1 Biological Neural Networks Example: The Visual System

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Biological Neural NetworksBiological Neural Networks

Example: The Visual SystemExample: The Visual System

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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

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Cross Section of the Human EyeCross Section of the Human Eye

Copyright (c) Allyn & Bacon 2004

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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


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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.


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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


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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.


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PhotoreceptorPhotoreceptor

BipolarBipolar

GanglionGanglion


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Major cell types of the retinaMajor cell types of the retina


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Receptive Fields of Ganglion CellsReceptive Fields of Ganglion Cells


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Color MixingColor Mixing


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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)


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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.


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Stimuli in Stimuli in receptive receptive field of field of neuronneuron

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Cat V1 (striate Cat V1 (striate cortex)cortex)

Orientation Orientation preference preference mapmap

Ocular Ocular dominance dominance mapmap


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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.

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“Data Flow Diagram”of Visual Areas inMacaque Brain

Blue:motion perception pathway

Green:object recognition pathway

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Receptive Fields in Hierarchical Neural NetworksReceptive Fields in Hierarchical Neural Networks

neuron A

receptive field of A

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Receptive Fields in Hierarchical Neural NetworksReceptive Fields in Hierarchical Neural Networks

receptive field of A in input layer

neuron Ain top layer

Documents

1 Biological Neural Networks Example: The Visual System