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6) The Visual System
In vertebrates:
 the eye detects colour and light
 but the brain assembles the information and perceives the image
Although light detection in the eye is the first stage in vision, remember that it is actually the brain that “sees
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The toad visual system has 3 levels of analysis:
st
 Retina (1 level)
nd
 Pretectal thalamus (2 level)
rd
 Tectum (3 level)
The human visual system also has 3 main levels:
 Retina
 Thalamus
 Cortex

A circuit level description of the visual system
This shows how complex our visual system is
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 The human eye is surrounded by a mucous membrane called the sclera
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The sclera forms
the outermost tissue layer of the eye and is composed of a tough white fibrous tissue
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 The choroid, a thin, pigmented layer, it gives the colour of the eye i
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it forms the coloured iris
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 By changing size, the iris regulates the amount of light entering the pupil, the hole in the centre of the iris
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It’s the only layer that contains neurons and photoreceptors
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It’s the region of the retina that lacks
photoreceptors
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 Ciliary body, a ring of tissue that around the lens produces the fluid that fills the front of the eye (the aqueous
humor)
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 In front of the lens lies, the anterior cavity is filled with the aqueous humor, a clear watery substance
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Humans and other mammals focus light by changing the shape of the lens: Focussing
The brain not only processes visual information, but also controls what information is captured
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Focusing in the mammalian eye
includes ciliary muscles that control the shape of the lens, which bends light and focuses it on the retina
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

When you focus on a close object, your lens becomes almost spherical
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By turning your head and pointing your eyes in a
particular direction, your brain also determines what lies
in your field of vision
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Overall,
the human retina contains about 125 million rods and about 6
million cones
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The ratio of rods to cones increases with distance from the fovea, with the
peripheral regions having only rods
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 At night, looking directly at a dimly lit object is ineffective, since the rods—the more sensitive light receptors—are found
outside the fovea
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Refractive Errors

Myopia (short sighted) is a
condition of the eye where the light
that comes in, does not directly
focus on the retina but in front of it
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It can be
caused by the corneal surface being
too curved, or by the eyeball being
too long
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Hyperopia (long sighted) occurs in people who are unable to focus on
near objects
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It can be caused
by the eyeball being too short or the refracting system too weak
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The human retina contains two types of
photoreceptors: rods and cones
 Rods are more sensitive to light but
do not distinguish colours; they
enable us to see at night, but only in
black and white
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There are three
types of cones
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These cones have these 3 pigments
called photopsins that detect light of
different wavelengths
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The most obvious feature of the retina viewed through an ophthalmoscope is the blood vessels on its surface
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It is
interesting to note that the sensation of light cannot occur at the optic disk because there are no photoreceptors here, nor can
it occur where the large blood vessels exist because the vessels cast shadows on the retina
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We are not aware of any holes in our field of vision because the brain fills in our perception of
these areas
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At the middle of each retina is a darker-coloured region with a yellowish hue
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It has an absence of large blood vessels
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The term is from the Latin for “pit,” and the retina is thinner in the fovea than elsewhere
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Thus, the part of the retina that lies closer to the
nose than the fovea is called nasal, the part that lies near the temple is called temporal, the part of the retina above the fovea
is called superior, and that below it is called inferior
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The neurons of the retina then relay visual information captured by the photoreceptors to the optic nerve and brain along the
pathways shown with red arrows
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Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor
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Many sensory receptors are very sensitive: they are able to
detect the smallest physical unit of stimulus
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Each rod or cone contains visual pigments consisting of a light-absorbing molecule called retinal bonded to a protein called an
opsin
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Once light activates rhodopsin, cyclic GMP breaks down, and Na+ channels close
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Photoreceptors don’t generate action potentials
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One such pigment is rhodopsin, a well-studied vertebrate visual pigment
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The entire rhodopsin molecule sits within the plasma membrane of a photoreceptor
cell, such as the rod cells of humans (see p
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When 11-cis-retinal absorbs a photon of light energy, it changes into a
different isomer of retinal, called all-trans-retinal
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In vertebrate eyes, the retinal and the opsin eventually
separate from each other (a process called bleaching), which causes the molecule to lose its photosensitivity
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This conversion activates rhodopsin, which activates a G protein, which in turn activates an enzyme that can hydrolyse
cyclic GMP
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 In the light, rhodopsin is activated and so the G protein dependent pathway is activated, cyclic GMP is broken down,
Na channels close, and the cell becomes hyperpolarized
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
In very bright light, however, rhodopsin remains active, and the response in the rods becomes saturated
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This is why you are temporarily blinded if you pass quickly from the bright sunshine into a movie theatre or other dark
environment
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”)
The Rhodopsin is found in the disks of rods and cones
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Photoreceptors are relatively depolarized in the dark (-40mV): at this level, neurotransmitter is released continuously
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Colour Vision and Red-green colour blindness

