Visual System - Essential Concepts Flashcards
What are the key tasks for the visual system (2)?
To gather information about the spatial & temporal distribution of light reflected from objects & the surrounding scene AND Reconstruct this information to form meaningful representations (perceptions) of the visual world.
What are the different methods of studying the visual system?
To study the anatomy via a microscope.
To study the anatomy via pathway tracing
To study via Functional imaging
To study Clinical neurology
Single Cell electrophysiology
What is functional imaging?
To examine brain activity in neurologically-intact (i.e., healthy) people while they perform specific visual tasks (e.g., functional Magnetic Resonance Imaging; fMRI)
What does clinical neurology involve?
Identifying the specific visual deficit(s) that the brain-damaged subjects experience & relating these to the location of the lesion (damage) site.
What concept is clinical neurology based on?
‘Loss-of-function’ is due to loss of the brain region specialized to carry out that function. (Idea different functions of the brain carry out different tasks)
What is the theory behind focusing on microscopic anatomy to study the visual system?
Neurons with different structures & function occupy different layers of the given tissue. Thus by learning and identifying these we can learn more about different parts of the visual system.
What is this?
A cross section of the retina.
[you should be able to label the cross section]
What happens in pathway tracing?
You inject a substance into a local region of the visual pathway & examine where the neurons there transport it to down their axons, thus revealing their connections with other parts of the system.
[Not usually done in humans]
What is the theory behind pathway tracing of the visual system?
Visual Pathways/Processing occurs in sequence: PhR (Photoreceptors)-to-Ganglion Cells-to-LGN (lateral geniculate nucleus) -to-Cortex
Do neurones with different structures have the same function?
No they have different functions
Do neurones of similar functions cluster together?
Yes they will do this for example in the same cell layer, vertical column or area.
What is over-represented in MAPS of the visual field?
The high acuity central (~15 degrees of vision) as it has the most photoreceptors (from which most stimuli is recieved).
[E.g. 66-75% of V1 processes information from the fovea and macula]
What does every neurone in your central nervous system have?
What does every visual neurone possess?
[same answer for both]
A Receptive Field
What do serial (step by step) processing/pathways in the visual system do?
Reconstruct the image.
“•Serial (step-by-step, sequential) pathways (e.g., cones-bipolar cells-ganglion cells-LGN-visual cortex) = Reconstructing the Image”
What do parallel (side by side) processing/pathways in the visual system do?
Allow for functional specialisation/division of labour
(In the visual system this would happen for things like colour or motion)
What is the primary visual pathway?
By definition the primary visual pathway involves the ganglion cells of your retina, that give rise to the axons that travel down the optic nerve, pass through the optic chiasm into the optic tract and by defintion make connections with neurons in the Lateral geniculate nucleus of the thalamus. Neurones in the lateral geniculate nuclei then give rise to axons of their own which travel in the optic radiations (this is white matter under the occipital lobes of the cerebral cortex) and end up making connections with neurones in the primary visual cortex (otherwise known as V1).
What happens at the optic chiasm?
Nasal axons cross but Temporal axons remain uncrossed
How does the retina of the macula compare to more periphery retina?
More ganglion cells (blobs in the picture represent the cell bodies) in the macula than in the periphery. (this directly corresponds with the over-representation of the macula in Visual MAPS)
Elaborate on receptive fields.
Take the scenerio of a red photo receptor.
When do receptive fields increase in size?
With Eccentricity - smaller receptive fields exist in central vision whereas larger receptive fields exist in the periphery (this relates to the idea of higher acuity in central vision).
They Increase in size at increasingly higher levels of the Visual System-
- e.g., cones have tiny RFs, but those of bipolar cells are larger, and those of ganglion cells are larger still
- with single cells in the highest visual cortical areas having RFs that cover more than 20o of visual space!
- Due to convergent inputs & spatial summation at each successive (serial) step in the visual pathway
What is the problem with photoreceptors from the perspective of the Visual system?
Why do Receptive Fields (RFs) get bigger as you go up the visual system?
Theres 126 million photoreceptors in one eye (that recieve input from about 0.01 degrees of the real world) each sending a message of whether there is light being picked up by that receptor or not.
Thus the Visual system has to RE-BUILD the visual image (via serial processing) to create meaningful representations of real-world objects.
Hence: RFs increase in size so that cells analyze progressively larger regions of the image & in complexity so that what they analyze increasingly resembles objects that we actually perceive
[should know the slide attached]
What is an example of parallel image processing?
Rods and cones as they are an example of functional specialisation.
Does parallel processing of rods and cones contiue across the visual system?
Yes and it even goes as far as involving different cortical cells .
How does parallel processing continue into ‘higher’ areas of the visual cortex?
[you don’t need to specifically know this just a general idea]
