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1 Biological Neural Networks Example: The Visual System.

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1 1 Biological Neural Networks Example: The Visual System

2 2 Copyright © Allyn & Bacon 2004 Visible light is just a part of the electromagnetic spectrum

3 3 Copyright © Allyn & Bacon 2004 Cross Section of the Human Eye

4 Copyright (c) Allyn & Bacon 2004 4 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

5 Copyright (c) Allyn & Bacon 2004 5 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.

6 Copyright (c) Allyn & Bacon 2004 6

7 7 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

8 Copyright (c) Allyn & Bacon 2004 8

9 9 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.

10 Copyright (c) Allyn & Bacon 2004 10 Copyright © Allyn & Bacon 2004 PhotoreceptorBipolarGanglion

11 Copyright (c) Allyn & Bacon 2004 11 Copyright © Allyn & Bacon 2004 Major cell types of the retina

12 Copyright (c) Allyn & Bacon 2004 12 Copyright © Allyn & Bacon 2004 Receptive Fields of Ganglion Cells

13 Copyright (c) Allyn & Bacon 2004 13 Copyright © Allyn & Bacon 2004 Color Mixing

14 Copyright (c) Allyn & Bacon 2004 14 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 (560nm)

15 Copyright (c) Allyn & Bacon 2004 15

16 16 Copyright © Allyn & Bacon 2004

17 Copyright (c) Allyn & Bacon 2004 17 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.

18 Copyright (c) Allyn & Bacon 2004 18 Copyright © Allyn & Bacon 2004

19 Copyright (c) Allyn & Bacon 2004 19

20 Copyright (c) Allyn & Bacon 2004 20 Stimuli in receptive field of neuron

21 21 Cat V1 (striate cortex) Orientation preference map Ocular dominance map

22 Copyright (c) Allyn & Bacon 2004 22

23 23 Structure of NNs (and some ANNs) In biological systems, neurons of similar functionality are usually organized in separate areas (or layers).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.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.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.

24 24 “Data Flow Diagram” of Visual Areas in Macaque Brain Blue: motion perception pathway Green: object recognition pathway

25 25 Receptive Fields in Hierarchical Neural Networks neuron A receptive field of A

26 26 Receptive Fields in Hierarchical Neural Networks receptive field of A in input layer neuron A in top layer

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