Visual System Circuitry
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An introductory lecture to the microscopic and macroscopic organization of the visual system.
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Visual System Circuitry
- 1. Cuteness. One of the many functions of the visual system Visual System Circuitry Csilla Egri, KIN 306, Spring 2012
- 2. Outline ๏จ Retinal circuitry ๏ค โSurroundโ receptive fields ๏จ Visual pathways ๏ค Projections to thalamus and cortex ๏จ Lesions in visual pathway 2
- 3. Retinal circuitry: review of cell types 3 ๏จ rods and cones synapse on bipolar cells and horizontal cells ๏จ horizontal cells make lateral inhibitory synapses with surrounding bipolar cells or photoreceptors ๏จ bipolar cells make synaptic connections with ganglion cells and amacrine cells ๏จ amacrine cells transmit signals from bipolar cells to ganglion cells or to other amacrine cells ๏จ ganglion cells transmit action potentials to the brain via the optic nerve B&L Figure 8-7
- 4. Retinal circuitry: key features 4 ๏จ 2 types of bipolar cells ๏จ On center: hyperpolarized by glutamate ๏จ Off center: depolarized by glutamate ๏จ Bipolar and horizontal cells play a role in lateral inhibition ๏จ Important for increasing visual contrast ๏จ Set up โsurroundโ arrangement of ganglion cell receptive fields B&L Figure 8-7
- 5. Receptive fields 5 ๏จ Photoreceptor receptive fields include retinal area that, when stimulated by light, results in hyperpolarization of individual photoreceptor ๏ค Small and circular ๏จ Ganglion cell receptive field size determined by ๏ค ganglion cell type ๏ค degree of convergence of photoreceptors and bipolar cells and field type by retinal circuitry (lateral inhibition) ๏ฎ On-center/off-surround ๏ฎ Off-center/on-surround Where in the retina is there is there a high degree of convergence?
- 6. Receptive fields 6 ๏จ On-center/off-surround ๏ค Light shines on center of ganglion cell receptive field ๏ ganglion cell increases AP firing ๏ค Light on surround region ๏ decreased AP firing ๏จ Off-center/on-surround ๏ค Light on center ๏ decreased AP firing ๏ค Light on surround ๏ increased AP firing B&L Figure 8-8
- 7. Neural circuits of retinal receptive fields 7 surround centre surround Ganglion cell receptive field P P P _ _ B B H H G G On-center bipolar and ganglion cells Off-center bipolar and ganglion cells
- 8. Neural Circuits of Retinal Receptive Fields 8 Light stimulus on center: ๏จ โ glu release from central photoreceptor ๏จ โ inhibition of on-center bipolar cell ๏ depolarization ๏จ โ NT release ๏ on-center ganglion cell excited ๏จ less glu available to excite off-centre bipolar cell ๏ hyperpolarization ๏จ โNT release๏ off-center ganglion cell inhibited light
- 9. Neural Circuits of Retinal Receptive Fields 9 light light Light stimulus on surround: ๏จ โ glu release from surround photoreceptor ๏จ โ excitation of horizontal cells ๏ โ inhibitory NT released ๏จ โ inhibition of central photoreceptor ๏ โ glu released ๏จ โ glu hyperpolarizes on-center bipolar cell and depolarizes off-center bipolar cell ๏จ On-center ganglion cell inhibited, off-center ganglion cell excited
- 10. Retinal receptive fields: outcome 10 ๏จ Surround arrangement and lateral inhibition allows ganglion cells to respond best to contrast borders in a visual scene ๏ค Ex. Reading dark letters against a white background ๏ค Respond only weakly to diffuse illumination B&L Figure 8-8
- 11. Ganglion cell types and projections 11 Lateral geniculate nucleus
- 12. Visual pathway 12 ๏จ Light from binocular zone strikes retina in both eyes ๏จ Monocular zone only strikes retina on same side as light The right visual field is projected to the ___________________ and ___________________ hemiretina The optic nerves segregate and carry information from ______________________ Each ___________________ crosses at the optic chiasm The optic tracts carry information from ______________________ to the brain Left visual field Right visual field Right temporal hemiretin a Left temporal hemiretina Left/right nasal hemiretin a Optic nerves Optic tracts B&L Figure 8-9
- 13. Visual pathway 13 ๏จ Major projections to the lateral geniculate nucleus in the thalamus, but also to: ๏จ Hypothalamus ๏จ Regulation of circadian rhythm ๏จ Pretectum between the thalamus and midbrain ๏จ Pupillary light reflex ๏จ Superior colliculus in the _________________ ๏จ Reflex movements of head and eyes towards stimulus ๏จ Right and left visual fields project to contralateral hemispheres of the visual (striate) cortex
- 14. Lateral geniculate nucleus 14 ๏จ LGN transmits info to 1ยบ visual cortex (area 17) ๏ค Gates transmission of signal to cortex ๏จ Divided into 6 nuclear layers: ๏ค 2 magnocellular layers (layers 1-2) ๏ฎ Input from M ganglion cells ๏ฎ concerned with location and movement of visual image (neurons respond to brightness) ๏ค 4 parvocellular layers (layers 3-6) ๏ฎ Input from P ganglion cells ๏ฎ Concerned with color and form of image (cells respond to color contrast) ๏จ each layer receives input from only one eye (maintains retinotopic organization) Kandel Figure 27-6
