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THE VISUAL SYSTEM

Visual PerceptionAditi Majumder, UCI

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Visual System

Eye - sensorLateral geniculate nucleusStriate cortex

Striped AppearanceExtra striate cortex

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Visual System

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Visual System

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Eye : The Sensor

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Eye : Structure

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Eye : Accommodation

Accommodation : Flexible focusing abilityCornea – 80% of focusing abilityLens – 20% adaptable focusing ability

Focus at one depth at a timeLimitation

Depth of fieldNear and far point

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Myopia/Presbyopia

Far and near point is different for different individuals

If near point is far: presbyopiaIf far point is near: myopia

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Iris

Muscle controlling the pupil sizeControls the illumination on the retina3mm – 7mmIncrease by 5 timesCannot explain the 10 orders of magnitude light sensitivity

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Other

Pigmented EpitheliumBehind the retina – darkAbsorbs light and avoid scatteringAllows high-contrast sharp retinal image

Fovea2 degress of angle subtendedMost densely populated with photoreceptorsImage fixated on the retinaStiles-Crawford effect

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Receptors in Eye

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Receptor Organization

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Receptors

Rods120 million Only in periphery

Cones6 million1% in fovea Rest in periphery

In periphery 20:1 ratio for rods and conesBlind Spot

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Receptor Distribution

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Receptor Distribution

Turned away from lightTo get nutrition from opaque pigment epitheliumThe other cells are transparent not to block light reaching the retinaBlock the axons of the ganglion cells from leaving the eye

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Blind Spot

Ganglion nerve fibers fold and leave by crossing a part of retinaThis part does not have any receptorsFilled out by the brainExperiment

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Receptor Functions

Cones – for vision in photopic conditionsRods – for vision in scotopic conditions

Saturate at high light levelsBoth – for vision in mesopic conditions

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Receptor Functions

Conversion of light to electrical energyLight sensitive chemical pigment in the receptorsAbsorbs photons and changes the shape to create a graded potential across the membrane of the outer segmentLogarithmic response supports Stevens LawPotential is transmitted down the outer membrane to other cells

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Receptor Functions

Pigment BleachingPigment Regeneration

30 minutes for rods and 6 minutes for cone

Effects of Visual Pigments on Perception

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Dark Adaptation Curve

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Dark Adaptation

Initial rapid increase 6 minutes

Slower further increase25-30 minutes

Rods have higher sensitivity than cones

Rods responsible for dark vision

Pigment Regeneration6 minutes for cones30 minutes for rods

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Spectral Sensitivity

Threshold Curve Sensitivity Curve

Relative threshold of light required to detect monochromatic light of each wavelength

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Rod and Cone Sensitivity

Rods have sensitivity to shorter wavelengthsPeak near 500nm being maximum sensitive to the blue-green regionShift of wavelengths in dark

Purkinje shiftThings appear bluish in darkGreen foliage seems to stand out in the dusk

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Cone Sensitivity

Cone curve is the combined sensitivity of three conesShort wavelength – S conesMedium wavelength – M conesLong wavelength – L cones

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Cone Sensitivity

Inconsistent with parts? – NORelative proportions of the S,M and L cones are differentS:M:L = 1:6:12Rods does not help in color vision. No color perception in dark

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Why do we need three cones?

What happens with one cone?10 photons of 500nm produce x reaction10 photons of 560nm produce 4x reaction40 photons of 500nm produce 4x reaction

No way to tell between the wavelength

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Why do we need three cones?

What happens with one cone?Two parameters: Which wavelength and what brightness?Cannot measure both with one sensor

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Why do we need three cones?

What happens with two cones?You can distinguish both intensity and wavelengthNeed more wideband sensitivity to cover all colorsMiss out completely parts of the spectrum

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Why do we need three cones?

With three conesWe do not need more than three since they cover the visible spectrum

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Why rods not considered as the fourth cone?

Spatial distribution and nature of convergence are completely differentThey are activated usually in completely different light conditions

Effects of Neural Processing on Perception

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Convergence

Measured number of neuron synapse on another neuron

Eye : Number of receptor neuron synapses on a ganglion cell

Each ganglion cell receives many synapsesRods to cones convergence ratio is 20:11 million ganglion cells, 120 million rods and 6 million conesRods have higher convergence than cones

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Rods are more sensitive in dark

Triggered

2 2 2 222

2

222

Threshold for ganglion triggering = 10

Not triggered

Rods have greater spatial summation than cones

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Cones give visual acuity

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Cones give visual acuity

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Neural Circuits and their Effects on Perception

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Neural Circuits

Many neurons connected through convergenceSmall – a few neuronsLarge – a few hundred thousand neurons

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Neural Processing by Excitation and Inhibition

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Neural Processing by Excitation and Inhibition

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Neural Processing by Excitation and Inhibition

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Receptive Fields

The area of the retina that when stimulated influences the firing rate of the ganglion cellsExcitatory centerInhibitory surroundLateral Inhibition

Excitatory Center

Inhibitory Center

Neutral Region

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Hermann Grid

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Explanation : Lateral Inhibition

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Mach Bands

Actual Intensity

Percieved Intensity

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Explanation : Lateral Inhibition

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Explanation : Lateral Inhibition

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Simultaneous Contrast

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Simultaneous Contrast

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Explanation : Lateral Inhibition

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Artifact of Mach Band

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Theory of Continuity

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Geometric Continuity

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Mach Bands

Artifacts when any patch or curve does not show C1 continuityThey are always avoided

ShadingBlending

Image editing, contrast compressing, tone reduction

Geometric Modeling

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Shading

Id = kd (L . N)Is = ks (2N (L . N) – L) . V

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Shading

Flat Shading[I(v1)+I(v2)+I(v3)]/3Is not C1 continuous

Interpolated ShadingLinear Interpolation of I(v1), I(v2) and I(v3) Is C1 continuous

v1

v2

v3

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Shading

Flat Shading – Mach Bands Interpolated Shading

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Intensity Blending

Proj1Proj2

Proj1Proj2

Overlap Region Overlap Region

Inte

nsity

0.0

Spatial Location

Depends onWidth of blendingBlending Function

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Intensity Blending

Proj1Proj2

Proj1Proj2

Overlap Region Overlap Region

Inte

nsity

0.0

Spatial Location

Depends onWidth of blendingBlending Function

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Intensity Blending

Proj1Proj2

Proj1Proj2

Overlap Region Overlap Region

Inte

nsity

0.0

Spatial Location

Depends onWidth of blendingBlending Function

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Intensity Blending

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Intensity Blending

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Image MosaicsMulti-Projector DisplaysImage Editing

Applications

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Geometric Modeling Primitives

Three propertiesContinuousContinuous with transformationsContinuous with subdivisions