Lecture 4: Visual Systems Flashcards
Retina:
the innermost layer of
the eye.
The retina is considered a part of:
The Nervous System.
The retina contains:
neurons that are
sensitive to light and convert
visual information to electrical
signal and transmit it to the
brain.
Photosensitive
(photoreceptor) neurons (2):
Rods
Cones
Rod shape:
rod-shaped (rectangular
head)
rod number:
120 millions
Rod location in the retina:
Periphery
Rod luminosity level
Dim light (high
sensitivity to light) –
Night vision
rod vision accuracy
Low resolution
Rod: Photosensitive pigments:
Rhodopsin (found in
carrots)
Rod: Type of cell:
1 type: monochromic
(absorb white color)
Rod : Color Vision?
No
Cone shape:
Cone-shaped (triangular
head)
Cones number:
8 millions
Cones: Location in the Retina:
Center (fovea)
Cones: Luminosity level:
Bright light (low
sensitivity to light) –
Day vision.
Cones: Vision Accuracy:
High resolution
Cones: Photosensitive pigments:
- Iodopsin
- Porphyropsin
- Cyanopsin
Cones: Type of cell:
3 types:
➢ S-cones (absorb blue)
➢ M-cones (absorb green)
➢ L-cones (absorb red)
Cones: Color vision:
yes
S-Cones absorb:
blue
M-Cones absorb:
green
L-cones absorb:
red
At the lowest levels of illumination, only __ are activated (high
sensitivity to light).
rods
__ begin to contribute to light perception at about the level of
__ (i.e. mesopic vision)
Rods,Starlight
are the only receptors that function under relatively
bright light spectrum (phototopic vision) (e.g. indoor lighting, sunlight).
Cones
5 Classes of Neurons:
- Photosensitive Cells (Rods + Cones)
- Bipolar Cells
- Ganglion Cells
- Horizontal Cells
- Amacrine cells
Bipolar cells connect:
photosensitive
cells to ganglion cells.
Ganglion cells:
Axons form optic nerve
Horizontal cells:
Regulate the activity of
photosensitive cells.
Amacrine cells:
Allow Lateral
communication between
neurons.
In dark (absence of light), rods/cones are :
Depolarized
Na+ & Ca2+ channels are open, allowing Na+ & Ca2+ enter to the cell
and depolarize it.
When stimulated by light, rods/cones are
hyperpolarized
Na+ & Ca+ channels are closed and K+ channels are open =
hyperpolarization.
In Dark (not stimulated by light):
➢ Rods/Cones are depolarized.
➢ Rods/Cones inhibit (block) Bipolar cells (hyperpolarized)
➢ Bipolar cells can not excite Ganglion cells (hyperpolarized)
➢ Visual information is not transmitted to CNS.
In Light (stimulated by light):
➢ Rods/Cones are hyperpolarized.
➢ Bipolar cells released from rods/cones inhibition
(depolarized),
➢ Bipolar Cell excite Ganglion cells (depolarized)
➢ Visual information is transmitted to CNS
Ganglion cells respond differently
to
light and dark depending on the
location of light/dark in the visual
field (center vs periphery)
Two classes of ganglion cells:
On-center cells
Off-center” cells
“On-center” cells:
active to light
spot in center of the visual field
and silent to dark spot in center.
“Off-center” cells:
silent to light
spot in center of the visual field
and active to dark spot in center.
The activity of On-center/Off-center
ganglion cells is controlled by
multiple
interactions between the
photosensitive cells, bipolar cells and
horizontal cells.
specific response to light
constitutes an efficient manner to
transmit visual information to the brain
(i.e. encoding visual information).
On the retinal surface, the image is
inverted top to bottom and reversed
right to left.
On the retinal surface, the image is inverted top to bottom and reversed
right to left.
The image is converted to it real position in
the visual cortex
Visual Field:
part of word seen by the eye
Visual Field: part of word seen by the eye:
Divided to 2 fields:
Left & Right
Divided to 2 fields: Left & Right
➢ Each field is divided to
Median hemi-field & Lateral hemi-field
Median hemi-fields : seen by the
Temporal retina.
Lateral hemi-fields seen by the
Nasal retina.
Visual fields of both eyes overlap in the central portion =
Binocular visual field.
Axons of the ganglion cells form
optic nerve (3rd
order neuron).
60% of the optic nerve fibres crosse (decussate) at
the __ to the opposite side of the brain (contralateral)
optic chiasm
40% of the fibres stay on
the same
side (ipsilateral) of the brain
Fibres from the nasal retina project to
the contralateral side of the brain (i.e. crosse at the
optic chiasm).
Fibres from the temporal retina project to
the
ipsilateral side (do not crosse).
After the optic chiasm, the optic fibres form the
tract
The majority of optic fibres terminate in the
Lateral
Geniculate Body (LGB) in the thalamus.
Neurons from the LGB form the
optic radiation (4th
order neuron) and terminate in the visual cortex
(occipital lobe).
Axons of the ganglion cells project also
to other midbrain areas (3):
(1) Pretectum
(2) Hypothalamus
(3) Superior colliculus
Pretectum:
Control of the pupillary light
reflex.
Hypothalamus:
Regulation of circadian
rhythms (biological clock).
Superior colliculus:
orienting the
movements of head and eyes (see the
vestibular system).
