Visual System

Question 1

What is the function of the eye?

Answer 1

to present an image of the external world to the photoreceptors of the retina

Question 2

What is light focused by?

Answer 2

The optical components (cornea, lens, and ocular media) of the eye onto the retinal surface where the photoreceptors transform the light energy into electrical signals which are transmitted along the visual pathways to the cortex.

Question 3

Retina: Name and describe the gross anatomy of the eyeball:

Answer 3

1) Optic Disk = produces “blind spot”; exit of the optic nerve which is the collection of the axons from the ganglion cells of the retina.
2) Macula Lutea = contains the central fovea (pit)
3) retina = has three components

Question 4

Describe the 3 components of the Retina:

Answer 4

1) layer of rods and cones: these are the light receptors. Rods = are located at the periphery of retina, for low light vision & perception of movement. Cones = are concentrated in central retina; The fovea, containing ONLY cones, is the area of maximum visual acuity, color & brightness discrimination.. 2) ganglion cell layer: myelinated axons of these cells form the Optic Nerve. Receptive fields are described for ganglion cells.
3) other cells in between we don’t need to know

Question 5

What are the 4 function - optical considerations?

Answer 5

1) Refraction
2) Lens function
3) Accommodation
4) Refractive Abnormalities

Question 6

Describe the function - optical consideration of Refraction:

Answer 6

in the normal eye, an inverted image of the object is focused on the retina

Question 7

Describe the function - optical consideration of Lens function:

Answer 7

The lens is a device for changing the refractive power. Changes in refractive power are accomplished by changing the shape of the lens (rounder for close viewing, flatter for distant viewing). This change in refractive power which allows the viewing of near objects is called ACCOMMODATION

Question 8

Describe the function - optical consideration of Accommodation:

Answer 8

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 flat shape by the suspensory fibers that connect it to the ciliary muscle. To accommodate for near vision, the ciliary muscle contracts, reducing the tension on the suspensory fibers. Due to the lens’ natural elasticity, it contracts into a more spherical shape. Contraction of the ciliary muscle is controlled by parasympathetic nerve fibers

Question 9

Describe the function - optical consideration of Refractive Abnormalities:

Answer 9

Emmetropia is the normally occurring condition in which the image of an object is focused on the retinal surface. Hypermetropia (far-sightedness) - the focal point falls behind the retinal surface. Myopia (near-sightedness) the focal point falls in front of the retina. Presbyopia is the loss of lens elasticity noted with age. Corrective prescriptions may then include a part of the lens that corrects vision for distance and a part that corrects for near vision = bifocals.

Question 10

Name & describe the two types of photoreceptors:

Answer 10

• rods: have a LOW EXCITATION threshold and are therefore used in dim conditions. Rod acuity is poor (but this doesn’t matter, because when light is low, you can’t see small objects anyway).
• Cones: have a HIGH EXCITATION threshold , and are used in high light conditions. Acuity is high and also provides color visions.

Question 11

Describe what Color Vision is:

Answer 11

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. The perception of other colors is due to the relative excitation of the different populations of cones. Color blindness is the result of the absence of one or more of the cone populations.

Question 12

Describe Coding Of Visual Signals:

Answer 12

• Ganglion Cell Function: Ganglion cells are the final stage of retinal processing, and transmit 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. [remember on-center and off-surround; surround inhibition]
  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”, while those that respond, while those that respond only when the light is turned on or off are termed “TRANSIENT” ganglion cells . These cell types roughly correspond to the small ganglion cells (P-cells), and large ganglion cells (M-cells), respectively. It is believed that M-Cells (associated more with Rods) are primarily concerned with signaling changes in the scene being viewed including movement, changes in light and dark contrast, and with basic form analysis, while P-cells (associated with cones) provide information about fine detail (high resolution analysis of image; acuity) and color.

Question 13

Describe Visual Fields:

Answer 13

• the part of space that is being viewed
• Info that is transmitted from the eye through 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 divided into PRIMARY and SECONDARY pathways. 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.

Question 14

Describe Topography of visual fields:

Answer 14

Each point on the retinal surface sees a particular point in the visual field, with neighboring retinal points seeing neighboring visual field points. 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.

Question 15

Describe what hemifields are in the visual fields:

Answer 15

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 only from the opposite (contralateral) hemifield. This separation is accomplished at the level of the optic chiasm.

Question 16

Describe Binocularity and Homonymity:

Answer 16

Images of the visual world are composed of information from the two eyes (binocular) which are merged to form a single image (homonymity).

