this is the last lecture in the series on the visual system and deals with the processing of visual information through the visual cortices and other pathways. visual information is received through 3 types of ganglionic cells- Y type cells receive input from rods and thus detect peripheral movement or light intensity changes. X type cells receive input from cones and as such detect fine detail and color, and W type cells receive peripheral rod input. the center-surround receptive field information then projects to the lateral geniculate nucleus, which then projects information to the primary visual cortex.
within the primary visual cortex there are "ocular dominance columns" that the LGN projects to that are cortical columns that receive input from one eye or another. within these ocular dominance columns there are distinct layers of cells: "simple cells" in the middle layers receive input from ganglionic cells that receive input from receptive fields that are aligned within the same line-- and are thus stimulated by bars of light in the particular orientation of the ganglionic cells. complex cells receive input from several simple cells and therefore detect movement of bars of light. simple cells in layer 4 also receive "monocular input" and complex cells in the upper and lower layers process "binocular" visual information, processing the disparity between retinal information of each eye.
the primary visual cortex then sends information to the neighboring visual cortices, V2 through V5, each cortex processing the visual information in a progressively complex fashion. for example, whereas V1 receives simple light/dark/color input, V2 processes the object/background differences as well as formation of illusory/incomplete boundaries. V2 then projects into two streams: the dorsal stream then goes to V3, then to the middle temporal cortex, and is the "motion processing" or "how and where" of visual processing. the ventral stream goes to V4, then to the inferotemporal cortex, and is the detail oriented "what" pathway of visual processing. (the right side of the inferotemporal cortex is involved in processing complex shapes such as faces, and the left side is involved in language processing).
a few other brain areas involved in visual processing: the superior colliculus is part of the midbrain that receives Y-type visual information, audition, and somatosensory information and is involved in orienting the head and eyes towards visual stimuli via the "extrageniculate" pathway- via the posterior thalamic nuclei and the amygdala. the superior colliculus is also involved in producing the saccadic eye movements (and thus modulates the cranial nerves that innervate the extrinsic eye muscles, CN III, IV, VI), rapid fixations that break up scanning movements and allow the brain to put together bits of visual information in a process analogous to a film movie. the superior colliculus is also involved in the phenomenon of "blindsight", where a lesion in V1 blocks the primary (primitive) visual processing pathway, and visual information flows through the intact "extrastriate" (mammalian) pathway through the superior colliculus.
the suprachiasmic nucleus is a thalamic nuclei that receives receptive field information from ganglionic cells and is involved in mediating the circadian clock. the circadian clock receives information about light/dark cycles and via transcription factor cycles mediates different autonomic regulatory pathways in the body. the factors in the positive transcriptional loop are BMAL1 and PER2; the ones in the negative loop are CRY1,2 and PER1,3. the "central heterodimers" are BMAL1 and CLOCK. the output of these transcriptional loops produces transcription of CLOCK controlled genes which produce proteins that ultimately alter cellular function. the SCN also projects to the liver, adrenals, and pineal, where is mediates melatonin release.
questions
pathways...
1. each side of the visual field projects to the ... side of the brain.
2. what can total blindness in one eye be caused by?
3. what is heteronymous hemianopsia and what can it be caused by?
4. what is homonymous hemianopsia and what is it caused by?
lateral geniculate nucleus...
5. what are the three types of ganglion cells that the lateral geniculate nucleus receives signals from?
6. describe the role of Y type ganglion cells.
7. describe the role of X type ganglion cells.
8. describe the role of W type ganglion cells.
9. lateral geniculate nucleus projects center-surround receptive fields to...
primary visual cortex...
10. what is the primary visual cortex surrounded by?
11. what is the role of simple, complex, and hypercomplex cells in the primary visual cortex?
12. describe how a simple cell in the visual cortex can sense a "bar of light".
13. what do complex cells do?
14. thalamic neurons project contralateral visual field input to...
15. why are ocular dominance columns so named?
16. which layer is monocular input found in the visual cortex?
17. which layer is binocular input found?
18. retinal disparity provides information regarding...
V2-V5...
19. what does the visual association cortex do?
20. describe the dorsal stream from the visual association cortex.
21. describe the ventral stream from the visual association cortex.
22. both streams converge into...
23. what is a difference between the cells of V2 and V1?
24. what is V4 tuned for?
25. what is the inferotemporal cortex tuned for?
wernicke's area, fusiform gyrus, superior colliculus...
