Transcript talk

Color vision
Different cone photoreceptors have opsin
molecules which are
differentially sensitive
to certain wavelengths
of light – these are the
physical basis of color.
The cones are
classified as S, M or L,
depending on their
color sensitivity.
Fig 15-23
Color vision
We already know that
rods are far more
sensitive to light, but
rods preferentially
respond to light at 500
nm wavelength. Light
at other wavelengths
must be stronger to
elicit the same
response.
Fig 15-24
Color vision
How do cones signal
color?
For a stimulus of any
wavelength, each type of
cone will have a different
response, and the
combination of these signals
will signal the perceived
color.
Fig 15-25
Color vision
How do cones signal
color?
Cones send color information
centrally by causing centersurround stimulation in retinal
ganglion cells; in these cases,
the center and surround are
responding to different colors,
and receive inputs from M
(medium) and L (long)
wavelength cones.
Fig 15-26
Color vision
How do cones signal
color?
The other class of color
retinal ganglion cells encodes
the antagonism of yellowblue, and this ganglion cell
receives information from all
3 types of cones.
Fig 15-26
Ganglion cells project the visual field
Retinal ganglion cells from
the retina transmit information
centrally in parallel pathways.
They are divided into
subtypes, called W, X and Y
in cats. The W cells are the
oldest cells phylogenetically,
and send projections to the
older portions of the brain
(mostly midbrain) and not to
the cortex.
Fig 16-5
Ganglion cells project the visual field
Y cells have large somata
and large dendritic branching
fields (and are viewing a
relatively larger portion of
the visual field). They are
called M cells, for
magnocellular, in primate
cortex. They respond rapidly,
thus specializing in the
movement of a stimulus
Fig 16-5
Ganglion cells project the visual field
X cells have smaller somata
and less branching fields.
They are called P cells, for
parvocellular, in primate
cortex. They have smaller
receptive fields and are more
concerned with color,
texture, detail and form
Fig 16-5
The visual field
At the optic chiasm, visual
information from the two sides
of the head cross.
In animals with eyes on the
sides of the head, the entire
visual field for each side is sent
to the opposite side of the brain
(to the tectum).
Fig 16-2
The visual field
In forward-looking animals,
the visual image is split
An object on the right side of the
visual field is seen by both left
hemi-retinae (but not by the right
hemi-retinae). The optic nerves
leave the retinae, and at the optic
chiasm, the two left hemi-retinae
projections go left, while the two
right hemi-retinae go right.
The visual field
There are binocular and
monocular zones
When both eyes are fixed on
a central point, much of the
visual field is seen by both
eyes and is therefore
binocular. However, in the
temporal regions, only one
eye can perceive the
peripheral fields on each
side; that is the monocular
zone.
Central projections
Targets – LGN and cortex
The retinal output then
travels to the mid-brain
(where pupillary reflexes are
mediated) and to the
thalamus, before going on to
the cortex.
Fig 16-1
Central projections
Each LGN serves the contralateral visual field
In this example, the left
nasal retina and the right
temporal retina view the
same visual field (except
for the monocular zone).
Because the ganglion
cell projections from the
left nasal retina cross to
the right side, all the
ganglion cells serving
the left visual field go to
the right LGN.
Fig 16-3
Projection to LGN
Each LGN layer is eye-specific
The projections from the retinal
ganglion cells maintain the field
of view as it was seen - this is
called a retinotopic map. The
LGN contains 6 layers of cell
bodies; each layer receives input
from only one eye. The two
most ventral layers receive M
(magno) ganglion cell inputs,
while the other 4 receive P
(parvo) inputs.
Projection to LGN
Differences between M cells and P cells in LGN
M cells do not see color differences, but perceive differences
in brightness. M cells are able to resolve images quickly, but
not precisely. P cells are sensitive to color, and are also able
to resolve detail in stimuli.
So, in general, the M cells respond to movement in the visual
field while P cells are better at discriminating color and
detail. The M and P cells project to different cortical layers.
Each LGN neuron receives input from only a few ganglion
cells. The receptive fields of LGN neurons are centersurround, much like those in the retina.
Projection to cortex
The visual field is projected in a
retinotopic way
The right visual field is
projected to the left cortex,
while the left visual field is
represented on the right.
The region of the fovea,
because of its high sensitivity
and density of cones, is
represented on a huge amount
of the cortex!
Projection to cortex
The striate cortex is a six-layered structure - layer 4 is
the major input layer.
LGN inputs
Cortical cells
Projection to cortex
Local cortical neurons distribute the inputs
LGN inputs
Cortical cells
Stellate cells in layers 4 receive the input and project it to
layers 2/3, which then project to layers 5 and 6. The output
of the cortex is via pyramidal cells.
Projection to cortex
LGN input is segregated
The magnocellular (Y) cells of
the LGN are located in layers 1
and 2 (layer 1 for the contralateral
eye, layer 2 for the ipsilateral).
The parvocellular (X) cells are in
layers 4 and 6 for the
contralateral eye, layers 3 and 5
for the ipsilateral eye. These
inputs remain strictly segregated
when they arrive at the cortex.
Fig 16-6
Projection to cortex
LGN input is segregated
The inputs from the magnocellular
layers synapse on stellate cells in
layer IVca in the cortex, but in
different positions, called ocular
dominance columns.
The parvocellular (X) layers
synapse in layers IVa and cb, but
still in separate ocular dominance
columns.
So, within layer IV, the visual
information is still eye-specific.
Fig 16-6
Cortical receptive fields
• Simple cells respond best
to oriented bars
Recording from cells in
the cortex demonstrated
that many “simple” cells
respond to bars of light of
particular orientation, not
spots as in the retina or
LGN.
Fig 16-7
Cortical receptive fields
• Simple cells respond best
to oriented bars
The simple cell response
can be constructed by
having several on-center
cells from the LGN project
onto one cell, causing the
simple cell to to respond to
a bar of light. Complex
cell receptive fields are
composed of converging
simple cells.
Orientation columns
• Cells with similar orientation
responses are in columns
This figure shows an area
of the cortex with
orientation columns. Each
column through the cortex
responds to the same
orientation, and represents
a small portion of the
visual field. Adjacent
columns are from
neighboring areas in the
visual field.
Fig 16-8
Ocular dominance columns
• Each visual field point is
represented by two eyes
The input from the right
visual field, which is coming
through the left LGN, is
represented on the left cortex
by both eyes.
Fig 16-8
Color
Color information from
specific LGN cells in
input to layers other
than layer IV, and cells
responding to color are
organized into regions
called “blobs”.
Fig 16-11
Higher order visual
processing
Information from the
primary visual cortex
(V1) is passed via several
pathways, to higher-order
portions of the visual
cortex (V2-V5).
Fig 16-14
Higher order
visual
processing
Different areas of
the visual cortex
are thus concerned
with different
aspects of vision.
Fig 16-15
Higher order visual
processing
Cells in V5 are concerned
with processing of visual
motion, and respond to
oriented movement with
changes in action potential
frequency.
Fig 16-16