phys6a0 (1) - VISION Chapter 6 System Design in Sensory...

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Unformatted text preview: VISION Chapter 6 System Design in Sensory Processes Separate Neural Tracts Selectivity Reliability vs. Rapidity Reliability redundancy (parallel processing) vs. Rapidity sensitivity (serial processing) Chapter 6 2 Sensory Processes & Experience A Goal of : understanding HOW our representation stimuli are related to the characteristics of information transformation at each level in a sensory pathway. X Receptors Brainstem Filters Thalamus Cortex Receptors Chapter 6 Transducers 3 Information Suppression Preventing "Overload" Resulting from Non-Relevant Information Reduces Metabolic Expense Failure increased metabolic expense sensory overload Chapter 6 4 Receptors Transduction Kinds of Receptors: enormous diversity Generator Potential Generator Potential (stimulus) Graded Potential Active Processing (e.g. stopped-image exp) Chapter 6 5 Nerve Impulse Coding Intensity frequency of neuronal firing "recruitment" range fractionation: cells specialized to fire in a particular segment or fraction of an intensity scale distinctive sensory experience = "modalities" Type of Stimulus labelled lines: particular nerve cells labelled for law of specific nerve energies (Johannes Muller) across fiber pattern theory: each receptor responds to a wide range of stimuli, & it contributes to the perception of every stimulus in its system Chapter 6 6 Vision Stimulus cl 1 . Wavelength ( ) .................... Hue 2. Intensity 3. Purity .................... Brightness ..................... Saturation Chapter 6 7 The Electromagnetic Spectrum Chapter 6 8 The Pathway for Vision scn LGN Receptors X Cortex superior colliculus Chapter 6 9 Chapter 6 10 Anatomy of the Eye Retina photoreceptors (125 million) rods cones synapse with bipolar cells bipolar cells amacrine & horizontal cells ganglion cells ganglion cells (~ 1 million) Chapter 6 axons = optic nerve 11 Structure of the Eye Chapter 6 12 Chapter 6 13 The Retina 120 Million Rods & 8 Million Cones Most Dense Concentration of Cones = Fovea ("Pit") Rods outside fovea very sensitive to light; not good at acuity cannot transduce color Cones inside fovea needed for acuity not as good as rods in detecting movement/light-dark Chapter 6 needed for color vision therefore, not good at night 14 Cross Section of the Retina Chapter 6 15 Recepotr Neural Connections Chapter 6 16 ReceptorNeural Connections Chapter 6 17 The Retina (cont) Light/Dark Adaptation cones ~ 10 mins rods ~ 20-30 mins photopic vs. scotopic systems cones: 3 pigments rods: 1 pigment-- rhodpsin Chapter 6 18 Photoreceptors Photopigment opsin retinal all-trans 11-cis Transduction Light bleaches photopigment 11-cis retinal alltrans retinal opsin activation of -------phosphodiesterases c-GMP Na+ permeability = hyperpolarization= membrane potential= generator (receptor) potential Chapter 6 19 The Phototransduction Process Chapter 6 20 Visual Pathways Receptor-to-Ganglion Cell Ratios rods (periphery): many converge on a single ganglion cell cones (fovea): equal number of ganglion cells & cones Many Parallel Pathways retino-geniculo-striate + 5 others Chapter 6 21 Color Vision: Photopigments 3 Different Types of Photopigments (Cones) & Rhodopsin (Rods) Absorption Characteristics Number: "red" & "green" about same & only ~ 8% "blue" cones Chapter 6 22 Color Vision: Theories Trichromatic vs. Opponent-Process Trichromatic each cone has one of three pigments but not restricted to only a narrow part of the visible spectrum: blue [419 nm] = S green [531 nm] = M yellow-green [559 nm] = L Under ordinary circumstances, almost any object stimulates cones of at least 2 kinds, providing for d visual acuity & good perception of form Chapter 6 23 Receptor Response to Wavelengths Chapter 6 24 Connections Between Cones & Bipolar Cells Chapter 6 25 Receptor Ganglion Cell Interaction Chapter 6 26 Color Vision: Theories (cont) Opponent-Process record from neurons in ganglion or LGN 4 main types of cells: (+R -G); (+G -R); (+Y -B); (+B -Y) Chapter 6 27 Neural Basis of Visual Perception Receptive Fields area of body which when stim. excites/inhibits a cell = that cell's receptive field visual field is an area of the retina or visual field which excites/inhibits a neuron present stimulus & record from cell a How to Document a Cell's Receptive Field change from baseline firing = response Fig. 6.18 Chapter 6 28 Simple Receptive Fields Chapter 6 29 Complex Receptive Fields Chapter 6 30 Receptive Fields of Ganglion / Cortical Neurons Chapter 6 31 Neural Basis of Visual Perception Lateral Inhibition scan image Greatest excitation occurs on the light side of the boundary. Lowest level of excitation occurs on the dark side of the boundary. Therefore, there is a contrast effect or sharpening of perception of the boundary. Chapter 6 32 Parallel Pathways in the Visual System 3 Separate, Partly Independent Pathways shape, color/brightness, movement & depth perception Characteristics of Ganglion Cells X, Y, & W cells X = mainly fovea, smaller Y = evenly throughout retina, larger synapse with LGN Chapter 6 33 Parallel Pathways in the Visual System Characteristics of LGN Cells 6 layers of neurons: layers 2, 3, & 5 = ipsilateral eye layers 1, 4, & 6 = contralateral eye contains a complete map of the retina 4 layers composed of "parvocellular" neurons; 2 layers of "magnocellular" neurons X cells parvocellular ("tonic"& smaller r.f.) Y cells magnocellular ("phasic"& larger ``) Chapter 6 34 Parallel Pathways in the Visual System Cerebral Cortex: 6 Layers parvocellular & magnocellular pathways become 3 "blobs" = columns within visual cortex (V1/striate) prefer color stimuli connected to parvocellular (and some magnocellular) "interblobs" = sensitive to SHAPE magnocellular path = sensitive to movement & broad outlines Chapter 6 35 Parallel Pathways in the Visual System V2 thin stripe (parv) V4 (Color) thick stripe (magnocel) V3 (Shape) pale stripe (parv) middle temp. cortex (Movement) Inferior Temporal Cortex (Form) Chapter 6 36 Cortical Visual Information Processing: 3 Pathways Chapter 6 37 Mechanism of Shape Detection Feature Detectors simple cells complex cells hypercomplex / end-stopped cells Table 6.3 summarizes Spatial Frequencies sine wave grating = cycles/degree of visual angle lines & bars = special case of Fourier analysis Chapter 6 38 Columnar Organization of Visual Cortex Topographical Organization ipsilateral & contralateral representation Columns cells "tuned" to specific orientation all cells within a column share the same preference for stimuli at a given orientation or azimuth within the visual field Chapter 6 39 Shape Analysis Inferior Temporal Cortex huge receptive fields cells that can actually respond to a hand or a face shape constancy ability to see same shape even though its image (shape) changes on the retina ability of cells in inf. temp. cortex to ignore changes in size & direction Chapter 6 40 Motion Perception MT (middle temporal cortex / V5) & MST (medial superior temporal cortex) Chapter 6 41 Ventral vs. Dorsal Stream Ventral Stream "what" stream Inferior temporal cortex Dorsal Stream "where" or "how" pathway Parietal cortex Overlapping properties of the 2 streams Chapter 6 42 ...
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