Question 17

Describe Homonymous:

Answer 17

corresponding halves. 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 also views the corresponding right half of the visual field; the upper visual field of the right eye corresponds to the upper visual field; etc. Therefore, the right primary visual cortex (area 17) is perceiving only the left half of the visual field (contralateral homonymous perception). The visual pathway organization is such that everything caudal to the optic chiasm is carrying only contralateral homonymous sensation.

Question 18

Describe Hemianopia (Hemianopsia):

Answer 18

anopia is a loss of visual perception, therefore hemianopia is a loss of visual perception of HALF of the entire visual field.

Question 19

Describe Heteronymous:

Answer 19

• different halves. This term as it applies to visual fields, describes each individual eye viewing different visual fields. For example, the right eye would transmit information from the right half of the visual field, while the left eye would transmit information from the left half of the visual field, (i.e., looking at the pencil as it approaches your face.) The most common damage that limits encoding different parts of the visual field for the two eyes is when a tumor impacts the optic chiasm, the crossing fibers from each eye. This ultimately results in the person only being able to transmit information from the ipsilateral temporal retina or the more central, binocular parts of the visual field, excluding peripheral vision from both eyes. This results in a Bitemporal Heteronymous Hemianopia, or Bitemporal Hemianopia

Question 20

Describe the first step of the Primary Visual Pathway:

Answer 20

optic nerve fibers from Temporal Retina (nasal visual hemifield) course caudally through OPTIC NERVE, OPTIC CHIASM, and OPTIC TRACT to the ipsilateral lateral geniculate nucleus (L.G.N.)

Question 21

Describe the second step of the Primary Visual pathway:

Answer 21

Fibers from nasal retina (temporal visual hemifield) course caudally through the optic nerve and then cross in the chiasm. These fibers from nasal retina , after crossing in the chiasm, travel along the optic tract to the contralateral LGN.

Question 22

Describe the third step of the Primary Visual Pathway:

Answer 22

After optic nerve fibers pass through chiasm they continue as the optic tract to end primarily (80%) in LGN with some (20%) terminating in the midbrain in or near the superior colliculus. They reach the midbrain via the brachium of the superior colliculus (BSC).

Question 23

Describe the fourth step of the Primary Visual Pathway:

Answer 23

After very specific (see below) synapses in the LGN, thalamocortical axons proceed to primary visual cortex (Area 17, calcimine cortex) by way of visual radiations (geniculocalcarine radiations, optic radiations). These fibers are the efferent fibers leaving the lateral geniculate nucleus and coursing into the primary visual cortex surrounding the calcimine fissure. These thalamocortical fibers are carrying visuotopic information to precise regions of primary visual cortex (Area 17).

Question 24

What is the Lateral Geniculate Nucleus?

Answer 24

is a relay center in the thalamus for the visual pathway. It receives a major sensory input from the retina. The LGN is the main central connection for the optic nerve to the occipital lobe.

Question 25

Describe Geniculocalcarine Fibers:

Answer 25

• fiber that are carrying visuotopic information from the upper halves of both retinae (lower visual hemifields) course directly backward around the lateral ventricle, as part of the internal capsule to reach the superior bank of the calcimine fissure - cuneus.
• fibers that are carrying visuotopic information from the lower halves of the retinae (upper visual fields) course forward toward the tip of the temporal horn of the lateral ventricle, then loop inferiorly then caudally in the temporal lobe to reach the inferior bank of the calcimine fissure - LINGUAL GYRUS. The long loop of the geniculocalcarine fibers is called MEYER’S LOOP, and can be damaged separately from the other geniculocalcarine fibers resulting in a Contralateral Upper Homonymous Quandrantanopia

Question 26

Describe the Retinotopic Organization of the Primary Visual Cortex:

Answer 26

contains an orderly map of the CONTRALATERAL visual field, which it receives from neurons of the LGN. The map is arranged so that the central visual field is represented in the most POSTERIOR PART of the occipital lobe, with the macula represented at the occipital pole. The peripheral visual fields are represented in a ROSTRAL direction along the calcimine fissure (cuneus and lingual gyrus).

Question 27

What part of the visual field do these gyro (cuneus and lingual) contain? What is the blood supply to these hemifield regions?

Answer 27

• the Cuneus gyrus receives information from the medial loop from the LGN that contains information from the upper halves of both retinae (lower visual hemifields)
• the Lingual gyrus receives information from the Meyer’s Loop (lateral loop) from the LGN that contains information from Lower halves of both retinae (upper visual hemifields)
• The calcarine fissure (containing the lingual and cuneus lobules) of the occipital lobe has a blood supply from the vertebral artery which turns into the Posterior Inferior Cerebellar Artery which then turns into the Calcarine artery.

