Visual System Flashcards
What is the optic disk?
exit of the CNII Optic Nerve
What does the optic disk produce?
a “blind spot”
What does the macula lutea contain?
central fovea (pit)
What are the four cell layers of the retina?
- Pigment Layer
2 and 3. Layer of Rods and Cones - Ganglion Cell Layer
What is the pigment layer derived from?
choroid
What does the pigment cell layer attach to the eyeball?
retina
What is the function of the pigment cell layer?
absorbs stray light
What is the funcion of the layer of rods and cones?
light receptors
Where are rods located?
periphery of retina
What is the function of rods?
for low light vision and perception of movement
Where are the cones located?
concentrated in central retina
Does the fovea contain only rods or cones?
cones
What is the significance of the fovea?
area of maximum visual acuity, color and brightness discrimination.
mylenated axons of the ganglion cell layer for what?
CNII Optic Nerve
Ganglion cells describe
receptive fields
In the normal eye, how is the orientation of the object focused on the retina?
inverted
What is light refracted by in the eye?
cornea, aqueous humor, lens, and vitreous humor
What is the function of the lens?
The lens is a device for changing the refractive power. Changes in refractive power are accomplished by changing the shape of the lens.
The lens is made ___ for close viewing, and ___ for distant viewing.
Rounder for close viewing and flatter for distant viewing.
The change in refractive power which allows the viewing of near objects is called ___
accomodation
To view a near object, the lens must increase its refractive power by becoming more
convex (rounder)
At rest, the lens is held in a relatively ___ shape
flat
At rest the lens is held in a relatively flat shape by the
suspensory fibers that connect it to the ciliary muscle.
To accomodate for near vision, the ciliary muscle ___
contracts reducing tension on the suspensory fibers
Due to the lens’ natural elasticity, it contracts to a more ___ shape.
spherical
Contraction of the ciliary muscles is controlled by
parasmypathetic nervie fibers
What is Emmetropia?
the normally occurring condition in which the image of an object is focused on the retinal surface.
What is Hypermetropia?
(far-sightnedness) - the focal point falls behind the retinal surface.
What is Myopia?
(near-sightnedness) - the focal point falls in front of the retina
What is Presbyopia?
is the loss of lens elasticity with age
What is the function of bifocals?
part of the lens that corrects for distance and a part that corrects for near vision due to loss of lens elasticity with age.
What are the two types of photoreceptors?
- Rods
2. Cones
Is excitation threshold low or high for rods?
low excitation threshold
Are rods used in dim or high light conditions?
dim conditions (low excitation threshold)
Is rod acuity poor or high?
Poor
Why doesn’t it matter that rod acuity is poor?
because when light is low, you can’t see small objects anyway.
Is excitation threshold low or high for cones?
high
Are cones used in dim or high light conditions?
high light (high excitation threshold
Is cone acuity low or high?
Acuity is high
Do rods or cones provide color vision?
cones
What is the ability to distinguish color due to?
the presence of three separate cone populations, each of which is maximally sensitive to different wavelength of light: blue, green, and red.
What is perception of other colors due to?
relative excitation of the different populations of cones.
What is colorblindness due to?
absence of one or more of the cone populations
For neural coding of visual signals, generator potentials originating in receptors are transmitted to what type of cell?
bipolar cells
For neural coding of visual signals, generator potentials originating in receptors are transmitted to bipolar cells and then
altered by other cells in the retina
What are the two separate systems in which the neural coding of visual signals can be visualized?
Vertical System
Horizontal System
What is the Vertical System for neural coding of visual signals?
signals pass from receptor to bipolar cells to ganglion cells
What is the Horizontal System for neural coding of visual signals
horizontal and amicrine cells provide lateral interactions (lateral inhibition) between the different vertical system components
What is the function of ganglion cells?
Ganglion cells are the final stage of retinal processing, and tasmit information to subcortical visual centers in the brain?
Axons of ganglion cells form the
optic nerve
Receptive Field Definition
the receptive field (RF) of a ganglion cell is defined as that area in visual space (or the corresponding area of retinal surface, which upon illumination, influences the signaling of that neuron.
