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NSCI 1051 (6)

Visual System Notes

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Temple University
Neuroscience - CLA
NSCI 1051

Visual System Fovea: depression in the center of the macula lutea – central area of retina Optic nerve: exit the left and right eyes at the optic disks; go through orbits and fatty tissue, then through holes in the floor of the skull Optic disc: entrance to the optic nerve Macula: a yellowish spot in the middle of the retina with relatively few large blood vessels; contains the fovea Phobia: Visual field: the entire region of space that can be seen with both eyes looking straight ahead • Visual Hemifields: left and right of the midline (of your visual field) • Binocular visual field: the central portion of both visual hemifields is viewed by both retinas Rod photoreceptor: peripheral vision (in peripheral retina) and vision under conditions of low illumination, not good at noticing fine details, absent from the central part of the fovea Cone photoreceptor: function in bright light and are responsible for central discriminative vision and color detection, absorbs light from different parts of the color spectrum (more numerous in central area – fovea) Optic chiasm: where the two optic nerves combine, lies at base of the brain anterior to pituitary gland Optic tract: axons of the retinofugal (leaving the retina) projections after they cross at the optic chiasm run just under the pia along the lateral surfaces of the diencephalon Optic radiation: a collection of axons coursing from the LGN to the visual cortex Myers loop Decussation: the crossing of a fiber bundle from one side of the brain to the other – only the axons originating in the nasal retinas cross, so there’s a partial decussation at the optic chiasm. Non-thalamic targets of optic tract – Hypothalamus: • Biological rhythms including sleep and wakefulness Non-thalamic targets of optic tract – Pretectum: • Size of the pupil, certain types of eye movement Non-thalamic targets of optic tract -- Superior Colliculus: • Orients the eyes in response to new stimuli • Brainstem nucleus that monitors stimuli Lateral Geniculate Nucleus (of the dorsal thalamus): • Most optic tract axons innervate the LGN. • Gateway to the primary visual cortex. • Input from visual cortex as well • Input from brainstem related to alertness and attentiveness, can modulate how LGN responds to visual stimuli • Responsible for conscious visual perception (what we see is influenced by how we feel) • LGN neurons receive synaptic input from the retinal ganglion cells • One left and one right LGN, major targets of the 2 optic tracts, input from the two eyes is kept separate • In the right LGN the right eye axons synapse on LGN cells in layer 2, 3, and 5. • ^LOOK AT DIAGRAM IN BOOK • In the left LGN the left eye axons synapse on cells in layers 1, 4, and 6. • 6 distinct layers of cells bent around the optic tract like a knee joint Magnocellular system: • Ventral layers of the LGN • M-type ganglion cells found in retina project to here • Relatively large center-surround receptive fields • Insensitive to differences in wavelength • Layer 1 & 2 • Large, monocular receptive fields with transient response • Color-blind, rapidly conducting axons responsible for perception of movement, high contrast-sensitivity responsive to small differences in brightness, and low resolution. Magnocellular neurons differ from parvocellular neurons because: 1) Shorter response latencies to optic chiasm stimulation, 2) Greater sensitivity to luminance contrast 3) Better temporal resolution. Parvocellular system: • Dorsal layers of the LGN • P-type ganglion cells found in retina project to here • Relatively small center-surround receptive fields and exhibit color opponency • Layer 3-6 • Small, monocular receptive fields with sustained response • High-resolution information about shape and others low- resolution information about color and shades of grey Koniocellular layers (of LGN): • Receive input from the non-M and non-P types of retinal ganglion cells and also project to visual cortex • Very small cells • Ventral to each principal layer parvo inter-blob pathway blob pathway Blob Receptive Fields: • Blob cells: Wavelength-sensitive; Monocular; No orientation; direction selectivity Primary visual cortex (V1 or Striate Cortex): • Single major synaptic target of the LGN • 9 layers, 3 sub-layers • Layer 1 consists almost entirely of axons and dendrites of cells in other layes • Responsible for orientation selectivity • V2 - Binocularity • Layer IVC innervates superficial layers Outputs: • – Layers II, III, and IVB: Projects to other cortical areas • – Layer V: Projects to the superior colliculus and pons • – Layer VI: Projects back to the LGN Its (orientation- selective V1) receptive fields consist of:  Simple cells: • Binocular; Orientation- selective; Elongated on-off region with antagonistic flanks responds to optimally oriented bar of light • Possibly composed of three LGN cell axons with center-surround receptive fields  Complex cells: • Binocular; Orientation- selective; ON and OFF responses to the bar of light but unlike simple cells, no distinct on-off regions Retinotopy: an organization whereby neighboring cells in the retina feed information to neighboring places in their target structures (the LGN and the Striate Cortex) – correspondence between what part of cortex will be activated by a stimulus coming through your visual field (eyes) Area 17 = Primary visual Cortex, lies on the medial surface of the hemisphere surrounding the calcarine fissure Areas 18 and 19 Ocular dominance columns: • How left and right eye inputs are segregated when they reach layer IVC of striate cortex • The distribution of axon terminals relaying information from the injected eye was not continuous in later IVC but split up into equally spaced patches • This shows left and right eye inputs to layer IV are split up into alternating bands
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