Question 28

Describe the Lateral Geniculate Nucleus:

Answer 28

The LGN is located at the termination of the optic tracts (retinogeniculate fibers) in the thalamus. It is composed of six layers in the human.

Question 29

Describe the Topography of the LGN:

Answer 29

each LGN contains a representation of the contralateral visual hemifield

Question 30

Describe the layers of the LGN:

Answer 30

the six layers of the LGN are divided into two groups; Magnocellular layers and Parvocellular layers. Each layer contains a map of the contralateral visual hemifield.

Question 31

Describe the Function of the LGN:

Answer 31

Cells in the Magnocellular layers are involved in the perception of dark and light contrast (M-cells, correspond to information form transient ganglion cells that are activated by rods), while those in the Parvocellular layers process fine spatial and color information (P-cells, correspond to information from sustained ganglion cells that are activated by cones).

Question 32

Describe the Secondary Visual Pathways:

Answer 32

Anatomical pathway - axons of retinal ganglion cells project to several areas within the midbrain, with the major projection being to the SUPERIOR COLLICULUS. The retinocollicular 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 fiber 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 relfex that adjusts pupil complex result in constriction of pupil; descending connections to T1, T2, intermediolateral cell column control dilation of pupil.

Question 33

What are the two Visual Field Deficits (effects of lesions in the visual pathways)?

Answer 33

1) Quadrant Defects - often first clue to some pathology in “silent” areas of cortex, especially temporal lobe cortex
It can be associated with a lesion of an optic radiation. While quadrantanopia can be caused by lesions in the temporal and parietal lobes, it is most commonly associated with lesions in the occipital lobe.
2) Macular Sparing - field defects which include everything except macular field (central vision); occurs when entire ipsilateral visual cortex is destroyed except for the occipital pole. The occipital pole (foveal representation) receives an overlapping blood supply.

Question 34

Where do axons arising from post-synaptic cells in subcortical visual nucleus ascend through what? synapse on what?
And where are the visual stimulations most focused on?

Answer 34

• Ascend through the optic radiations to synapse with cells in the visual cortex
• focused on two areas: 1) Primary Visual (striate) cortex or Area 17, and 2) Secondary & Tertiary Visual (prestriate) cortex or Areas 18 & 19.

Question 35

Describe the Retinotopic Organization of the Primary Visual Cortex:

Answer 35

• primary visual cortex contains an orderly map of the contralateral visual hemifield, which it receives from neurons of the LGN. The map is arranged so that the central visual field is represented in the posterior pole of the occipital lobe. The peripheral visual fields are represented in the more anterior portions of the calcarine sulcus. The foveal representation is greatly magnified and occupies a disproportionally large portion of the cortical tissue.
• The info is then distributed through areas 18 and 19 (V2, v3) and then to areas in the Temporal lobe (object recognition), to areas in the posterior parietal lobe for perception of motion, rotation and depth, and to the parieto-temporal lobe junction for perception of color and to a lesser extent motion, rotation and depth.
• 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). The information derived from M-cells of LGN is responsible for relaying information concerning gross form and motion to parietal lobe areas (M pathway)

Question 36

Describe Lesions of the Primary Visual Cortex:

Answer 36

most evidence regarding visual deficits following lesion of Area 17 come from experiments in monkeys, although some human evidence is also available.
- depending on location and extent, these lesions result in CONTRALATERAL visual field deficits (visual field cut). 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 function from the Superior Colliculus of the midbrain.

Question 37

Describe other Visual Areas:

Answer 37

1) Prestriate (Extrastriate) Cortex Areas 18 & 19: Called visual association cortex, receives projections from three sources; ipsilateral Primary Visual Cortex, ipsilateral Superior Colliculus, and the contralateral Prestriate Cortex. Lesions of this area are not as devastating as those to area 17, and cause deficits in discriminating between objects or patterns.
2) Posterior Half of the Middle and Inferior Temporal Gyri: Receives projections from prestriate cortex & superior colliculus. 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 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). 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.
3) Posterior Part of the Temporal Lobe and its Junction with the inferior Parietal Lobe: Area V5 (P Pathway). This region of the cortex, 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 translational movements.
4) Anterior Half of the Middle and Inferior Temporal Gyri, Brodmann’s Area 20 and 21: receives projections from the posterior half of these gyri (P Pathway) The physiological properties of cells in this area are similar to those found in the posterior area, especially form of objects. 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).
5) Posterior Parietal Lobe, Area V3 Brodmann’s Area 7: receive projections from visual association areas (M Pathway). This region 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).