PPT
advertisement
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 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. © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002 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). © Stephen E. Palmer, 2002 © Stephen E. Palmer, 2002 Visual Cortex What/Where Pathways Dorsal Pathway ("Where" System) Parietal Frontal Occipital Primary Visual Cortex Temporal A Ventral Pathway ("What" System) Evidence from lesions of monkey cortex Dorsal Lesion Ventral Lesion B Object Discrimination C 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 Depth Form Color Featural Pathways Separate neural pathways in which different features are processed. Motion V2 Form Depth Motion MT V1 Color Form Depth Motion LGN Color Form 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 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) 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?
