Biological Neural Networks Example The Visual System 1
Biological Neural Networks Example: The Visual System 1
Visible light is just a part of the electromagnetic spectrum 2 Copyright © Allyn & Bacon 2004
Cross Section of the Human Eye 3 Copyright © Allyn & Bacon 2004
Anatomy of the Visual System § The Eyes § Cornea: § Transparent outer covering of the eye that admits light § Pupil: § Adjustable opening in the iris that regulates the amount of light that enters the eye § Iris: § Pigmented ring of muscles situated behind the cornea Copyright (c) Allyn & Bacon 2004 4
Anatomy of the Visual System § Photoreceptors § Retina: § The neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye. § Rod: § Photoreceptor cells of the retina, sensitive to light of low intensity. § Cone: § Photoreceptor cells of the retina; maximally sensitive to one of three different wavelengths of light and hence encodes color vision. Copyright (c) Allyn & Bacon 2004 5
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Anatomy of the Visual System § The Eyes § Lens: Consists of a series of transparent, onion-like layers. Its shape can be changed by contraction of ciliary muscles. § Accommodation: § Changes in the thickness of the lens, accomplished by the ciliary muscles, that focus images of near or distant objects on the retina Copyright (c) Allyn & Bacon 2004 7
Copyright (c) Allyn & Bacon 2004 8
Anatomy of the Visual System § The Eyes § Fovea: § Area of retina that mediates the most acute vision. Contains only color-sensitive cones. § Optic Disk: § Location on retina where fibers of ganglion cells exit the eye. Responsible for the blind spot. Copyright (c) Allyn & Bacon 2004 9
Photoreceptor Bipolar Ganglion Copyright (c) Allyn & Bacon 2004 10 Copyright © Allyn & Bacon 2004
Major cell types of the retina Copyright (c) Allyn & Bacon 2004 11 Copyright © Allyn & Bacon 2004
Receptive Fields of Ganglion Cells Copyright (c) Allyn & Bacon 2004 12 Copyright © Allyn & Bacon 2004
Color Mixing Copyright (c) Allyn & Bacon 2004 13 Copyright © Allyn & Bacon 2004
Coding of Visual Information in the Retina § Photoreceptors: Trichromatic Coding § Peak wavelength sensitivities of the three cones: Blue cone: Short. Blue-violet (420 nm) Green cone: Medium. Green (530 nm) Red Cone: Long. Yellow-green (560 nm) Copyright (c) Allyn & Bacon 2004 14
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16 Copyright © Allyn & Bacon 2004
Analysis of Visual Information in Striate Cortex § David Hubel and Torsten Wiesel § 1960’s at Harvard University § Discovered that neurons in the visual cortex did not simply respond to light; they selectively responded to specific features of the visual world. Copyright (c) Allyn & Bacon 2004 17
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Stimuli in receptive field of neuron Copyright (c) Allyn & Bacon 2004 20
Cat V 1 (striate cortex) Orientation preference map Ocular dominance map 21
Copyright (c) Allyn & Bacon 2004 22
Structure of NNs (and some ANNs) • In biological systems, neurons of similar functionality are usually organized in separate areas (or layers). • Often, there is a hierarchy of interconnected layers with the lowest layer receiving sensory input and neurons in higher layers computing more complex functions. • For example, neurons in macaque visual cortex have been identified that are activated only when there is a face (monkey, human, or drawing) in the macaque’s visual field. 23
“Data Flow Diagram” of Visual Areas in Macaque Brain Blue: motion perception pathway Green: object recognition pathway 24
Receptive Fields in Hierarchical Neural Networks neuron A receptive field of A 25
Receptive Fields in Hierarchical Neural Networks neuron A in top layer receptive field of A in input layer 26
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