
Rod-cone dark adaptation curves
Approximate dark‐
adapted photoreceptor sensitivities.
4
Log10 quantal sensitivity
3
Rods
2
1
Rod‐cone break
L
0
S
-1
M
-2
-3
400
500
600
700
Wavelength (nm)
Achromatic and chromatic vision, rods and cones.
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Andrew Stockman
Rod-cone dark adaptation curves
Cone plateau
Temporal differences
Rods take much longer to recover after a bleach than cones
From Hecht, Haig & Chase (1937)
Suction electrode recording
Photocurrent responses
Greater temporal integration improves rod sensitivity (but reduces temporal acuity)
Achromatic and chromatic vision, rods and cones.
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Andrew Stockman
Highest flicker rates that can just be seen (c.f.f.) FLICKER INVISIBLE
Rods
Spatial differences
(visual acuity)
Cones
FLICKER VISIBLE
Photopically
(cone) equated scale
Rod and cone
visual acuities
Rod and cone
visual acuities
König (1897)
1/1.6=.63
Greater spatial integration improves rod sensitivity but reduces acuity
1/1=1
1/.2=5
König (1897)
Rods
Rods
The loss must be postreceptoral because the rods are smaller than cones in the periphery)
Rods
Rods
The acuity here is defined as the reciprocal value of the size of the gap (measured in arc minutes) that can be reliably identified.
Achromatic and chromatic vision, rods and cones.
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Directional sensitivity
differences
Stiles-Crawford
effect
Rod vision is achromatic
Vision at the photoreceptor stage is relatively simple because the output of each photoreceptor is:
UNIVARIANT
Why?
What does univariant mean?
Achromatic and chromatic vision, rods and cones.
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UNIVARIANCE
UNIVARIANCE
Crucially, the effect of any absorbed photon is independent
of its wavelength.
Crucially, the effect of any absorbed photon is independent
of its wavelength.
Rod
Once absorbed a photon produces the same change
in photoreceptor output whatever its wavelength.
UNIVARIANCE
Rod
So, if you monitor the rod output, you can’t tell which “colour” of photon has been absorbed.
UNIVARIANCE
Crucially, the effect of any absorbed photon is independent
of its wavelength.
What does vary with wavelength is the probability
that a photon will be absorbed.
Rod
This is reflected in what is called a “spectral sensitivity function”.
All the photoreceptor effectively does is to count photons.
Achromatic and chromatic vision, rods and cones.
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Rod spectral sensitivity function
Rod spectral sensitivity function (V)
More
sensitive
-1
Less
sensitive
-2
-3
CIE V'
-4
-5
-6
0
-1
10V’
-2
CIE V'
-3
log(V)
-4
-5
-6
Much more detail at
lower sensitivities
-7
400
-7
400
500
600
700
Linear sensitivity plot
Logarithmic sensitivity plot
500
600
700
800
Relative sensitivity (energy units)
0
Log relative sensitivity (energy units)
Log relative sensitivity (energy units)
(also known as the scotopic luminosity curve, CIE V)
1.0
CIE V'
0.8
0.6
0.4
0.2
0.0
400
Wavelength (nm)
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
In order of rod sensitivity:
0
> > > >
-1
> > > >
-2
-3
CIE V'
-4
-5
-6
-7
400
500
600
700
Wavelength (nm)
Achromatic and chromatic vision, rods and cones.
800
Log relative sensitivity (energy units)
Log relative sensitivity (energy units)
Rod spectral sensitivity function (V)
0
-1
So, imagine you have four
lights of the same intensity
(indicated here by the height)
-2
-3
The green will look brightest,
then blue, then yellow and lastly
the red will be the dimmest
CIE V'
-4
-5
-6
-7
400
500
600
700
800
Wavelength (nm)
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Log relative sensitivity (energy units)
Andrew Stockman
0
-1
We can adjust the intensities to
compensate for the sensitivity
differences.
-2
-3
When this has been done, the
four lights will look completely
identical.
