Transcript The Visual System

The Visual System
Gross structure of the eye
lens
Cornea
–does
most of
the
focusing
Suspensory
ligaments
Central
fovea –
~4,000
receptors –
105 times
better
resolution
than TV
Ciliary
muscle
Optic
nerve
Aqueous
humor
Vitreous
humor
Facts about refraction
• The refraction system (cornea and lens) focuses
an inverted image on the retina – most of the
refraction is corneal, but the lens is the part of
the system whose refraction can be altered for
distance accomodation.
• Accomodation for near objects: ciliary muscles
contract – tension in suspensory ligaments
decreases – lens assumes a more spherical
shape – greater light bending
The retina
• Consists of:
– Photoreceptors (two basic types) – modified
epithelial cells
– 4 major types of interneurons:
• Bipolar, horizontal, amacrine (2nd order cells)
• Ganglion cells (3rd order cells whose axons pass
into the optic nerve)
Microscopic anatomy of the retina
Note that this
picture is
oriented so that
the eyecup
faces
downwards and
light would
strike the retina
from the bottom
of the picture.
At the fovea, the overlying layers of the retina are swept aside to
improve illumination of the photoreceptors. The receptive fields of
ganglion cells in this are quite small, so perception of fine detail is
high.
Photoreception: primates are tricromats
• Visual pigments confer wavelength
specificity:
– Rhodopsin (rod receptors) 498 nm
– Blue cones 437 nm
– Green cones 533 nm (but there are some
genetic variants)
– “Red” cones 564 nm (actually, peak is in
yellow-orange part of spectrum)
Most mammals are functionally
colorblind
• Most other mammals that have been
tested lack hue discrimination, even
though some of them appear to have the
retinal equipment to do so.
• In particular, most mammals do not
perceive red light
Scotopic and photopic systems
• In primates, rod coupled system (scotopic
system) operates in dim light, conecoupled system (photopic system)
operates in bright light.
• In terms of photoreceptor distribution, the
photopic system dominates in the central
retina and fovea centralis; the scotopic
system dominates toward the periphery of
the retina.
What you need to know about photoreceptor
function
• Photoreceptors are depolarized in the dark
and hyperpolarize when illuminated.
• The photoreceptor’s membrane potential
modulates synaptic vesicle release in the
way that you would expect.
• The transmitter released by
photoreceptors can have inhibitory effects
on some 2nd order cells and excitatory
effects on others.
How do we map receptive fields of retinal ganglion
cells?
• An anesthetized animal (which doesn’t track moving
stimuli with eye movements) views a video screen on
which a stimulus spot is moved. The image of the spot
falls on corresponding parts of the retinal surface.
• An electrode is advanced into the optic nerve of the
stimulated eye. It randomly contacts a single ganglion
cell’s axon. In the absence of an appropriate stimulus,
the ganglion cell is spontaneously active at a low rate.
• The investigator moves the stimulus spot around widely
on the screen until there is a response from the ganglion
cell. The investigator then moves the spot over small
distances and records the changes in the ganglion cell’s
activity.
Two kinds of ganglion cells can be detected
in the retinas of most mammals
• Static cells with annular receptive fields, either
on center/off surround or off-center/on surround.
The optimum stimulus for such cells is either a
light spot on a dark background or a dark spot
on a light background.
• Direction-sensitive cells that respond most
vigorously to spots that move across their
receptive fields in a particular direction.
Retinal ganglion cells are spot detectors
Static cells
Dynamic cells
Lateral inhibition in
the retina determines
annular receptive
fields of ganglion
cells
The orange
ganglion cell is
an off center/on
surround cell –
the tan one is an
on center/off
surround cell.
Notice that lateral
inhibition is
mediated by
horizontal cells.
Neurons in the visual pathway
Retina
108 photoreceptors
Optic Nerve
106 axons of ganglion
cells
Lateral Geniculate
Nucleus (thalamus)
107 interneurons
Primary visual cortex
109 neurons
Anatomic pathway from the retina to the primary
visual cortex
In this partly dissected
brain the green line
traces the visual pathway
from the optic chiasm to
the lateral geniculate
nucleus of the thalamus
(loop), and then to the
primary visual cortex
The left cortex is a map of the right side of
the visual field, and vice versa.
