Visual System Flashcards
1
Q
The human visual system can sense light fluxes of as little as ________________________________________ and can resolve a point separation of _________________.
A
one quantum (the equivalent of seeing a candle from one mile away)
of 100 microns (1/10th of a mm).
2
Q
The function of the eye is to present an image of the external world to the photoreceptors of the retina. The light is focused by the optical components (cornea, lens and ocular media) of the eye onto the retinal surface where the photoreceptors transform the light energy into _________________ signals which are transmitted along the visual pathways to the cortex.
A
electrical
3
Q
optic disk =
A
produces “blind spot”; exit of the optic nerve.
4
Q
macula lutea =
A
contains central fovea (pit)
5
Q
What are the 4 layers of the retina?
A
- pigment cell layer
- layer of
a. rods &
b. cones - ganglion cell layer
6
Q
pigment cell layer =
A
derived from choroid, attaches retina to eyeball, absorbs stray light
7
Q
layer of rods & cones =
A
light receptors.
8
Q
rods =
A
are located at the periphery of retina, for low light vision & perception of movement.
9
Q
cones =
A
are concentrated in central retina.
10
Q
fovea =
A
containing only cones, is the area of maximum visual acuity, color & brightness discrimination.
11
Q
ganglion cell layer =
A
myelinated axons of these cells form the optic nerve. Receptive fields are described for ganglion cells
12
Q
Refraction =
A
In the normal eye, an inverted image of the object is focused on the retina.
13
Q
Lens function =
A
The lens is a device for changing the refractive power.
14
Q
What structures refract light?
A
cornea, aqueous humor, lens (where the image is inverted) and vitreous humor
15
Q
Changes in refractive power are accomplished by:
A
changing the shape of the lens (rounder for close viewing, flatter for distant viewing).
16
Q
The change in refractive power which allows the viewing of near objects is called:
A
accommodation
17
Q
Accommodation - To view a near object, the lens must:
A
increase its refractive power by becoming more convex (rounder).
18
Q
At rest, the lens is held in a relatively ___________ shape by the suspensory fibers that connect it to the ______________ muscle.
A
flat
ciliary
19
Q
To accommodate for near vision, the ciliary muscle ______________, reducing the tension on the suspensory fibers.
A
contracts
20
Q
Due to the lens’ natural elasticity, it contracts into a more ___________ shape. Contraction of the ciliary muscle is controlled by ___________________________ nerve fibers.
A
spherical
parasympathetic
21
Q
What are 3 refractive abnormalities =
A
- Emmetropia
- Hypermetropia
- Presbyopia
22
Q
Emmetropia =
A
is the normally occurring condition in which the image of the an object is focused on the retinal surface.
23
Q
Hypermetropia (far-sightedness) =
A
the focal point falls behind the retinal surface; myopia (near-sightedness) it falls in front of the retina.
24
Q
Presbyopia =
A
is the loss of lens elasticity noted with age.
25
Corrective prescriptions may then include a part of the lens that corrects vision for distance and a part that corrects for near vision =
bifocals
26
Rods =
Have a low excitation threshold and are therefore used in dim conditions
27
Cones =
Have a high excitation threshold, and are used in high light conditions. Acuity is high and also provide color vision.
28
Rod acuity is _______.
poor (but this doesn't matter, because when light is low, you can't see small objects anyway).
29
Color Vision =
The ability to distinguish color is due to the presence of three separate cone populations, each of which is maximally sensitive to a different wavelength of light: blue, green, and red.
30
The perception of other colors is due to:
the relative excitation of the different populations of cones.
31
Color blindness is the result of:
the absence of one or more of the cone populations
32
Generator potentials originating in receptors are transmitted to ______________ and then altered by the other cells in the retina.
bipolar cells
33
This pathway can be visualized as two separate systems; a vertical system in which signals pass from ______________ to _________________ to _____________, and a horizontal system.
from receptors to bipolar cells to ganglion cells
34
A horizontal system in which horizontal and amacrine cells provide:
lateral interactions (lateral inhibition) between the different vertical system components.
35
Ganglion Cell Function =
Ganglion cells are the final stage of retinal processing, and transmit information to subcortical visual centers in the brain.
36
Axons of Ganglion cells form the:
optic nerve.
