The Brain

1
The Brain Vision
2
The Eye is Nothing Without The Brain

• Certainly the eye in mammals has evolved through
  a very sophisticated evolutionary course in order
  to let us sense our environment.
• But without the brain to interpret that sensory
  information, we would be unable to make sense of
  the visual world.
• Information conveyed to us through our sensory
  systems is raw, unrefined, impression.
• In the visual system, our visual apparatus is
  obscured by blood vessels, and the cells bodies
  of our receptors and ganglion cells.

3
Light

• This slides shows a cross section of the cells in
  the human retina
• Blood vessels obscure light entering the retina
  and break it up, prior to its hitting the
  photoreceptors.
• Thus we must reconstruct a coherent picture of
  the visual world based on a very fragmented
  initial pattern of stimulation. Visual
  processing is information processing, not image
  transmission!
• How do we do this?

4
Bottom up top down processing
5
Sensation Perception
Sensation is a bottom up phenomenon, it is
stimulus dependant. Perception is a top down
phenomenon it involves the organization and
interpretation of stimuli which necessitates the
use of previous experiences
But how do we process information so quickly?
6
The Concept of Parallel Processing

• While the brain was thought for years to be
  compartmentalized (e.g. specific functions
  localized to specific areas of the brain,
  movement to the motor cortex), evidence now
  suggests that this is not the case.
• The brain seems to function is a very holistic
  way in which massive simultaneously activation of
  many areas occurs.
• This massive activation (for even a stimulus as
  simple as a spot of light) has been referred to
  in cognitive psychology as Parallel Distributed
  Processing (PDP).
• PDP proposes that the brain functions by
  distributing information processing throughout in
  a parallel fashion, rather than processing
  information in a serial fashion (stepwise).
• This means that once basic sensory information
  has been transduced and encoded in basic sensory
  areas it is sent to many different association
  areas simultaneously.
• This theory explains the speed at which
  information may be processes in humans, despite
  the finite limits on neuronal speed and firing
  rate.

7
Perception Demonstration

• If your on the RIGHT side of the room close your
  eyes
• IF your on the LEFT side of the room, focus on
  the screen

The following demonstration is based on two
processes Feature detection, a bottom up process
perceptual organization, a top-down process
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What do you see? Please remain silent
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What do you see?
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Open your Eyes! What do you see in the next slide?
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What do you see?
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Parallel processing Begins in the eye

• There are actually two different types of
  ganglion cells, that are gathering information
  from retinal rods cones (via Bipolar
  Horizontal cells).
• These two ganglion cells are referred to as Large
  Small, Based on the size of their dendritic
  arbors.
• The larger the dendritic arbor, the more signals
  the cell can get from bipolar and horizontal
  cells.
• The spread of the dendrites corresponds to the
  region of the photoreceptor mosiac sampled by the
  cell.
• Therefore the receptive field of the small
  ganglion cell would be smaller than that of the
  larger one.

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Axons from the ganglion cells come together to
form the optic nerve. The optic nerve caries
visual information to the thalamus
The large ganglion cells synapse with cells in
the lower two layers of the thalamus (the
Magnocellular layers). The smaller ganglion cells
synapse with the upper four layers (The
Parvocellular pathway).
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Crossing visual fibers
15
The What and the Where Pathways

• The two pathways carry different types of
  information.
• The Magnocellular pathway carries information
  about perception of motion, space, position,
  depth, figure ground separation, and the overall
  organization of the scene (the where system).
• The Parvocellular is responsible for our ability
  to recognize objects, including faces, in color
  and fine complex detail (the what system).
• Note that the Magnocellular system is color
  blind, but has very high contrast sensitivity.

