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what's Sclera

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the outermost layer of the eye and is a tough white fibrous tissue Sclera muscles

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Pupil

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The pupil is an opening that allows light to enter the eye.

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Cornea

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A transparent gelatinous structure in front of Iris which allows light in.it is nourished by the aqueous humour

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The anterior chamber.

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The space beteen lens & cornea filled with the aqueous humour

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Posterior chamber

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• Between lens & Iris.

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Canal of schelm

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A canal at junction of cornea and iris through which aqueous humour flows

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Vitreous humour

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The middle of eye , jelly-like Vitreous humor makes up 80% of eye and keeps eyeballs spherical . need constant replacement + has phagocytes which breakdown and remove obstacles/ floaters.

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Macula

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• The macula is a spot at the back of the eye responsible for central vision.

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whats at the centre of macula?

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fovea

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Optic disk

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• A region at the back of eye continuous with the retina which is the origin of blood vessels and where the optic nerve axons exit eye

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why is the optic disk called a blind spot

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nophotoreceptors

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what causes glaucoma

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disorder caused by imbalance of aqueous humour due to increased pressure reducing blood supply to the eye thus damaging retina

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Transduction of light

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• Light passes pass through the cornea, lens, and anterior and posterior chambers and reach the photoreceptors (rods and cones) located in the retina • Focusing of images on the photoreceptors depends on refraction (bending) of light rays as they pass through the cornea and the lens

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photoreceptor types

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rods and cones

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do photoreceptors producs APs?

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no,They don't not produce APs. They respond with graded membrane potentials .

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describe cones

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- responsible for daylight, colour and acuity vision -concentrated at fovea- central vision -They have a fast integration time and response and are sensitive to direct axial light due to the cone shape. -least numerous (5 million) -• Cell bodies are located in outer nuclear layer of retina have 3 photopigments with different peak sensitivities ( SML cones)

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what happens due to loss of cones

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decreased visual acuity and colour vision deficits

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describe rods

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- responsible for night vision have more discs to absorb light. Their disks contain lightabsorbing photopigments. -more numerous (100 million) -slow integration -1 photopigment -Cell bodies are located in outer nuclear layer of retina -dominate peripheral retina -contain rhodopsin pigment which is highly sensitive to light allowing night vision its peak sensitivity ~ 496 nm (blue/green)

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how is colour perceived

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Comparison of two areas of equal light intensity reflecting different wavelengths

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types of cones, colour and max nm activation

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S-cone -- short wavelength (430nm max) ---> blue M cone --> medium wavelength (530 nm) --> green L cone---> large walength (560max) ---> red

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Trichromacy

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the condition of possessing three independent channels for conveying color information, derived from the three different cone types

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describe the inner nuclear layer of the eye

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• Contains the cell bodies of amacrine cells, horizontal cells, and bipolar cells which function as interneurons. • Horizontal and bipolar cells process signals from photoreceptors.

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phototransduction

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• The right and left lateral geniculate nuclei, located in the dorsal thalamus, are the major targets of the two optic tracts. • Light ---> eye retina - to photoreceptors • Absorbed by pigments complex (rhodopsin (rods)/ conopsin cones) & cis-retinal ) • Light causes conformational change from cis-RETINAL to trans-retinal • Complex - GDPGTP - activates cGMP phosphodiesterase which Breaks down cGMP tp gmp results in closure of sodium channels • "dark current" prevented - Na+ leaves but cannot enter • Cell hyperpolarises - reduces transmitter released

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what happens when light hits rod

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it turns off via closure of na+ channels

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describe the Visual hemifieldfield production

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• light from periphery hits the nasal retina and travels contralaterally crossing at the optic chiasm to the LGN • light from the visual field hits the temporal retina and travels ipsilateraly to the LGN,then optic radiation • Objects within binocular field (B) of left hemi-field are imaged by nasal retina of left eye but also temporal retina of right eye • Objects outside of left binocular field (A) imaged by nasal retina of left eye • Fibres from nasal retina cross (decussate) at optic chiasm • NB Left visual hemi-field is viewed by right hemi-sphere and vice versa

