The Visual System

• Purpose of the Visual System:

  • Vision is much more complex than the just detecting and processing waves of light - involves colour vision, depth perception, motion perception, localisation
  • Light waves are transverse, faster than sound waves and can travel through vacuum
  • Visible light = 400nm - 700nm

  *Red = longest wavelength

  *Violet = shortest wavelength

  • Objects appear as a particular colour by reflecting a particular wavelength of light whilst absorbing all of the others.

  *White = reflects all wavelengths

  *Black = absorbs all wavelengths
• Components of The Eye: (1)

  • the eye is a sensory processing/transduction organ, transferring light waves into nerve impulse and sending a signal to brain via optic nerve CN II
  • Conjunctiva = transparent membrane lining inside of eyelid + folds back on itself (only covers sclera) - lubricates + protect the eyeball
  • Lens + ciliary body divide eye into 2 - anterior chamber + vitreous body

  *Anterior chamber = filled w/ aqueous humour that supplies oxygen and nutrients to the lens & cornea

  *Vitreous chamber = vitreous humour (98% water) , keeps shape, holds retina in place + transmits light

  
• Layer 1 of Eyeball: FIBROUS LAYER

  • made up of cornea and sclera
  • Cornea = transparent part at the front of the eye which refracts light to pass into the eye
  • Sclera = opaque/white part of the eye essential for protecting the contents of the eye, enables innervation and muscle attachment site
• Layer 2 of the Eyeball: VASCULAR LAYER

  • made up of the choroid, ciliary body + iris
  • Choroid = rich in blood vessels and supplies essential nutrients to all layers of the eye
  • Ciliary body = ciliary muscles and suspensory ligaments which connect to the lens to hold it in place and to control its shape
  • Iris = coloured part of the eye which is made up of smooth muscle to control the amount of light entering the eye through the pupil 

  *Pupil dilation = dilator pupillae

  *Pupil constriction = sphincter pupillae

  *innervated by CN III (oculomotor nerve, Edinger–Westphal nucleus)
• Layer 3 of the Eyeball: INNER LAYER

  • Retina = contains millions of photoreceptors that transform light energy into nerve signals which are sent to the brain via the optic nerve CN II
  • Macula = lens focuses light onto a central portion of the retina (approximately 5mm in diameter)  

  *central (i.e. straight-ahead) vision

  *visual acuity

  *most of our colour vision

  • Optic Disc = point of entry for the optic nerve which is lacking photoreceptors , creating a "blind spot" in our vision

  *this blind spot is filled in by what the brain thinks should be there
• Problems:

  • Of aqueous humour - continually forms and drains but if not correctly drained from the anterior chamber, can lead to an increase in intraocular pressure which can cause compression of the retina and optic nerve

  *Glaucoma = leads to blindness if not treated

  • Of the lens - transparent flexible structure that changes shape to help focus light on the retina and is made up of tightly packed fibers which are continually added throughout our lifetime, which causes the lens to become denser and less flexible

  *presbyopia

  *cataracts - clouding of the lens
• Optical Properties of the Eye:

  • Light slows down as it passes into a denser medium

  *Refraction = light hits denser medium at an oblique angle rather than straight on

  • As light passes into the eye, it is refracted three times:

  *Entering cornea - most amount of refraction, but not able to adjust for focussing on objects at varying distances

  *Entering lens -  high elasticity alters focal point to view objects at varying distances

  *Exiting lens

  • Far point = distance where no change in lens shape is required for focussing on far objects, approaches eyes as parallel rays (6m for emmetropes)
• Focusing on Distant Objects:

  • Parallel rays from object
  • Sympathetic input relaxes ciliary muscle
  • Tightens ciliary zonule
  • Flattens lens (reduces accommodation)
  • Dilation of pupils
  • Divergence of eyeballs

  *convex lens will produce an inverted, laterally-reversed image on retina
• Focusing on Near Objects:

  • Diverging rays from object
  • Parasympathetic input contracts ciliary muscle
  • Relaxes ciliary zonule
  • Bulges lens (more accommodation) - causes greater amount of refraction and focusses the image on the fovea
  • Constriction of pupils - prevents divergent rays of light from passing through the very edges of the lens 
  • Convergence of eyeballs - keeps image focused on retina

  *Near point of vision = closest point that we can focus on clearly (greatest bulge)

  *In emmetropes this distance is 10cm, but increases w/ increasing age (presbyopia)
• The Retina:

  • millions of photoreceptors (neurones) that convert light energy into an electrical signal
  • neurones that process response to light (knowing when daytime/nightime)

  *Neurones process light back to front - HOE BITCH ATE GYATT

  • Sclera - Choroid - Pigment Epithelium - Photoreceptors - Horizontal Cells - Bipolar Cells - Amacrine Cells - Ganglion Cells
• Retina is divided into 2 layers:

  • Pigmented layer (outermost) - 1 cell thick consisting of pigmented cells that absorb light , prevent it scattering in the eye, provide support and nutrients (vit A) for photoreceptors
  • Neural layer (transparent innermost layer) - composed of 3 types of neurones (photoreceptors, bipolar cells, ganglion cells)

  *Photoreceptors and bipolar cells = light energy is converted to a signal and passed on

  *Ganglion cells = action potentials generated here and leave eye as as CN II
• Horizontal Neurones in the Retina:

  • Horizontal cells = create the centre-surrond receptive field of the bipolar cell and are responsible for shifting the spectral sensitivity of the bipolar cells to adapt to levels of ambient light 
  • Amacrine cells = send/receive signals b/w bipolar cells, ganglion cells and other amacrine cells, detect/code moving objects and the onset and offset of illumination.
• Photoreceptors:

  • Cones = central vision and colour vision, each cone cell only synapses with a single bipolar cell, connected to a single ganglion cell

  *less sensitive to light (they need bright light for activation), but have very high visual acuity and produce sharp images of a small amount of the visual field

  • Rods =  perception in dim-light and for peripheral vision, 100 rods converge onto a single ganglion cell

  *more sensitive to light (react to a single photon) and low level of acuity/resolution
• The Fovea:

  • centre of macula where images are viewed in detail
  • The fovea only contains cone cells
  • Rod cells outnumber cone cells by approximately 20:1, and are located only in the periphery of the retina.