16
Evolution and The What and the Where Pathways

• The Parvocellular pathway is well developed only
  in Primates, and it is not immediately oblivious
  why this subdivision would occur, and why the
  pathways would differ color, acuity, speed, and
  contrast.
• It seems that this is an evolutionary
  development.
• Lower mammals are much less sensitive to color,
  but they are more sensitive to motion.
• Because it would be an advantage to sense things
  that move, prey or predators, as well as to
  operate in a 3 directional environment, the older
  system would code for these variables.
• As The newer primate system evolved, it was
  probably easier to just maintain the old system
  and add on

17
What would the perception of art be like without
the development of a color system?

• If evolution have even given us a different
  color spectrum, how would it have effected our
  development of art.
• Obliviously our system did not develop to allow
  us to appreciate art, but to operate in our
  visual environment
• Were are just fortunate we can use to develop
  and appreciate art.

18
Cellular Properties in Information Processing

• A Center surround cell, responds when its center
  is stimulated, but not when its surround is
  stimulated.
• This is how we are able to see lines Contours.
• Center surround organization, occurs in bipolar,
  ganglion, and cortical areas

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On and Off Center Arrangement

• Center surround arrangement occurs in two forms
• On-center cells are stimulated by light in the
  center and inhibited by energy in the surround
• Off-Center cells are stimulated by light in the
  surround and inhibited by light in the center

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Why a center surround cell responds as it does

• This slide shows responses of an on-center
  surround cell in response to spots of light.
• The largest response in to a medium spot of
  light.
• Diffuse light gets the least response

21
On Off Center Bipolar Cells

• On and off center arrangement of cells occurs as
  early as the second stage of visual processing
  (e.g. directly behind the visual receptors).
• This arrangement continues into thalamic and
  cortical areas as well.

22
So why do center surround cells allow us to do
edge detection?
23
Illusions based on Center surround

• A Herman Grid demonstrates an illusion based on
  center surround responding
• (Ludimar Herman, 1870).

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Illusions based on Center surround

• The Cornsweet illusion is also based on edge
  detection.
• This is caused by greater sensitivity to abrupt
  than to gradual change
• Does the entire left half of the panel look
  lighter?
• The luminance is actually the same in the outer
  halves.

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A Demonstration of the Cornsweet illusion in real
life
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Center surround organization Color

• Center surround organization also occurs for
  color, not just luminance.
• This slide demonstrates on and off center
  arrangement in cells found in the thalamus

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First 3 stages of visual processing
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Visual information from the thalamus finally
reaches the primary visual areas in the posterior
portion of the brain

• There are over 100 million receptors in the
  cortex that are sensitive to visual input
• Many of these cells are sensitive over to vary
  specific input, we refer to these cells as
  simple cells
• Simple cells respond to basic stimuli that cross
  there receptive fields. These cells were first
  described by Hubel Wiesel in the visual cortex
  of a cat in 1968.
• But the Magnocellular and Parvocellular divisions
  remain, and these pathways leave the primary
  visual cortex and project to different
  association areas.

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Higher order processing (Cognition?)

• Of course once information has reached the
  primary visual areas of the brain, and been sent
  to association areas it must be integrated and a
  response initiated.
• For viewing complex visual stimuli, A feedback
  loop between the brain and the visual system is
  necessary higher order processing.
• Once the basic components of a stimulus has been
  encoded identified, the system must
  automatically keep scanning for salient features
  in the scene.

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Right Left Hemisphere Processing

• It is important to note that information must be
  rapidly transferred back between the two
  hemispheres of the brain.
• This occurs through the comissures of the brain
  (e.g. the corpus collosum and the anterior
  comissure). The main comissure is the corpus
  collosum.

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The Sperry Experiments

• If this flow of information is interrupted, a
  failure to integrate information correctly will
  occur.
• In the 1970s some epileptic patients who did not
  respond well to medications underwent a procedure
  know as a comissurotomy which demonstrated just
  how important the flow of information between
  hemispheres is.
• Experiments by Roger Sperry confirmed the
  different functions of the right and left
  hemispheres and the need for information
  integration through the comissures.