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Physiology of Vision: a swift overview LGN Thalamus Parietal visual cortex Dorsal Stream Striate cortex (V1) Some figures from Steve Palmer Eye Optic nerve Temporal visual cortex Extrastriate cortex Ventral Stream Pixels to Percepts A. Efros, CMU, Spring 2011

Understanding the Brain

Anatomy versus Physiology Anatomy: The biological study of the physical structure of organisms.

Physiology: The biological study of the functional structure of organisms.

The Brain is tricky business • Aristotle thought it’s for cooling the blood • Localized or distributed?

Phrenology How bad Machine Learning got started…

Localized or Distributed?

Evidence from patients with partial brain damage • Lots of useful data from soldiers in the Russo-Japanese war But also evidence for distributed nature of processing • E.g. [Lashley] showed “graceful degradation” of memory performance in rats

The Visual System

Both eye and brain are required for functional vision Two kinds of blindness: Normal blindness (eye dysfunction) Cortical blindness (brain dysfunction) © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002

The Visual System

Eyes register optical information Pathways to occipital cortex Two pathways from V1 “What” pathway to temporal cortex “Where” pathway to parietal cortex Convergence on frontal cortex © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002

Pathways to the Brain

Anatomy of Pathway to Visual Cortex © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002

Pathways to the Brain

The Lateral Geniculate Nucleus (LGN) Waystation in Thalamus Projections from both eyes Six layers Projects to cortical area V1 © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002

Visual Cortex

Map of Visual Areas in Cortex “Unfolded” view of visual areas in the macaque cortex (sizes not to scale).

Visual Cortex

What/Where Pathways Dorsal Pathway ("Where" System)

Parietal

Evidence from lesions of monkey cortex

Frontal

Temporal Occipital

Primary Visual Cortex Ventral Pathway ("What" System) Dorsal Lesion B Ventral Lesion C Object Discrimination Landmark Discrimination © Stephen E. Palmer, 2002

Visual Cortex

What/Where Pathways Evidence from Neuropsychology

Visual agnosia

: Inability to identify objects and/or people Caused by damage to inferior (lower) temporal lobe Disruption of the “what” pathway

Visual neglect

: Inability to see objects in the left visual field Caused by damage to right parietal lobe Disruption of the “where” pathway © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002

Visual Cortex

Feature-based Pathways Hypothesis

Visual Features

Color Shape Depth Motion

Color Form Depth

Motion

Color Form Depth Motion

Featural Pathways

Separate neural pathways in which different features are processed.

Color Form Color Form Depth Motion

LGN

Depth Motion

RETINA © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002

The Gross Summary

The visual system is composed of many interactive functional parts: Eye (optics of image formation) Retina (light transduction) LGN (waystation?) Area V1 (hypercolumns) Higher cortical areas (features) Cortical pathways (what/where) © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002

Image Formation Film Digital Camera The Eye

Monocular Visual Field: 160 deg (w) X 135 deg (h) Binocular Visual Field: 200 deg (w) X 135 deg (h)

The Eye is a camera The human eye is a camera!

• •

Iris

colored annulus with radial muscles

Pupil

the hole (aperture) whose size is controlled by the iris • What’s the “film”?

– photoreceptor cells (rods and cones) in the

retina

The Retina Cross-section of eye Cross section of retina Pigmented epithelium Ganglion axons Ganglion cell layer Bipolar cell layer Receptor layer

Retina up-close Light

Two types of light-sensitive receptors

C on es

cone-shaped less sensitive operate in high light color vision

Rods

rod-shaped highly sensitive operate at night gray-scale vision © Stephen E. Palmer, 2002

Rod / Cone sensitivity The famous sock matching problem…

Distribution of Rods and Cones Fovea Blind Spot 150,000 100,000 50,000 Rods Rods Cones Cones 0 80 60 40 20 0 20 40 60 80 Visual Angle (degrees from fovea) Night Sky: why are there more stars off-center? © Stephen E. Palmer, 2002

Electromagnetic Spectrum Human Luminance Sensitivity Function http://www.yorku.ca/eye/photopik.htm

Visible Light Why do we see light of these wavelengths?

…because that’s where the Sun radiates EM energy © Stephen E. Palmer, 2002

The Physics of Light

Any patch of light can be completely described physically by its spectrum: the number of photons (per time unit) at each wavelength 400 - 700 nm.

# Photons (per ms.) 400 500 600 700 Wavelength (nm.) © Stephen E. Palmer, 2002

The Physics of Light

Some examples of the spectra of light sources A. Ruby Laser B. Gallium Phosphide Crystal 400 500 600 700 Wavelength (nm.) C. Tungsten Lightbulb 400 500 600 700 Wavelength (nm.) D. Normal Daylight 400 500 600 700 400 500 600 700 © Stephen E. Palmer, 2002

The Physics of Light

Some examples of the reflectance spectra of surfaces Red Yellow Blue Purple 400 700 400 700 400 700 400 700 Wavelength (nm) © Stephen E. Palmer, 2002

The Psychophysical Correspondence

There is no simple functional description for the perceived color of all lights under all viewing conditions, but …...

A helpful constraint: Consider only physical spectra with normal distributions mean # Photons 400 area variance 500 600 Wavelength (nm.) 700 © Stephen E. Palmer, 2002

The Psychophysical Correspondence Mean Hue

blue green yellow Wavelength © Stephen E. Palmer, 2002

The Psychophysical Correspondence Variance Saturation

hi.

high med.

medium low low Wavelength © Stephen E. Palmer, 2002

The Psychophysical Correspondence Area Brightness

B. Area Lightness bright dark Wavelength © Stephen E. Palmer, 2002

Physiology of Color Vision

Three kinds of cones: 440 530 560 nm.

100 S M L 50 400 450 500 550 600 650 WAVELENGTH (nm.) • Why are M and L cones so close?

Retinal Processing