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Higher Processing of Visual Information: Lecture I --- April 2, 2007 by Mu-ming Poo 1. Overview of the Mammalian Visual System 2. Structure of Lateral Geniculate Nucleus (LGN) 3. LGN Receptive Fields 4. Retinotopic Maps Three subcortical areas in the visual pathway Pretectal area, superior colliculus, and lateral geniculate nucleus (LGN) Pretectal area mediates pupillary light reflex Retina – pretectal area – Edinger- Westphal nuclei (on both sides) – IIIrd cranial nerve – pupillary constrictor muscles. Superior colliculus controls saccadic eye movements coordinates visual, somatic and auditory information and adjusts movement of the head and eyes towards a stimulus 1. Superior colliculus – brain stem – eye muscles (oculomotor reflex) 2. Superior colliculus – tectospinal and tectopontine tracts – head and neck muscles Lateral Geniculate Nucleus (a subregion of the thalamus) relays visual information from retina to cortex Projection from retina to LGN fixation point • Nasal RGC: axons crossover at optic chiasm, project to contralateral LGN fovea • Temporal RGC: axons stay on the same side (ipsilateral) • Left visual field: right LGN, right V1 • Right visual field: left LGN, left V1 1-6: lesion that produce distinct visual defects Structure of LGN Layers 1,3,6 receive contralateral input and Layers 2,3,5 receive ipsilateral input. Each LGN serves the contralateral visual field. The retinotopic maps from two eyes are in register across the layers. (Axons from RGCs responding to the same visual field innervate LGN cells that are aligned vertically across the layers) Layers of LGN Magnocellular layers (motion) Layer 1 & 2, large cells, color blind, low spatial resolution (large RF), high temporal resolution (good for processing motion stimuli), receive inputs from M type RGC cells. Parvocellular layers (form and color): Layer 3 to 6, small cells, color sensitive, high spatial resolution (small RF), low temporal resolution (does not see fast flickers of light). receive inputs from P type RGC cells. Interlaminar koniocellular (K) Layers between each of the M and P layers. K cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex, receive inputs from blue/yellow RGCs. Lesion studies (after selective lesion) Receptive Fields of LGN neurons Receptive field -- Part of the retina (visual field) in which light can evoke response from a cell. - Circular with antagonistic surround ON or OFF center ( 1o in diameter) - Each LGN cells receives only a few retinal ganglion cells (no transformation) - + - + + - + + Note: Only 20% of inputs to LGN are from retina, the rest from other areas, e.g. brain stems and cortex. -M Layers (1 &2) receive feedback inputs from extrastriate cortex Receptive Fields of RGCs Leaving the retina ON-OFF Center-surround receptive field Color opponency M cells P cells B/Y (K) cells LGN receptive fields Each LGN neuron receives input from only a few ganglion cells. The receptive fields of LGN neurons are concentric with a stronger surround antagonism than that in RGCs, suggesting the LGN is not a simple relay station. (Hubel & Wiesel, 1962) Retinotopic Maps Adjacent neurons in the retina project to adjecent neurons in higher-order brain regions. Mapping of LGN: 1. Recording parallel to the layer showed that adjacent cells are excited by adjacent RGCs of the same retina 2. Recording perpendicular to the layers showed that cells in different layers are excited by RGCs in either right or left retina but having the same receptive field location. LGN Cells in different layers are in “topographic register”. FP visual field 1 23 left right retina LGN 1 23 V2 V1 From visual field to V1 medial visual field lateral V1 lower visual field anterior V1 upper visual field posterior V1 V2