1 biological neural networks example: the visual system
Post on 19-Dec-2015
225 Views
Preview:
TRANSCRIPT
1
Biological Neural NetworksBiological Neural Networks
Example: The Visual SystemExample: The Visual System
2
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
Visible light is just a part of the Visible light is just a part of the electromagnetic spectrumelectromagnetic spectrum
3
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
Cross Section of the Human EyeCross Section of the Human Eye
Copyright (c) Allyn & Bacon 2004
4
Anatomy of the Visual SystemAnatomy of the Visual System The EyesThe Eyes
Cornea:Cornea: Transparent outer covering of the eye that Transparent outer covering of the eye that
admits lightadmits light
Pupil:Pupil: Adjustable opening in the iris that regulates Adjustable opening in the iris that regulates
the amount of light that enters the eyethe amount of light that enters the eye
Iris:Iris: Pigmented ring of muscles situated behind Pigmented ring of muscles situated behind
the corneathe cornea
Copyright (c) Allyn & Bacon 2004
5
Anatomy of the Visual SystemAnatomy of the Visual System PhotoreceptorsPhotoreceptors
Retina:Retina: The neural tissue and photoreceptive cellsThe neural tissue and photoreceptive cellslocated on the inner surface of the posteriorlocated on the inner surface of the posteriorportion of the eye.portion of the eye.
Rod:Rod: Photoreceptor cells of the retina, sensitive to Photoreceptor cells of the retina, sensitive to
light of low intensity.light of low intensity.
Cone:Cone: Photoreceptor cells of the retina; maximally Photoreceptor cells of the retina; maximally
sensitive to one of three different wavelengths sensitive to one of three different wavelengths of light and hence encodes color vision.of light and hence encodes color vision.
Copyright (c) Allyn & Bacon 2004
6
Copyright (c) Allyn & Bacon 2004
7
Anatomy of the Visual SystemAnatomy of the Visual System The EyesThe Eyes
Lens:Lens:Consists of a series of transparent, onion-like Consists of a series of transparent, onion-like
layers. Its shape can be changed by layers. Its shape can be changed by contraction of ciliary muscles.contraction of ciliary muscles.
Accommodation:Accommodation: Changes in the thickness of the lens, Changes in the thickness of the lens,
accomplished by the ciliary muscles, that accomplished by the ciliary muscles, that focus images of near or distant objects on the focus images of near or distant objects on the retinaretina
Copyright (c) Allyn & Bacon 2004
8
Copyright (c) Allyn & Bacon 2004
9
Anatomy of the Visual SystemAnatomy of the Visual System The EyesThe Eyes
Fovea:Fovea: Area of retina that mediates the most acute Area of retina that mediates the most acute
vision. Contains only color-sensitive cones.vision. Contains only color-sensitive cones.
Optic Disk:Optic Disk: Location on retina where fibers of ganglion Location on retina where fibers of ganglion
cells exit the eye. Responsible for the blind cells exit the eye. Responsible for the blind spot.spot.
Copyright (c) Allyn & Bacon 2004
10
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
PhotoreceptorPhotoreceptor
BipolarBipolar
GanglionGanglion
Copyright (c) Allyn & Bacon 2004
11
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
Major cell types of the retinaMajor cell types of the retina
Copyright (c) Allyn & Bacon 2004
12
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
Receptive Fields of Ganglion CellsReceptive Fields of Ganglion Cells
Copyright (c) Allyn & Bacon 2004
13
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
Color MixingColor Mixing
Copyright (c) Allyn & Bacon 2004
14
Coding of Visual Information in the RetinaCoding of Visual Information in the Retina Photoreceptors: Trichromatic CodingPhotoreceptors: Trichromatic Coding
Peak Peak wavelengthwavelength sensitivities of the three cones: sensitivities of the three cones:Blue cone:Blue cone: Short-Short- Blue-violet (420 nm) Blue-violet (420 nm) Green cone:Green cone: Medium-Medium- Green (530 nm)Green (530 nm)Red Cone:Red Cone: Long-Long- Yellow-green (560nm)Yellow-green (560nm)
Copyright (c) Allyn & Bacon 2004
15
16
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
Copyright (c) Allyn & Bacon 2004
17
Analysis of Visual Information in Striate CortexAnalysis of Visual Information in Striate Cortex
David Hubel and Torsten WieselDavid Hubel and Torsten Wiesel
1960’s at Harvard University1960’s at Harvard University
Discovered that neurons in the visual cortex did Discovered that neurons in the visual cortex did not simply respond to light; they selectively not simply respond to light; they selectively responded to specific features of the visual world.responded to specific features of the visual world.
Copyright (c) Allyn & Bacon 2004
18
Copyright © Allyn & Bacon 2004Copyright © Allyn & Bacon 2004
Copyright (c) Allyn & Bacon 2004
19
Copyright (c) Allyn & Bacon 2004
20
Stimuli in Stimuli in receptive receptive field of field of neuronneuron
21
Cat V1 (striate Cat V1 (striate cortex)cortex)
Orientation Orientation preference preference mapmap
Ocular Ocular dominance dominance mapmap
Copyright (c) Allyn & Bacon 2004
22
23
Structure of NNs (and some ANNs)Structure of NNs (and some ANNs)• In biological systems, neurons of similar In biological systems, neurons of similar
functionality are usually organized in separate functionality are usually organized in separate areasareas (or (or layerslayers).).
• Often, there is a Often, there is a hierarchyhierarchy of interconnected layers of interconnected layers with the lowest layer receiving sensory input and with the lowest layer receiving sensory input and neurons in higher layers computing more complex neurons in higher layers computing more complex functions.functions.
• For example, neurons in macaque visual cortex For example, neurons in macaque visual cortex have been identified that are activated only when have been identified that are activated only when there is a there is a face face (monkey, human, or drawing) in the (monkey, human, or drawing) in the macaque’s visual field.macaque’s visual field.
24
“Data Flow Diagram”of Visual Areas inMacaque Brain
Blue:motion perception pathway
Green:object recognition pathway
25
Receptive Fields in Hierarchical Neural NetworksReceptive Fields in Hierarchical Neural Networks
neuron A
receptive field of A
26
Receptive Fields in Hierarchical Neural NetworksReceptive Fields in Hierarchical Neural Networks
receptive field of A in input layer
neuron Ain top layer
top related