Week 6: Vision Part II Central Processing Flashcards
What is the optic nerve composed of?
Composed of axons of retinal ganglion cells, which exit the eye through the optic disc. Eventually enters the brain.
Where is the suprachiasmatic nucleus located?
Hypothalamus
How much axons does the suprachiasmatic nucleus receive from the retina
A small number
What is the suprachiasmatic nucleus involved in?
Controlling circadian rhythms
What does the pretactal nuclei consist of? And where is it located?
Cluster of small nuclei
Midbrain
What is the pretactal nuclei involved in?
Controls visual reflexes
What are the 3 types of visual reflexes controlled by the pretactal nuclei?
1) optokinetic nystagmus - observed when a repetitive visual stimulus moves fast in 1 direction
2) pupilary light reflex - pupil becomes larger or smaller in response to intensity of light reaching the eye. More light, pupil contracts, allowing less light to enter. Dark, pupil relaxes (larger), allowing more light to enter.
3) Vestibule-ocular reflex
Where is the superior colliculus located?
Midbrain
How much axons does the superior colliculus receive from the retina?
Large number
What is the superior colliculus essential for? What could it be involved in?
Visuomotor transformations (eg. fixing on a visual item) May play a role in visual processing. Believed to underlie newborns' visual capacities. Primary target of retina in many mammals.
Where is the lateral geniculate nucleus (LGN) located?
Thalamus
_____ is the primary retinal target in primates.
Lateral Geniculate Nucleus (LGN)
What is the LGN essential for?
Conscious vision
It is the primary input to the visual cortex
There are 6 layers in the LGN.
Layers 1-2 are the _____ layers.
Layers 3-6 are the _____ layers.
Magno
Parvo
Where do the parvocellular layers of the LGN receive input from?
Midget cells in the retina
What are midget cells characterised by?
- very small dendritic trees
- largest population of retinal ganglion cells
- small receptive fields
- slow at propagating action potentials
Where do magnocellular layers of the retina receive input from?
Parasol cells in the retina
What are parasol cells characterised by?
- large dendritic trees
- larger receptive fields
- small proportion of retinal ganglion cells
- fast transmission of information, action potentials move faster down axons
What does fixating on an object mean?
Positioning both eyes such that the object is located right in the center of your fovea.
Why does the fovea have the highest resolution for vision?
It has the highest density of photoreceptors
Light from the right visual field falls on the ___ side of the left eye and ___ side of the right eye.
temporal; nasal
Light from the left visual field falls on the ___ side of the left eye and ___ side of the right eye.
nasal; temporal
What is the optic chiasm?
The point where fibres in the optic nerve cross over the other hemisphere of the brain.
right hemifield –> left thalamus (LGN)
left hemifield –> right thalamus (LGN)
Layers 1, 4, 6 of the LGN receive _____ projections.
Layers 2, 3, 5 of the LGN receive ______ projections.
contralateral (left eye, right thalamus etc.)
ipsilateral (left eye, left thalamus etc.)
LGN is often considered a relay station between ___ and ____?
retina ; visual cortex
Why is the LGN often considered a relay station between retina and visual cortex?
Because the response properties of its neurons are similar to those found in the eye.
The LGN projects to V1, where they are processed in different “columns”, what are they called?
Ocular dominance columns
What is the difference between an orientation selective complex cell vs simple cell?
Complex cell has no inhibitory cell unlike the simple cell. Complex cells also tend to have larger receptive fields.
What will evoke an activity in a complex cell?
A bar located anywhere in the receptive field in the right orientation will evoke an activity
simple cells and complex cells are neurons found in ____?
V1
Besides orientation selectivity, what other property do complex cells possibly have?
direction-selectivity
The dorsal (where) /magnocellular pathway spans which part of the brain?
posterior parietal cortex
The ventral (what) / parvocellular pathway spans which part of the brain?
temporal lobe
What is the magnocellular pathway involved in?
involved in processing spatial information (where things are)
What is the parvocellular pathway involved in?
Involved in processing object identity (what they are)
What is the input to magnocellular layers in the magnocellular pathway?
parasol cells, characterised by very fast transmission speed
dorsal pathway processes motion, which require very fast responses
What is the input to parvocellular layers in the parvocellular pathway?
midget cells. characterised by smaller receptive fields, which is important for shape perception, since we need high resolution to determine identity of objects.
Where does the neurons in the dorsal pathway project to?
Area MT (middle temporal area)
What do neurons in the dorsal pathway consist of?
complex direction-selective cells with very large receptive fields
What condition arises from the damage of MT area?
Akinetopsia – the inability to perceive motion. person only sees flashes of stationary stimuli, very disturbing since motion is an important dimension of vision.
What condition results from damage to dorsal parietal visual areas?
visuospatial neglect – inability to pay attention to a part of space when another competing stimulus is present on the other hemisphere.
right parietal damage –> hemineglect of left visual field
vice versa
Which two conditions can arise from damage to the inferior temporal area?
1) agnosia – inability to recognise objects
2) prosopagnosia – inability to recognise faces
What is agnosia characterised by?
Can recognise borders and maybe shape, but unable to link these visual features with the identity of objects he has learned throughout his life. can recognise faces!
What is prosopagnosia characterised by?
inability to recognise faces, including own face :(
can learn to cope by using other visual cues, like smile, hair colour etc.