CIE V'
-4
Rod
-5
-6
Changes in light intensity are confounded with changes in colour (wavelength)
-7
400
500
600
700
800
Wavelength (nm)
UNIVARIANCE
A consequence of univariance is that we are colour‐
blind when only one photoreceptor operates…
A change in photoreceptor output can be caused by a change in intensity or by a change in colour. There is no way of telling which. Colour or intensity
change??
Each photoreceptor is therefore ‘colour blind’, and is unable to distinguish between changes in colour and changes in intensity.
Examples: SCOTOPIC VISION, cone monochromacy
Achromatic and chromatic vision, rods and cones.
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Spectral sensitivities and the Purkinje shift Peak rod sensitivity
Peak overall cone (L&M) sensitivity
The Purkinje Shift
Log10 quantal sensitivity
0
-1
A change in the relative brightness of colours as the light level changes because of the difference in spectral sensitivity between rod and cone vision (e.g., reds and oranges become darker as rods take over)
-2
-3
L
-4
Rods
S
-5
400
450
500
550
600
650
M
700
Wavelength (nm)
Simulated: Dick Lyon & Lewis Collard at Wikimedia
With three cone photoreceptors, our colour vision is chromatic…
Cone spectral sensitivities
Log10 quantal sensitivity
0
L
M
-1
S
-2
-3
400
450
500
550
600
650
700
Wavelength (nm)
Achromatic and chromatic vision, rods and cones.
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At the photoreceptors, colour is encoded by the relative cone outputs
Or to put it another way: How is colour encoded?
Blue light
0
Log10 quantal sensitivity
So, if each photoreceptor is colour‐
blind, how do we see colour?
L
-1
S
M
-2
-3
400
450
500
550
600
650
700
Wavelength (nm)
Colour is encoded by the relative cone outputs
Colour is encoded by the relative cone outputs
Blue light
Blue light
0
L
-1
S
M
Red light
-2
-3
Log10 quantal sensitivity
Log10 quantal sensitivity
0
L
-1
S
Green light
M
-2
Red light
-3
400
450
500
550
600
650
700
Wavelength (nm)
Achromatic and chromatic vision, rods and cones.
400
450
500
550
600
650
700
Wavelength (nm)
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Colour is encoded by the relative cone outputs
Blue light
Red light
Green light
Purple light
Yellow light
White light
But what happens next (i.e., how is colour encoded
after the photoreceptors)?
POSTRECEPTORAL
COLOUR VISION
Colour phenomenology
Can provide clues about how colours are processed after the photoreceptors…
Which pairs of colours coexist in a single, uniform patch of colour?
Which pairs never coexist?
WHY?
Achromatic and chromatic vision, rods and cones.
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Reddish‐yellows?
Reddish‐greens?
Achromatic and chromatic vision, rods and cones.
Reddish‐blues?
Bluish‐yellow? 19
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The colour opponent theory of Hering
The colour opponent theory of Hering
Reds can get bluer or yellower but not greener
Yellows can get greener or redder but not bluer
The colour opponent theory of Hering
The colour opponent theory of Hering
Greens can get bluer or yellower but not redder
Blues can get greener or redder but not yellower
Achromatic and chromatic vision, rods and cones.
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The colour opponent theory of Hering
The colour opponent theory of Hering
is opposed to
R-G
is opposed to
Y-B
How might this be related to visual processing after the cones?
Some ganglion cells are
colour opponent
Imagine that this is the region
of space that the cell “sees” in
the external world
Achromatic and chromatic vision, rods and cones.
Some ganglion cells are
colour opponent
A red light falling on the central area
excites the cell (makes it fire faster)
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Some ganglion cells are
colour opponent
Some ganglion cells are
colour opponent
A green light falling on the surround area
inhibits the cell (makes it fire slower)
RED On-centre
GREEN Off-surround
Red-green colour opponency
Some ganglion cells are
colour opponent
Four
variants
GREEN On-centre
RED Off-surround
Achromatic and chromatic vision, rods and cones.
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Blue/yellow pathway
Source: David Heeger
Summary
LGN cell
responses
Trichromatic stage
LESS COMMON
Achromatic and chromatic vision, rods and cones.
