Biological Neural Networks
Example: The Visual System
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Visible light is just a part of the
electromagnetic spectrum
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Cross Section of the Human Eye
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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
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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.
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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
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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.
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Photoreceptor
Bipolar
Ganglion
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Major cell types of the retina
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Receptive Fields of Ganglion Cells
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Color Mixing
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Coding of Visual Information in the Retina
 Photoreceptors: Trichromatic Coding
 Peak wavelength sensitivities of the three cones:
Blue cone:
ShortBlue-violet (420 nm)
Green cone:
MediumGreen (530 nm)
Red Cone:
LongYellow-green (560nm)
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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.
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Stimuli in
receptive
field of
neuron
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Cat V1 (striate
cortex)
Orientation
preference
map
Ocular
dominance
map
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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.
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“Data Flow Diagram”
of Visual Areas in
Macaque Brain
Blue:
motion perception
pathway
Green:
object recognition
pathway
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Receptive Fields in Hierarchical Neural Networks
neuron A
receptive field of A
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Receptive Fields in Hierarchical Neural Networks
neuron A
in top layer
receptive field of A in input layer
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