The visual system is unique as much of visual processing occurs outside the brain within the retina of the eye. The previous chapter described how the light-sensitive receptors of the eye convert the image projected onto the retina into spatially distributed neural activity in the first neurons of the visual pathway (i.e., the photoreceptors). Within the retina, the receptors synapse with bipolar and horizontal cells, which establish the basis for brightness and color contrasts. In turn, the bipolar cells (the 2° visual afferent) synapse with retinal ganglion cells and amacrine cells, which enhance contrast effects that support form vision and establish the basis for movement detection. The information from the eye is carried by the axons of the retinal ganglion cells (the 3° visual afferent) to the midbrain and diencephalon. This chapter will provide more information about visual pathway organization and the visual processing that occurs within the brain. Show
15.1 The Visual Pathway from Retina to Cortex As noted previously in the somatosensory sections, all sensory information must reach the cerebral cortex to be perceived and, with one exception, reach the cortex by way of the thalamus. In the case of the visual system, the thalamic nucleus is the lateral geniculate nucleus and the cortex is the striate cortex of the occipital lobe. The Optic Nerve
The axons of the 3° visual afferents (the retinal ganglion cells) form the optic nerve fiber layer of the retina on their course to the optic disc. At the optic disc, the 3° visual afferents exit the eye and form the optic nerve. The fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina (i.e., the nasal hemiretina) decussate in the optic chiasm to the opposite optic tract (Figure 15.1). Consequently, each optic tract contains retinal ganglion cell axons that originate in the nasal half of the contralateral retina and the temporal half of the ipsilateral retina. Recall that the ipsilateral temporal hemiretina and the contralateral nasal hemiretina have projected on them the images of corresponding halves of their visual fields. For example, the temporal (left) hemiretina of left eye and the nasal (left) hemiretina of right eye both have projected on them the right halves of their respective visual fields. Consequently, each optic tract has within it axons representing the contralateral half of the visual field. The axons in the optic tract terminate in four nuclei within the brain (Figure 15.2):
The Lateral Geniculate Nucleus The vast majority of optic tract fibers terminate on neurons in the lateral geniculate nucleus (LGN) of the thalamus (Figure 15.3A). Like the retina, the lateral geniculate nucleus is a laminated structure, in this case, with six principal layers of cells (Figure 15.3B).
The optic tract fibers (3° visual afferents) from each eye synapse in different layers of the LGN. Consequently, each LGN neuron responds to stimulation of one eye only.
The functional properties of LGN neurons are similar to those of retinal ganglion cells. The LGN neurons are monocular (i.e., respond to stimulation of one eye only) and have concentric (center-surround) receptive fields. The LGN neurons are segregated into three major groups:
The axons of these different types of LGN neurons terminate in different layers or sublayers of the primary visual cortex. Visual Cortical Areas The primary visual cortical receiving area is in the occipital lobe. The primary visual cortex is characterized by a unique layered appearance in Nissl stained tissue.
Consequently, it is called the striate cortex. It includes the calcarine cortex, which straddles the calcarine fissure, and extends around the occipital pole to include the lateral aspect of the caudal occipital lobe (Figure 15.4, Area 17).
The LGN neurons (4° visual afferents) send their axons in the internal capsule to the occipital lobe where they terminate in the striate cortex (Figure 15.5).
The striate cortex (Figure 15.6) is considered to be the primary visual cortex or V1, as
The striate cortex is involved in the initial cortical processing of all visual information necessary for visual perception and its damage results in loss of vision in the contralesional hemifield.
The color (kLGN), shape (pLGN) and movement (mLGN) information from the thalamus are sent to different neurons within V1 for further processing in V1 and then sent onto different areas of the extrastriate visual cortex.
V1 blob cells: Some V1 cells resemble kLGN neurons. They are
The P-stream information processed by the V1 blob cells is used in color perception, color discrimination and the learning and memory of the color of objects. The blob cells are the "color" processing cells of V1. V1 interblob cells: Most V1 interblob cells are
Location specific V1 interblob cells: One subset of V1 interblob cells responds best when the stimulus is in a specific location of the receptive field (i.e., they also exhibit location specificity). The P-stream information processed by the V1 interblob cells that exhibit orientation and location specificity but are not motion sensitive is used in object perception, discrimination, learning and memory or in spatial orientation. These interblob cells are the "shape/form" processing cells and the "location" processing cells of V1. Movement sensitive V1 interblob cells: A second subset of interblob cells respond best to moving stimuli (i.e., exhibit movement sensitivity, Figure 15.8) without a preference for the direction of movement.