Circuitry responsible for the pupillary light reflex (4 steps):
- Optic nerves from both eyes project to the pretectum to control the pupillary light reflex
- Pretectum neurons project to the Edinger-Westphal nucleus
- Edinger-Westphal neurons send their axons via the oculomotor nerve to terminate on the
ciliary ganglion neurons - The ciliary ganglion neurons (parasympathetic) innervate the constrictor muscle in the iris
and cause iris muscle to adjust the size of the pupil.
What are other names for the visual cortex?
= striate cortex = primary visual cortex = V1 = Brodmann’s area 17.
Visual cortex processes
the basic of sensory information sent by the retina (e.g. place back
the inverted image to its original position, binocular vision, 3D image, ect).
Each region of the retina projects to
a dedicated region of the visual cortex (i.e. map of
sensory representation).
macula
fovea
central region of the retina
Sensory information from the central region of the retina (macula,
fovea) have
a large representation in the visual cortex.
he central region of the retina sends
the large portion of sensory information to
the visual cortex (binocular vision, HD vision)
There are __ cellular layers in the visual cortex “Cytoarchitecture”.
6
In the visual cortex, layer __ is the main input ( receives LGN fibers)
Layer 4
Layers 5 and 6 in the primary visual (striate) cortex are
descending outputs (i.e. outputs to sub-cortical regions).
Layers 2 and 3 in the primary visual (striate) cortex are:
ascending outputs (i.e. outputs to other cortical regions)
The lateral geniculate nucleus (LGN)
receives input from
both eyes
Geniculate
neurons are
monocular (i.e. receive the
images of each eye separately).
LGN fibres project to
layer 4 of the striate
cortex.
LGN inputs are segregated in layer 4 in
ocular dominance columns
The neurons of layer 4 are:
monocular
Layer 4 neurons send their outputs to
other cortical layers; it is at this stage
where:
the images from both eyes
converges to form one single image
(cyclopean representation of the external
world = binocular vision).
Neurons in the primary visual cortex respond selectively to
oriented edges
Different neuron types respond selectively to
different visual information (i.e.
edge orientation, shape, color, …).
Primary visual cortex projects to
other cortical areas “Extrastriate cortex’’ (secondary visual
cortex) involved in complex visual perception:
Dorsal pathway is the __ pathway
where/action pathway
dorsal pathway projects to:
the parietal lobe
The dorsal pathway is responsible for :
Spatial aspect of vision (e.g. analysis of motion, positional relationship between objects, etc)
The ventral pathway is the __ pathway
what pathway, perception
ventral pathway projects to:
The temporal lobe
The ventral pathway is responsible for:
object recognition
The eye is a fluid-filled sphere enclosed by:
3
layers of tissue:
The eye is a fluid-filled sphere enclosed by 3
layers of tissue, the sclera is the __ layer
outermost
The eye is a fluid-filled sphere enclosed by 3
layers of tissue, the choroid is the __ layer
intermediate
The eye is a fluid-filled sphere enclosed by 3
layers of tissue, the retina is the __ layer
innermost
The eye has two fluid compartments:
(1) Aqueous humour
(2) Vitreous humour
aqueous humour can be found in:
The anterior chamber
Aqueous humour in the anterior chamber is a:
Watery liquid that supplies nutriments to surrounding
structures.
Vitreous humour is a thick or thin fluid?
thick gelatinous
Vitreous humor, a thick gelatinous that fills the
space of
posterior chamber (i.e. between the back
of the lens and the retina)
What is the function of the vitreous humour?
It maintains the shape
of the eye and supplies nutriments to surrounding
structures.
What is the function of the lens in the eye
refracts (bends) light rays
Sclera:
white fibrous tissue
forming the outermost layer
of the eye.
At the front of the eye, Sclera becomes the __
Cornea
Cornea:
transparent tissue that
separates and protects the
eye from the external
environment and allows light
rays into the eye.
Choroid:
Intermediate layer. Is a
capillary bed that nourishes the
retinal cells.
Ciliary body:
a ring of tissue that
encircles the lens and consists of
a muscular and vascular part:
Ciliary muscle controls
the
shape of the lens
cilliary muscles connected to the lens by:
the Zonule fibers.
Ciliary processes produces
the aqueous humor in the
anterior chamber.
Iris:
coloured portion of the eye
seen through the cornea, contains
2 sets of muscles that contract and
adjust the size of the pupil.
Function of the lens:
refracts (bends)
light rays coming into the
eye and generates a
focused image on the
surface of the retina.
Accommodation
dynamic
changes in the refractive power of
the lens to form a shape image on
the retina
Accomodation changes are made by:
the contraction of the
Ciliary muscle.
Distant vision: lens becomes
thin
and flat
Distant vision: __ refractive power
LESS
Near vision: lens becomes:
thick
and round
near vision: __ refractive power
most
Emmetropia:
In the normal eye,
the lens has a right accommodation
level, the image is focused right on
the retina
Myopia:
he lens over
accommodates (i.e. high
refractive power),
In myopia, the image is focused
in front of the retina
in myopia, it is hard to see:
distant objects
myopia is corrected by __ eye glasses
myopia is corrected by concave (diverging) eye glasses
Hyperopia: the lens __
does not
accommodate enough (i.e. weak
refractive power)
In hyperopia, the image is focused:
beyond the retina
Hyperopia makes it hard to see:
nearby objects
Hyperopia is corrected by:
convex (converging) eye glasses
