Lateral InhibitionLateral Inhibition - Lina...

Lateral InhibitionLateral Inhibition

Copyright 2010 by Lina J. Karam

Lateral InhibitionLateral InhibitionMach Band Effect


Lateral InhibitionLateral Inhibition Mach Band Effect ‐ Luminance versus brightness

Luminance

ray level

Brightness

Distance

Gr

Distance


Lateral Inhibition Simultaneous Contrast – Luminance versus Brightness

All the center squares have the same intensity.


Luminance and Brightness: Contrast Sensitivity g yContrast sensitivity experiment

L

L+ΔL

The smallest ΔL for which dot is visible is the just‐noticeable difference (JND) or detection threshold.

C t t iti it i l ti l t JND ΔLContrast sensitivity inversely proportional to JND ΔLWeber’s law: ΔL/L= constant (~0.01)

• Equal increments in log L should be perceived to be ll d ff l l b hequally different: linear relation between Brightness B

and log of luminance• Log relation between Brightness B and Luminance L

i t d b f ti


approximated by a power function

Luminance and Brightness: Contrast Sensitivity

Relation between Brightness and Luminance

g y

4.0LB =B1.0

0.8

0.6

0.40.4

0.2

L0.2 1.00.4 0.6 0.8


Luminance and Brightness: Contrast Sensitivity g yContrast sensitivity experimentVariability between subjects result in

LVariability between subjects result in different JND value per subject Psychometric function: probability

L+ΔL

y p yof detection for a background illumination L

xx

Probabilityof Detection, P

β

β

) |/|(

) |/|(

21

1JND

JND

LL

LL

P

ePΔΔ−

ΔΔ−

−=

−=

x

ΔLΔL


ΔLΔLJND

Contrast Sensitivity Contrast sensitivity experiment:sinusoidal grating over flat L

β

y

background of constant luminance L+ΔL

β

L is mean luminance of sinusoidal grating and background ΔL is amplitude of sinusoidal grating β is orientation of sinusoidal grating The smallest contrast ΔL for which sinusoid is visible is the just‐

noticeable difference (JND) or contrast sensitivity detection threshold at the considered sinusoid’s frequency and orientation

The JND varies with the spatial and angular (orientation) frequencies of the sinusoidal grating for a given background luminance L.


Contrast SensitivityContrast SensitivityVisual acuity: ability to detect spatial details Contrast Sensitivity (spatial): how visual acuity changesContrast Sensitivity (spatial): how visual acuity changes with spatial frequency and orientation for a given surrounding luminance


Contrast SensitivityContrast SensitivityVisual acuity: ability to detect spatial details Contrast Sensitivity (spatial): how visual acuity changesContrast Sensitivity (spatial): how visual acuity changes with spatial frequency and orientation for a given surrounding constant luminance


Contrast SensitivityContrast SensitivitySpatial frequency

i l d h l h f h li h• is not related to the wavelength of the light • is the number of oscillations in a given space

0o

10o

30o

0 1o10o0

30o

0o 1o

4 cycles/retinal arc

30o

10o0o

10o

30o

0o 0.5o

4 cycles/half retinal arc 8 l / ti l


30o = 8 cycles/retinal arc

Contrast SensitivityContrast SensitivitySpatial contrast sensitivity function varies with orientation

Spatio temporal contrast sensitivity function: how visual acuitySpatio-temporal contrast sensitivity function: how visual acuity changes with spatial frequency, orientation, and temporal frequency for a given surrounding constant luminanceA h ti h ti t t iti itCopyright 2010 by Lina J. Karam

Achromatic versus chromatic contrast sensitivity

Luminance Masking/Light AdaptationLuminance Masking/Light AdaptationContrast sensitivity function varies in function of the background luminance levels. gLuminance masking experiments:•Vary luminance level of flat background over which the sinusoidal grating is superimposed and repeat “contrastsinusoidal grating is superimposed and repeat contrast sensitivity” experiment for each luminance level•Obtain luminance-adjusted contrast sensitivity JND threshold

L

β

L

β

L

β

L+ΔLL+ΔLL+ΔL


Base CSF (e.g., neutral gray flat background)

Contrast MaskinggConstant background replaced with a “masking stimulus” (masker) on

TargetMasker

g ( )which the target signal (to be detected) is superimposed.