Among vertebrates, most fishes, amphibians, and reptiles, including birds, have very good colour vision
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Many mammals are nocturnal, and
having a high proportion of rods in the retina is an adaptation that gives these animals keen night vision
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In humans, the
perception of colour is based on three types of cones, each with a different visual pigment—red, green, or blue
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Slight differences in
the opsin proteins are sufficient for each photopsin to absorb light optimally at a different wavelength
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For this reason, the brain’s perception
of intermediate hues depends on the differential stimulation of two or more classes of cones
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Abnormal
colour vision typically results from alterations in the genes for one or more photopsin proteins
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For this reason, colour blindness is more common in males than in females
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Males only have one X chromosome, so a mutation in that gene will lead to this
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Green is about 500-600nm
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Our vision is from about 400-650nm
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However, not all normal trichromats perceive colours
exactly the same
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However, significant abnormalities of colour vision
extend well beyond this range of normal trichromatic vision
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The most
common abnormalities involve red green colour vision, and they
are much more common in men than women
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Men
will have abnormal red-green vision if there is a defect on the
single X chromosome they inherit from their mother
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Gene Therapy for Colour Vision

Squirrel monkeys have just 2 cones: blue and red (or green)
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After 20 weeks, the new opsin allele was
being expressed in cones and the monkeys had begun to
distinguish red from green in a field of coloured dots
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There are five basic classes of neurons in the retina:
 photoreceptors
 Rods
 Cones
 bipolar cells
 ganglion cells
 horizontal cells
 amacrine cells
The human retina is organized into layers of neurons that receive visual information and process it before sending it to the
brain
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The
layers of cells between the photoreceptors and the ganglion cells process information about the visual field
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A three-neuron chain - photoreceptor cell to bipolar cell to ganglion cell - is the most direct pathway of information flow from
photoreceptors to the optic nerve
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GANGLION: From our discussion of rod cells, we know that the photoreceptor cells at the back of the retina hyperpolarize
in response to light and do not generate action potentials
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The much larger axons of the ganglion cells form the optic nerve and carry information about retinal
stimulation to the rest of the central nervous system
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Changes in the membrane potential of rods and cones in response to light alter the rates at which the rods and cones
release neurotransmitter at their synapses with the bipolar cells
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The rate
of neurotransmitter release from the bipolar cells determines the rate at which ganglion cells fire action potentials
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BIPOLAR CELLS: A given bipolar cell can receive input from multiple rods or multiple cones, but not from both
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Most of these cells have graded potentials
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Horizontal cells form synapses
with neighbouring photoreceptors and bipolar cells
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This lateral flow of information enables the retina to sharpen the perception of contrast between
light and dark patterns
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AMACRINE CELLS: They are also interneurons that communicate laterally across the retina
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Some amacrine cells are highly sensitive to changing
illumination or to motion
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Processing of Visual Information

The processing of visual information begins in the retina itself, where
both rods and cones form synapses with bipolar cells
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 Which of the two responses a bipolar cell exhibits depends on
the type of glutamate receptor on its surface at the synapse
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Those that are
hyperpolarized by glutamate now depolarize
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In other cases, horizontal cells carry signals from one rod
or cone to other photoreceptors and to several bipolar cells
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The result is that the region receiving light appears lighter and the dark surroundings even darker
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Amacrine cells distribute
some information from one bipolar cell to several ganglion cells
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A single ganglion cell receives
information from an array of rods and cones, each of which responds to light coming from a particular location
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The fewer rods or cones that supply a single ganglion cell, the smaller the receptive field
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The ganglion cells of the fovea have very small receptive fields, so visual acuity (sharpness) in the fovea is high
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He found that each ganglion cell responds to stimulation of a small circular patch of the retina, which
defines the cell's receptive field
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 Off–centre: The same stimulus applied to the receptive field centre of an off-centre ganglion cell reduces the rate of
discharge, and when the spot of light is turned off, the cell responds with a burst of action potentials
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Upper
panels indicate the time sequence of stimulus
changes; note the overlapping receptive fields
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 B) Effects of dark spot in the
receptive field centre
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The functional differences between these two cell types can be understood in terms of both their anatomy and their
physiological properties and relationships
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Like most other cells in the retina, bipolar cells have graded
potentials rather than action potentials
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 Photoreceptor synapses with off-centre bipolar cells are called sign-conserving, since the sign of the change in
membrane potential of the bipolar cell (depolarization or hyperpolarization) is the same as that in the
photoreceptor
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Remember that photoreceptors hyperpolarize in response to light which decreases their release of neurotransmitter
(Glutamate)
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 A) Functional anatomy of cone inputs to the centre of a ganglion cell receptive field
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 B) Responses of the various cell types to the presentation of a light spot in the centre of the ganglion cell receptive
field
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Glutamate receptors on bipolar cells:
 mGluR6 (G-protein coupled metabotropic) are on on-centre bipolar cells that cause hyperpolarisation when
glutamate from photoreceptors are bound to the cell (in dark) because these receptors activate an intracellular
cascade that closes cGMP-gated Na+ channels
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 AMPA (ionotropic) are on off-centre bipolar cells that cause depolarisation when glutamate from photoreceptor
is bound to the cell (in dark)
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 Kainate (ionotropic) are on off-centre bipolar cells that cause depolarisation when glutamate from
photoreceptor is bound to the cell the (in dark)
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Therefore, glutamate hyperpolarises on-centre bipolar cells and depolarises off-centre bipolar cells
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Although the details of their actions are not entirely clear, horizontal cells are thought to exert their influence via the release of
neurotransmitter directly onto photoreceptor terminals, regulating the amount of transmitter that the photoreceptors release
onto bipolar cell dendrites
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So more glutamate is released
Leading to hyperpolarisation in the
bipolar cell
So less AP in the ganglion cell

