Chapter Six Vision

1
Chapter SixVision
2
Visual Coding and Retinal Receptors

• Reception-absorption of physical energy by
  receptors
• Transduction-the conversion of physical energy to
  an electrochemical pattern in the neurons
• Coding- one-to-one correspondence between some
  aspect of the physical stimulus and some aspect
  of the nervous system activity

3
Visual Coding and Retinal Receptors

• From Neuronal Activity to Perception
• coding of visual information in the brain does
  not duplicate the stimulus being viewed
• General Principles of Sensory Coding
• Muller and the law of specific energies-any
  activity by a particular nerve always conveys the
  same kind of information to the brain
• Qualifications of the Law of Specific Energies
• the rate of firing or pattern of firing may
  signal independent stimuli
• timing of action potentials may signal important
  information indicating such things as movement
• the meaning of one neuron depends on what other
  neurons are active at the same time

4
Visual Coding and Retinal Receptors

• The Eye and Its Connections to the Brain
• Pupil-opening in the center of the eye that
  allows light to pass through
• Lens-focuses the light on the retina
• Retina-back surface of the eye that contains the
  photoreceptors
• The Fovea-point of central focus on the retina
• The Route Within the Retina
• photoreceptors-rods and cones
• bipolar cells-receive input from rods and cones
• ganglion cells-receive input from bipolar cells
• optic nerve-made up of axons of ganglion cells
• blind spot-the point where the optic nerve leaves
  the eye

5
Figure 6.2  Cross section of the vertebrate
eye Note how an object in the visual field
produces an inverted image on the retina.
6
Figure 6.4  Visual path within the eyeball The
receptors send their messages to bipolar and
horizontal cells, which in turn send messages to
the amacrine and ganglion cells. The axons of the
ganglion cells loop together to exit the eye at
the blind spot. They form the optic nerve, which
continues to the brain.
7
Figure 6.6  Two demonstrations of the blind spot
of the retina Close your left eye and focus your
right eye on the o in the top part. Move the page
toward you and away, noticing what happens to the
x. At a distance of about 25 cm (10 inches), the
x disappears. Now repeat this procedure with the
bottom part. At that same distance what do you
see?
Animation
8
Visual Receptors Rods and Cones

• Rods
• abundant in the periphery of the retina
• best for low light conditions
• see black/white and shades of gray

• Cones
• abundant around fovea
• best for bright light conditions
• see color

9
Transduction

• Both Rods and Cones contain photopigments
  (chemicals that release energy when struck by
  light)
• 11-cis-retinal is transformed into
  all-trans-retinal in light conditions
• this results in hyperpolarization of the
  photoreceptor
• the normal message from the photoreceptor is
  inhibitory
• Light inhibits the inhibitory photoreceptors and
  results in depolarization of bipolar and ganglion
  cells

10
Color Vision

• The Trichromatic (Young-Helmholtz) Theory
• we perceive color through the relative rates of
  response by three kinds of cones, each kind
  maximally sensitive to a different set of
  wavelengths
• The Opponent-Process Theory
• we perceive color in terms of paired opposites
• The Retinex Theory
• When information from various parts of the retina
  reaches the cortex, the cortex compares each of
  the inputs to determine the brightness and color
  perception for each area

11
Figure 6.12  Possible wiring for one bipolar
cell Short-wavelength light (which we see as
blue) excites the bipolar cell and (by way of the
intermediate horizontal cell) also inhibits it.
However, the excitation predominates, so blue
light produces net excitation. Red, green, or
yellow light inhibit this bipolar cell because
they produce inhibition (through the horizontal
cell) without any excitation. The strongest
inhibition is from yellow light, which stimulates
both the long- and medium-wavelength cones.
Therefore we can describe this bipolar cell as
excited by blue and inhibited by yellow. White
light produces as much inhibition as excitation
and therefore no net effect. (Actually, receptors
excite by decreasing their usual inhibitory
messages. Here we translate that double negative
into excitation for simplicity.)
12
Color Vision Deficiency

• Color Vision Deficiency-inability to perceive
  color differences
• Generally results from people lacking different
  subsets of cones

13
Neural Basis of Visual Perception

• An Overview of the Mammalian Visual System
• Rods and Cones synapse to amacrine cells and
  bipolar cells
• Bipolar cells synapse to horizontal cells and
  ganglion cells
• Axons of the ganglion cells leave the back of the
  eye
• The inside half of the axons of each eye cross
  over in the optic chiasm
• Pass through the lateral geniculate nucleus
• Transferred to visual areas of cerebral cortex

14
Processing Visual Stimuli

• Mechanisms of Processing in the Visual System
• Receptive Field-the part of the visual field to
  which any one neuron responds
• They have both excitatory and inhibitory regions
• Lateral Inhibition-the reduction of activity in
  one neuron by activity in neighboring neurons
• Heightens contrasts-those receptors just inside
  the border are most excited and those outside the
  border are the least responsive