26. what is Wernicke's area associated with?
27. how is Wernicke's area connected with Broca's area?
28. inappropriate activity in the fusiform gyrus can lead to...
29. what is synesthesia and how is it related to the fusiform gyrus?
30. describe the superior colliculus: what it is, what it receives, projects to, what function it is involved in.
31. what are saccadic eye movements and how is the superior colliculus involved in producing them?
32. which cranial nerves does the SC modulate?
blindsight, SCN, circadian clock...
33. lesion of primary visual cortex leads to...
34. what is the pathway for visual information in blindsight?
35. what is the suprachiasmatic nucleus and what is it involved in?
36. circadian rhythms depend on...
37. positive transcriptional loops are mediated by...
38. negative transcriptional loops are mediated by...
39. what are the central heterodimers that are activated or inactivated by these transcriptional loops?
40. output from the circadian clock manifests in...
41. where does the SCN project to? which neurotransmitters does it use to do so?
42. what do the hypothalamic nuclei that the SCN projects to involved in?
43. SCN contains neurons that specifically target...
44. what does effect does the SCN have on the pineal gland?
answers
1. contralateral
2. damage to the optic nerve of one eye distal to the optic chiasm.
3. loss of opposite visual fields due to lesions of the optic chiasm.
4. loss of same side visual fields due to a lesions between the optic chiasm and the visual cortex on one side.
5. Y type, X type, W type.
6. receive input from rods in the periphery of the retina and thus are more sensitive to rapid movement or light intensity changes.
7. receive input from cones in the fovea and thus are more sensitive to fine details of objects, and color.
8. also receive input from rods, detecting movement in periphery of visual field.
9. primary visual cortex, V1.
10. secondary, association, and tertiary visual cortices.
11. developing perception of form, color, direction of movement, and binocular vision through hierarchical organization of these cells.
12. it can receive the receptive fields for ganglion cells that are all oriented along the same line and thus the simple cell can be maximally stimulated only when a bar of light hits the retina at the same location and orientation.
13. detect information about movement of bars of light by combining input from several simple cells.
14. ocular dominance columns in the primary visual cortex.
15. because they only receive input from one eye or the other.
16. simple cells in layer 4.
17. the complex cells in the upper and lower layers.
18. the depth of vision.
19. projects visual information into two streams, dorsal and ventral.
20. the dorsal stream is loosely involved with the "how" and "where" of visual processing. the dorsal stream goes from V2 to V3 to the middle temporal cortex to the posterior parietal cortex.
21. the ventral stream is loosely involved with the "what" of visual processing- identifying complex shapes, forms, colors. the stream goes from V2 to V4 and into the inferotemporal cortex, then to the posterior parietal cortex.
22. prefrontal cortex to form working memory.
23. V2 cells are tuned to more complex stimuli, such as the formation of illusory boundaries and distinguishing between object and background.
24. color, simple geometric shapes.
25. recognition of faces and other complex shapes on the right side, perception of language on the left side.
26. understanding symbolic systems.
27. via the arcuate fasciculus.
28. visual hallucinations.
29. the phenonemon in which a subject experiences different sensory modalities simultaneously such as sound and color, possibly related to cross-activation of color and number neurons in fusiform gyrus.
30. a midbrain structure that has an "extrageniculate" visual pathway involved in orienting the head and eyes towards visual stimulus. receives Y type ganglionic signals, audition, somatosensory input, projects to posterior thalamus (pulvinar nuclei) and amygdala.
31. saccadic eye movements are coordinated movements of the eyes which breaks scanning motion into quick
fixations on different points; the superior colliculus mediates these movements along with the frontal eye field in the motor cortex.
32. III, IV, VI.
33. lack of consciousness of visual information.
34. because the primary pathway to the visual cortex is blocked, the extrastriate pathway takes over and compensates.
35. the nucleus that receives light information from the melanopsin ganglion cells and mediates the circadian rhythm.
36. positive and negative transcriptional loops.
37. BMAL1 and PER2
38. CRY1/2 and PER1/3
39. CLOCK and BMAL1.
40. CLOCK-controlled gene expression, which leads to altering of cellular function.
41. to the dorsomedial and paraventricular hypothalamic nuclei, using vasopressin, GABA, and glutamate.
42. autonomic activity, and HPA axis hormone release.
43. liver, pineal, adrenal.
44. mediates release of melatonin which regulates sleep wake cycles, temperature, cortisol release.
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