LCS 11: Cognitive Science Lorraine Tyler Thursday at 4:15PM, Edmunds - - PowerPoint PPT Presentation

Pomona College

LCS 11: Cognitive Science

Vision

Jesse A. Harris April 17, 2013

Jesse A. Harris: LCS 11: Cognitive Science, Vision 1

Agenda

֠ Upcoming talks Lorraine Tyler Thursday at 4:15PM, Edmunds 101 William Marslen-Wilson Friday at noon, Edmunds 101 ֠ The eye and the visual system ֠ Hoffman’s visual rules ֠ GQ 5.1 due Sunday, 9PM ֠ Signup for group meetings

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The problem

Vision feels easy, but it’s not at all.

Constraints on vision

• 1. Limited perceptual field
• 2. Physical blindspots
• 3. Subject to all kinds of illusions and perceptual error

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But this problem is nothing compared to ...

The miracle of vision!

The visual system transforms a fragmented 2D image into a coherent 3D construct. Miracle starts with the eye itself ...

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Visible eye

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Cornea Curved transparent lens through which light

• enters. Focuses light onto the retina. Light bent

and slowed by cornea. Iris Provides an adjustable aperture (colored part of eye). Pupil Hole in middle of iris, adjusts for amount of light Smaller when more light, limiting the amount let through to another lens.

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Inner eye

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Lens Contracts to give give more refractive power, needed for focusing on close objects. Power to accommodate by contraction decreases with age. Retina Light sensitive layer lining back of eye, containing multiple types of photoreceptors (rods and cones) and a chain of cells that provide the first, rough processing, ending in the retinal ganglion cells. Fovea Area with dense population of cones for acuity

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Lens

Contraction

Lens contracts to focus image on fovea when images are close (a).

Correction

When the muscles cannot contract the lens enough (b), corrective lenses allow for adjustment (c).

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Age related decline Image projection Image projection

Psychologist G. M. Stratton wore lens over his eyes that inverted the images - to right side up. After about 8 days, adjusted well (but not entirely) to the inverted image.

• G. M. Stratton

(1865–1957)

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Neural retina Neural retina LGN

• 1. Signals sent from retinal ganglion cells to lateral

geniculate nucleus (LGN) Cells in LGN responsible for retinotopic mapping - arrangement of receptive field according to an orderly map.

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Retinotopic maps

◮ Light gathered from retina maintains its spatial features

• f its representation in other areas of the brain

◮ Simplistic representation of how information is relayed

from retina to the primary visual cortex

• 1. Light activates electrical impluses in retina
• 2. Carried via fibers of the optic nerve to highly specific

areas in the lateral geniculate nucleus (LGN)

• 3. Transmitted to primary visual cortex (V1) in occipital

region (back) of the brain

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Stereopsis

Stereopsis

The perception of depth that occurs with normal, binocular vision.

◮ Slightly different visual inputs

to each eye due to different locations

◮ Gives rise to depth perception ◮ Also to 3D movies ◮ 10% of population do not

perceive depth in 3D movies,

• ften due to damage to eye

during development

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Random dot stereogram

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Random dot stereogram

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Primary visual area

After basic processing, signals sent to V1 for visual processing

Selectivity

Cells in V1 tuned to respond to very selective features

◮ orientation ◮ edges ◮ light

Classical Hubel & Wiesel demonstration of selectivity in V1: http://www.youtube.com/watch?v=IOHayh06LJ4

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Fundamental problem of vision

The image at the eye has countless possible interpretations.

◮ Visual input can matched up in any number of ways with

the world

◮ Visual system applies rules and defaults to transform

ambiguous or indeterminate cues into a coherent percept (mental construction what is perceived).

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Rotated tables

http://www.michaelbach.de/ot/sze_shepardTables/Q

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Construction of percept

The fundamental role of visual rules

You construct visual worlds from ambiguous images in conformance to visual rules. Constants in construction implies constants among visual interpretation - rules.

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Rules

Rule of generic view

Construct only those visual worlds for which the image is stable (i.e., generic) view.

Rule 1

Always interpret a straight line in an image as a straight line in 3D.

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Rules

Rule of generic view

Construct only those visual worlds for which the image is stable (i.e., generic) view.

Rule 2

If the tips of two lines coincide in an image, then always interpret them as coinciding in 3D.

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Rules

Rule of generic view

Construct only those visual worlds for which the image is stable (i.e., generic) view.

Rule 2

If the tips of two lines coincide in an image, then always interpret them as coinciding in 3D.

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Penrose triangle

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Penrose triangle

East Perth, Australia

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Kopfermann figure

Rules 1 and 2 allow us to prune many possible interpretations

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