The Visual Brain in Action: Chapter 1 - Computer Science

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30 Νοε 2013 (πριν από 4 χρόνια και 1 μήνα)

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Early Vision

Bruce Draper

Department of Computer
Science

Colorado State University

Overview


“The Biomimetic Vision Trilogy”

1.
Selective Attention


Understanding the problem


Last week

2.
Early Vision


Understanding the literature


Today

3.
Ventral vision


Understanding object recognition


March 9 (next week)

General Theme


Vision evolved to serve the needs of animals


Vision is
action oriented
(it
guides behavior
)


Actions may be immediate (e.g. grasp, navigate)


Actions may be delayed (“perception”)


Vision is not one system


As animals became more complex, more and more visual
capabilities evolved in separate systems

Note: this is not a new idea. See
The Visual Brain in Action
by
Milner & Goodale 1996;
The Metaphorical Brain
by Arbib 1972; or
even
Cybernetics
by Weiner 1948.

Input: The Eye(s)

Start at the beginning:


Lens focuses light


Iris serves as aperture


Retina contains receptors


Optic nerve transmits to brain


Lens, iris are controlled by muscles under the control of
the brain

S. Palmer.
Vision Science.
P. 27

Retina as
Processor



Five cell types:



receptor (rod/cones)

Species dependent



horizontal



bipolar cells



amacrine cells



ganglion cells




Its inside out!



Blind spot where optic
nerve passes through
retina

S. Palmer.
Vision Science
. P. 30

Retina (cont.)



Ganglion Cells


The first cells to produce spike discharges


Other retinal cells use graded potentials


Spikes are needed for long distance communication


On
-
center off
-
surround


Off
-
center on
-
surround

Two Types of Ganglion Cells


P


ganglion cells in primates (like Y cells in cats):


Large receptive fields (low frequency?)


Transient response


Fast transmission


Receive inputs from all colors and from rods


P


ganglion cells (like X cells in cats):


Small receptive fields (high frequency?)


Sustained response


Medium transmission


Color opponent channels




Fields of View & Stereo


Right hemisphere
receives the left visual
field from both eyes


And vice
-
versa


Splitting the field of view
supports disparity
computations


High resolution in fovea,
lower elsewhere


Fovea is
±
2
°

(thumbnail at
arms length)


Visual Projections


The eye + optic nerve is a shared device


There are eleven projections (“endpoints”) of the
optic nerve:


Retinogeniculate Projection


Onto LGNd (Lateral Geniculate Nucleus, dorsal) and then to V1
(a.k.a. primary visual cortex, striate cortex)…


This path is dominant in people; barely evident in non
-
mammals


Retinotectal Projection


Onto Superior Colliculus, then the Pulvinar Nucleus, then LGN,
V1, MT, higher level centers…


This path is dominant in non
-
mammals; evolutionarily older


Involved in eye movements, motion, tracking


Projections (LGN & S.C.)

S. Palmer.
Vision Science
. P. 25.

Projections (II)


At least 8 more (minor) projections!


Retina

Suprachiasmatic Nucleus


Circadian rhythms


SN is part of the hypothalamus (like LGNd)


SN receives multi
-
modal projections


Retina

Nucleus of the Optic Tract (NOT)


Optokinetic nystagmus


Retina


Accessory Optic Nuclei


Visual control of posture, locomotion


Retina


Pretectum


Pupillary Light Reflex

Two LGNd Channels


P


cells in the retina project to the two magnocellular
(“large cell”) layers in the LGN.


Livingstone & Hubel: color
-
blind, fast, high contrast
sensitivity, low spatial resolution


P


cells in the retina project to the four parvocellular
(“small cell”) layers in the LGN.


L&H: color selective, slow, low contrast sensitivity, high
spatial resolution


LGNd also has interlaminar layers with unknown
role/properties


Receives projections from optic nerve, S.C.

Right at the levels of cells; wrong at the level of populations

Primary Visual Cortex (V1)


First cortical visual area


Columnar (like all cortex)


Retinotopically mapped


Ocular dominance columns


Edges (Gabor filters), color,
disparity & motion maps


Connects to other
retinotopic areas (V2, V3,
MT)


http://webvision.med.utah.edu/imageswv/capas
-
cortex.jpg

Proof of Retinotopic Mapping

Pattern flashed (like a
strobe) in front of monkey
injected with sugar dye

Left primary visual
cortex of the same
monkey

Tootell, et al. 1982.

Single Cell Recordings in V1

Two Channels in V1


P




Magnocellular LGN Layers


layer 4C


of V1.


Projects further to layer 4B


Motion direction selective


Orientation selectivity & binocular


No color


P




Parvocellular LGN


4C


of V1


Projects to layers 2 & 3.


Layers 2 & 3 subdivide into “blobs” and “interblobs”:


Blobs

are color selective, simple receptive fields


No orientation/movement direction selectivity, binocularity.


Prefer low frequencies, have small Magnocellular input


Interblobs

have reduced (non
-
zero) color selectivity


Binocular, high
-
frequency, orientation selective

Simple Cells


The first orientation selective cells found in
V1 were labeled “simple cells”


Well approximated by Gabor functions with fixed
orientations, scales and phases

Jones & Palmer 1987

Complex Cells


The second set of orientation selective cells
were called complex cells


Well modeled as combining 90
°

out
-
of
-
phase
Gabor responses (quadrature pairs)


Captures energy at a particular orientation &
scale

even

filter

odd

filter





+



Organization of cells in V1

Hubel & Weisel proposed the following
organization for cells in V1

Well, a little more complex…

Ocular dominance

columns

Color
-
coded orientation

Sensitivity columns

More on V1


Only about 27% of V1 cells are orientation selective


About 70% of orientation selective cells are complex
cells


Orientation selective cells also include end
-
stopped
cells (a.k.a. hypercomplex cells), and grating cells.


Non
-
orientation selective cells include cells that
respond to:


Color (Hue/Sat maps?)


Disparity


Motion


V1 Connections

V1 is the starting point
of cortical visual
processing.


Dorsal projections lead
to somatosensory and
motor control areas


Ventral projections lead
toward associative
memories

From Van Essen 1992. Image can be found at

http://webvision.med.utah.edu/imageswv/Visual
-
Cortex1.jpg

Anatomical Maps of Visual Cortex

1983 Version

1990 Version

Two Visual Subsystems


In 1982, Ungeleider & Mishkin propose that
there are two primary visual pathways in
humans and primates:



The dorsal (or “what”) pathway


Ends in the posterior parietal cortex



The ventral (or “where”) pathway


Ends in the inferotemporal cortex

Visualizing Two Subsystems

D. Milner & M. Goodale,
The Visual Brain in Action
, p. 22