ΔL is amplitude of target sinusoidal gratingTh ll ΔL f hi h h i i ibl iThe smallest ΔL for which the target is visible is contrast

masking detection (JND) threshold.

Repeat “contrast sensitivity” like experiment for different maskers (background sinusoids with different frequencies, i i d li d )


orientations, and amplitudes)

Color VisionColor VisionCones in the human retina contains one of 3 pigments

• sensitive to short wavelengths (blue)sensitive to short wavelengths (blue)• sensitive to middle wavelengths (green)• sensitive to long wavelengths (red)g g ( )

Comparison of outputs of different cones

nce

S‐ConeM‐Cone L‐Cone1

ized

abs

orba

S Cone

Nor

mal

i

0

Higher Frequency Lower Frequency


500nm wavelength (nm), λ400nm 600nm 700nm

Color VisionVisible range of electromagnetic spectrum

λ380nm 780nm

γ rays x rays ultraviolet visible infrared microwaves radioλ

T T: time period in secondsλT T: time period in secondsf : frequency in cycles/sec f = 1/T cycles/second (Hertz)

λ: spatial period in metersλ: spatial period in metersf = c/λ cycles/second (Hertz)λ = c/f = c .Tf


TimeSpace

f

Color VisionColor VisionCones in the human retina contains one of 3 pigments

• sensitive to short wavelengths (blue)sensitive to short wavelengths (blue)• sensitive to middle wavelengths (green)• sensitive to long wavelengths (red)g g ( )

Majority of cones are “red” and “green” cones; less than bl10% “blue”

“Blue” cones have high color discrimination sensitivityBlue cones have high color discrimination sensitivity than the other cone types – humans can distinguish between a variety of blue hues


y

Color VisionColor VisionRelative color sensitivity of cones

• Typical normalizes absorption spectra of cones also

ance

1

Typical normalizes absorption spectra of cones, also known as spectral response curves

SB(λ) SG(λ) SR(λ)

lized

abs

orb

wavelength (nm) λ500nm

Nor

mal

400nm 600nm0

Higher Frequency Lower Frequency

700nm wavelength (nm), λ500nm400nm 600nm 700nm


Color VisionColor Vision Cortical Pegsconcerned with color

Visual Cortexcolor


Color Sensitive CellSingle‐Opponent Color Sensitive Cell

G

+‐ ‐

‐‐

Red

Green +

Green

Red

‐

‐‐

‐


Color Sensitive CellSingle‐Opponent Color Sensitive Cell

Y ll Bl

+‐ ‐

‐‐

Yellow

Blue +‐

‐‐

‐Blue

Yellow


Color Sensitive CellDouble‐opponent Color‐Contrast Cortical Cell

R+G+R‐

G‐R

G+R‐

GreenRed

Best Stimulus

R

R+G‐

Double‐opponent red‐green color‐contrast cortical cells with concentric receptive fields are highly sensitive to

Some Double‐opponent simple cells also exhibit

GreenRedG‐


receptive fields are highly sensitive to simultaneous color contrast

simple cells also exhibit orientation selectivity

Color VisionRelative color sensitivity of cones

3 types of cones: each has a different peak absorption– 3 types of cones: each has a different peak absorption frequency

– Approximately, 6 millions cones and 100 millions rods – Majority of cones are “red” and “green” cones; less than

10% “blue” cones“Bl ” h hi h l di i i i i i i– “Blue” cones have high color discrimination sensitivity than the other cone types – humans can distinguish between a variety of blue huesy


Color VisionLight

• Electromagnetic radiation • It is described as a function P(λ) of optical power intensity asIt is described as a function P(λ) of optical power intensity as

a function of wavelength, λ• This function is called Spectral Power Distribution (SPD)