  *why we can detect dimly lit or moving objects better if they are viewed in the periphery rather than looking directly at them. 

  • the retina receives a blood supply from the central artery and choroid layer of the retina, which enters through the centre of the optic nerve

  *can be checked for signs of hypertension and diabetes.
• Photopigments:

  • outer segments of rods and cones contain photopigments that change shape as they absorb light

  *embedded in areas of the plasma membrane which form a disk shape to increase their surface area for capturing light.

  • consist of a light-absorbing molecule retinal that binds w/ opsins. (4 different types of opsins, so 4 different types of photopigments)

  *Retinal absorbs different types of wavelengths of light depending on what type of opsin it is bound to
• 3 Cone Cell Opsins:

  • Blue cones = 420nm
  • Green cones = 530nm
  • Red cones = 560nm

  *absorb these wavelengths

  • When all cone cells activated simultaneously = see white light
  • When cones' absorption spectra overlap = activation of different types cone cells at the same time allows us to see intermediate hues 

  1 Rod Cell Opsin:

  • Rhodopsin (deep purple pigment) = inputs are only ever perceived in grey tones as only one type of visual pigment
• Colour-Blindness:

  • congenital lack of one or more cone pigments - people may be unaware of it b/c have learnt to rely on other cues (different intensities of the same colour) to distinguish b/w different colours

  *Most common type =  red-green blindness (loss of red or green pigments)

  Night-Blindness:

  • rod cells are not functioning correctly, caused by...

  *Malnutrition = vitamin A needed for regeneration of rhodopsin

  *Retinitis pigmentosa = disease that destroys rod cells
• Phototransduction in the Retina (using rods): DARK

  • cGMP gated channels open - influx of cation (+ve) causes rod to depolarise (stimulate)
  • Voltage-gated Ca2+ channels open in synaptic terminals
  • Neurotransmitter released continuously
  • Causes inhibitory post-synaptic potential in the bipolar cell - causes it to hyperpolarise and prevent any signal being sent further
  • Closes voltage-gated Ca2+ channels and inhibits neurotransmitter release
  • No excitatory post-synaptic potential in ganglion cell = no action potentials along CN II
• Phototransduction in the Retina (using rods): LIGHT

  • cGMP channels closed, so cation (+ve) influx stops - rod hyperpolarises
  • Voltage-gated Ca2+ channels close in synaptic terminals
  • No neurotransmitter released
  • Lack of inhibitory post-synaptic potential in bipolar cell causes depolarisation
  • Opens voltage-gated Ca2+ channels and neurotransmitter is released
  • Excitatory post-synaptic potential in ganglion cell = action potentials propagate along CN II

  ! When photopigment aborbs a photon of light, the retinal molecule breaks down into a new form
• Lateral Geniculate Nucleus:

  • majority of the optic tracts synapse with neurons in the LGN of the thalamus
  • LGN consists of 6 layers with each layer receiving input from either the ipsilateral (same side) or contralateral (opposite side) eye 

  *neurons from parvocellular and magnocellular layers form optic radiation in cerebral white matter - projects to the primary visual cortex (V1) in the occipital lobes of the brain

  • 1+2 inner layers = magnocellular layer = detection of movement.
  • 3,4,5,6 outer layers = parvocellular layer = detection of patterns and colour
• Visual Pathways:

  • optic nerves = axons of the retinal ganglions cells which exit the eye

  *Medial fibres of eye (nasal) = cross over to the opposite side at the chiasm and continue via the optic tract

  *Lateral fibres of eye (temporal) do not cross over at the optic chiasm and continue down the optic tract on the same side

  Visual Fields:

  • due to lens flipping image at the retina...

  *lateral visual field = projected onto the medial/nasal retina in each eye

  *medial visual field = projected on the lateral/temporal retina in each eye
• Optic Chiasm Problems:

  • from optic chiasm onwards, each optic tract carries a complete representation of its alternate visual field
  • Lesions to the optic nerve in front of the optic chiasm = deficits in visual field in one eye
  • Lesions to the optic tract after optic chiasm = deficits in vision field in both eyes

  
• Visual Processing:

  • as each human eye sees a slightly different, but overlapping, view of the world, we are able to perceive depth

  *Loss of an eye or unilateral damage to the optic nerve can lead to a loss of depth perception.

  • depth processing in V1 (occipital lobe) = neurones deconstructing images into light and dark edges
  • object form ,colour and movement processing = more specialised visual areas (beyond occipital lobe)
  • Decision making/reasoning = Output from the ventral and dorsal streams projects to the frontal lobe for further processing

  
• The Ventral and Dorsal Visual Streams:

  • Ventral = the what pathway for object processing (identification, colour, texture, detail, shape, size)

  *occipital - temporal - frontal lobe

  *damage here = visual object agnosia (inability to recognise objects)

  • Dorsal = the where pathway for spatial processing (location, movement , spatial transformations + relations)

  *occipital - parietal - frontal lobe

  *damage here = optic ataxia (inability to reach and grasp object correctly)

  ! Blindsight (damage to V1) = inability of seeing visual targets but formal testing shows they can spatially locate them