Summary of Retina

Phototransduction: The light that falls on
photoreceptors is transformed by retinal
circuitry into a pattern of action potentials
that ganglion cell axons convey to the
visual centres in the brain
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It also allows
the retina to adapt, such that it can
respond effectively over the enormous
range of illuminant intensities in the world
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As we saw in Chapter 11, ganglion cell axons exit the retina through a circular region in its nasal part called the optic disk (or
optic papilla), where they bundle together to form the optic nerve
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Once past the optic chiasm, the ganglion cell axons on each side form the optic tract
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The ganglion cell axons in the optic tract reach a number of structures in the diencephalon and midbrain
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Neurons in the LGN, like their counterparts in the
thalamic relays of other sensory systems, send their axons to the cerebral cortex
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A second major target of ganglion cell axons instead of the LGN, is a collection of neurons that lies between the thalamus and
the midbrain in a region known as the pretectum which is involved in the reduction of the diameter of the pupil that occurs
when sufficient light falls on the retina
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Remember, the visual cortex is in
the occipital lobe

Binocular Vision

The figure shows a projection of the binocular field of view onto the two
retinas and its relation to the crossing of fibres in the optic chiasm
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 Points in the binocular portion of the right visual field (C) fall on
the nasal retina of the right eye and the temporal retina of the
left eye
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The axons of ganglion cells in the nasal retina cross
in the optic chiasm, whereas those from the temporal retina do
not
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Visual Field Deficits

The figure shows how lesions at different points along the primary visual pathway can lead to visual field deficits
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(A) Damage to right optic nerve leads to loss of vision in right eye
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Which is the loss of vision of the temporal
visual field of each eye
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Which is loss of sight in the left visual field
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(E) Damage to right striate cortex leads to left homonymous hemianopsia with macular sparing
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 P-ganglion cells (small neurons ) terminate in the Parvocellular layers
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M ganglion cells:
 Have larger receptive fields
 Axons have faster conduction velocities
 Don’t transmit info about colour because they don’t have cone specificity so are insensitive to differences in
wavelength of light
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P ganglion cells:
 Have smaller receptive fields
 Axons have slower conduction velocities

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Do transmit info about colour because they do have cone specificity so are sensitive to differences in wavelength of
light
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Involved in the detailed analysis of the shape, size, and colour of objects
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Shows the visiuotopic organization of the striate cortex in the right occipital lobe, as seen in midsagittal view
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The area of central vision (the fovea) is represented over a
disproportionately large part of the caudal portion of the lobe, whereas peripheral vision is represented more anteriorly
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Calcarine sulcus

Thalamus to Cortex: Depth Sensitivity

Binocular disparities (large difference) are generally thought to be the basis
of stereopsis (the sensation of depth that arises from viewing nearby
objects with two eyes instead of one)
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When these disparities are small, the images are fused and the disparity is
interpreted by the brain as small differences in depth
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Blue = Right
eye
Green = Left
eye
Cortex V1

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Thalamus to Cortex: Orientation Tuning

Scientists used microelectrode recordings in anesthetized animals to examine the responses of
individual neurons in the lateral geniculate nucleus and the cortex to various patterns of
retinal stimulation
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However, the small spots of
light that were so effective at stimulating neurons in the retina and lateral geniculate nucleus
were largely ineffective in visual cortex
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The responses of cortical
neurons are thus tuned to the orientation of edges, much like cone receptors are tuned to
the wavelength of light
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(A) An anesthetized animal is fitted with contact lenses to focus the eyes on a screen, where images can be projected; an
extracellular electrode records the neuronal responses
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Topographic Organization of Visual Tuning Properties

Hypercolumns/cortical modules were used to study the V1
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You have the whole 360 degrees orientations in one hypercolumn
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One hypercolumn represents one part
of the visual field
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Colours indicate the average preferred orientation of columns at a given location
 red indicates the location of columns that respond preferentially to horizontal orientations
 blue those that respond preferentially to vertical orientations
The smooth progression of preferred orientations is interrupted by point discontinuities (pinwheel centres; circle)
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This smooth progression is interrupted periodically by point discontinuities, where neurons with disparate orientation
preferences lie close to each other in a pattern resembling a child's pinwheel
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 Ventral (what) pathway (ITC) goes to the
inferior temporal cortex
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Many higher level visual areas are multimodal, motor related
or cognitive
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Ventral Path (ITC)

Involved in face detection
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