EEE 508 - Lecture 4

Organization in the Visual Cortex

• Information in the visual cortex is represented within an orientation- andfrequency-selective modular neural bank.

• A hypercolumn consists of two ocular dominance columns. Each of the twoocular dominance columns receive inputs from the same retinal location (butfrom a different eye) and contains a complete set of orientation columns.

• Each hypercolumn corresponds to a particular region. Adjacent hypercolumnsanalyze for adjacent but overlapping retinal patches.

Hypercolumnbased structure ofthe striate cortex

EEE 508 - Lecture 4

Binocular Vision

EEE 508 - Lecture 4

Binocular Vision

Random-DotStereograms

EEE 508 - Lecture 4

Binocular Vision

• Binocular stereo is not the only means for depth perception• Spatial depth perception possible through monocular cues:

! Head movement parallax! Linear perspective! Shading and texture gradients! Occlusion of more distant objects by near ones! Accomodation (muscular tension to focus objects)

EEE 508 - Lecture 4

Color Vision• Color vision involves the analysis of the object in relation to its

background! Color not experienced if eye in uniform color field without any

pattern! A point source of white light can appear any color! Totally different wavelength combinations can produce identical

colors (red + green ? yellow)

EEE 508 - Lecture 4

Color Vision

EEE 508 - Lecture 4

Color Vision• Cones in the human retina contains one of 3 pigments

! sensitive to short wavelengths (blue)! sensitive to middle wavelengths (green)! sensitive to long wavelengths (red)

• Comparison of outputs of different cones• Chromatic aberration at shorter wavelengths

! short wavelengths cones not found in fovea

Fourth root of wavelength (nm)

Nor

mal

ized

abs

orba

nce

EEE 508 - Lecture 4

Single-opponent Color Sensitive Cell

EEE 508 - Lecture 4

Double-opponent Color-Contrast Cortical Cell

Double-opponent red-green color-contrastcortical cells with concentric receptive fieldsare highly sensitive to simultaneous colorcontrast

Some Double-opponent simple cellsalso exhibit orientation selectivity

EEE 508 - Lecture 4

Perception

• Not the inevitable result of a set of stimulus pattern• Best interpretation of sensory data based on past experience

and a set of generic criteria• Visual perception must involve analysis and decomposition of

a scene into coherent and recognizable entities

Shape Constancy-an Ellipse Seen as a Circle

EEE 508 - Lecture 4

Perception

• HSV partitioning laws

(c) Symmetry(a) Proximity (b) Similarity

(d) Closure (f) Simplicity(e) Good Continuation

EEE 508 - Lecture 4

Perception

• HSV partitioning laws! The law of common fate: moving dots with uniform velocity

through a field of similar but stationary dots• The laws are not absolute; e.g. proximity law influenced by

prior experience

EEE 508 - Lecture 4

Perception• Visual illusion and ambiguity

EEE 508 - Lecture 4

Perception

• Visual ambiguity! there is more than one reasonable interpretations of the imaged data! there are no cues to cause one interpretation to dominate! reasoning process carried out by the perceptual system at the level below

our conscious awareness• The Ponzo and arrow illusions

Arrow illusion

EEE 508 - Lecture 4

Perception

EEE 508 - Lecture 4

Perception

• Subjective contours

• The impossible triangle

EEE 508 - Lecture 4

Perception

• Impossible Patio, Balcony

EEE 508 - Lecture 4

Perception

• Several higher mental processes associated with vision! visual representations often used in problem solving! recall ability for iconic info better than that for symbolic info! visual perception and representation are culture dependent