Perceptual Systems
This presentation is the property of its rightful owner.
Sponsored Links
1 / 39

Perceptual Systems PowerPoint PPT Presentation


  • 79 Views
  • Uploaded on
  • Presentation posted in: General

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

Download Presentation

Perceptual Systems

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Perceptual systems

Perceptual Systems


Perceptual systems

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


Perceptual systems

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


Perceptual systems

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


Perceptual systems

Major transformations of the light signal in the retina:

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


Perceptual systems

The Eye and Retina

iris

pupil

Pigment epithelium reduces scatter

Important vegetative functions

Note – blind spot - cf damage to peripheral retina

Visualize retinal blood vessels.


Perceptual systems

Visual Angle

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

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

Note: peripheral cones are fatter. Rods similar to

foveal cones

Note: color vision in peripheral retina


Perceptual systems

Visual Acuity matches photoreceptor density

Relative visual acuity

Receptor density

1 foveal cone= 0.5 min arc


Perceptual systems

  • 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)


Perceptual systems

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


Figure 2 9 photoreceptor density across the retina1

Figure 2.9 Photoreceptor density across the retina

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


Perceptual systems

Transduction: light into electrical signals

“dark light”

Note sluggish response


Perceptual systems

Major transformations of the light signal in the retina:

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


Perceptual systems

Probability of absorption of a photon depends on wavelength

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


Perceptual systems

Note: peak sensitivity in day about

the same wavelength as maximum

output of sun.

Peak

night day

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


Perceptual systems

Midget system

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.


Perceptual systems

Horizontal and amacrine

cells form inhibitory

surrounds of ganglion cells.

Why ON and OFF

cells?


Perceptual systems

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 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.


Perceptual systems

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


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.


Figure 3 7 the contrast sensitivity function red line the window of visibility

Figure 3.7 The contrast sensitivity function (red line): the window of visibility


Perceptual systems

Perceptual consequences of center surround antagonism

Brightness is coded by the differences in illumination between adjoining regions

This results from center-surround organization.


Perceptual systems

Perceptual consequences of center surround antagonism

Brightness is coded by the differences in illumination between adjoining regions


Perceptual systems

Major transformations of the light signal in the retina:

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


Perceptual systems

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


Perceptual systems

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 Dark adaptation curve

Sensitivity recovers when the retina is in the dark, rapidly for cones, slowly for rods.

(afterimages)


Perceptual systems

Major transformations of the light signal in the retina:

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)


Perceptual systems

Retinotopic 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


Perceptual systems

Signals from each eye are

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


Perceptual systems

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


Figure 2 17 dark adaptation curve1

Figure 2.17 Dark adaptation curve


Perceptual systems

Cone Photoreceptors are densely packed in the central fovea

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

same across the visual field.


Perceptual systems

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)


  • Login