Dorso-Lateral Geniculate Nucleus and Parallel Processing

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  • A striking features of the dlGN is its division into three distinct sections, each constituted of a different types of neuron. The two most ventral layers in Fig 13.1 consist of large neurons referred to as magno cells, and the dorsal four layers consist of smaller neurons referred to as parvo cells. In between these principal layers , in interlaminar regions are collections of yet smaller cells called konio cells.
  • Retinotopic map= each point in visual space represented along a line perpendicular to the layers is precise with the each point in the retina
  • Single receptive field centre of midget ganglion cells constituted single cone contributing highly developed visual acuity
  • Color opponency- wave length based discrimination is good
  • Color opponency- wave length based discrimination is good
  • Dorso-Lateral Geniculate Nucleus and Parallel Processing

    1. 1. Dorsal Lateral Geniculate Nucleus & Parallel Processing GS Shrestha, M.Optom, FIACLE Lecturer
    2. 2. Introduction• The most significance – Number of optic nerve fibres project to the Lateral Geniculate Nucleus (LGN) in the Thalamus • Relay to the form vision – From LGN, the visual pathway proceeds to the primary visual cortex, V1 • Complex processing of visual signals – Visual processing and object recognition is enhanced by more than 30 extrastriate cortical areas
    3. 3. Targets of the retinal projection Retinal ganglion cell axonsMajor Minor •Several small hypothalamic nuclei •Suprachiasmatic , supra optic, paraventricular nuclei •Accessory optic system •Nucleus of optic tract •Dorsal, medial and terminal nuclei
    4. 4. dLGN90% retinal ganglion cells project to dLGNIs Laminated and shows Retinotopic OrganizationEach layer receives input from a specific eye and class of ganglion cell
    5. 5. Superior ColliculusA midbrain structure conjunction with cortical frontal eye fields and the brain stem reticular formation Is laminated and retinotopically organized nucleus. Visually guided saccadic eye movements Retinal projection segregates with alternating columns of left and right eye terminals. 10% of all retinal ganglion cells project to the SC Are small caliber originate from ganglion cells with small dendritic fields and do not project to other retinal targets
    6. 6. The PretectumA group of small midbrain nuceui is just rostral to the SC Receives signals from a group of small diamter retinal ganglion cells with large receptive fields Involve with the control of the pupillary light reflex by means of a projection to the Edinger- Westphal nucleus of oculomotor complex. Show consensual response
    7. 7. The Pulvinar nucleus Largest nucleus mass Receive projections from the small caliber fibres from the optic nerve and the SC It projects to several visual cortical area including V1 and extrastriate, parietal areas Represents second pathway that can bypass the LGN to get the visual V1 and may plays a role in processing from vision Code importance of visual stimuli-silence or attention ◦ Eg, the eye hand co-ordination
    8. 8. Hypothalamic nucleusReceives direct sparse retinal projection that leave the dosal surface of the optic chiasma and has been implicated in the synchronization of circadian rhythms
    9. 9. The paraventricular and supraopticnucleiInvolve with the regulation of the light dark cycle for neuroendrocrine functions
    10. 10. The Accessory optic systemsThe lateral terminal nucleusThe medial terminal nucleus,the dorsal terminal nucleusNOT in the mid brainImportant role in optokinetic nystagmus in viewing with prolong large field motion
    11. 11. Overview of dLGNKey gateway to visual signals entering the cortexLess agreement in role of vision ◦ Receptive field properties of dLGN cells= retinal ganglion cell inputRegulate the flow and strength of visual signals sent to cortex
    12. 12. Structural organizationLayers 2,3,5 receives input from Ipsilateral eye IpsiLayers 1,4,6 receives input from Contralateral eye ContraDorsal four layers-small neurons-P layers cells=Parvocellular layer (midget cells)Ventral two layers-Large neurons M cells=magnocellylar layers (parasol cells)Between P and M cells=very small bistratified cells- konio cellsCombination of all these layers=Parallel Processing
    13. 13. K6 K5 K4 Konio cells K3 K2 K1Coronal section of dorsolateral nucleus of the monkey
    14. 14. Structural Organizations Superior hemifield in retina=Lateral zone Inferior hemifield= medial zone Central (foveal)= posterior zone Peripheral-anterior zone Each layer receives monocular input contalateral input is received from contralateral eye only (nasal fibres) Ipsilateral input receives input from ipsilateral eye only (temporal fiblres)
