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Copyright © 2007 by Allyn and Bacon
Chapter 4The Visual System
How We See
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Copyright © 2007 by Allyn and Bacon
What you see is not necessarily what you get Somehow a distorted and upside-
down 2D retinal image is transformed into the 3D world we perceive
2 types of research needed to study vision Research probing the components of the
visual system Research assessing what we see
Copyright © 2007 by Allyn and Bacon
Light Enters the Eye and Reaches the Retina No species can see in the dark, but some
are capable of seeing when there is little light
Light Photons of energyWaves of electromagnetic radiation
Humans see light between 380-760 nanometers in length
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Light enters the eye
Wavelength – perception of color Intensity – perception of brightness Light enters the eye through the pupil
whose size is regulated by the irises Sensitivity – the ability to see when light
is dim Acuity – the ability to see details
Copyright © 2007 by Allyn and Bacon
Light enters the eye
Lens – focuses light on the retina Ciliary muscles adjust the shape of
the lens as needed Accommodation – the process of
adjusting the lens to bring images into focus
Copyright © 2007 by Allyn and Bacon
Binocular cues
Convergence – eyes must turn slightly inward when objects are close
Binocular disparity – difference between the images on the 2 retinas
Both are greater when objects are close – provides brain with distance information
Copyright © 2007 by Allyn and Bacon
The Retina
Built inside-outLight passes through several cell layers
before reaching its receptors Vertical pathway – receptors > bipolar
cells > retinal ganglion cells Lateral communication
Horizontal cellsAmacrine cells
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
The Retina
No receptors where information exits the eyeCreates the blind spot
FoveaAt the center of the retina, high acuityReduced light distortion
Copyright © 2007 by Allyn and Bacon
Cone and Rod Vision
Duplexity theory of vision – cones and rod mediate different kinds of vision
Cones – photopic visionHigh-acuity in good lighting
Rods – scotopic visionHigh-sensitivity, allowing for low-acuity
vision in dim light
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Cone and Rod Vision
More convergence in rod system, increasing sensitivity while decreasing acuity
Which receptors are found at the fovea?
Cones
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Eye Movement
We continually scan the world with small and quick eye movements – saccades
These bits of information are then integrated
Stabilize retinal image – see nothing Visual system responds to change
Copyright © 2007 by Allyn and Bacon
The Conversion of Light to Neural Signals Transduction –conversion of one
form of energy to another Visual transduction – light energy to
neural signals by visual receptors Pigments absorb light Absorption spectrum determines
spectral sensitivity
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Rhodopsin
The pigment found in rods, a G-protein-linked receptor that responds to light
In the dark, sodium channels are partially open Rods depolarized
When light strikes, sodium channels close Rods hyperpolarize in response to light
Copyright © 2007 by Allyn and Bacon
Retina-geniculate-striate pathway
~90% of axons of retinal ganglion cells Information from the left visual field of
each eye projects to the right lateral geniculate nucleus (LGN) and vice versa
Most LGN neurons that project to primary visual cortex (V1, striate cortex) terminate in the lower part of cortical layer IV
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Retinotopic organization
Information received at adjacent portions of the retina remains adjacent
More cortex is devoted to areas of high acuity – like the disproportionate representation of sensitive body parts in somatosensory cortex
About 25% of primary visual cortex is dedicated to input from the fovea
Copyright © 2007 by Allyn and Bacon
The M and P Channels
Magnocellular layers (M layers)Big cell bodies, bottom 2 layers of LGNParticularly responsive to movement Input primarily from rods
Parvocellular layers (P layers)Small cell bodies, top 4 layers of LGNColor, detail, and still or slow objects Input primarily from cones
Copyright © 2007 by Allyn and Bacon
The M and P Channels
M layers – movement, rods P layers – color and detail, cones Project to slightly different areas in
lower layer IV in striate cortex, M neurons just above the P neurons
Project to different parts of visual cortex beyond V1
Copyright © 2007 by Allyn and Bacon
Lateral Inhibition and Contrast Enhancement Visual system detects change Mach bands – nonexistent stripes
that visual system creates to enhance the contrast and make edges easier to see – an example of contrast enhancement
A consequence of lateral inhibition
Copyright © 2007 by Allyn and Bacon
Receptive Fields of Visual Neurons
The area of the visual field within which it is possible for a visual stimulus to influence the firing of a given neuron
Hubel and Wiesel looked at receptive fields in cat retinal ganglion, LGN, and lower layer IV of striate cortex
Copyright © 2007 by Allyn and Bacon
Receptive Fields of Visual Neurons
Similarities seen at all 3 levels:Receptive fields of foveal areas smaller than
those in the peripheryCircular receptive fieldsMonocularMany had an excitatory area and an inhibitory
area separated by a circular boundary
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Receptive Fields in Striate Cortex
Neurons of lower layer IV are an exception – circular receptive fields (as in retinal ganglion cells and LGN)
Most neurons in V1 are eitherSimple – receptive fields are rectangular with
“on” and “off” regionsComplex – also rectangular, larger receptive
fields, respond best to a particular stimulus anywhere in its receptive field
Copyright © 2007 by Allyn and Bacon
Receptive Fields in Striate Cortex
SIMPLE Rectangular “on” and “off” regions,
like cells in layer IV Orientation and
location sensitive All are monocular
COMPLEX Rectangular Larger receptive fields Do not have static
“on” and “off” regions Not location sensitive Motion sensitive Many are binocular
Copyright © 2007 by Allyn and Bacon
Columnar Organization of Primary Visual Cortex Cells with simpler receptive fields send
information on to cells with more complex receptive fields
Functional vertical columns exist such that all cells in a column have the same receptive field and ocular dominance
Ocular dominance columns – as you move horizontally, the dominance of the columns changes
Retinotopic organization is maintained
Copyright © 2007 by Allyn and Bacon
Seeing Color – 2 Theories
Trichromatic theory (component theory)
Proposed by Young, refined by Helmholtz
3 types of receptors, each with a different spectral sensitivity
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Seeing Color – 2 Theories
Opponent-process theory Hering 2 different classes of cells encoding
color, and another class encoding brightness
Each encodes two complementary color perceptions
Copyright © 2007 by Allyn and Bacon
Seeing Color – 2 Theories
Both are correct – coding of color by cones seems to operate on a purely component basis, opponent processing of color is seen at all subsequent levels
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Color Constancy and the Retinex Theory Color constancy – color perception is not
altered by varying reflected wavelenths Retinex theory – color is determined by
the proportion of light of different wavelengths that a surface reflects
Relative wavelengths are constant, so perception is constant
Copyright © 2007 by Allyn and Bacon
Visual Cortex
Primary – receives most of its input from the LGN
Secondary – receives most of its input from primary visual cortex
Visual association cortex – receives input from secondary visual cortex and other secondary sensory systems
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Scotomas: Completion
Damage to an area of primary visual cortex produces a scotoma, an area of blindness
Completion prevents many patients of from being aware of their deficit – the mind fills in the blanks
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Prosopagnosia
Visual agnosia for faces Agnosia – a failure of recognition How is it that you can have an agnosia
for a particular type of information? What does this say about how information is organized in the brain?