Direction specific V1 interblob cells: A third subset displays a preference for movement in a particular direction (i.e., some also exhibit directional sensitivity, Figure 15.9). The M-stream of information processed by the motion sensitive V1 interblob cells is used to detect object movement and direction/velocity of movement and to guide eye movements. These motion-sensitive interblob cells are the "motion detecting” cells of V1. Extrastriate Visual Cortex. The extrastriate cortex includes all of the occipital lobe areas surrounding the primary visual cortex (Figure 15.4, Areas 18 & 19). The extrastriate cortex in non-human primates has been subdivided into as many as three functional areas, V2, V3, and V4. The primary visual cortex, V1, sends input to extrastriate cortex and to visual association cortex. The information from the “color”, “shape/form”, "location" and “motion” detecting V1, neurons are sent to different areas of the extrastriate cortex (Figure 15.10). Damage to extrastriate cortex does not result in a “simple loss of vision”; rather it results in higher order visual perceptual deficits including the failure to recognize objects, colors and/or movement of objects.
Visual Association Cortex. The visual association cortex extends anteriorly from the extrastriate cortex to encompass adjacent areas of the posterior parietal lobe and much of the posterior temporal lobe (Figure 15.4, Areas 7, 20, 37 & 39). In most cases, these areas receive visual input via the extrastriate cortex, which sends color, shape/form, location and motion information to different areas of the visual association cortex (Figure 15.10). The Dorsal Stream: The neurons in the parietal association cortex and superior and middle temporal visual association cortex (Areas 7 and 39 and the superior part of Area 37 in Figure 15.4) have binocular receptive fields and process P-channel information about object location and M-channel information about object movement. These dorsally located visual association neurons are responsible for producing our sense of
The dorsal stream processes information about the “where” of the visual stimulus (Figure 15.10). Damage the dorsal visual association cortex results in deficits in spatial orientation, motion detection and in guidance of visual tracking eye movements. The Ventral Stream: The neurons in the inferior temporal visual association cortex (Area 20 and the inferior part of Areas 37 & 39 in Figure 15.4) process P-channel information about object color and form. These ventrally located visual association neurons are responsible for processing information necessary for our abilities to
This ventral stream processes information about the “what” of the visual stimulus (Figure 15.10). Damage to the inferior visual association cortex produces deficits in complex visual perception tasks, attention and learning/memory. 15.2 Retinotopic Organization in the Visual Pathway Clinical Examples The topographic (spatial) relationships of retinal neurons are maintained throughout the visual system, which preserves the retinotopic map of the visual world. That is, the retina is mapped onto the LGN and striate cortex in an organized (topographic) fashion. Consequently, neighboring parts of retina project to neighboring parts of LGN and neighboring parts of LGN project to neighboring parts of the striate cortex. This retinotopic organization in the visual pathway results in a spatial representation of the visual field in the LGN and visual cortex. Spatial Representation of the Retinal Image You should recall the following regarding the spatial representation of the retinal image within the visual pathway.
Visual Field Defects Visual field defects are areas of loss of vision in the visual field. Visual field defects
are detected by perimetry testing, during which the patient fixates his eyes on a target and his ability to detect a small object in specific positions in space is determined.
Figure 15.11 illustrates perimetry test results for the two eyes of someone with normal vision. The bottom panel of Figure 15.11 is a simplified illustration of the monocular visual fields used in the following examples of visual field defects. A visual field defect provides clues to the structure(s) affected. That is, the area(s) of visual field loss and eye(s) exhibiting the visual field loss offer clues about the site of the damage. The following examples of visual field losses should help you determine how well you can utilize what you have learned thus far about the visual system. 15.3 Clinical Example #1
Symptoms: The patient is having his semiannual physical examination. As he is diabetic, the physician examines his retinas and performs a confrontation test of his visual fields. An abnormality is detected in his left fundus (Figure 15.12) but the confrontational field test detects nothing. Perimetry testing is requested.