15
Figure 6.16  Receptive fields The receptive field
of a receptor is simply the area of the visual
field from which light strikes that receptor. For
any other cell in the visual system, the
receptive field is determined by which receptors
connect to the cell in question.
16
Figure 6.17  Blocks on a surface of gelatin,
analogous to lateral inhibition Each block pushes
gelatin down and therefore pushes neighboring
blocks up. Blocks at the edge are pushed up less
than those in the center.
17
Figure 6.18  An illustration of lateral
inhibition Do you see dark diamonds at the
crossroads?
18
Neural Basis of Visual Perception

• Concurrent Pathways in the Visual System
• In the Retina and Lateral Geniculate
• Two categories of Ganglion cells
• Parvocellular-smaller cell bodies and small
  receptive fields, located near fovea detect
  visual details, color
• Magnocellular-larger cell bodies and receptive
  fields, distributed fairly evenly throughout
  retina respond to moving stimuli and patterns
• In the Cerebral Cortex
• V1-Primary Visual Cortex-responsible for first
  stage visual processing
• V2-Secondary Visual Cortex-conducts a second
  stage of visual processing and transmits the
  information to
• additional areas
• Ventral stream-visual paths in the temporal
  cortex
• Dorsal stream-visual path in the parietal cortex

19
Figure 6.20  Three visual pathways in the
cerebral cortex (a) A pathway originating mainly
from magnocellular neurons. (b) A mixed
magnocellular/parvocellular pathway. (c) A mainly
parvocellular pathway. Neurons are heavily
connected with other neurons in their own pathway
but only sparsely connected with neurons of other
pathways. Area V1 gets its primary input from the
lateral geniculate nucleus of the thalamus the
other areas get some input from the thalamus but
most from cortical areas. (Sources Based on
DeYoe, Felleman, Van Essen, McClendon, 1994
Tso Roe, 1995 Van Essen DeYoe, 1995)
20
Neural Basis of Visual Perception

• The Cerebral Cortex The Shape Pathway
• Hubel and Wiesels Cell Types in the Primary
  Visual Cortex
• Simple Cells
• has fixed excitatory and inhibitory zones in its
  receptive field
• Complex Cells
• receptive fields cannot be mapped into fixed
  excitatory and inhibitory zones
• Respond to a pattern of light in a particular
  orientation
• Hypercomplex cells (End-stopped cells)
• Resemble complex cells but have a strong
  inhibitory area
• at one end of its bar-shaped receptive field

21
Figure 6.23  The receptive field of a complex
cell in the visual cortex It is like a simple
cell in that its response depends on a bar of
lights angle of orientation. It is unlike a
simple cell in that its response is the same for
a bar in any position within the receptive field.
22
Neural Basis of Visual Perception

• The Columnar Organization of the Visual Cortex
• Column are grouped together by function
• Ex cell within a given column respond best to
  lines of a single orientation
• Are Visual Cortex Cells Feature Detectors?
• Feature Detectors-neurons whose responses
  indicate the presence of a particular feature
• Shape Analysis Beyond Areas V1 and V2
• Inferior Temporal Cortex (V3)-detailed
  information about stimulus shape
• (V4)-Color Constancy Visual Attention
• (V5)-Speed and Direction of Movement

23
Neural Basis of Perception

• Disorders of Object Recognition
• Visual Agnosia-Inability to Recognize Objects
• Prosopagnosia-Inability to recognize faces

24
Neural Basis of Visual Perception

• The Cerebral Cortex The Color Pathway
• Parvocellular to V1 (blobs) to V2, V4, and
  Posterior Inferior Temporal Cortex
• The Cerebral Cortex The Motion and Depth
  Pathways
• Structures Important for Motion Perception
• Middle-temporal cortex-V5-speed and direction of
  movement
• Motion Blindness-Inability to detect objects are
  moving

25
Neural Basis of Visual Perception

• Visual Attention
• Attentional Processes can determine what is seen
• The Binding Problem Revisited Visual
  Consciousness
• How are all aspects of an object brought
  together?

Animation
26
Development of the Visual System

• Infant Vision
• See better in the periphery than in the center of
  vision
• Great difficulty in shifting attention

27
Experience and Visual Development

• Early Lack of Stimulation of One Eye-blindness
  occurs in that one eye
• Early Lack of Stimulation of Both Eyes-if this
  occurs over a long period of time, loss of sharp
  receptive fields is noted
• Restoration of Response and Early Deprivation of
  Vision-deprive stimulation of the previously
  active eye and new connections will be made with
  the inactive eye
• Uncorrelated Stimulation in Both Eyes-each
  cortical neuron becomes responsive to the axons
  from just one eye and not the other

28
Experience and Visual Development
Early Exposure to a Limited Array of
Patternsmost of the neurons in the cortex become
responsive only to the patterns that the subject
has been exposed to Lack of Seeing Objects in
Motion-become permanently disable at perceiving
motion Effects of Blindness on the Cortex-parts
of the visual cortex become more responsive to
auditory and tactile stimulation