Colour opponent
stage
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Colour…
So that’s colour (chromatic) vision, but what about “luminance” (achromatic) vision?
ACHROMATIC COMPONENTS
CHROMATIC COMPONENTS
Split the image into...
CHROMATIC COMPONENTS
By itself chromatic information provides relatively
limited information…
Achromatic and chromatic vision, rods and cones.
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ACHROMATIC COMPONENTS
Achromatic and chromatic cone
vision (colour and luminance)
Achromatic information important for fine detail …
In addition to neural pathways that signal colour there are also
pathways that signal intensity or luminance:
Luminance is encoded by summing the L‐ and M‐cone signals:
Blue light
L+M
+
Green light
L+M
+
Yellow light
L+M
Achromatic and chromatic vision, rods and cones.
+
Red light
+
Purple light
+
White light
+
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Colour is in many ways secondary to
luminance
Colour
after-effects
+
Achromatic and chromatic vision, rods and cones.
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+
+
+
+
Achromatic and chromatic vision, rods and cones.
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+
Rob van Lier, Mark Vergeer & Stuart Anstis
Boynton
illusion
Achromatic and chromatic vision, rods and cones.
Boynton
illusion
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Boynton
illusion
Boynton
illusion
Watercolour
effect
A wave-line
colour illusion
Seiyu Sohmiya
Achromatic and chromatic vision, rods and cones.
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Neon Disk
Neon Spreading
www.blelb.com
Interesting artistic effects occur when vision
depends only on colour (and not on luminance)
Detail from 'Plus Reversed', Richard Anuszkiewicz, 1960
from Viperlib
Achromatic and chromatic vision, rods and cones.
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Andrew Stockman
What are the postreceptoral neural substrates of the chromatic and luminance pathways?
Red‐green chromatic pathways have been linked to the parvocellular retinal stream for L‐M.
Parvocellular
midget
ganglion cell
midget
bipolars
L
These cells are chromatically opponent simply by virtue of the fact that they have single cone inputs to the centre of their receptive fields!
M
midget
ganglion cell
Achromatic and chromatic vision, rods and cones.
From Rodieck (1998)
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Koniocellular
Blue‐yellow chromatic pathways have been linked to the koniocellular stream…
diffuse
bipolar
diffuse
bipolar
S-cone
bipolar
blue-yellow
bistratified
ganglion cell
From Rodieck (1998)
Koniocellular
diffuse
bipolar
S-cone
bipolar
diffuse
bipolar
blue-yellow
bistratified
ganglion cell
Luminance pathways, which produce achromatic percepts, have been linked to the magnocellular stream.
From Rodieck (1998)
Achromatic and chromatic vision, rods and cones.
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Magnocellular
Austin Roorda, 2004
CONE ARRAY ON RETINA
diffuse
bipolar
To be able to resolve this E, the image must be sampled at enough points.
The parvocellular pathway, with its one‐to‐one cone to bipolar connections, provides enough samples.
parasol
ganglion cells
From Rodieck (1998)
The parvocellular pathway must be double‐duty supporting finely detailed luminance vision as well as much more coarse colour vision.
The magnocellular pathway, with diffuse bipolar cells and many‐to‐one cone to bipolar connections, does not.
Multiplexing colour and
luminance information
Austin Roorda, 2004
Achromatic and chromatic vision, rods and cones.
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Chromatic pathways, which produce chromatic percepts, have been linked to the parvocellular retinal stream.
Luminance pathways, which produce achromatic percepts, have been linked to the magnocellular stream, but also depend on the parvocellular stream.
Parvocellular pathway:
High spatial frequencies (spatial detail)
Low temporal frequencies
Chromatic
Lower contrast sensitivity
Parvocellular
pathway
Koniocellular
pathway
Magnocellular
pathway
Magnocellular pathway:
High temporal frequencies (motion/flicker)
Low spatial frequencies
Achromatic
Higher contrast sensitivity
From Rodieck (1998)
Achromatic and chromatic vision, rods and cones.
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