Side & Retinotopicity of damage: The visual loss
So you conclude that the visual defect involves
Retinal Damage: A defect involving only the visual field of one eye indicates possible damage in the retina or optic nerve. If the visual loss is confined to one eye, it is called a monocular visual field defect. Often retinal lesions are small and do not follow the boundaries of the visual field quadrants. Such a visual field disorder is called a scotoma. A retinal visual field defect is most severe when vision in the central field is affected, as in the case of macular degeneration. In macular degeneration, the patient will report difficulty reading and seeing clearly and visual field testing will demonstrate that the patient has a central scotoma (i.e., is blind in the visual field center). 15.4 Clinical Example #2
Symptoms: The patient complains of a sudden headache and loss of vision in his left eye. Ophthalmoscope examination does not reveal abnormalities in the left eye1. However, confrontation testing indicates a severe loss of vision in the left eye. The patient is referred
for immediate neuroradiographic tests and perimetry testing. Side & Retinotopicity of damage: The visual loss
So, you conclude that the visual defect is
Neural imaging results indicate an aneurysm on the left ophthalmic artery, which is compressing the left optic nerve (Figure 15.16). Compression of the nerve prevents action potentials from the retina to travel to the lateral
geniculate nucleus of the thalamus. Long-term compression may damage the nerve, however, of greater concern is the potential rupture of the aneurysm, which could cause extensive brain damage.
Optic Nerve Damage: Each optic nerve contains the axons of retinal ganglion cells from one eye, e.g., the right nerve from the right eye. Damage to one optic nerve will produce a monocular visual field defect. Destruction of one optic nerve (e.g., crushed by a tumor on the orbital surface of the frontal cortex) will result in the total loss of vision in the ipsilesional eye. 15.5 Clinical Example #3
Symptoms: At his annual physical exam, the patient complains of a general malaise and changes in his vision that he noticed while playing soccer. He said he was often "blindsided" on the playing field because he "couldn't see players approaching him from the side". Ophthalmoscope examination does not reveal abnormalities in either
eye2. Confrontation field testing indicates a constriction of the temporal hemifields of both eyes. The patient is referred for neuroradiographic tests and perimetry testing. Side & Retinotopicity of damage: The visual loss
You conclude that the visual field defect is related to damage that
Neural
imaging results (Figure 15.18) indicate a pituitary adenoma that is compressing the optic chiasm. Compression of the decussating nerve fibers prevents action potentials from the nasal hemiretina to reach the contralateral lateral geniculate nucleus of the thalamus. As the tumor grows larger it will crush the optic chiasm, destroying it and eventually compromising the remaining optic nerve fibers.
Optic Chiasm Damage: The fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina decussate in the optic chiasm to the opposite optic tract (Figure 15.1). The crossing fibers of the optic chiasm may be crushed by a pituitary tumor. Damage to the optic chiasm produces a unique form of visual field deficit, a bitemporal hemianopia (Figure 15.17). Recall that the fibers of the optic chiasm carry information about objects in the
temporal hemifields of both eyes (i.e., the right hemifield of the right eye and the left hemifield of the left eye). Consequently section of the optic chiasm produces a visual loss in only the temporal half of the visual field of each eye. When the patient views the world out of both eyes, the boundary of his binocular visual field is narrower than normal. 15.6 Clinical Example #4
Symptoms: A patient is brought to the emergency room complaining of a severe headache and nausea. He is conscious and coherent when examined in the ER. Ophthalmoscope examination does not reveal abnormalities in either eye. Confrontation field testing indicates a visual loss in the right hemifield of both eyes. The patient is referred for neuroradiographic tests and perimetry testing.
You conclude that the visual field defect is related to damage that
Neural imaging results indicate injury to the rostral half of the left calcarine cortex, which receives
blood from the left posterior cerebral artery (Figure 15.20). Recall that the rostral calcarine cortex processes information from the visual field periphery, whereas the caudal and lateral striate cortex process information derived from the visual field center.