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Visual pathways

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• light goes to the retina lateral geniculate nucleus primary visual cortex

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whats a full visual field (F)

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area viewed by both eyes

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Visual Hemifields

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• Full visual field (FVF) is area viewed by both eyes • Left/Right hemi-field divides FVF into two • Binocular VF is area of overlap between left & right hemi-fields

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fixation points

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• Fixation point (b) - represents focus point for object • Near (a) & far (b) disparities - areas in front and behind where two images are perceived. • Perceptual disparities -interpreted as 'depth' by brain

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what is interpreted as depth?

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perceptual disparities

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Retinofugal projection

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• Pathway of optic nerve to brain • R & L optic nerves to optic chiasm • The chiasm is site of a decussation (crossing) so that the L visual field from both eyes projects to the R cortical hemisphere and R field to L hemisphere. • The nasal part of the retina crosses to contralateral side the temporal part remains ipsilateral • Axons project to R and L Lateral geniculate nucleus (LGN) in R and L thalamus • R and L optic radiations to primary visual cortex (aka, area 17, V1 or striate cortex) of R and L occipital lobes of cerebral cortex

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Blindsight

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a type of vision "which is not dependent on primary areas responsible for processing vision" • "Thought to result from the projections of the optic nerves to subcortical areas, including the superior colliculus and the amygdala" • "Part of our vision that's for orienting and doing rather than for understanding"

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LGN projections

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• Neurones of LGN project to visual cortex via optic radiation

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LGN target

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• Main target is primary visual cortex Also referred to as Brodmann's area 17, the striate cortex or V1 • Lies in occipital lobe • Laminar structure

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striate cortex

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Striate" cortex - striped- has neuronal bodies arranged in layers which can be seen by staining the cortex . named can also be seen in fresh tissue caused by • Due to horizontal pyramidal cell connections in layer III

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Visual Association Cortex

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• Early studies identified Brodmann's areas 17 (V1), 18 & 19 as 'visual' • Actually > 50 % primate cortex subserves vision, V1-V5 • Dorsal stream - motion processing • Ventral stream - visual details, face selective cells

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RF 'centre' contact with bipolar cell

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direct

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RF 'surround' contact with bipolar cell

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via horizontal cell

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what's a receptive field

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area of retina that alters bipolar Vm in response to light

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describe off centre bipolar cells

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-they depolarise in the dark when light hits surround they receive indirect input from receptive field centre the photoreceptor is hyperpolarised and this acts on horizontal cell also hyperpolarises horizontal cell acts on bipolar cells and hyperpolarises them

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On centre cells

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-depolarised when light falls on centre -light on surround hyperpolarises cells • Reduces horizontal cell output • Reduces GABA inhibition of centre • More glutamate released in centre so ON cell hyperpolarises

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types of ganglion cells

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• Magnocellular ganglion cells (M-type) • Parvocellular ganglion cells (P-type): • K ganglion cells (non-M, non-P type)

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•Magnocellular ganglion cells (M-type)

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o 5% of ganglion cell population o larger, more numerous, for scotopic vision & motion detection o respond to combined M & L cones and rods o brightness contrast only

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• Parvocellular ganglion cells (P-type):

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o 90% of ganglion cell population o small, fewer, for colour vision o respond to single cones or groups of same cones o colour opponent cells

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• K ganglion cells (non-M, non-P type)

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o small cell bodies, medium-sized dendritic fields, colour selective o 5% ganglion population

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Ganglion cell RF's

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• Have antagonistic centre-surround receptive fields (RF) • 'On-centre' and 'Off-centre' ganglion cell subtypes • 'On-centre' - depolarize in light, hyperpolarize in dark • 'Off-centre' - hyperpolarize in light, depolarize in dark • Optimal encoding of differences in illumination across RF

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Off" ganglion cells

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• Shadow in 'surround' RF only - hyperpolarisation & decreased firing • As 'edge' moves over 'centre' get depolarization & increased firing • When shadow completely covers 'centre' & 'surround' then firing decreases again • Conclude that optimal stimulus is dark-light border across 'centre' & 'surround' RF i.e. spatial variation

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What purpose does this centre-surround antagonistic organisation serve?