    15. 15. Difference in M,P,K cellsMorphology of dendritesCalcium binding protein contentPhysiologic propertiesAxonal projection within visual cortex
    16. 16. Difference in cell structuresP cells orients • M cells complex • K cells orients perpendicular to radially branching parallel to the the cell layers dendrites dLGN layers Maintain compact • Sample more • A few long profile widely within M dendrites Small receptive layers • Larger receptive field centres • Large receptive field Calcium binding field • Calcium binding protein- • Calcium binding protein- calbindin parvalbumin protein- D 28K parvalbumin
    17. 17. Cell Classes Two principal cell classes• Relay cells: send axon cells • Interneurons whose to visual cortex axons remais with in• Glutamic acid -neuro the dLGN trasmitter • γ-aminobutyric acid-• 4:1 neurotransmitter • 1:4
    18. 18. X cells and y cellsFirst evidence of parallel processing in the mammalian retina (Enroth- Cugell and Robson, 1966)-cat ganglion cells to spatial stimuli specifically sine wave gratings
    19. 19. X- and Y-cellsX- cells linear cells ◦ For an X-cell a spatial gratings can be positioned within the cell’s receptive field such that no response is elicited. ◦ Excitation and inhibition are linearly summed and cancel each other. The excitation is equal to the inhibition.Y-cells nonlinear cells ◦ Y- cells doesn’t sum spatial information in a linear fashion.
    20. 20. Afferent axons80% input from midget ganglion cells7-9% input from parasol ganglion cellsRetinal input for K cells?
    21. 21. Efferent axons In primates- efferent out put from LGN terminates within the primary visual cortex and the visual sector of the thalamic reticular nucleus A minor efferent projection from LGN terminate in several extrastriate ares-originates from K LGN cells ◦ Implicates as residual vision in Blind Sight (loss of primary visual cortex) Inconclusion, most K cells and all P and M cells send axons to primary visual cortex
    22. 22. Efferent AxonsP cells send efferent axons to 4Cß of Primary Visual CortexM cells send efferent axons to 4Cά more sparesly to layer 6K cels send their axons to cortical layer 3B where they terminate in patches of cells and some k cells also send axons to cortical layer to 1
    23. 23. Receptive field propertiesOn and OFF centre with opposing surroundsK relay cells appear to have nonstandard visual receptive field
    24. 24. Wave length baseddiscrimination of P cells
    25. 25. Response TimeParvo ◦ Sustained response when presented with a long duration stimulus ◦ Sustained neurons respond to a stimuli for a longer period of time they are better suited to code Low Temporal Frequency StimuliMagno ◦ Transient response to the same stimulus with only  Brief burst at stimulus onset and offset (transient amacrine cells) ◦ Transient respond to rapid illumination changes give M-neurons the capability to resolve high temporal frequency stimuli
    26. 26. Receptive FieldsParvo ◦ Smaller Receptive Fields ◦ Higher Spatial Frequency Resolution ◦ Parvo cells make up the great majority of retinal ganglion cells, both foveal and nonfoveal.Magno ◦ Larger Receptive Fields
    27. 27. Conduction VelocityParvo ◦ SlowerMagno ◦ Faster ◦ Larger Diameter Axons transmit information (action potentials) faster
    28. 28. Retinal ConcentrationParvo ◦ Represents 90% of Foveal Ganglion CellsMagno ◦ Concentration is constant outside the fovea ◦ Represents 10% of non-Foveal Ganglion Cells
    29. 29. Functions of the PathwaysMagno System ◦ “Where” System ◦ Alerts us that a visual event has occurred ◦ Detects movement with rapid transmission ◦ Dorsal cortical processing streamParvo System ◦ “What” System ◦ Details of the event are analyzed ◦ Ventral cortical processing stream
    30. 30. Characteristics of Parvo and Magno neuronsCharacteristics P Cell M Cell K cellSome size Medium large smallReceptive field Centre Centre variableorganization surround surroundDendrite field size Small Medium largeContrast sensitivity Low/ weak High but Intermediat saturated eCortical projection 4Cß 4Cά 3b and 1Color coding Color Non color Some blue opponent opponent onSpeed of transmission Slow Fast
    31. 31. Characteristics of Parvo and Magno neuronsCharacteristics P Cell M Cell K cellTMTFs low high variablePreferred spatial High Low LowfrequencySpeed of transmission medium (4 Fast (2msec) Low (5msec) msec)Spatial linearity Linear Linear or - nonlinearColor vision and contrast Poor at high Poor lowsensitivity spatial frequency frequency contrastTemporal Sustained Transient Both typeresponsiveness
    32. 32. Thank you

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