37
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. [Remember on-center and off-surround]
38
(In addition to the above classification, ganglion cells have been subdivided on the basis of their response duration or morphology.) Cells that respond as long as the stimulus remains within the receptive field are termed:
"Sustained" ganglion cells
39
Those that respond only when the light is turned on or off are termed:
"Transient" ganglion cells.
40
These cell types roughly correspond to the small ganglion cells (_________), and large ganglion cells (_________), respectively.
P-cells
| M-cells
41
At present it is believed that M-cells are primarily concerned with _____________ changes in the scene being viewed including ________________, and changes in ___________________________ and with ______________ analysis.
signaling
movement
light and dark contrast
basic form
42
P cells provide information about:
fine detail (high resolution analysis of image) and color.
43
Information that is transmitted from the eye thorough the optic nerve must eventually reach the cortex to be perceived. In order to do this, the signals are transmitted through the central visual pathways that are be divided into __________________________________. To some extent these subdivisions serve different functions, however, there are areas of both anatomical and functional overlap between them. Effective functioning of the visual system requires proper interaction between both divisions.
primary and secondary pathways
44
Definition of Topography =
Each point on the retinal surface sees a particular point in the visual field, with neighboring retinal points seeing neighboring visual field points.
45
Thus, the visual field is represented on a corresponding area on the retina (retinal field) with the topographical representation being maintained throughout the rest of the visual system. Closely associated with the concept of topography is the concept of dividing the visual field into __________________.
hemifields
46
Definition of Hemifields =
The visual field can be divided by a vertical line at the fixation point into a left and right hemifield.
47
Each half of the brain receives information only from the _________________________ hemifield. This separation is accomplished at the level of the optic chiasm.
opposite (contralateral)
48
Definition of Binocularity and Homonymity =
Images of the visual world are composed of information from the two eyes (binocular) which are merged to form a single image (homonymity).
49
Homonymous =
corresponding halves.
50
This term, as it applies to visual fields, describes both eyes viewing the same or corresponding visual field. For example, as the right eye views the right half of the visual field, the left eye _________________________________________; the upper visual field of the right eye corresponds to the ____________________________________; etc. Therefore, the right primary visual cortex (Area 17) is perceiving only the ______________________________________.
also views the corresponding right half of the visual field
upper visual field of the left eye
left half of the visual field (contralateral homonymous perception).
51
The visual pathway organization is such that everything caudal to the optic chiasm is carrying only :
contralateral homonymous sensation.
52
Hemianopia (Hemianopsia) =
anopia is a loss of visual perception, therefore hemianopia is a loss of visual perception of half of the entire visual field.
53
Heteronymous =
different halves. This term, as it applies to visual fields, describes each individual eye viewing different visual fields.
54
For example (of heteronymous), the right eye would view the ________________ of the visual field, while the left eye would view the _______________ of the visual field, i.e., looking at a pencil as it approaches your face.
right half
left half
55
NOTE: this term is not always necessary for a specific description of a visual field deficit, i.e., when the terms binasal or bitemporal are included. Thus for the example above, when each eye is only able to see the _____________________ visual field and not the _______________visual fields, the deficit would be classified as a Binasal Heteronymous Hemianopia. (Binasal Hemianopia is also correct and preferred.)
ipsilateral temporal
nasal
56
Primary Visual Pathway_
Optic nerve fibers from temporal retina (nasal visual hemifield) course caudally along the _________ edge of the optic nerve, optic chiasm and optic tract to the ______________________________
lateral
ipsilateral lateral geniculate nucleus (L.G.N.).
57
Primary Visual Pathway_ Fibers from nasal retina (temporal visual hemifield) course caudally along the ___________ edge of the optic nerve and then ____________ in the chiasm.
medial
cross
58
Primary Visual Pathway_
These fibers from nasal retina, after crossing in the chiasm, course along the ____________ edge of the optic tract to the ______________________.
medial
contralateral LGN.
59
Primary Visual Pathway_
Fibers from the upper retina (inferior or lower visual hemifield) course caudally along the __________ edge of the optic nerve, optic chiasm, optic tract. Fibers from the lower retina (superior or upper visual hemifield) course caudally along the __________ edge of this _____________________ pathway.
dorsal
ventral
retinogeniculate
60
Primary Visual Pathway_ Retinogeniculate fibers carrying information from the more peripheral visual fields are located most __________________ and those carrying information from the central (fovea) visual fields are located more __________________within the optic nerve, optic chiasm, optic tract.
superficially
internally
61
Primary Visual Pathway_ After optic n. fibers pass through chiasm they continue as the optic tract to end primarily _____________________ with some (____) terminating in the __________________________________. They reach the midbrain via the _____________________________________
(80%) in LGN
20%
midbrain in or near the superior colliculus
brachium of the superior colliculus (BSC).
