Major transformations of the light signal in the retina:
1. Temporal filtering – visual response slower than input signal.
2. Spatial filtering – local signals are combined across space to varying degrees.
3. Light adaptation – retina modifies responsiveness depending on average light level.
4. Color coding – trichromacy and color opponency
Temporal Filtering: Transduction: light into electrical signals
“dark light”
Note sluggish response
Spatial Filtering: Loss of fine detail + loss of low frequencies.
Loss of Fine detail:
Two of the factors limiting visual acuity are
- optics of the eye
- size and spacing of photoreceptors
- (in central fovea, a cone is about 0.5 min arc)
- Grating versus vernier acuity: Snellen (letter chart versus
threading a needle)
Vernier acuity is an order of magnitude better than grating acuity.
How can this be?
Sine wave gratings
Acuity is the highest frequency pattern that is just visible – ie the narrowest stripes
A similar measure is made by the Snellen letter chart:
E
Major transformations of the light signal in the retina:
1. Temporal filtering – visual response slower than input signal.
photoreceptor response is slow – increases sensitivity
2. Spatial filtering – local signals are combined across space to varying degrees.
Acuity for fine patterns determined by optics and photoreceptor layout.
3. Light adaptation – retina modifies responsiveness depending on average light level.
4. Color coding – trichromacy and color opponency
Spatial Filtering: Loss of fine detail + loss of low frequencies.
Center-surround organization of bipolar and ganglion cells
Light spot excites cell
Dark spot excites cell
Biggest response to a spot in center
Center-surround organization means that responses to uniform lights are reduced
Horizontal and amacrine
cells form inhibitory
surrounds of ganglion cells.
Why ON and OFF
cells?
Much greater convergence
for rods. Larger RF’s
Difference of Gaussian model
(DOG)
Perceptual consequences of center surround antagonism
Brightness is coded by the differences in illumination between adjoining regions
This results from center-surround organization.
Perceptual consequences of center surround antagonism
Brightness is coded by the differences in illumination between adjoining regions
Major transformations of the light signal in the retina:
1. Temporal filtering – reduced response to high temporal frequencies – Temporal
integration – a strong 1 msec flash is equivalent to a weaker 50 msec flash.
2. Spatial filtering:
- Anatomical organization of photoreceptors provides high acuity in
fovea with rapid fall-off in the periphery. (photoreceptor density)
-Convergence of photoreceptors onto ganglion cells also leads to
acuity limitations in the peripheral retina. (1 cone per midget cell in fovea)
- Center-surround antagonism reduces sensitivity to uniform fields.
3. Light adaptation
4. Color coding
Light adaptation: the problem
Need to respond over a range of 1010 – but ganglion cells can only signal 0-200 spikes/sec
Ganglion cells change
sensitivity as well as
photoreceptors.
Response on different
background intensities
tvi curve
ΔI/I = 1
Receptor adaptation
Perceptual consequence of light adaptation: hard to tell ambient light intensity
Figure 2.17 Dark adaptation curve
Sensitivity recovers when the retina is in the dark, rapidly for cones, slowly for rods.
(afterimages)
Major transformations of the light signal in the retina:
1. Temporal filtering – reduced response to high temporal frequencies – Temporal
integration – a strong 1 msec flash is equivalent to a weaker 50 msec flash.
2. Spatial filtering:
- Anatomical organization of photoreceptors provides high acuity in
fovea with rapid fall-off in the periphery. (photoreceptor density)
-Convergence of photoreceptors onto ganglion cells also leads to
acuity limitations in the peripheral retina. (1 cone per midget cell in fovea)
- Center-surround antagonism reduces sensitivity to uniform fields.
3. Light adaptation – sensitivity regulation - adjustment of operating range to mean
light level. (Light level 1010 range, ganglion cells, 102 range.)
4. Color opponency. Organization of 3 cone photoreceptors into color opponent
signals (Luminance, Red-Green, Yellow-Blue)
Exercises
Foveal blind spot
Purkinje shift
Afterimages – persistence and fading.
Probability of absorption of a photon depends on wavelength
(but receptor doesn’t know what wavelength it absorbed)
Retinotopic Organization and Cortical Magnification
Adjacent points in the world
Project to adjacent points in cortex
The brain uses more physical space
for signals from the fovea than
the periphery
Two kinds of cells in retina project
to different layers in LGN
M=magno=big
P=parvo=small
K= konio
Signals from each eye are
adjacent in LGN but remain
segregated in different layers.
Convergence occurs in V1.
Magno and parvo cells have different spatial and temporal sensitivities.
Function of the different
M and P pathways is
unclear.
Note: attempts to
Isolate a pathway
psychophysically were
unsuccessful
Hecht, Schlaer, & Pirenne, 1942
A single quantum is sufficient to excite a rod photoreceptor.
A few quanta within a small area is sufficient to give a sensation of light.
Measure number of quanta for a just detectable sensation of light – about 100 quanta.
Of those 100 quanta, about 90 are lost on the way to the retina from scatter in the eye.
So 10 quanta incident on the retina lead to a sensation of light.
Light has a Poisson distribution, so the probability that more than one photon falls on
a single rod is very small. Therefore, a single photon must excite a rod, and 10 photons excite a
retinal ganglion cell. This signal is transmitted to the brain with minimal loss and generates a
sensation of light.