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Perceptual Systems. Sources: Wolfe, J, Kluender , K, Levi, D. et al Sensation & Perception 2012 3 rd ed Sinauer – 15% discount and free shipping if ordering online from Sinauer Kandel , Schwartz & Jessel Principles of Neural Science McGraw-Hill 5 th ed

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Sources:

Wolfe, J, Kluender, K, Levi, D. et al Sensation & Perception 2012 3rdedSinauer – 15% discount

and free shipping if ordering online from Sinauer

Kandel, Schwartz & Jessel Principles of Neural Science McGraw-Hill 5thed

Gazzaniga, Ivry, Mangun Cognitive Neuroscience Norton, 3rded

Readings, Class 1:

Wolfe Ch 1,2

Kandell et al Chapters 26


Visual Perception: what do we want to explain?

How do we get visual information from the world and use it to control

behavior?

Traditional sub-areas - visual sensitivity

color vision

spatial vision

temporal vision

binocular vision/ depth perception

texture perception

motion perception

surfaces, segmentation

object perception

attention

perceptual learning

spatial orientation

eye movements


The constructive nature of perception: a process of guessing the state of the world

from sometimes incomplete sensory data.

Constructive in the sense that it relies on memory representations of past experience


Major transformations of the light signal in the retina: the state of the world

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.

Color coding– trichromacy and color opponency


The Eye and Retina the state of the world

iris

pupil

Pigment epithelium reduces scatter

Important vegetative functions

Note – blind spot - cf damage to peripheral retina

Visualize retinal blood vessels.


Visual Angle the state of the world

x

a

d

tan(a/2) = x/d

a = 2 tan-1 x/d

Most of the optical power of the eye is accomplished by the cornea

18mm

1 diopter = 1/focal length in meters

55 diopters = 1/.018

0.3mm = 1 deg visual angle


Optical correction errors
Optical correction errors the state of the world

Blur circle

Presbyopia = stiffening of lens with age so it is no longer variable


Figure 2 9 photoreceptor density across the retina
Figure 2.9 Photoreceptor density across the retina the state of the world

Note: peripheral cones are fatter. Rods similar to

foveal cones

Note: color vision in peripheral retina


Visual Acuity matches photoreceptor density the state of the world

Relative visual acuity

Receptor density

1 foveal cone= 0.5 min arc


  • Two of the factors limiting visual acuity the state of the worldare

  • - 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 the state of the world 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


Figure 2 9 photoreceptor density across the retina1
Figure 2.9 Photoreceptor density across the retina the state of the world

Question: Rods are small and dense. Why isn’t acuity better in the peripheral retina?


Transduction: light into electrical signals the state of the world

“dark light”

Note sluggish response


Major transformations of the light signal in the retina: the state of the world

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.

Color coding– trichromacy and color opponency


Probability of absorption of a photon depends on wavelength the state of the world

(but receptor doesn’t know what wavelength it absorbed)


Note: peak sensitivity in day about the state of the world

the same wavelength as maximum

output of sun.

Peak

night day

Why blue flowers are brighter and red flowers are darker at dusk.


Midget system the state of the world

preserves acuity in

the central fovea

M= magnocellular, P= Parvocellular

Convergence: many rods converge onto a single rod bipolar cell, and several cones converge

onto a diffuse bipolar cell. This allows the signal to be amplified.


Horizontal and the state of the worldamacrine

cells form inhibitory

surrounds of ganglion cells.

Why ON and OFF

cells?


Hecht, the state of the worldSchlaer, & 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 form 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.


Center-surround organization of bipolar and ganglion cells the state of the world

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


Figure 3 6 sine wave gratings illustrating low a medium b and high c spatial frequencies
Figure 3.6 Sine wave gratings illustrating low (a), medium (b), and high (c) spatial frequencies

These grating stimuli are called “Gabor patches”. Spatial frequency is measured in

Cycles per degree, and contrast is a measure of the difference in intensity between

light and dark bars.



Perceptual consequences of center surround antagonism the window of visibility

Brightness is coded by the differences in illumination between adjoining regions

This results from center-surround organization.


Perceptual consequences of center surround antagonism the window of visibility

Brightness is coded by the differences in illumination between adjoining regions


Major transformations of the light signal in the retina: the window of visibility

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

Color coding


Light adaptation: the problem the window of visibility

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


Loss of sensitivity at low temporal frequencies (slow rate of change of intensity) is a

consequence of light adaptation (sensitivity changes with average light level)

(afterimage fading)


Figure 2 17 dark adaptation curve
Figure 2.17 of change of intensity) is a 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: of change of intensity) is a

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.)

Color opponency. Organization of 3 cone photoreceptors into color opponent

signals (Luminance, Red-Green, Yellow-Blue)


Retinotopic of change of intensity) is a Organization and Cortical Magnification

The brain uses more physical space

for signals from the fovea than

the periphery

Adjacent points in the world

Project to adjacent points in cortex


Signals from each eye are of change of intensity) is a

adjacent in LGN but remain

segregated in different layers.

Convergence occurs in V1.

Two kinds of cells in retina project

to different layers in LGN

M=magno=big

P=parvo=small

K= konio


Magno of change of intensity) is a 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


Figure 2 17 dark adaptation curve1
Figure 2.17 Dark adaptation curve of change of intensity) is a


Cone Photoreceptors are densely packed in the central fovea of change of intensity) is a

Note: despite lower density of cones in peripheral retina, color vision is basically the

same across the visual field.


Figure 2.11 Blue, green, and red represent the S-, M-, and L-cones, respectively, of a living human being in a patch of retina at 1 degree from the fovea

  • 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)


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