T2 L8: Physiology of Vision Flashcards
How does the Fovea appear through an Ophthalmascope?
As a yellow spot
How much ray bending is done by the Cornea?
2/3. The rest is done by the lens
What is Hypermetropia and what causes it?
Long sightedness caused by a short eyeball or a lens system that is too weak
What is Myopia and what causes it?
Short sightedness caused by the eyeball being too long or the system being too strong
What is refractive power measured in?
Diopters (D)
What are rods for in the retina?
Detecting dim light
What are cones for in the retina?
For detecting bright light and colour
What are Muller cells for?
Responsible for the homeostatic and metabolic support of retinal neurons
What are Amacrine cells for?
The major carriers of rod signals to the ganglion cells in the retina
How many receptors converge into a single optic nerve?
About 125 (There are 125 times for receptors that neurones in the eye)
How many rods are there compares to cones in the retina?
120 million rods and 5 million cones
What is Rhodopsin?
The photosensitive pigment in rods
Describe the process by which rods detect light
When Rhodopsin is hit by a photon, it flips from 11-cis to all-trans.
This results in chemical changes that cause the cGMP-gated nonselective cation channels to close.
This leads to hyperpolarisation of the photoreceptor and a reduction in release of glutamate.
What do ganglion cells respond to in the retina?
To contrast (on centre and off-centre) rather than changes in light intensity
Which wavelength do red cones detect?
560nm
Which wavelength do green cones detect?
530nm
Which wavelength do blue cones detect?
420nm
Which wavelength do rods detect?
500nm
What are the 3 types of cones and what is their abundance?
Red (most abundant)
Green
Blue (Least abundant)
What is colour blindness?
The loss or modification of one or more of the three cone visual pigments (Cone opsins)
Where is the gene for red/green colour blindness found and what does that mean?
It’s on the X chromosome so blue/green colour blindness affects 7% of men and 0.5% of women
Where is the gene for blue colour blindness found and what does that mean?
It found of chromosome 7 so is paired in both sexes. This type is very rare
What is Central Achromatopsia and what causes it?
A rare type of colour blindness that had nothing to do with the pigments but is caused by damage to the cortical colour processing areas (V4) in the brain
What is the function of the lateral geniculate nucleus?
It’s the area that receives input from both eyes to build a representation of the contralateral visual hemifield. It’s found on both sides
What is the function of the superior colliculus in the mid-brain?
It’s the area where visual, auditory and somatosensory information are integrated to initiate motor commands. It’s attached to the midbrain
What is the function of the striate nucleaus?
Output from both the lateral geniculate nuclei goes directly towards the striate nucleus found in the occipital love (V1)
This is the area where images from both hemispheres are combined
The area from the fovea is greatly exaggerated
What are the 3 areas of the visual cortex and what are their functions?
- Ocular dominance columns driven by the left and right eye, not both at the same time
- Smaller orientationial columns which are organized regions of neurons that are excited by visual line stimuli of varying angles.
- Colour blobs for colour information
What does a hypercolumn in the visual cortex contain?
It contains each of the 3 areas together
What causes left eye blindness?
A lesion in an area before the optic chiasm
What is Homonymous hemianopia and causes it?
A defect involving either the two right or the two left halves of the visual fields of both eyes caused by a lesion on one side after the nerves reach the chiasm
What is Bilateral hemianopia and what causes it?
A defect involving either the two right or the two left halves of the visual fields of both eyes caused by a lesion at the optic chiasm
What is Scotoma and what causes it?
A blind spot that obstructs part of your vision caused by damage, pressure, or lesions in an area of the retina. Eg, methyl alcohol, MS, Hypertension. It can affect both eyes
What is the dorsal stream from the occipital to the parietal cortex concerned with?
With location, motion, and action
What is the ventral stream from the occipital to temporal cortex concerned with?
Object and face identity, and conscious perception
What is visual agnosia and what causes it?
A condition in which a person can see but cannot recognize or interpret visual information. It’s caused by a lesion in the ventral steam
What is Optic ataxia?
When someone can see but not act on what they see
What is Prosopagnosia and what causes it?
The inability to recognise familiar faces associated with damage to the fusiform gyrus on the underside of the temporal lobe
What is blindsight?
The ability to respond to visual stimuli without consciously perceiving them
What is the Vestibulo-ocular reflex (VOR) for and what is the mechanism behind it?
It stabilises gaze by countering movement of the head
Mechanism: Oculomotor nuclei - vestibular nucleus - semi-circular canals
What is the Optokinetic reflex (OKR) for and what is the mechanism behind it?
It stabilises the image of a moving object on the retina
Mechanism: Oculomotor nuclei - nucleus of the optic tract
What is the pupillary reflex and why does it happen?
The fact that both pupils will contract if light is shone only into one
It happens because the Edinger Westphal nuclei receive signals from both eyes. So even if the nerve is damaged on one eye, the other nerve will cause contraction
