Physiology of vision Flashcards
Role of iris
- iris acts as a diaphragm, varying its diameter by 4x, and thus retinal intensity by 16x
What is retinal pigmented epithelium
- A pigment layer behind the retina that absorbs unwanted light
What is the optic disc
- Where the optic nerve leaves the eye, and blood vessels enter and leave the retina
what is the fovea
- In a tiny pit located in the macula of the retina that provides the clearest vision of all
- Only in the fovea are the layers of the retina spread inside to let light fall directly on the cones, the cells that give the sharpest image
- AKA fovea centralis or central fovea
Ratio of bending of light carried out between cornea and lens
- Cornea - 2/3 of the ray bending
- Lens - other 1/3 but also allows the focus to vary(accommodation)
What is presbyopia
- Long sightedness caused by old age
What is hypermetropia?
- Long sightedness. eyeball too short or lends system too weak
- Image forms behind the retina
What is myopia
- Short sightedness
- Eyeball too long or lens system too strong
Describe the structure of the retina
- Vertebrae retina evolved back to front, ganglion cells and blood vessels are in the light path to the photoreceptors(except in the fovea)
Rough numbers of receptors in the retina
- 120 million rods(dim light)
- 5 million cones(3 types - bright light and colour)
What are the processing layers of the retina
- 3 direct layers(receptors, bipolars and ganglion cells)
- 2 transverse layers(horizontal and amacrine cells) - signal processing including lateral inhibition
Process of light detection by retinal cells
- When hit by a photon, the retinal in the rhodopsin molecule flips from 11-cis to all-trans
- This sets off a series of biochemical events which results in an electrical change(hyperpolarisation) in the cell membrane
How do ganglion cells respond to light
- Unlike the receptors, ganglion cells respond very weakly to changes in overall light intensity
- Instead, they respond to local contrast: light on a dark background or dark on a light background
Describe the pattern of ganglion cell response to light
- Basic pattern is either on-centre or off-centre
- This is due to lateral inhibition
- Fields tend to be circular
- Ganglion cells send action potentials down the optic nerve: receptors and bipolars have only graded electrical potentials
Relative levels of each type of cone
- There are typically more red cones than green cones, and far fewer blue cones than either of the other two
What causes colour blindness
- Results from a loss or modification of one or more of the three cone visual pigments
- The genes for the red and green pigments are on the X chromosome and damage to one of these genes results in red/green colour blindness
Why is colour blindness more common in males than females
- Males have only one x chromosome, but females have two(ie an intact spare)
Why is blue colour blindness more rare than red-green colour blindness
- The blue pigment gene is on chromosome 7, which is paired in both sexes
- Blue colour blindness is consequently much rarer than red/green
What is central achromatopsia
- Has nothing to do with the pigments
- Is caused by damage to the cortical colour processing(V4)
Describe the central visual pathways
- Optic nerve from each retina divides into L and R halves
- In the optic chiasm L halves combine, same for R halves
- Optic tracts relay in the lateral geniculate nuclei of the thalamus
- Part of each optic tract goes to the superior colliculus in the mid-brain
Where does the output of each lateral geniculate go to
- The output of each lateral geniculate goes almost exclusively to the striate cortex in the occipital lobe(V1)
- Thereafter, the cortical input passes on to areas that process depth, motion, colour etc
How are simple cell responses constructed
- Simple cell responses are constructed from tows of ganglion cell(or LGN) on and off-centre fields
How is the visual cortex organised
- Is organised in three overlapping patterns
- Ocular dominance columns
- Smaller orientational columns in which the orientation of optimal stimuli varies systematically across the surface
- Colour ‘blobs’. Colour info is kept separate from orientation, and passed on to other regions such as V4
What is a hypercolumn
- A hypercolumn contains one complete set of everything
What is homonymous hemianopsia
- Is a visual field loss on the left or right side of the vertical midline
- Usually occurs in both eyes but can just occur in one eye
What is bitemporal hemianopsia
- Is the medical description of a type of partial blindness where vision is missing in the outer half of both the right and left visual field
What is a scotoma
- A scotoma is an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity
What causes scotomas
- They may be caused by retinal damage, lesions in the visual cortex, or by pressure from tumours restricting the optic nerve, chiasm, optic tract or optic radiation
What is the dorsal stream responsible for
- The dorsal stream, from occipital to parietal cortex, is concerned with location, motion and action
What is the ventral stream responsible for
- The ventral stream, from occipital to temporal cortex, is concerned with object(and face) identity, and with conscious perception
What is visual agnosia
- A condition in which a person can see but cannot recognise or interpret visual information, due to a disorder in the parietal lobes
What is prosopagnosia
- Is a disorder characterised by the inability to recognise faces
- AKA facial agnosia or face blindness
What is the area most associated with prosopagnosia
- The area most associated with prosopagnosia is the fusiform gyrus, on the underside of the temporal lobe
What is the striate cortex and what can damage to it cause
- Is part of the visual cortex that is involved in processing visual information
What is the vestibulo-ocular reflex
- Stabilizes gaze by countering movement of the head
What is the optokinetic reflex
- Stabilizes the image of a moving object on the retina
What is the pupillary reflex
- Normally, if one is illuminated, both pupils will contract, because both the pretectal nuclei and the edinger westphal nuclei receive signals from both eyes
- Damage to one optic nerve will prevent light in that eye from closing the pupil(direct response), but light in the other eye will still do so(the consensual response)
- Damage to one oculomotor nerve will prevent pupil contraction in that eye, but stimulation of either eye will cause contraction in the pupil in the second eye