How have ganglion cells been subdivided?
on the basis of their response duration or morphology.
What are “sustained” ganglion cells?
cells that respond as long as the stimulus remains within the receptive field.
What are “transient” ganglion cells?
cells that respond only when the light it turned on of off
P-cells are (SMALL/LARGE) ganglion cells of the (TRANSIENT/SUSTAINED) type
small ganglion cells of the sustained type
M-cells are (SMALL/LARGE) ganglion cells of the (TRANSIENT/SUSTAINED) type
large ganglion cells of the transient type
At present it is believed that M-cells are primarily concerned with
signaling changes in the scene being viewed including movement
and changes in light and dark contrast and with basic form analysis
P-cell provide information about
fine detail (high resolution analysis of image)
and color.
Information that is transmitted from the eye through the optic nerve must eventually reach ___ the to be perceived.
cortex
Each point on the retinal surface sees a particular point in the visual field, with neighboring retinal points seeing neighboring visual filed points. Thus the visual field is represented on a corresponding are on the
retina (retinal field) with topographical representation being maintained throughout the rest of the visual system.
The visual field can be divided by a vertical line at the fixation point into a
left and right hemifield
Each half of the brain receives information from (IPSILATERA/CONTRALTERAL/BILATERAL) hemifield.
Each half of the brain receives information only from the opposite (CONTRALTERAL) hemifield.
The separation of the brain receiving information from only the opposite (contralteral) hemifield happens at the level of the ___
optic chiasm
Define homonymity
Images of the visual world are composed of information from the two eyes (binocular) which are merged to form a single image.
What is the general definition of Homonymous? As the term applied to the visual fields?
corresponding halves. This term as it applies to the visual fields, describes both eyes viewing the same or corresponding visual field.
Define Contralteral homonymous perception
The right primary visual cortex (area 17) is perceiving only the left half of the visual field.
The visual pathway organization is such that everything (CAUDAL/ROSTRAL) to the optic chiasm is carrying only contralteral homonymouse perception
The visual pathway organization is such that everything CAUDAL to the optic chiasm is carrying only contralteral homonymouse perception
Define anopia
loss of visual perception
Define hemianopia
loss of visual perception of half of the entire visual field
What is the general definition of Heteronymous? As the term applies to the visual fields?
different halves. As it applies to visual fields, describes each eye viewing different visual fields.
Define Binasal Heteronymous Hemianopia or Binasal Hemianopia.
When each eye is only able to see the ipsilateral temporal visual field and not the nasal visual fields.
Optic nerve fibers from temporal retina (nasal visaul hemifield) course caudally along the ___ edge of the optic nerve, optic chiasm, and optic tract
caudally along the LATERAL edge
Optic nerve fibers from the temporal retina (nasal visual hemifield) course (IPSILATERA/CONTRALTERAL/BILATERAL) to the lateral geniculate nucleus (LGN).
Optic nerve fibers from the temporal retina (nasal visual hemifield) course to the to the IPSILATERAL lateral geniculate nucleus (LGN).
Optic nerve fibers from nasal retina (temporal visual hemifield) course caudally along the ___ edge of the optic nerve
caudally along the MEDIAL edge
Optic nerve fibers from the nasal retina (temporal visual hemifield) course (IPSILATERA/CONTRALTERAL/BILATERAL) to the lateral geniculate nucleus (LGN).
Optic nerve fibers from the nasal retina (temporal visual hemifield) course to the to the CONTRALATERAL lateral geniculate nucleus (LGN).
Optic fibers from the nasal retina (temporal visual hemifield) course caudally along the medial edge of the optic nerve and then
cross in the chaism and then course along the medial edge of the optic tract
Optic fibers from the upper retina (inferior or lower visual hemifield) course caudally along the ___ edge of the edge optic nerve, optic chiasm, optic tract.
dorsal
Optic fibers from the lower retina (superior or lower visual hemifield) course caudally along the ___ edge of this retinogeniculate pathway.
ventral
Retinogeniculate fibers carrying information from the more peripheral visual fields are located more
superficially within the optic nerve, optic chiasm, and optic tract.