Visible LightVisible Light• Electromagnetic radiation that stimulates our visual response• SPD of visible light contain wavelengths in the range 370 to

730 nanometers (nm)730 nanometers (nm)• Color of light depends on the distribution of the power

intensities over the visible spectrum• Different wavelengths appear different colorsg pp• Each wavelength is associated with a color


Color VisionExamples of Spectral Power Distributions (SPDs)

1Fluorescent Light

wavelength, λ (nm )400 500 600 7000

1 Equal Power White (also known as

0

Equal Energy White)


wavelength, λ (nm )400 500 600 7000


11Blue color

wavelength λ (nm )400 500 600 7000

• Spectral power distributions consisting of a single wavelengths correspond to a pure, saturated (undiluted), color – known as

h ti li ht

wavelength, λ (nm )400 500 600 700

monochromatic light



11Green color

wavelength λ (nm )400 500 600 7000


h ti li ht

wavelength, λ (nm )400 500 600 700

monochromatic light



1 Red color1 Red color

wavelength λ (nm )400 500 600 7000


h ti li ht

wavelength, λ (nm )400 500 600 700

monochromatic light



1 h l1 White color

wavelength λ (nm )400 500 600 7000

• The three above monochromatic colors viewed together look white.

wavelength, λ (nm )400 500 600 700


Color VisionColored Objects

• Color of object defined by two spectras rface reflectance of an object‒ surface reflectance of an object

‒ light source shining on object1

Obj

1 Incandescent Light

Object

×

λ (nm )400 500 600 7000

λ (nm )400 500 600 7000

1

=Object Color

Copyright 2010 by Lina J. Karamλ (nm )400 500 600 700

0

Color Vision


Color VisionMetamerism property of vision

• Two different spectra can give the same color

11

λ (nm )400 500 600 7000

λ (nm )400 500 600 7000


Color VisionColor Sensation

1 S (λ) SG(λ) (λ)

Cone Spectral Response

1SPD

1 SB(λ) SG(λ) SR(λ)1

×

λ500nm400nm 600nm0

700nmλ (nm )400 500 600 700

0

λ500nm400nm 600nm700nm

1 ∫=max

min

)()(λ

λ

λλλ dSPC jj

=

0

Integrate each curve toget area under each curve


λ (nm )400 500 600 7000

Color Vision1

SPD

λ (nm )10 50 100 1500

“Indeed rays, properly expressed, are not colored”I N t 1675Isaac Newton, 1675

Power distributions exist in the physical worldColor exists only in the eye and the brain


Color exists only in the eye and the brain

Color Vision

Color sensation - perceptual attributes1. Brightness – perceived Luminance2. Hue – color3. Saturation – amount of white light diluting the color


Luminance Computation Luminance measured by:

• Weighting the light intensity distribution (given by SPD) by h l i l f hthe relative spectral responses of the cones

• Summing the resulting weighted intensities

Relative spectral responses of the cones

0.2

by each

R RR

t absorbe

d b RG

RR

same intensity

RB0.01

ction of light

e of con

e

same intensity


400 700550 λ (nm)Frac

typ

Luminance Computation i ffi i f i f h (λ)Luminous efficiency function of the eye - E(λ)

• Normalized sum of the cones’ relative spectral responses

• Describes the relative “brightness” sensitivity of the eye at each wavelength

Peak at 555nm in the green‐yellow part

0 8

1.0

0.4

0.6

0.8

380 460 540 620 700 780 λ (nm)0

0.2

0.4


380 460 540 620 700 780 λ (nm)

Luminance and Brightness i f li h i h (λ )Luminance Y of a light source with SPD P(λ )

1 1.0

Luminous Efficiency, E(λ)

0 4

0.6

0.81.0

×

λ (nm )400 500 600 7000

380 460 540 620 700 780 λ (nm)00.20.4

1)()( λλ EP

∫=max

i

)()(λ

λ

λλλ dEPYY

λ (nm )400 500 600 7000

Integrate curve toget area under curve

minλ


λ (nm )400 500 600 700

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