62
Primary Visual Pathway_ After very specific synapses in the LGN, ____________________ axons proceed to primary visual cortex, (Area 17, calcarine cortex) by way of visual radiations (geniculocalcarine radiations, optic radiations). These fibers are the efferent fibers leaving the ___________________ and coursing into the ______________________ surrounding the calcarine fissure.
thalamocortical
lateral geniculate nucleus
primary visual cortex
63
Primary Visual Pathway_ These thalamocortical fibers are carrying:
visuotopic information to precise regions of primary visual cortex (area 17).
64
Primary Visual Pathway_ geniculocalcarine fibers that are carrying ________________ information from the upper halves of both retinae (lower visual hemifields) course directly backward around the _______________, as part of the internal capsule to reach the _____________________________
visuotopic
lateral ventricle
superior bank of the calcarine fissure - cuneus.
65
Primary Visual Pathway_ geniculocalcarine fibers that are carrying visuotopic information from the lower halves of the retinae (upper visual hemifields) course forward toward the _________________________________, then loop inferiorly then caudally in the temporal lobe to reach _____________________________________.
tip of the temporal horn of the lateral ventricle
the inferior bank of the calcarine fissure - lingual gyrus.
66
Primary Visual Pathway_ This long loop of geniculocalcarine fibers is called ___________________.
Meyer's Loop
67
Primary Visual Pathway_ Meyer's Loop can be damaged separately from:
the other geniculocalcarine fibers resulting in a Contralateral Upper Homonymous Quandrantanopia.
68
Primary Visual Pathway_The retinotopic organization of the primary visual cortex contains an orderly map of the __________________ visual field, which it receives from neurons of the ____________.
contralateral
LGN
69
Primary Visual Pathway_The map is arranged so that the central visual field is represented in the most _____________ part of the occipital lobe, with the _____________ represented at the occipital pole.
posterior
macula
70
Primary Visual Pathway_The peripheral visual fields are represented in a __________ direction along the calcarine fissure (cuneus and lingual gyrus). What part of the visual field do these two gyri contain? What is the blood supply to these hemifield regions?
rostral
??? (lower visual field, upper retina --> cuneus) (upper visual field, lower retina --> lingual gyrus)
???? (posterior cerebral artery)
71
Lateral Geniculate Nucleus is located at the:
termination of the optic tracts (retinogeniculate fibers) in the thalamus.
72
How many layers is the LGN composed of in the human?
6
73
Each LGN contains a representation of the __________________________ visual hemifield.
contralateral
74
What are the 2 groups of the 6 layers of the LGN?
1. magnocellular layer
2. parvocellular layer
*each layer contains a map of the contralateral visual hemifield
75
Cells in the magnocellular layers are involved in the perception of _____________________________.
dark and light contrast
76
M-cells, correspond to information from:
transient ganglion cells that are activated by rods
77
While those in the parvocellular layers process ______________________________.
fine spatial and color information
78
P-cells, correspond to information from:
sustained ganglion cells that are activated by cones
79
Anatomical Pathway - Axons of retinal ganglion cells project to several areas within the midbrain, with the major projection being to the ________________________.
superior colliculus
80
The retino-collicular 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).
81
A small bundle of fibers branches off in the brachium of the superior colliculus and terminates in the nuclei of the _______________ area just rostral to the superior colliculus. This connection is the afferent limb of the ____________________ reflex that adjusts pupil diameter specific to amount of light entering the eye.
pretectal
pupillary light
82
Connections to the oculomotor complex result in constriction of pupil; descending connections to ___________________________________ control dilation of pupil.
T1, T2 intermediolateral cell column
83
Quadrant defects =
often first clue to some pathology in "silent" areas of cortex, especially temporal lobe cortex.
84
Macular sparing =
field defects which include everything except macular field (central vision)
85
Macular sparing occurs when:
occurs when entire ipsilateral visual cortex is destroyed except for the occipital pole. The occipital pole (foveal representation) receives an overlapping blood supply.
86
After you have studied cranial nerves & light reflexes try to ascertain how you might distinguish between lesions at #5 versus 8 on this handout.
