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

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)

Spatial Filtering: Loss of fine detail + loss of low frequencies.

Sine wave gratings

Acuity is the highest frequency pattern that is just visible – ie the narrowest stripesA similar measure is made by the Snellen letter chart: E

Vernier acuity is an order of magnitude better than grating acuity.How can this be?

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

Center-surround organization of bipolar and ganglion cells

Center-surround organization means that responses to uniform lights are reduced

Light spot excites cellDark spot excites cell

Biggest response to a spot in center

Spatial Filtering: Loss of fine detail + loss of low frequencies.

Horizontal and amacrine cells form inhibitory surrounds of ganglion cells.

Why ON and OFFcells?

Much greater convergencefor 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 regionsThis 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 problemNeed to respond over a range of 1010 – but ganglion cells can only signal 0-200 spikes/sec

Receptor adaptation

Response on different background intensities

tvi curve

ΔI/I = 1

Ganglion cells change sensitivity as well as photoreceptors.

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

The brain uses more physical space for signals from the fovea thanthe periphery

Adjacent points in the worldProject to adjacent points in cortex

Signals from each eye areadjacent in LGN but remainsegregated in different layers.Convergence occurs in V1.

Two kinds of cells in retina projectto different layers in LGN

M=magno=bigP=parvo=smallK= konio

Magno and parvo cells have different spatial and temporal sensitivities.

Function of the differentM and P pathways isunclear.

Note: attempts to Isolate a pathwaypsychophysically 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 aretinal ganglion cell. This signal is transmitted to the brain with minimal loss and generates a sensation of light.