Vision lecture Flashcards
How is an focus varied?
Focus is varied by changing the shape (and power) of the lens
What is the purpose of the iris?
The iris acts as a diaphragm, varying its diameter by 4x, and thus retinal intensity by 16x
What is found behind the retina?
Behind the retina is a pigment layer which absorbs unwanted light
Describe what the retina looks like through an ophthalmoscope
The main feature is the optic disc, where the optic nerve leaves the eye, and blood vessels enter and leave the retina
What is the fovea?
Small yellow spot
What proportion of the ray bending does the cornea do?
2/3
What proportion of the ray bending does the lens do?
1/3
What is Hypermetropia (long sightedness)?
eyeball too short or lens system too weak
What is Myopia (short sightedness)?
eyeball too long or lens system too strong.
Describe the structure of the retina
Vertebrate retina evolved back to front: ganglion cells and blood vessels are in the light path to the photoreceptors (except in the fovea).
Receptors and processing layers (3 direct layers and 2 transverse layers)
What is Rhodopsin?
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
What are the basic responses in ganglion cells
Off centre
On centre
Describe the arrangement of red, green and blue cones
There are typically more red cones than green cones, and far fewer blue cones than either of the other two.
Describe colour blindness genetics
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.
Males have only one X chromosome, but females have two (i.e. an intact spare) which is why red/green colour blindness is much more common in males (7%, versus 0.5% in females).
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
There is another, much rarer kind of colour blindness, which has nothing to do with the pigments, but is caused by damage to the cortical colour processing areas (V4).
Describe the columnar organization of the visual cortex
The primary visual cortex (v1, or Brodmann area 17) is organized 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 information is kept separate from orientation, and passed on to other regions such as V4
What causes a scotoma?
Scotomas come in many kinds: 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?
The dorsal stream, from occipital to parietal cortex, is concerned with location, motion and action.
What is the ventral stream?
The ventral stream, from occipital to temporal cortex, is concerned with object (and face) identity, and with conscious perception.
What is associated with damage to specific parts of the temporal lobe?
Prosopagnosia - fusiform gyrus, on the underside of the temporal lobe.
What leads to blindsight?
Destruction of the striate cortex (V1) leads to blindness in the the part of the visual field that corresponds to the damaged area. The area of blindness (scotoma) may be small, or an entire hemifield.
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 eye 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.