After you have studied cranial nerves & light reflexes try to ascertain how you might distinguish between lesions at #5 versus 8 on this handout.
87
Axons arising from post-synaptic cells in the subcortical visual nuclei ascend through the ______________________ to synapse with cells in the visual cortex. Although there are many areas of cortex that respond to visual stimulation, most attention is focused on two areas:
optic radiations
1. primary visual (striate) cortex or area 17, and secondary and
2. tertiary visual (prestriate) cortex, or areas 18 and 19. Each of these areas has its own topographic map of the visual world.
88
Retinotopic Organization - Primary visual cortex contains an orderly map of the _________________ visual hemifield, which it receives from neurons of the _______________.
contralateral
LGN
89
The map is arranged so that the central visual field is represented in the ____________________ of the occipital lobe. The peripheral visual fields are represented in the _________________________________________.
posterior pole
more anterior portions of the calcarine sulcus
90
The foveal representation is greatly magnified and occupies a disproportionally large portion of:
the cortical tissue.
91
The information is then distributed through areas 18 and 19 (V2, V3) and then -->
to areas in the temporal lobe (object recognition)
92
to areas in the temporal lobe (object recognition) --->
to areas in the posterior parietal lobe for perception of motion, rotation and depth
93
to areas in the posterior parietal lobe for perception of motion, rotation and depth -->
the parieto-temporal lobe junction for perception of color and to a lesser extent motion, rotation and depth.
94
These three general regions are extensively interconnected to allow us to _________________________ (depth and movement) along with _____________________ (color and form) within the visual space.
perceive visual space and
object specificity
95
The information derived from p-cells of LGN is responsible for relaying information on form and color of objects to: ___________________________________.
the temporal lobe regions (P pathway).
96
The information derived from m-cells of LGN is responsible for relaying information concerning:
gross form and motion to parietal lobe areas (M pathway).
97
Lesions of Primary Visual Cortex - Most evidence regarding visual deficits following lesions of area 17 come from experiments in monkeys, although some human evidence is also available.
Lesions of Primary Visual Cortex - Most evidence regarding visual deficits following lesions of area 17 come from experiments in monkeys, although some human evidence is also available.
98
Depending on location and extent, these lesions result in ___________________ visual field deficits (visual field cut).
contralateral
99
If the lesion involves all of area 17 bilaterally, the subject will:
appear blind;
however, under special test conditions, some limited visual capabilities are demonstrable, such as the pupillary light reflex. These capabilities are thought to reflect collicular function.
100
Prestriate (Extrastriate) Cortex (Areas 18 and 19) called visual association cortex, receives projections from three sources:
1. ipsilateral primary visual cortex
| 2. ipsilateral superior colliculus 3. contralateral prestriate cortex.
101
Lesions of this area (prestriate cortex, 18 and 19) are not as devastating as those to area 17, and cause deficits in:
discriminating between objects or patterns.
102
Posterior Half of the Middle and Inferior Temporal Gyri receives projections from ____________________________. Cells in these areas have very large receptive fields when compared to striate and prestriate cortex. The receptive fields of these cell may extend across the midline thus, the cell is responsive to stimuli from ______________________.
prestriate cortex and superior colliculus.
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).
103
(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 lesions of this area have considerable difficulty in identifying the salient features of objects or patterns that distinguish them.
104
Posterior part of the temporal lobe and its junction with the inferior parietal lobe:
area V5 (P pathway)
105
This region of the cortex, including parts of the superior temporal sulcus, is involved in ________________________________. A part of this region is also involved in movement, especially with respect to _______________________ movements.
form and color recognition.
translational
106
Anterior Half of the Middle and Inferior Temporal Gyri, Brodmann’s areas 20 and 21, receives projections from the:
posterior half of these gyri (P pathway)
107
The physiological properties of cells in this area (20 and 21) are similar to those found in the _____________________, especially form of objects.
posterior area
**Animals with lesions of this area are greatly impaired in their ability to recognize a previously seen pattern or object. Thus, this area has been suggested to function in "memorizing" a visual pattern (object recognition).
108
Posterior parietal lobe, area V3 (Brodmann’s area 7), receives projections from:
visual association areas (M pathway).
109
This region (Brodmann's area 7) is involved in the ability to ______________________________________. It is also involved in the ability to identify:
identify rotational and directional movements
visual spatial orientation, or position of body in space (depth perception).
