• Foveal cones are about 2.4 μm in
diameter (0.7 min of arc)
• Peripheral cones are about 5.8 μm in
diameter (1.7 min of arc)
Ganglion cells
Ganglion cells
• Claim that ganglion cells are not
photosensitive. (But, recall Pritchard.)
• Ganglions cells fire action potentials,
where photoreceptors use graded
potentials.
Ganglion cells
~ 1.25 million ganglion cells
10-15 types of RGC
~ 100 million rods
~ 5 million cones
Recall that, in photoreceptors, more light
means less electrical current.
• Ganglion cells are responsive only to
limited areas of the visual field; they have
limited receptive fields.
• Ganglion cells can be ON-center with an
OFF-surround.
• Ganglion cells can be OFF-center with an
ON-surround.
Lateral Inhibition
• Stimuli in the surround suppresses activity
in the center.
• In other words, surround stimuli are
antagonistic to center stimuli.
Ganglion cells with this concentric
arrangement (i.e., ON-center, OFFsurround or OFF-center, ON-surround)
found primarily in mammals with a fovea.
Retinas lacking foveas, unlike most
mammalian retinas, can process image
motion and direction of motion.
How do photoreceptors, amacrine, bipolar,
and horizontal cells work to make ganglion
cells respond as they do?
Blake & Sekuler are pretty sketchy on
amacrine cells.
Amacrine cells are inhibitory.
Amacrine cells come in about 50
morphological types.
Horizontal cells
Each horizontal cell connects to numerous
photoreceptors.
Each photoreceptor connects to numerous
horizontal cells.
Horizontal cells inhibit inactive
photoreceptors more than active
photoreceptors.
Wässl, H. (2004). Parallel Processing in the Mammalian Retina. Nature Reviews
Neuroscience, 5, 1-11.
Bipolar cells
• These contact photoreceptors.
• Some bipolars: More current, less active.
• Some bipolars: More current, more active.
• More light, less current, more active.
• Less light, more current, more active.
• Inverting
• Non-inverting
Some numerical details
• In the fovea, the receptive field center of a
retinal ganglion cell is connected to one
cone cell.
• In the fovea, the surround of retinal
ganglion cell is connected to around 1-3
cone cells.
Receptive Field Size
• The farther from fovea, the larger the field.
• At each degree of eccentricity, there are
“big field” and “small field” cells. The “big”
fields are roughly three times the area of
the “small” fields.
Three types of retinal ganglion cells
• M cells (magnocellular) (large)
• P cells (parvocellular) (small)
• K cells (koniocellular) (very small)
* Footnote 2, p. 82 has the terminology backward.
M cells are parasol; P cells are midget.
M cells vs. P cells
• M are bigger, hence conduct more quickly.
• P cells are more numerous (80% of ganglion
cells in primates are P cells)
• For any degree of eccentricity, P cells have
smaller receptive fields.
• M cells are more responsive to luminance
differences
• M cells are faster responders than are P cells.
• M cells are color indifferent; P cells are color
selective.
Analysis of Perceptual Phenomena
in Terms of Processes in Ganglion
Cells
• Hermann Grid Illusion
• Mach bands
Questions about the Hermann Grid
Illusion
• Why are spots only at the intersection?
• Why are there no spots in central vision?
Problems for the RGC Theory
• Blake & Sekuler:
– Illusion varies with orientation.
– Illusion varies with spatial extent of grid
– Illusion varies with regularity of grid
Problems for the RGC Theory
Schiller & Carvey, 2005:
1. Illusion is perceived over a large range of
sizes.
2. Illusion is reduced when the grid is rotated
by 45º.
3. Illusion can be reduced by manipulations
that do not alter the antagonistic/surround
activation of retinal ganglion cells.
4. The ratio of the square size to the width of
the intersecting bars is an important factor.
Problems for the RGC Theory
Schiller & Carvey, 2005:
5. Enhancing center/surround antagonism at the
intersections of bars does not enhance the
illusory effect.
6. Varying contrast and color produces illusory
effects not readily handled by the theory.
7. The spatial arrangement of RGC receptive
fields is not what has been assumed by the
theory.
• Lightness contrast: Physically identical
materials appear to differ in lightness.
– Previous example
• Lightness constancy: Physically distinct
materials appear to be the same in
lightness.
– Text in bright sunlight and indoors
Does RGC activity explain
lightness constancy?
• Hans Wallach, (1963): An RGC will
produce the same response to the same
ratio of center-surround illumination.
Convergence
• Overall 100 million photoreceptors to 1.25
million RGC
• But …
– Convergence varies systematically with eccentricity
– In the fovea, one cone per RGC center.
– In the periphery, 100’s of photoreceptors per RGC
center.
– The more receptors an RGC has, the larger its
receptive field
Extreme convergence: All
photoreceptors to 1 RGC
• The RGC would be unable to resolve
anything in its visual field. (Unable to
distinguish differences in the spatial
distribution of light.)
• The RGC would be very light sensitive.
Less extreme convergence
The Duplex Eye
There is a trade-off between spatial resolution and
light sensitivity.
The eye meets this trade off by having two
systems, one for spatial resolution and one for
light sensitivity.
Many RGC gather inputs from both rods and
cones and switch between which make the most
input to them.
• Scotopic vision = low light vision = roddriven vision
• Mesopic vision = middle light vision
• Photopic vision = bright light vision = conedriven vision
Foveal convergence
• 33% of RGC receive cone input from retina.
• Retina = ~2% of retinal cells
• Blake & Sekuler: ratio of cones to RGC = 1:1.
• Schiller & Carvey: “in the central retina the
number of retinal ganglion cells outnumbers the
cone photoreceptors by a factor of 2 to 4).
• Perceptual resolution closely matches degree of
convergence
Illustrations of blurred peripheral
vision
Scotopic vision
• Under ideal conditions, a rod can react to
a single photon.
• But, the response of a single rod is
insufficient to yield a percept.
Scotopic vision
Most effective when:
1. The rods are completely dark adapted (~35
minutes), i.e. when all the opsin molecules in
the plasma disks have been refilled with
retinal.
2. The photons strike a rod rich area of the retina.
(~3mm from the edge of the fovea.)
3. The light entering the eye must be at the peak
rod sensitivity
Photopic vision
1. The cones are completely light adapted
(?)
2. The photons strike the cone rich area of
the retina, i.e., the fovea.
3. The light entering the eye must be at the
peak cone sensitivity
Purkinje shift
• During the day, 550nm light appears
brighter than 500 nm light.
• During the night, 500 nm light appears
brighter than 550 nm light.
Dark Adaptation
• Partially a function of pupil size.