Download Lecture S&P

Chapter 6
The Visual System
From Your Eyes to Your Cortex
This multimedia product and its contents are
protected under copyright law. The following are
prohibited by law:
• any public performance or display, including
transmission of any image over a network;
• preparation of any derivative work, including the
extraction, in whole or in part, of any images;
• any rental, lease, or lending of the program.
What do we see?


Somehow a distorted and upsidedown 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
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 energy
 Waves of electromagnetic radiation

Humans see light between 380-750
nanometers
The Conversion of Light to Neural
Signals
–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
 Transduction
The 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

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

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
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
Receptive Fields of Visual Neurons
 Similarities
Receptive
seen at all 3 levels:
fields of foveal areas smaller
than those in the periphery
Circular receptive fields
Monocular
Many had a center-surround
organization
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 either

– receptive fields are rectangular with
“on” and “off” regions
 Complex – also rectangular, larger receptive
fields, respond best to a particular stimulus
anywhere in its receptive field
 Simple
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
Columnar Organization of V1




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
Seeing Color – 2 Theories
 Trichromatic
theory (component
theory)
 Proposed by Young, refined by
Helmholtz
 3 types of receptors, each with a
different spectral sensitivity
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
Seeing Color – 2 Theories
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
 Both
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
