Vision: Subjective and Objective Issues

Oleh Tretiak

Medical Imaging Systems

2002

Why Study Vision

• Understand how to display images

• Understand what is seen

• Understand how vision works

Sources of Information

• Physics of vision

• Neurophysiology of vision

• Psychology of vision

• Psychophysics

References

• David Hubel, Eye, Brain, and Vision, Henry Hold & Company, 1995

• James P. C. Southall, Physiological Optics, Dover, 1961

• Vicki Bruce, Patrick R. Green, Mark A. Georgeson, Visual Perception: Physiology, Psychology, and Ecology, Psychology Press, 1996

Visual Pathway - Anatomy

Visual Pathway - Symbolic

The Retina and the Eye

The Eyeball

Vision and Eye Fixation (motion)

Half-retina maps

Layers of the Geniculate Nucleus

Section through Striate Cortex

Map From Retina to Striate Cortex

Striate Cortex Input/Output

Index of refraction

Cornea (n2) 1.376

Aqueous humor and vitreous body (n3 = n7)

1.336

Outer portion of lens (n4 = n6) 1.386

Core-lens (n5) 1.406

The model is due to Gullstrand (1924). The power of the eye in this model ranges from 58.64 dptr (diopters) to 70.57 dptr. Most of the refraction (43 dptr) is due to the cornea.

Subjective Intensity (Contrast) Models

• In a model for subjective intensity, let x by the brightness (power/area) and y the subjective brightness (contrast). Both x and y range from 0 to 100.

• Logarithmic model:

• Power law model:

• On the next slide we show thirty four steps designed to produce uniform contrast with (1) linear, power law with gamma = 1, (2) power law, gamma = 0.7, and (3) logarithmic. Note that the appearance of this depends on the transfer function of the display.

€

y = 50 log10 (x), 1 ≤ x ≤100

€

y =100(x /100)γ

Simultaneous Contrast Examples

Background = 245, circles = 210, 225, and 235

Background = 70, circle = 60

The circles have the same objective intensity.

‘Typical’ Visual Spatial Response

Objective value (intensity)

Subjective (perceived) value

Mach Bands

‘Typical’ visual temporal response

Boundaries

BrightnessTexture

Shape Perception

Size Perception