- 15. Primary visual cortex 15 ๏จ LGN neurons representing each eye project to primary visual cortex ๏จ Retinotopic map for monocular and binocular visual fields maintained B&L Figure 8-10
- 16. Extrastriate Cortex 16 ๏จ Thalamus projects to layer 4 of primary visual cortex (Broadmannโs area 17, or V1), info processed and sent to diffuse locations in the extrastriate cortex ๏ค Broadmannโs area 18 (V2) โ analysis of visual meaning ๏ค Dorsal stream (MT) โ spatial recognition ๏ฎ Perception, analysis of visual scene ๏ค Ventral stream (V4) object recognition ๏ฎ Action, guided movement and spatial characteristics of the environment Monkey brain
- 17. Lesions in visual pathway 17 Kandel Figure 27- 20 Level of lesion can be determined by specific deficit in the visual field 1.Right optic nerve ๏ง Loss of vision in right eye 1.Optic chiasm ๏ง Loss of vision in temporal visual field of both eyes 1.Right optic tract ๏ง Loss of vision in left visual field of both eyes
- 18. Objectives After this lecture you should be able to: ๏จ Describe the components of retinal circuitry ๏ค Relate these connections to synaptic transmissions in center surround receptive fields ๏จ Trace the pathway from the retina to the primary visual cortex ๏จ Describe the structure and function of the lateral geniculate nucleus and its projections ๏ค List the major functions of projections to the extrastriate cortex ๏จ Determine the level of a lesion in the visual pathway based on a specific deficit in the visual field or visa versa 18
- 19. 19 Test your knowledge 1. Axons from the __________________ hemiretina cross at the optic chiasm 2. For the following schematic diagram of the cells of the retina, name each of the cells. Explain how the firing rate of cell (c) is affected by light shining on the surround if this arrangement represents an off centre-on surround receptive field. Include in your answer a description of the events that occur at each synapse involved. a) b) c) d)
Editor's Notes
- Interplexiform cells: transmit signals in the retrograde manner from the inner plexiform layer to the outer plexiform layer. Signals are inhibitory and control lateral spread of visual signals by horizontal cells in the outer plexifrom layer. Role may be to help control the degree of contrast in the visual image. Amacrine cells help analyze visual signals before they leave the retina. There are two type of bipolar cells: โon typeโ have excitatory receptors โoff-typeโ have inhibitory receptors Amacrine cells: transform sustained bipolar cell output into transient responses of ganglion cells act as interneurons in pathway from rod bipolar cells to ganglion cells
- Direct path: Photoreceptor ๏ bipolar cell ๏ ganglion cell Indirect path: Photoreceptor ๏ horizontal, amacrine, bipolar cells ๏ ganglion cells cones in center of ganglion cell receptive field influence ganglion cell activity by direct pathway cones in surround of ganglion cell receptive field influence ganglion cell activity by indirect pathway
- i.e., response in center of receptive field is opposite to response in surround, due to opposite effects of direct and lateral pathways depolarized by glutamate (opening of Na+ channels) hyperpolarized by glutamate (opening of K+ channels or closing of Na+ channels)
- Always have a tonic release of AP, but their frequency is mediated by center/surround receptive fields
- On center bipolar cells hyperpolarized by glutamate Off center bipolar cells depolarized by glutamate Center photoreceptors always synapse onto bipolar cells of each type, on center and off center Surround photoreceptors synapse on horizontal cells which mediate signals via lateral inhibitory connections
- On center bipolar cells hyperpolarized by glutamate
- Light impinging on both center and surround of bipolar cell may result in cancellation of center and surround effects. Responses of amacrine cells depend on pattern of convergence from on-center and off-center bipolar cells (response involves increase or decrease in firing rate). Firing rate of ganglion cells is determined by input from bipolar and amacrine cells dominant input from amacrine cells can produce uniform or mixed responses across receptive field dominant input from bipolar cells produces center-surround responses
- Fibers from the nasal hemiretina of each eye cross to the opposite side at the optic chiasm, whereas fibers from the temporal hemiretina do not cross. In the illustration, light from the right half of the binocular zone falls on the left temporal hemiretina and right nasal hemiretina. Axons from these hemiretinas thus contain a complete representation of the right hemifield of vision (see Figure 27-6).
- The visual cortex is area 17 Superior colliculi located in midbrain, part of the tectum - orienting the head to visual (or other) stimuli, and in certain kinds of eye movements.