Retinogeniculate fibers carrying information from the central (fovea) visual fields are located more
internally within the optic nerve, optic chiasm, and optic tract.
After optic nerve fibers pass through the chiasm they continue as the
optic tract
After optic nerve fibers pass through the chiasm they continue as the optic tract to end primarily (80%) in the ___ with some (20%) terminating in the ___.
After optic nerve fibers pass through the chiasm they continue as the optic tract to end primarily (80%) in the LGN with some (20%) terminating in the MIDBRAIN IN OR NEAR THE SUPERIOR COLLICULUS.
Optic nerve fibers reach the midbrain via the
brachium of the superior colliculus (BSC)
After very specific synapses in the LGN, thalamocortical axons proceed to
primary visual cortex (area 17, calcarine cortex)
After very specific synapses in the LGN, thalamocortical axons proceed to primary visual cortex (area 17, calcarine cortex) by way of
visual radiations (geniculocalcarine radiation, optic radiations)
geniculocalcarine fibers that are carrying visuotopic information from the upper halves of both retinae (lower visual hemifields) course ___ to reach the ___.
course directly backward around the lateral ventricle as part of the internal capsule to reach the superior bank of the calcarine fissure - CUNEUS
geniculocalcarine fibers that are carrying visuotopic information from the lower halves of the retinae (upper visual hemifields) course
forward toward the tip of the tempooral horn of the lateral ventricle, then loop inferiorly then caudally in the temporal lobe to reach the inferior bank of the calcarine fissure - lingual gyrus.
What is Meyer’s Loop?
long loop of geniculocalcarine fibers carrying visuotopic information from the lower halves of the retinae (upper visual hemifields)
What is Contralateral Upper Homonymous Quadrantanopia?
damage to meyers loop - affects the contralateral upper quadrant of the whole visual field.
What part of the visual field does the right cuneus contain?
contralateral (left) lower quadrant from both eyes
right temporal upper eye and left nasal upper eye
What part of the visual field does the left lingual gyrus contain?
contralateral (right) upper quadrant from both eyes
left nasal upper eye and right temporal upper eye
Where is the Lateral Geniculate Nucleus (LGN) located?
at the termination of the optic tracts (retinogeniculate fibers) in the thalamus
How many layers is the LGN composed of in the human?
six
Each LGN contains a representation of
the contralateral visual hemifield (topography)
The six layers of the LGN are divided into two groups:
- Magnocellular Layers
2. Parvocellular Layers
Cells in the magnocellular layer are involved in the perception of
dark and light contrast (M-cells, correspond to information from transient ganglion cells that are activated by rods)
Cells in the parvocellular layers process
fine spatial and color information (P-cells, correspond to information from sustained ganglion cells that are activated by cones.
Axons of retinal ganglion cells project to several areas within the midbrain, with the major projection being to the
superior colliculus
the reinocollicular fibers travel through the
brachium of the superior colliculus
the reinocollicular fibers travel through the brachium of the superior colliculus to terminate in the
superior colliculus
From the superior colliculus, post-synaptic fibers project to the
prestriate visual areas that surround primary visual cortex (Areas 18 and 19)
A small bundle of fibers branches off in the brachium of the superior colliculus and terminates in the
nuclei of the pretectal area just rostral to the superior colliculus.
A small bundle of fibers branches off in the brachium of the superior colliculus and terminates in the nuclei of the pretectal area just rostral to the superior colliculus. This connection is the afferent limb of the
pupillary light reflex that adjusts pupil diameter specific to amount of light entering the eye.
connections to the oculomotor complex result in
constrinction of pupil
descending connection to T1,T2 intermediolateral cell column control
dialation of pupil
This type of defect is often first clue to some pathology in “silent” areas of cortex, especially temporal lobe cortex
quadrant defects
Define macular sparing
field defects which include everything EXCEPT macular field (central vision)
When does macular sparing occur?
when entire ipsilateral visual cortex is destroyed except for the occipital pole. The occipital pole (foveal representation) receives an overlapping blood supply.
