Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ANAT 321 Dr. Brawer November 26, 2010 Lecture 22 ANAT 321 Lecture 22 – Visual System 2 Dr. James Brawer– November 26, 2010 NOTE: This NTC is meant to be used as a study aid to supplement your own class notes. Hence, not all of the text contained in the lecture slides will be reproduced here. Please send comments or questions about NTCs to us through e-mail: [email protected]. Announcements: - If you get an A in the class, you can apply to be a TA for next year. - On the lecture final, circle the answers on the question sheet, not just the scantron. - Please do the course evaluation! - Lab exam: be there before 8:15 (to the histology lab 1/56) - Final is 72 MCQs - Do not email any questions to Dr. Brawer. Call him at (514) 398-6331 or drop by his office Transmission of Visual Information Two rules of thumb: o Wiring of visual system (from retina to cortex), top is top, bottom is bottom, left is left and right is right o Retinal image is upside down and reverse of reality Upper left visual field will end up on lower right retina Far left visual field ends up on far right retina of both eyes o Upper visual field ends up on lower retina o This is due to the lens, which inverts images All axons from the ganglion cells in the left eye condense at the optic disk (blind spot) and contain fibers from both temporal (lateral) and nasal (medial) retina o All axons enter the optic nerve (really a tract) o Optic nerve leaves retina and nasal fibers will cross over to other side at optic chiasm, while temporal fibers keep going ipsilaterally o After chiasm both right visual fields are on the left post-chiasmatic optic tract Optic chiasm is purely composed of nasal fibers Lesion in left optic tract loss of right visual field in both eyes Lesion in optic chiasm loss of temporal visual field in both eyes (tunnel vision) o Medial retinas are denervated, so lateral visual fields are gone Lesions on the left side can occur near the optic chiasm where only the temporal fibers are affected (after left nasal fibers split off, but before right nasal fibers join up) 1 ANAT 321 November 26, 2010 Dr. Brawer Lecture 22 o This will cause loss of loss of right visual field in left eye only Optic tracts end up in the lateral geniculate nucleus in the thalamus, which relays to cortex on the same side left geniculate nucleus receives entire right visual field o Optic tract also gives connections to the superior colliculus (some collaterals from axons going to the Lateral Geniculate, while others are direct axons from the retina) o Not very clear what the superior colliculus does because you can have damage with no detectable visual defects Optic tract is very compact, whereas the optic radiations (projections from the lateral geniculate) are extremely spread out has consequences for lesions and normal vision Lateral Geniculate Body and Primary Visual Cortex A layered structure has 6 layers o Layers 6, 4, and 1 receive fibers from contralateral eye o Layers 5, 3, and 2 receive from ipsilateral eye o Layers are retina-specific o Outer 4 layers are parvocellular (small cells) o Inner 2 layers are magnocellular (large cells) Parvocellular layers receive information primarily about colour and form o We expect these layer to receives axons mainly from retinal ganglion cells in and around the fovea (where most cones are) even though fovea is so small, it takes up a large area in the lateral geniculate o This is why we only have focus in a very small area in the center of the retina if we had more, we would require an extensive amount of thalamic volume We compensate for a small region of focus by constantly moving our eyes (we are not conscious of these movements) o Macular degeneration: a neurodegenerative disease of age when macula/fovea of the retina degraded leads to functional blindness Magnocellular layers receive information primarily about movement and contrast Therefore, differential processing of visual information begins occurring at the level of the lateral geniculate nucleus (before even reaching the cortex) The lateral geniculate nucleus projects to the primary visual cortex, which is in the banks of the calcarine fissure on the medial surface of the occipital lobe The primary visual cortex has four quadrants: upper- and lower-right, upper- and lower-left o Upper-right quadrant receives the upper-right retinas of both eyes and lower-left part of the visual field of both eyes o Lesions to the entire right visual cortex would lead to a loss of left visual fields in both eyes Primary visual cortex is organized in a retinotopic order o Retina is sequentially represented in primary visual cortex o Foveal regions (central regions of retina) end up most posterior in the primary visual cortex; peripheral regions of retina end up more anterior o Foveal sparing: lesions to occipital lobe often spare the macular region (unsure why) Pathway from lateral geniculate to primary visual cortex is very diffuse o Fibers from upper part of the nucleus fan out towards the upper quadrant in the cortex, whereas fibers from lower part of nucleus fan out to the lower quadrant 2 ANAT 321 November 26, 2010 Dr. Brawer Lecture 22 o Lesion to temporal lobe may affect these radiations, and can affect the visual field o Optic radiations that are more ventral project into the temporal lobe and loop around to the lower bank of the calcarine fissure (this is called Meyer’s loop) Visual cortex receives information that has been processed in the retina and lateral geniculate body, and then dissects and distribute information to various cortices o Visual association cortex: region of cortex surrounding the primary visual cortex o When we see a red car driving by, the visual cortex will split up this input into colour (red), form (car) and movement and send it to various regions of association cortex Somehow, our brains have to later consolidate this information o Bilateral lesions in particular spots of the visual association cortex can lead to loss of the ability to perceive movement (sees movements as snap shots) no problem with identifying objects These visual cortices are plastic to some degree Prosopagnosia: inability to recognize faces Superior Colliculus It receives projections from the retina, but also from the spinal cord o Some axons in the anterolateral column (spinomesencephalic tract) end up in superior colliculus and periaqueductal gray can be stimulated by pain in foot or hand Also receives input from primary visual cortex Superior colliculus gives of a tract (tectospinal tract) that crosses the midline, stay medial and stems down to the spinal cord o Involved in visual reflexes (adjustments of head and shoulder) Blindsight: destruction of visual cortex causes absolute blindness o But, if put in front of a computer with a yellow dot moving around the screen, they can use their finger to follow the dot with their trajectory o This is likely mediated by the superior colliculus (which plays a big role in movement perception in lower animals) o Superior colliculus talks to the cortex via the thalamus Pulvinar nucleus receives information from the superior colliculus Pulvinar widely distributes to visual association cortices around the occipital lobe Visual Reflexes Consensual light reflex: light-to-dark reflex o Pupils must constrict to restrict the amount of incoming light o Light enters the eye and hits the retina o Retina projects to the pretectum (neurological tissue anterior to the superior colliculi, which are in the tectum) o Pretectum projects bilaterally to nucleus of Edinger-Wesphal (visceral III) o Nucleus of Edinger-Wesphal projects to the ciliary ganglion, which projects to ciliary body and iris causes sphincter to contract o Stimulating only one eye should cause an equal reflex in both eyes o Reflex occurs even if cortically blind 3 ANAT 321 November 26, 2010 Dr. Brawer Lecture 22 Accommodation reflex: far-to-near vision o Lens must fatten to increase refraction; pupil constricts slightly o Ciliary body must contract to take tension off of the lens o This reflex requires a cortex o We follow the visual pathway through the lateral geniculate to the visual cortex o Visual cortex projects to the superior colliculus (both colliculi talk to each other) o Superior colliculus projects to nucleus of Edinger-Wesphal, which projects to ciliary ganglion o Ciliary ganglion projects to eye to cause accommodation o Cortically blind individuals cannot do this reflex 4