Chapter 10.docx

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Department
Neuroscience
Course
NROC64H3
Professor
Matthias Niemeier
Semester
Winter

Description
Chapter 10 The visual system Introduction How the information extracted by the retina is analyzed by the central visual system Pathway for conscious visual perception: 1. Lateral geniculate nucleus of the thalamus 2. Primary visual cortex a. AKA i. Area 17 ii. V1 iii. Striate cortex Information funneled through this geniculocortical pathway is processed in parallel by neurons specialized for the analysis of different stimulus attributes. How do neurons represent the different facets of the visual world? How is the task of analyzing the visual world initially divided and organized within certain structures of the brain stem? The retinofugal projection The retinofugal projection: Also known as the neural pathway that leaves the eye, beginning with the optic nerve. ‘Fugal’ describes a pathway that is directed away from a structure (i.e. retinofugal  away from the retina) The optic nerve, optic chiasm, and optic tract: Three components to the retinofugal projection. The optic nerve: 1. Exits the left and right eyes at the optic disks. 2. Travels through the fatty tissue behind the eyes in their bony orbits 3. Pass through holes in the floor of the skull The optic chiasm: The optic nerves from both eyes combine to form the optic chiasm which lies at the base of the brain, just anterior to the pituitary gland. At the optic chiasm, the axons originating in the nasal retinas cross from one side to the other (known as a decussation). Because only the nasal retinas cross from one side to the other, a partial decussation occurs at the optic chiasm. Decussation: the crossing of a fiber bundle from one side of the brain to the other. The optic tract: Follows the optic chiasm. The optic tracts run just under the pia mater along the lateral surfaces of the diencephalon. Right and left visual hemifields: 1. Objects in the left visual hemifield will be projected to the nasal portion of the left eye and temporal portion of the right eye, eventually progressing to the right brain 2. Objects in the right visual hemifield will be projected to the temporal portion of the left eye and the nasal portion of the right eye, eventually progressing to the left brain Targets of the optic tract: 1. Hypothalamus 2. Midbrain 3. Lateral geniculate nucleus (LGN) of dorsal thalamus a. Give rise to optic radiations: i. Axons that project to the primary visual cortex (i.e. projections from LGN to cortex) b. Lesions anywhere in the pathway from eye to LGN to cortex cause blindness  therefore we know that this pathway mediates conscious visual perception Nonthalamic targets of the optic tract: 1. Projections to hypothalamus: a. Play a role in synchronizing a variety of biological rhythms i. Sleep and wakefulness ii. Dark-light cycle 2. Projections to the midbrain (pretectum): a. Control the size of the pupil and certain types of eye movement 3. Projections to the midbrain (superior colliculus): a. AKA optic tectum b. Pathway from retina to superior colliculus often called the retinotectal projection c. Commands eye and head movements to bring the image of a point in space onto the fovea d. Involved with orienting the eyes in response to a new stimuli in the visual periphery The lateral geniculate nucleus Arrangement of the LGN: Six distinct layers of cells, each layer labelled 1-6 (starting with the most ventral layer). In 3D space the LGN looks like pancakes stacked on top of one another bent around the optic tract (like a knee; geniculate). The LGN is the gateway to the visual cortex – conscious visual perception. The segregation of input by eye and by ganglion cell type: LGN neurons receive input from retinal ganglion cells. LGN neurons project output to primary visual cortex via the optic radiations. The segregation of the LGN into layers suggests that different types of retinal information are being kept separate at this synaptic relay: 1. Axons arising from M-type 2. Axons arising from P-type 3. Axons arising from nonM-nonP type Synapse on different LGN layers: 1. Left visual field projects to the right LGN a. The right eye (ipsilateral) axons synapse on LGN cells in layers 2,3,5 b. The left eye (contralateral) axons synapse on LGN cells in layers 1,4,6 2. Right visual field projects to the left LGN Ventral vs dorsal layers: 1. Two ventral layers (1,2) a. Contain larger neurons b. AKA magnocellular LGN layers i. M type ganglion cells in the retina project entirely to the magnocellular LGN 2. Four dorsal layers (3,4,5,6) a. Contain smaller neurons b. AKA parvocellular LGN layers i. P type ganglion cells in the retina project exclusively to the parvocellular LGN 3. Sub-layers ventral to each major layer a. AKA koniocellular LGN layers i. nonM-nonP ganglion cells project exclusively to the koniocellular LGN Functional implications of segregated LGN layers: Suggests that the retina gives rise to streams of information that are processed in parallel Receptive fields: The visual receptive fields of LGN neurons are almost identical to those of the ganglion cells that feed them: 1. Magnocellular LGN neurons a. Have relatively large center surround receptive fields b. Respond to stimulation of the receptive field centers with a transient burst of action potentials c. Are insensitive to differences in wavelength 2. Parvocellular LGN neurons a. Have relatively small center surround receptive fields b. Respond to stimulation of their receptive field centers with a sustained increase in the frequency of action potentials c. Many exhibit color opponency 3. Koniocellular LGN neurons a. Center surround b. Have either light/dark or color opponency Within all layers of the LGN, the neurons are activated only by one eye (i.e. they are monocular) and ON center and OFF center cells are intermixed Nonretinal inputs to the LGN: 1. 