Two areas of the cortex that respond to visual stimulation
primary visual (striate) cortex or area 17 secondary and tertiary visual (prestriate) cortex or areas 18 and 19
____ visual field is represented on the posterior pole of the occipital lobe
central visual field
____ are represented in the more anterior portions of the calcarine suclus
peripheral visual fields
the ____ representation is greatly maginified and occupies a disproportionately larger portion of the cortical tissue.
foveal
The information is then distributed through areas 18 and 19 (V2, V3) and then to areas in the temporal lobe for
object recognition
The information is then distributed through areas 18 and 19 (V2, V3) and then to areas in the posterior parietal lobe for
perception of motion, rotation ad dpeth
The information is then distributed through areas 18 and 19 (V2, V3) and then to areas in the parieto-temporal lobe junction for
perception of color and to a lesser extent motion, rotation and depth.
The temporal lobe, posterior parietal lobe, and parito-temporal lobe junction are extensively interconnected to allow us to perceive
visual space (depth and movement) along with object specificity (color and form) within the visual space.
The information derived from p-cells of LGN is responsible for relaying _____ to the temporal lobe regions (P pathway).
information on form and color of objects
The information derived from m-cells of LGN is reponsible for relaying ____ to parietal areas (M pathway).
information concerning gross form and motion
Depending on location and extent, a lesion in the Primary Visual cortex area 17 results in
contralateral visual deficits (visual field cut)
If a lesion involves all of area 17 bilaterally,
the subject will appear blind, however under special test conditions, some limited visual capabilities are demonstratabe, such as the pupillary light reflex.
Prestriate (Extrastriate) Cortex (areas 18 and 19) called visual association cortex perceives projections from three sources:
- ipsilateral primary visual cortex
- ipsilateral superior colliculus
- contralateral prestriate cortex
Lesions to the prestriate (extrastriate) cortex (areas 18 and 19)
are not devastating as those to area 17, and cause deficits in discriminating between objects or patterns.
The posterior half of the middle and inferior temporal gyri receives
projections from prestriate cortex and superior colliculus.
The cells in the the posterior half of the middle and inferior temporal gyri have ____ when compared to stiriate and prestriate cortex.
very large receptive fields
The receptive fields of the cells in the posterior half of the middle and inferior temporal gyri may extend across the mindline and thus,
the cell is responsive to stimuli from both hemifields
Because the receptive fields of the cells in the posterior half of the middle and inferior temporal gyri may extend across the midline, the cell is responsive to stimuli from both hemifields. This indicates
a convergence of information from both hemispheres into this cortical region (i.e. it receives a projection from the contralateral hemisphere).
What is the main sinificance of the posterior half of the middle and inferior temporal gyri?
This is the first level of the visual system in which visual stimuli from both hemispheres are merged to form a single image.
Animals with bilateral leasions to the posterior half of the middle and inferior temporal gyri have
considerable difficulty in identifying the salient features of objects or patterns that distinguish them.
The posterior part of the temporal lobe and its junction with the inferior parietal lobe (area V5 - P-pathway), including parts of the superior temporal sulcus is involved in
form and color recognition.
A part of this region is also involved in movement, especially with respect to transnational movements.
The anterior half of the middle and inferior temporal gyri, broadmann’s areas 20 and 21, receives
projections from the posterior half of these gyri (p pathway)
The physiological properties of the cells found within the anterior half of the middle and inferior temporal gyri, broadmann’s areas 20 and 21, are similar to thouse found in the
posterior area, especially form of objects.
Animals with lesions to the anterior half of the middle and inferior temporal gyri, broadmann’s areas 20 and 21, are greatly impaired in
their ability to recognize a previously seen pattern or object.
Because the anterior half of the middle and inferior temporal gyri, broadmann’s areas 20 and 21, experiences lesions impaires ability to recognize a perviously seen pattern or object, this are has been suggested to function in
“memorizing” a visual pattern (object recognition)
The posterior parietal love, area V3 (Broadmann’s area 7) receives
projections from visual association areas (M pathway).
The posterior parietal love, area V3 (Broadmann’s area 7) is involved in the ability to
identify rotational and directional movements.
It is also involved in the ability to identify visual spatial orientation, or position of body in space (depth perception)
