Transcript PowerPoint Ch. 6

Chapter Six
Vision
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
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
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
Figure 6.2 Cross section of the vertebrate eye
Note how an object in the visual field produces an inverted image on the retina.
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.
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
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
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
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
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.)
Color Vision Deficiency
Color Vision Deficiency-inability to perceive color differences
Generally results from people lacking different subsets of cones
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
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
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.
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.
Figure 6.18 An illustration of lateral inhibition
Do you see dark diamonds at the “crossroads”?
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
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; Ts’o & Roe,
1995; Van Essen & DeYoe,
1995)
Neural Basis of Visual Perception
The Cerebral Cortex: The Shape Pathway
Hubel and Wiesel’s 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
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 light’s 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.
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
Neural Basis of Perception
Disorders of Object Recognition
Visual Agnosia-Inability to Recognize Objects
Prosopagnosia-Inability to recognize faces
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
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
Development of the Visual System
Infant Vision
See better in the periphery than in the center of vision
Great difficulty in shifting attention
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
Experience and Visual Development
Early Exposure to a Limited Array of Patterns—most 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