- LGN relays info to the visual cortex by optic radiation. Exact point to point transmission with a high degree of spatial fidelity all the way from retina to visual cortex. Layers 2,3,5 receive input from lateral half of ipsilateral retina. Layers 1,4,6 receive input from medial half of contralateral retina. Gate transmission means controls how much of a signal is allowed to pass to the cortex. Highlight visual information that is allowed to pass. magnocellular pathway is concerned with location and movement of visual image (neurons respond to brightness) since it receives input from color insensitive retinal ganglion cells (M cells have same cone inputs to center and surround) parvocellular pathway is concerned with color and form of image (cells respond to color contrast) since it receives input from color sensitive retinal ganglion cells (P cells have different cone inputs to center and surround) LGN receptive fields: broad band cells sense contrast or brightness but do not contribute to color perception (respond to all wavelengths in center-surround manner) single-opponent cells receive opposite input from different cone types in center and surround, e.g., if center is excited by red then surround is inhibited by green Projections of eye to different layers: contralateral nasal hemiretina projects to layers 1, 4 and 6 ipsilateral temporal hemiretina projects to layers 2, 3 and 5 LGN neurons representing each eye terminate in primary visual cortex in alternating patches called ocular dominance columns LGN and primary visual cortex have retinotopic organization (receptive fields of adjacent regions arise from adjacent regions of retina)
- Local interneurons in visual cortex combine information from different LGN inputs resulting in more complex responses cortical neurons generally respond best to particular orientation of visual stimulus, i.e., orientation of bar of light, forming orientation columns where all neurons in column respond best to same stimulus orientation neurons in middle temporal area (MT) respond selectively to direction of moving edge without regard for color; form part of dorsal pathway leading to parietal lobe responsible for spatial analysis, e.g., motion, relative position of objects in visual scene neurons in V4 respond selectively to color without regard to direction of movement; form part of ventral pathway leading to inferior temporal lobe responsible for high-resolution form vision and object recognition
- Damage to dorsal stream: action pathway disrupted, canโt grasp objects but can recognize them Damage to ventral stream: perception pathway disrupted, canโt recognize objects but can grasp them Local interneurons in visual cortex combine information from different LGN inputs resulting in more complex responses cortical neurons generally respond best to particular orientation of visual stimulus, i.e., orientation of bar of light, forming orientation columns where all neurons in column respond best to same stimulus orientation neurons in middle temporal area (MT) respond selectively to direction of moving edge without regard for color; form part of dorsal pathway leading to parietal lobe responsible for spatial analysis, e.g., motion, relative position of objects in visual scene neurons in V4 respond selectively to color without regard to direction of movement; form part of ventral pathway leading to inferior temporal lobe responsible for high-resolution form vision and object recognition
- Deficits in the visual field produced by lesions at various points in the visual pathway. The level of a lesion can be determined by the specific deficit in the visual field. In the diagram of the cortex the numbers along the visual pathway indicate the sites of lesions. The deficits that result from lesions at each site are shown in the visual field maps on the right as black areas. Deficits in the visual field of the left eye represent what an individual would not see with the right eye closed rather than deficits of the left visual hemifield. 1. A lesion of the right optic nerve causes a total loss of vision in the right eye. 2. A lesion of the optic chiasm causes a loss of vision in the temporal halves of both visual fields (bitemporal hemianopsia). Because the chiasm carries crossing fibers from both eyes, this is the only lesion in the visual system that causes a nonhomonymous deficit in vision, ie, a deficit in two different parts of the visual field resulting from a single lesion. 3. A lesion of the optic tract causes a complete loss of vision in the opposite half of the visual field (contralateral hemianopsia). In this case, because the lesion is on the right side, vision loss occurs on the left side. 4. After leaving the lateral geniculate nucleus the fibers representing both retinas mix in the optic radiation (see Figure 27-19). A lesion of the optic radiation fibers that curve into the temporal lobe (Meyerโs loop) causes a loss of vision in the upper quadrant of the opposite half of the visual field of both eyes (upper contralateral quadrantic anopsia). 5, 6. Partial lesions of the visual cortex lead to partial field deficits on the opposite side. A lesion in the upper bank of the calcarine sulcus (5) causes a partial deficit in the inferior quadrant of the visual field on the opposite side. A lesion in the lower bank of the calcarine sulcus (6) causes a partial deficit in the superior quadrant of the visual field on the opposite side. A more extensive lesion of the visual cortex, including parts of both banks of the calcarine cortex, would cause a more extensive loss of vision in the contralateral hemifield. The central area of the visual field is unaffected by cortical lesions (5 and 6), probably because the representation of the foveal region of the retina is so extensive that a single lesion is unlikely to destroy the entire representation. The representation of the periphery of the visual field is smaller and hence more easily destroyed by a single lesion.
- Nasal hemiretina