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Physiology of Vision: a swift overview Parietal visual cortex Dorsal Stream Striate cortex (V1) LGN Thalamus Extrastriate cortex Eye Optic Ventral nerve Temporal Stream visual cortex Some figures from Steve Palmer 16-721: Learning-Based Methods in Vision A. Efros, CMU, Spring 2009 Image Formation Digital Camera Film The Eye Monocular Visual Field: 160 deg (w) X 135 deg (h) Binocular Visual Field: 200 deg (w) X 135 deg (h) What do we see? 3D world 2D image Point of observation Figures © Stephen E. Palmer, 2002 What do we see? 3D world Point of observation Painted backdrop 2D image The Plenoptic Function Figure by Leonard McMillan Q: What is the set of all things that we can ever see? A: The Plenoptic Function (Adelson & Bergen) Let’s start with a stationary person and try to parameterize everything that he can see… Grayscale snapshot P(q,f) is intensity of light • Seen from a single view point • At a single time • Averaged over the wavelengths of the visible spectrum (can also do P(x,y), but spherical coordinate are nicer) Color snapshot P(q,f,l) is intensity of light • Seen from a single view point • At a single time • As a function of wavelength Spherical Panorama See also: 2003 New Years Eve http://www.panoramas.dk/fullscreen3/f1.html All light rays through a point form a ponorama Totally captured in a 2D array -- P(q,f) Where is the geometry??? A movie P(q,f,l,t) is intensity of light • Seen from a single view point • Over time • As a function of wavelength Space-time images t y x Holographic movie P(q,f,l,t,VX,VY,VZ) is intensity of light • Seen from ANY viewpoint • Over time • As a function of wavelength The Plenoptic Function P(q,f,l,t,VX,VY,VZ) • Can reconstruct every possible view, at every moment, from every position, at every wavelength • Contains every photograph, every movie, everything that anyone has ever seen! it completely captures our visual reality! Not bad for a function… 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 Cones 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 # Receptors/mm2 . Fovea 150,000 Rods Blind Spot Rods 100,000 50,000 0 Cones Cones 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 . B. Gallium Phosphide Crystal # Photons # Photons A. Ruby Laser 400 500 600 700 400 500 Wavelength (nm.) 700 Wavelength (nm.) D. Normal Daylight # Photons C. Tungsten Lightbulb # Photons 600 400 500 600 700 400 500 600 700 © Stephen E. Palmer, 2002 The Physics of Light % Photons Reflected Some examples of the reflectance spectra of surfaces Red 400 Yellow 700 400 Blue 700 400 Wavelength (nm) Purple 700 400 700 © 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 area # Photons 400 500 variance 600 700 Wavelength (nm.) © Stephen E. Palmer, 2002 The Psychophysical Correspondence # Photons Mean blue Hue green yellow Wavelength © Stephen E. Palmer, 2002 The Psychophysical Correspondence # Photons Variance Saturation hi. high med. medium low low Wavelength © Stephen E. Palmer, 2002 The Psychophysical Correspondence Area Brightness # Photons B. Area Lightness bright dark Wavelength © Stephen E. Palmer, 2002 Physiology of Color Vision Three kinds of cones: 440 RELATIVE ABSORBANCE (%) . 530 560 nm. 100 S M L 50 400 450 500 550 600 650 WAVELENGTH (nm.) • Why are M and L cones so close? © Stephen E. Palmer, 2002 Retinal Processing © Stephen E. Palmer, 2002 Single Cell Recording Microelectrode Amplifier Electrical response (action potentials) mV Time © Stephen E. Palmer, 2002 Single Cell Recording © Stephen E. Palmer, 2002 Retinal Receptive Fields Receptive field structure in ganglion cells: On-center Off-surround Response Time Stimulus condition Electrical response © Stephen E. Palmer, 2002 Retinal Receptive Fields Receptive field structure in ganglion cells: On-center Off-surround Response Time Stimulus condition Electrical response © Stephen E. Palmer, 2002 Retinal Receptive Fields Receptive field structure in ganglion cells: On-center Off-surround Response Time Stimulus condition Electrical response © Stephen E. Palmer, 2002 Retinal Receptive Fields Receptive field structure in ganglion cells: On-center Off-surround Response Time Stimulus condition Electrical response © Stephen E. Palmer, 2002 Retinal Receptive Fields Receptive field structure in ganglion cells: On-center Off-surround Response Time Stimulus condition Electrical response © Stephen E. Palmer, 2002 Retinal Receptive Fields Receptive field structure in ganglion cells: On-center Off-surround Response Time Stimulus condition Electrical response © Stephen E. Palmer, 2002 Retinal Receptive Fields RF of On-center Off-surround cells Neural Response Response Profile Receptive Field Center Firing Rate on-center Surround off-surround On Off Horizontal Position © Stephen E. Palmer, 2002 Retinal Receptive Fields RF of Off-center On-surround cells Neural Response Surround Center Receptive Field Response Profile Firing Rate on-surround Surround Center off-center On Off Horizontal Position © Stephen E. Palmer, 2002 Retinal Receptive Fields Retinal Receptive Fields Receptive field structure in bipolar cells Light © Stephen E. Palmer, 2002 Retinal Receptive Fields Receptive field structure in bipolar cells LIGHT Receptors Horizontal Cells Direct Path Indirect Path Direct excitatory component (D) Indirect inhibitory component (I) D+I Bipolar Cell A. WIRING DIAGRAM B. RECEPTIVE FIELD PROFILES © Stephen E. Palmer, 2002 Visual Cortex Cortical Area V1 Parietal visual cortex aka: Primary visual cortex Striate cortex Brodman’s area 17 Dorsal Stream Striate cortex (V1) LGN Thalamus Extrastriate cortex Eye Optic Ventral nerve Temporal Stream visual cortex © Stephen E. Palmer, 2002 Cortical Receptive Fields Single-cell recording from visual cortex David Hubel & Thorston Wiesel © Stephen E. Palmer, 2002 Cortical Receptive Fields Single-cell recording from visual cortex Time © Stephen E. Palmer, 2002 Cortical Receptive Fields Three classes of cells in V1 Simple cells Complex cells Hypercomplex cells © Stephen E. Palmer, 2002 Cortical Receptive Fields Simple Cells: “Line Detectors” B. Dark Line Detector Firing Rate Horizontal Position © Stephen E. Palmer, 2002 Cortical Receptive Fields Simple Cells: “Edge Detectors” C. Dark-to-light Edge Detector Firing Rate D. Light-to-dark Edge Detector Firing Rate Horizontal Position Horizontal Position © Stephen E. Palmer, 2002 Cortical Receptive Fields Constructing a line detector Retina Receptive Fields LGN CenterSurround Cells © Stephen E. Palmer, 2002 Cortical Receptive Fields Complex Cells STIMULUS NEURAL RESPONSE 00o Time © Stephen E. Palmer, 2002 Cortical Receptive Fields Complex Cells STIMULUS NEURAL RESPONSE o 60 0 Time © Stephen E. Palmer, 2002 Cortical Receptive Fields Complex Cells STIMULUS NEURAL RESPONSE o 90 0 Time © Stephen E. Palmer, 2002 Cortical Receptive Fields Complex Cells STIMULUS NEURAL RESPONSE o 120 0 Time © Stephen E. Palmer, 2002 Cortical Receptive Fields Constructing a Complex Cell Retina Receptive Fields Cortical Area V1 Simple Cells © Stephen E. Palmer, 2002 Cortical Receptive Fields Hypercomplex Cells © Stephen E. Palmer, 2002 Cortical Receptive Fields Hypercomplex Cells © Stephen E. Palmer, 2002 Cortical Receptive Fields Hypercomplex Cells © Stephen E. Palmer, 2002 Cortical Receptive Fields Hypercomplex Cells “End-stopped” Cells © Stephen E. Palmer, 2002 Cortical Receptive Fields “End-stopped” Simple Cells © Stephen E. Palmer, 2002 Cortical Receptive Fields Constructing a Hypercomplex Cell RETINA Receptive Fields CORTICAL AREA V1 Complex Cell End-stopped Cell © Stephen E. Palmer, 2002 Mapping from Retina to V1 Why edges? So, why “edge-like” structures in the Plenoptic Function?
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