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: It helps the retina find edges in images. Objects are usually distinguished from their background by sudden changes in reflected light

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Colour Opponency

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•Colour perception is based on relative activity of ganglion cells whose RF centres receive inputs from red, green & blue cones • Looking at red box fatigues some of red cones • When we switch our vision to the white box, the green cones (for the red square) are unopposed, so we see green • Same for blue box - blue cones become fatigued so we see yellow when we switch our vision • Ganglion cells - important for providing spatial comparison information about light/dark, red/green, and blue/yellow

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what type of ganglion cell is Red ON centre green OFF surround

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P-cell

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describe the stimuli of red and green on P cells

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• RF - red ON centre; green OFF surround (R+G-) • Most effective stimulus is red light onto RF centre • Response strongly inhibited by green light onto RF surround • Some cells classed as blue ON & yellow OFF (B+Y-)

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what layers of the LGN do magnocellular g cells target

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: layers 1 & 2, inputs

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what layers of the LGN do parvocellular g cells target

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layers 3-6

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LGN targets: organisation

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• Retinotopic projection • Neighbouring locations on the retina project to neighbouring locations on the LGN • The projection pattern is preserved in the LGNV1 projection

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in what layer of the cortex are stellate cells found

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IV C

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in what layer of the cortex are pyramidal cells found

answer

III, IV B, V and VI

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Visual cortex - outputs

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• Pyramidal cells send projections to different areas of the cortex. • Layer II, III and IV B have axons projecting to other cortical areas • Layer V axons project to superior colliculus and pons • Layer VI give rise to the very large projection BACK to the LGN • Pyramidal cells in all layers also have local cortical connections.

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Intracortical connections

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• Radial connections - most connections run perpendicular to cortical surface, from white matter to layer 1. Retinotopic organisation is maintained. • Horizontal connections - collateral branches from pyramidal cells in layer III form connections with cells on either side

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Ocular dominance columns

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• Across layer IV, every 0.5mm or so, the response of the neurons switches from one eye to the other (so across 1mm cortex, we have responses for both eyes). • Retains segregation of eye inputs as seen in LGN • Inputs combined at next level (binocular processing) • Layer IV - major target of LGN neurons • LGN axons of left & right eye inputs segregated within layer IV • Autoradiography - inputs from one retina bright stripes on dark background

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Orientation selectivity

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• RF's in retina/LGN - circular, respond to spot of light • Neurons in V1 respond best to bar of light rather than spot • Orientation of bar is critical • Diagrams show responses of V1 cell to a stimulus with a specific orientation

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Columnar orientation selectivity

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• All neurons in vertical column display same orientation specificity • Neurons in oblique row display heterogeneous orientation specificity

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Parallel pathways: Magnocellular

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• M-type ganglion cells→ magnocellular LGN layers → IVCα →IVB • Pyramidal IVB cells → binocular RF's (simple and complex) • Orientation-selective • Many are direction-sensitive • Generally not wavelength-sensitive • Transient responses, large RF's, highest number of direction-sensitive RF's • ANALYSIS OF OBJECT MOTION

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Area MT

answer

• (located in middle temporal lobe in some monkeys) - also known as V5 • Neurons respond selectively to direction of moving edge, with no regard for colour • Direct input layer IVB - magnocellular pathway, also from V2 and V3 • Cells have large RF's, almost all direction-sensitive • Some MT cells respond to perceived rather than actual direction of movement of stimuli

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area mst (parietal lobe)

answer

• area MST - linear, radial and circular motion • Possibly used for navigation, directing eye movements and motion perception

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