Calcarine Cortex Damage. An infarct created by obstruction of, or a hemorrhage in, branches of the posterior cerebral artery may result in damage to the rostral calcarine cortex. Damage to the calcarine cortex on one side may produce a binocular, contralateral homonymous hemianopia with macular sparing (Figure 15.20). A collateral blood supply from branches of the middle cerebral artery is believed to spare the cortical neurons in the caudal and lateral regions of the striate cortex, which receive information from the macular area. 15.7 Clinical Example #5
Symptoms: A patient, who is stabilized after suffering a stroke two months earlier, is referred to a neuro-ophthalmologist for evaluation. The patient does not appear to be blind but has problems with processing visual information. For example, the patient cannot describe the color of an object presented to him or recognize faces. He has normal spatial orientation and motion detection. The patient is referred for perimetry testing. Perimetry
Test Results: The results indicate no consistent loss of vision. However, it is difficult to obtain consistent results because the patient tires easily and his attention appears to wander. Side & Retinotopicity of damage: The patient
You conclude that the neurological defect is
Neural imaging results indicate damage to the caudal portion of the inferior temporal lobe, which normally receives blood from branches of the posterior cerebral artery. Extrastriate or Association Cortex Damage: While destruction of the primary visual cortex produces blindness in the contralesional hemifield, damage to cortical areas surrounding the striate cortex does not Instead, they may produce profound deficits in the higher order-processing of visual information. For example, bilateral damage to a small area of the inferior temporal gyrus (Figure 15.21) produces a loss in the ability to recognize faces. Damage to more superior areas of the temporal lobe (area 39 in Figure 15.4) produces an inability to recognize or comprehend written words and/or passages. Damage to areas in the parietal cortex may result in the inability to see motion (i.e., a moving object will be seen in “frames’’ in one place at one point in time and at another place in a following period of time). The object does not appear to move; rather it appears to have jumped from one place to the next. Damage to large areas involving the posterior parietal cortex and superior temporal cortex may result in the symptom of "neglect", wherein objects in parts of the visual field are ignored or denied existence. 15.8 Summary In this chapter, you have learned how the visual system is organized in the brain. You have learned that stimulus features extracted by the retinal neurons (color, brightness contrast, movement) are kept segregated in separate “information channels” and processed in parallel by different cells at all levels of the visual system. Information coded and carried by one million retinal ganglion cells are distributed to hundreds of millions of cortical neurons in the occipital, parietal and temporal lobes. The perception of a coherent visual image is recomposed out of these fragments of information by the simultaneous activation of large areas of cortex. You have also learned how the spatial representation of the visual image is maintained by the retinotopic organization of the visual system and learned how this information is useful in determining the location and extent of damage to the visual system by examining the visual fields. Finally, you have learned that neuronal responses in visual cortex exhibit plasticity at different time scales, short term (as adaptation and dynamics) and long term (as learning) – this plasticity allows visual cortex to construct an accurate picture of the world that can rapidly adapt to match the changes in the environment. Test Your Knowledge
Which of the following are characteristic of the primary visual cortex "blob" neurons? They:
Which of the following are characteristic of the primary visual cortex "blob" neurons? They:
Which of the following are characteristic of the primary visual cortex "blob" neurons? They:
Which of the following are characteristic of the primary visual cortex "blob" neurons? They:
Which of the following are characteristic of the primary visual cortex "blob" neurons? They:
Which of the following are characteristic of the primary visual cortex "blob" neurons? They:
Make the best match between the below listed condition and the visual field defect. Match: occlusional of the left posterior cerebral artery
Make the best match between the below listed condition and the visual field defect. Match: occlusional of the left posterior cerebral artery
Make the best match between the below listed condition and the visual field defect. Match: occlusional of the left posterior cerebral artery
Make the best match between the below listed condition and the visual field defect. Match: occlusional of the left posterior cerebral artery
Make the best match between the below listed condition and the visual field defect. Match: occlusional of the left posterior cerebral artery
Make the best match between the below listed condition and the visual field defect. Match: occlusional of the left posterior cerebral artery
Make the best match between the below listed condition and the visual field defect. Match: trauma to the left temporal lobe
Make the best match between the below listed condition and the visual field defect. Match: trauma to the left temporal lobe
Make the best match between the below listed condition and the visual field defect. Match: trauma to the left temporal lobe
Make the best match between the below listed condition and the visual field defect. Match: trauma to the left temporal lobe
Make the best match between the below listed condition and the visual field defect. Match: trauma to the left temporal lobe
Make the best match between the below listed condition and the visual field defect. Match: trauma to the left temporal lobe
Make the best match between the below listed condition and the visual field defect. Match: lesion of the optic chiasm
Make the best match between the below listed condition and the visual field defect. Match: lesion of the optic chiasm
Make the best match between the below listed condition and the visual field defect. Match: lesion of the optic chiasm
Make the best match between the below listed condition and the visual field defect. Match: lesion of the optic chiasm
Make the best match between the below listed condition and the visual field defect. Match: lesion of the optic chiasm
Make the best match between the below listed condition and the visual field defect. Match: lesion of the optic chiasm
What nerve is a bundle of ganglion axons that carries information to the brain?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.
What structure of the eye is a bundle of ganglion axons that carries information to the brain multiple choice question?Optic Nerve, Chiasm, and Tracts
The optic nerve of each eye is composed of a group of unmyelinated axons of the retinal ganglion cells which emerge from the optic disc.
What nerve bundle carries the information from the eye towards the visual cortex of the brain?optic nerve, second cranial nerve, which carries sensory nerve impulses from the more than one million ganglion cells of the retina toward the visual centres in the brain.
What part of the eye carries information to the brain?The optic nerve carries signals from the retina to the brain, which interprets them as visual images.
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