psychology essay

02_VisualPerception.pdf

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10/2/2019

Vision

What is vision?

The Human Condition René Magritte

What is vision?

• Construction of the world based on incoming information provided by light interacting with photoreceptors on the retina.

• Most important/informative sense for humans • Greatest amount of sensory cortex is devoted to vision

• Conscious experience dominated by visual information

• Evolutionary adaptations • Early notification over rich detail (speed/accuracy trade)

• Collection of earlier adaptations

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Why must vision be constructed?

• Problem of inverse optics • Our eyes receive a 2D view of

the world • We live in a 3D world • Infinite amount of potential 3D

scenes that result in same 2D image

• Viewpoint invariance • Objects should be recognized

regardless of view • “Accidental” views

• Perception = sensation + experience

Basic roadmap of visual system

• Points of light ->

• Larger “circles” of light ->

• Lines and edges ->

• Shapes / Color / Motion ->

• Objects ->

• Complete visual image

• Feedforward AND feedback

Light • Comprised of photons – particle and wave

• Color (and visibility) of light determined by its wavelength

• Intensity of light determined by its amplitude

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The Eye

Focusing Light

• Light entering the eye must be focused on the retina

• Lens (and also cornea) responsible for focusing image properly • Lens adjusts focal point by becoming more or less

spherical (accommodation)

• Cornea helps to direct light into the lens

• Nearsightedness (myopia)

• Farsightedness (hyperopia)

Accommodation and Convergence

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Retina

• Light stimulates photoreceptors at far end of retina

• Neural signals travel to bipolar cells, where they are converged and sent to ganglion cells

• Ganglion cells further converge information and connect to brain

Horizontal cells Amacrine cells

Rods and Cones

Bipolar Cells

Ganglion Cells

Rods and Cones • Rods

• Low light vision (scotopic)

• Sensitive to fast motion

• High convergence (low spatial resolution)

• Concentrated at peripheries of eye

• ~120 million

• Stargazing

• Cones • Requires bright light (photopic)

• Three types – short, medium, and long – correspond to wavelength most sensitive to

• Low convergence (high spatial resolution)

• Concentrated in the fovea

• ~6 million

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Ganglion cells • Parvo cells

• Connections mostly from cones

• Color sensitive

• Good at edge detection

• Good spatial resolution

• Magno cells • Connections mostly from rods

• Sensitive to motion over wide areas

• Bad spatial resolution

• “Fast track”

From Eye to Brain

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Visual Fields

Left Visual Field Right Visual Field

Primary Visual Cortex

• Information from the optic nerve splits to both hemispheres in the optic chiasm

• Routed through the lateral geniculate nucleus (LGN) in the thalamus

• Projected to primary visual cortex, area V1 in the Occipital lobe

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Visual Cortex

• Area V1 (primary visual area)

• Back of occipital lobe

• Individual neurons responsive to edges of specific orientations

• Retinotopic organization

• Cortical magnification

Mapping Visual Cortex

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Visual Cortex and Edges

• What is the shape of receptive fields in V1?

• Hubel and Wiesel’s cat study

• Cells in V1 respond best to edges of particular orientations

Optimal Contrast Sensitivity

• Visual acuity • Smallest contrast able to be

seen reliably

• Snellen chart

• Visual system is most sensitive to contrast at certain frequencies

• About 4-6 cycles per visual angle

Subjective Edges

• Subjective edge – edge that exists for an observer, but not actually an edge

• Continuation – straight edges continue while occluded

• Closure – Edges are assumed to be “closed” and solid

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Onward from V1

http://www.intechopen.com/source/html/39288/media/image1.png

“What”

“Where”

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Information from LGN is routed to multiple pathways through the brain simultaneously! Dorsal stream becomes active earlier than ventral…why?

Color

• Physics definition • Wavelength of light

• Can be emitted from a light source or reflected off a surface

• Perceptual definition • Experience of “color”

• Based on wavelength – but only partially

• Color constancy effects, brightness/lightness interplay

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Cones and Color

• Three types of cones, corresponds to size of wavelength most sensitive to • Short (Blue)

• Medium (Green)

• Long (Red)

• Combination of information from three cones is unique for all visible wavelengths

• S-cones are less common, and absent in fovea

• M-cones are a more recent development (cf. dog vision)

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Red sensitive ganglion cell

Green sensitive ganglion cell

Yellow sensitive ganglion cell

Blue sensitive ganglion cell

Red

Green

Blue

Opponent-Process Theory

• Color “opposites”

• Red-Green, Blue-Yellow, Black-White

• Color afterimage – refractory period

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Color Constancy

• Compensation for lighting when determining color

• Different lightings can cause vastly different color sensations

• But perception accounts for this, stabilizing the world

http://xkcd.com/1492/

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Color Contrast / Averaging

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How far away are things?

Measuring Size

• The size of an object on the retina is determined by both the size of the object and the distance it is from the observer.

• To account for this, objects can be measured in terms of visual angle.

Size and Distance

• Why do we use visual angle to measure things? • Retinal size is determined by object size and distance

• Visual angle is known • If size of object is known, distance can be calculated • If distance is known, size can be calculated • Of course, there is ambiguity…

• What if neither are known? • Heuristics based on visual cues • Physiological cues

• Accommodation and convergence • Binocular disparity – eyes don’t capture exact same image

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Constructing 3D

• Scene from image

• Monocular depth cues • Requires one eye only

• Can be represented in 2D art/pictures

• Binocular depth cues • Requires both eyes

• Different images in each eye (why?)

• Distance information is provided from both

Monocular Cues

• Close one eye – can you still see depth?

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Occlusion

• Closer objects occlude objects further away

Proximity to Horizon (Relative Height) • Distance from object origin to horizon

• Closer proximity = further away

Relative Size / Height

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Vanishing Point

Ponzo Illusion

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Relative size

• Similar objects appear smaller as they are further away

Texture Gradient

• Similar to relative size

• Pattern gets smaller as it recedes - compression

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Aerial Perspective

• Objects far away are blurred and hazy

• Light scattering over long distances (and moisture)

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Ames Room

Ebbinghaus Illusion

• Size constancy

• Size is partially determined by context!

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Binocular Depth Cues

• Each eye gets a slightly different view of the visual field (check it yourself)

• Binocular disparity

• For a given object, the degree of offset is determined by how far away it is

Binocular Depth Cues

Left Eye Right Eye

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Inducing binocular 3D

• Anaglyph 3d (red-cyan glasses)

• Polarized 3d • This is the kind at 3d movies

• Polarized beam split by glasses – 2 images in one

• HMDs (head mounted displays) • One screen per eye

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But our cues only go so far…

Inverse Optics Problem

• Theoretically infinite amount of 3d visual scenes giving rise to 2d retinal image

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Correspondence Problem

Motion Effects

• Apparent Motion • Movies and animation

• Santa Cruz Fishhook

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Santa Cruz Fishhook Sign

Motion Effects

• Apparent Motion • Movies and animation

• Santa Cruz Fishhook

• Structure from motion • Rotating objects provide information about 3d shape

• Ballerina illusion

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Motion and Depth

• Motion parallax • Movement of scene when

you are moving

• Closer things move by faster