Visual pathway Flashcards
What is 50% of the cerebral cortex?
Visual
What are examples of the large no. of submodalities in vision?
- Colour
- Depth
- Motion
- Spatial localisation
What are the properties of visual acuity?
- Minimum separable images
- Depends on cone density in photopic vision
- Falls of with increasing eccentricity from fovea
What is used for acuity in central vision?
Letter or other symbol
What is most commonly used to assess peripheral vision?
Sensitivity to spots of light
Define retinal eccentricity?
When objects are projected directly onto fovea are the sharpest
How is visual acuity measured?
Smallest letters that can be distinguished on a chart and is governed by anatomical spacing of mosaic of sensory elements on retina
What is found only in the fovea?
Cone photoreceptors
Photoreceptors that deal with higher light levels, non-functioning at low light level - subserve colour
What is found at the periphery retina?
- Cones
2. Rods taking over at low light levels
What does visual acuity use?
Fovea
Width of 2 foveal cones = 1 arc minute
What is 360 degrees in vision?
Entire visual world around the subject
Divide into 60 arc/minute
What is the resolution limit of visual acuity?
60c/deg or a line resolution of 30 arc seconds
What is hyperacuity measured in and what does it detect?
3 arc seconds
Detect displacement of 2 light: Higher processing
Define visual acuity
The ability to read a standard test pattern at a certain distance, usually measured in terms of a ratio to ‘‘normal’’ vision
What is the test of foveal vision based on?
Gaps that are used by humans to identify those letters
- same no. of letters on each line
- Same level of difficulty
Where is fovea located?
Optical centre of eye
What is found at the optic nerve?
Physiological blindspot
What is not present in the fovea?
Rods
What is Perimetry?
A test of the entire visual field
Central 30 degrees
What does perimetry measure?
All areas of your eyesight, including your side, or peripheral vision
Where are Rods and cones found?
Rods: Periphery of the eye
Cones: Throughout the fovea [high concentration]
What happens at high light levels?
Rods doesn’t function
What happens at low light levels?
Cones doesnt function
What does numbers in perimetry represent?
Threshold detecting a bright spot on the background
What is visual world created from?
Electromagnetic radiation falling on the retina
What is Kanizsa triangle?
illusory contours
A white equilateral triangle can be clearly be perceived even though there are no explicit lines or enclosed spaces to indicate such a triangle
What does visual perception do?
Takes the information and does Bayesian probability prediction as to find the appropriate physical explanation for the image that is falling on the retina
What is perception thought to be?
Dynamic
What is filling in?
An empty region of visual space appears to be filled with colour, brightness or texture of its surround
What is the brain capable of filling in?
The blindspot, borders, surfaces and objects
What is fading?
When one fixates on a particular point for even a short period of time, an unchanging stimulus away from the fixation point will fade away and dissapear [peripheral fading]
What are photoreceptors?
Analysis of colour
What are the spectral responsitivites of 3 cone types?
- Blue, green or red
2. Short, medium and long wavelength photoreceptors
What can we not discriminate?
Any colour that cannot be created by different outputs from those 3 cone types
Why does retina have to be transparent?
Photoreceptors are at the back of retina therefore requires high energy requirements
What varies with eccentricity?
Dendritic spread and cell body size and cell type
What is the basic arrangement of RGC called?
Centre-surround
simplest form of receptive field
What are all RGC divided into?
50% on centre and 50% off centre
What is on-centre?
Light falling on centre
What is off-centre?
Light falling on surrounding regions
What is magno, parvo and Konzo?
A neuron with a small cell body that is located in koniocellular layer of LGN in primates, including humans
What does magno subserve?
Detection of high temporal frequencies - motion high contrast sensitivity
Chromatic information does not contribute to hue discrimination
What does parvo subserve?
slower temporal response - low contrast sensitivity but with spectral opponency
High acuity foveal cone system
What does konzo subserve?
Sluggish temporal response - variable morphology and response function
coneo-cellular pathway
What are the properties of magnocellular system?
Large RGCs include parasol ganglion cells
30 micrometre dendritic spread at Fovea
axonal diamater large
10% total population
projects to layer 4c alpha of V1
high contrast sensitivity
Chromatic information present but would not contribute to hue discrimination
What are the properties of Parvocellular system?
Small RGC include foveal midget ganglion cells
5-10 micrometre dendritic spread at fovea
Axonal diameter small
80% total population
Projects to layer 4cB ‘interblob’ and ‘blob’ regions of VI
Low contrast sensitivity
spectral opponency
What is the property of koniocellular system?
Small dendritic spread, varied morphology
Axonal diameter small
10% total population
Projects to layer interlaminar region of LGN+ to midbrain
Projects to layer 3 and ‘blob’ regions of VI
Intermediate constrast sensitivity
Variable spectral opponency
What are the properties of Intrinsically photosensitive RGCs?
3rd photoreceptor Large dendritic spread Axonal diameter small 1% total population Project to midbrain light sensors Input from rods and cones aswell as intrinsic sensitivity to light
What is the role of IPRGCs?
Circadian rhythms, pupil light reflex
What is optic nerve surrounded by?
cranial meninges
Chiasm
All information coming from one hemi-field is going to contralateral side of brain
How are magnocellular and parvocellular arranged in the optic nerve?
Magnocellular are ventral
Parvocellular are dorsal that go into LGN also crossing over where fibres are coming from retina - move to opposite optic tract in chiasm
What does visual input remain?
Monocular
How were blobs discovered?
Cytochrome oxidase staining of the visual cortex
higher metabolic activity