• In the human visual system, and other animals
where there is substantial overlap of the visual
fields of both eyes, axons of ganglion cells in the
medial (nasal) side of each retina decussate at
the optic chiasm, whereas the ones on the
lateral (temporal) side of each retina do not
decussate.
• Thus, those parts of the retinas of both eyes that
view the left side of the visual field pass
information to the right visual cortex, and vice
versa for the right side of the visual field.
Decussating and nondecussating retinothalamic
pathways deal with the overlap
Thought Question
• What differences would you expect in the
retinothalamic connections for those
animals that do not have much overlap in
the visual fields of the two eyes, compared
to those that have significant overlap?
Visual deficits can arise from disease processes
The pituitary tumor has
destroyed the
decussating fibers at the
optic chiasm, causing
tunnel vision.
The cerebral infarct has
destroyed the primary
visual cortex on one
side, causing loss of all
information from the
contralateral side of the
visual field.
This is a summary of the visual deficits that could be expected from
lesions at various points in the visual pathway
Form analysis in the primary
visual cortex
Ocular dominance
within the cortex can be
revealed by a method
that exploits the brain’s
energy metabolism
A monkey which had had one
eye masked was injected with
3H-labeled 2-deoxy D glucose.
This glucose analogue is taken
up by cells as if it were glucose,
but can’t be metabolized. After a
few minutes the animal was
sacrificed and the visual cortex
sliced for autoradiographs. The
stimulated cortical columns
accumulated the radiolabel at a
higher rate than the unstimulated
ones.
The experimental evidence for
cortical organization
A. single-cell records from cortical cells
while specific stimuli are being delivered to
the retina
B. isotopic measurements that give a big
picture of neuronal excitation patterns
across the whole visual cortex in response
to particular visual stimuli
Single-cell records: cortical cells are
orientation detectors
• Cortical cells are most sensitive to lines, borders
and edges. In some cases, the length of the bar
is also critical. This receptive field structure is
established by wiring each cortical cell to receive
projections from a row of ganglion cells on the
retinal surface.
• Simple cortical cells can be activated by static
bars or edges of particular orientations
• Complex cortical cells are activated by flashing
or moving bars of the correct orientation.
Orientation columns are revealed by a
stimulus pattern of repeating stripes
In this experiment a pattern
of vertical stripes was
presented in one part of
the visual field,
corresponding to the left
part of the autoradiogram,
while randomly oriented
stripes were presented in
the other part (right part of
the autoradiogram). In the
left part of the picture, only
those columns that are
specific for vertical
orientations respond; in the
right part, all columns
respond.
The basic functional unit of the visual cortex is a
hypercolumn
• In any single cortical column, all cells prefer the same
orientation and are dominated by input from one retina.
• Adjacent columns in an ocular dominance band prefer
slightly different orientations.
• A hypercolumn consists of two ocular dominance bands
(one for each eye) each containing enough columns to
account for all possible stimulus orientations.
A hypercolumn contains all of the information needed to
analyze for all orientations from a single point on the
cortical map, using input from both eyes
In this sketch we are looking at
the cortical surface
This hypercolumn
contains a total of 10
columns: 5 for each
orientation
dominated by the left
eye and 5
corresponding ones
dominated by the
right eye
Developmental assembly of
the visual cortex
In the cat, correct assembly of the cortical
map depends on early visual experience
• Elimination of all visual stimulus to both eyes
during a 1-2 week critical period soon after birth
results in failure to form orientation-specific
columns- the animal has severe visual deficits
and visual experience after the critical period
• Restricting exposure to one particular
orientation causes all columns to prefer that
orientation – the animal can see vertical
orientations but is blind to other orientations.
Thought question
• What do you suppose happens to the
cortical wiring of the left cortex if the right
eye is masked during the critical period?
A link to an interesting website that lets you
experience optical illusions (and sometimes tries to
explain them)
• http://www.michaelbach.de/ot/
A link to an on-line anatomical atlas – the
neurosyllabus portion may be helpful for the
present course segment
• http://da-atlases.biostr.washington.edu/da.html