80% of excitatory synaptic input comes from the primary visual cortex. The role of this corticofugal feedback pathway is not clearly identified. 2. The LGN also receives synaptic inputs from neurons in the brain stem whose activity is related to alertness and attentiveness Thus the LGN is more than a simple relay form retina to cortex; it is the first site in the ascending visual pathway where what we see is influenced by how we feel. Anatomy of the striate cortex Primary visual cortex: AKA V1 and striate cortex. It is located in brodmann’s area 17 and is located in the occipital lobe of the primate brain. Much of area 17 lies on the medial surface of the hemisphere, surrounding the calcarine fissure. Retinotopy: An organization whereby neighboring cells in the retina feed information to neighboring places in their target structures (i.e. the LGN and striate cortex). The 2D surface of the retina is mapped onto the 2D surface of the subsequent structures. Three important features of retinotopy: 1. The mapping of the visual field onto a retinotopically organized structure is often distorted because visual space is not sampled uniformly by the cells in the retina. a. The central few degrees in the visual field are overrepresented (magnified) in the retinotopic map 2. A discrete point of light can activate many cells in the retina and often many more cells in the target structure due to the overlap of receptive fields 3. Perception is based on the brain’s interpretation of distributed patterns of activity Lamination of the striate cortex: The striate cortex has cell bodies arranged in about 6 layers. The cell layers are largely devoid of neurons and consist almost entirely of axons and dendrites of cells in other layers. Layers of the striate cortex: 1. I 2. II 3. III 4. IV a. A b. B c. C i. Alpha ii. Beta 5. V 6. VI The cells of different layers: Two principal neuronal shape types 1. Spiny stellate cells a. Small neurons with spine covered dendrites that radiate out form the cell body b. Primarily seen in the two tiers of layer IVC c. Only make local connections within the striate cortex 2. Spiny pyramidal cells a. Covered with spines b. Characterized by a single thick apical dendrite that branches as it ascends toward the pia mater c. Characterized by multiple basal dendrites that extend horizontally d. Only pyramidal cells send axons out of the striate cortex to form connections with other parts of the brain. 3. Aspinous inhibitory neurons a. No spines b. Form only local connections Inputs and outputs of the striate cortex: 1. Only a subset of the layers receives input from the LGN or sends output to a different cortical or subcortical area. 2. Axons from the LGN terminate in several different cortical layers a. The largest number going to layer IVC 3. Magnocellular LGN neurons project to layer IVCa 4. Parvocellular LGN neurons project to layer IVCb 5. Koniocellular LGN axons bypass layer IV to make synapses in layers II and III Ocular dominance columns: PAGE 354-355 Innervation of other cortical layers from layer IVC: There are two patterns of intracortical connections: 1. Radial connections a. Run from white matter to layer I b. Maintains the retinotopic organization established in layer IV i. E.g. a cell in layer VI receives information from the same part of the retina as a cell above it in layer IV. 2. Horizontal connections a. Axons of some layer III pyramidal cells extend collateral branches that make horizontal connections within layer III Monocular vs binocular input: 1. Layer IVC stellate cells project axons radially up mainly to layers IVB and III where information from left and right eyes begin to mix for the first time a. Axons in layers III and IVB may form synapses with the dendrites of pyramidal cells of all layers 2. All layer IVC neurons receive only monocular input 3. Most neurons in layers II and III receive binocular input coming from both eyes a. Even so, there continues to be anatomical segregation of the magnocellular and parvocellular processing streams i. Layer IVCa 1. Receives magnocellular LGN input 2. Projects mainly to cells in layer IVB ii. IVCb 1. Receives parvocellular LGN input 2. Projects mainly to layer III Striate cortex outputs: Pyramidal cells in different layers innervate different structures: 1. Layer II / III / IVB pyramidal cells send their axons to other cortical areas 2. Layer V pyramidal cells send axons down to the superior colliculus and pons 3. Layer VI pyramidal cells give rise to the massive axonal projection back to the LGN 4. Pyramidal cell axons in all layers also branch and form local connections in the cortex Cytochrome oxidase blobs: PAGE 356 Physiology of the striate cortex Receptive fields: The receptive fields of neurons in layer IVC are similar to the magnocellular and parvocellular LGN neurons providing their input: 1. In layer IVCa a. Neurons are insensitive to wavelength of light 2. In layer IVCb a. Neurons exhibit center-surround color opponency Outside layer IVC, receptive field characteristics not observed in the retina or LGN are found: Binocularity: Each neuron in layers IVCa and IVCb receives afferents from a layer of the LGN representing either eye. PAGE 356 Orientation selectivity: Most of the receptive fields in the retina, LGN and layer IVC are circular and give their greatest response to a spot of light matched in size to the receptive field center. Outside layer IVC, other patterns emerge: 1. Many neurons in V1 respond best to an elongated bar of light moving across their receptive fields, but the orientation of this bar of light is critical. 2. The greatest response is given to a bar with a
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