1Higher Processing of Visual Information Lecture I---April 2, 2007 by Mu-ming Poo1. Overview of the Mammalian Visual System2. Retinotopic Maps and Cortical Magnification3. Structure Lateral Geniculate Nucleus (LGN)4. LGN Receptive Fields5. Organization of Mammalian Visual Cortices6. Structure of the Primary Visual Cortex Part I. Overview of Visual PathwayProjections of RGCs to the central nervous system2. Mid-brain projections3. Central projections LGN axons project to the primary visual cortex (striate cortex, V1) on the same sideRGC axons project to pretectum, which controls pupillary reflex. RGC axons project to superior colliculus, which controls saccadic eye movement.1. Optic nerve and optic chiasmOptic nerve consists of axons of retinal ganglion cells (RGCs).At the optic chiasm, RGC axons from nasal hemiretina cross and enter the contralateral hemisphere. RGC axons from temporal hemiretina enter the ipsilateral hemisphere.2In cortex, the area devoted to the foveal region of the retina is enlarged relative to the rest of the eye.Retinotopic mapAll structures in the geniculo-cortical system have retinotopic maps.Cortical magnificationEvery point on the map corresponds to a point on the retina.Cortex Visual fieldFovea: 1 cone => 1 RGC1 RGC => 2-3 LGN cells1 LGN cell => 1000 V1 cells10operiphery: 2 cone => 1 RGC2 RGCs => 1 LGN cell1 LGN cell => 100 V1 cellsVisual fieldCortexLGNKoniocellular LGN layers – Inter-layers just ventral to each numbered layer - inputs from blue/yellow ganglion cellsStructure of the primate LGN123456Magnocellular LGN layers -Layers 1 & 2 - inputs from M-type ganglion cells; Parvocellular LGN layers - Layers 3,4,5,6 - inputs from P-type ganglion cells;Each LGN has 6 layers labeled 1 thru 6 from ventromedial to dorsolateral. 50μm3Layers 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)Properties of P, M, and K cellsOnly <30% inputs to LGN are from the retina. >70% are feedback inputs, mostly from V1.Cortical Feedback to LGN: The corticogeniculate pathwayThe corticogeniculate axons terminate in all LGN layers in two main patterns (Ichida & Casagrande, 2002)(1) in individual M or P LGN layers.(2) Terminate in pairs of M, P, or K layers.Common functional patternsof corticogeniculate connectivity (Sillito et al, Science 1999):Feedback to LGN cells along the preferred orientation, enhancing the buildup of orientation selectivity within the geniculo-cortical circuit.Feedback to LGN cells along the axis of stimulus movement, enhancing responsiveness in anticipation of stimulus movement.4Receptive Fields of Receptive Fields of RGCsRGCsLeaving the retinaLeaving the retinaColor opponencyON-OFFCenter-surround receptive fieldM cells P cells B/Y (K) cellsEach LGN neuron receives input from only a few ganglion cells. The receptive fields of LGN neurons are concentric with a far stronger surround antagonismthan that in RGCs, suggesting the LGN is not a simple relay station.(Hubel & Wiesel, 1962)3. LGN receptive fieldsBrodmann’s cytoarchitectonic maps of cerebral cortex Side view of left hemisphereInside view of right hemisphere from the midline5The organization of mammalian visual corticeshumanmonkeymonkeyV1 has six layersAnatomy of the primary visual cortex (V1, striate cortex)Layer 1 is nearly aneuronal, composed predominantly of dendriticand axonal connections. Layers 2/3 contain excitatory neurons which project to extrastriate cortical regions Layer 4 is divided into 4A, 4B, 4Ca, and 4Cb. Layers 4Ca and 4Cb are the major recipients of LGN projections. Layers 5/6 contain excitatory projection neurons which provide feedback to LGN. Blobs --Cytochrome oxidase (CO) labeling of V1 shows CO rich regions in layer II & III termed "blobs"Layer 3Cortical cell typesExcitatory cells: Spiny pyramidal & stellate cellsInhibitory cells: >40 typesMost LGN cells terminate in layer 4, M in 4Ca, P mostly in 4Cb. K cells and some P cells terminate in blobs in layers 2 & 3.Inputs from LGNCollaterals (branches) of M & P terminate in 6.Layers II, III & IVB - feedward to extrastriate cortical areasLayer V - feedback to ponsand superior colliculusLayer VI - feedback to LGNOutput from