early vision bruce draper department of computer science colorado state university
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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 &
scaleevenfilter
oddfilter
+
Organization of cells in V1
Hubel & Weisel proposed the following organization for cells in V1
Well, a little more complex…
Ocular dominancecolumns
Color-coded orientationSensitivity 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 athttp://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