The human visual system Flashcards
What is the sclera and what is its function?
The white of the eye
Protective outer layer of collagen and elastin
What does the choroid do?
Provide oxygen and nutrients to the outer retina, especially the fovea
Where are cones concentrated?
The fovea (red and green, not blue)
Describe horizontal cells
Interneurons that connect the photoreceptors laterally
The connections it makes are called plexiform layers
What do bipolar cells do?
Transmit signals from photoreceptors to retinal ganglion cells via graded potentials rather than action potentials.
What do amacrine cells connect?
Retinal ganglion cells (laterally)
Describe the three types of retinal ganglion cells
M type: large receptive field, processes motion, not sensitive to colour
P type: small receptive field, detects fine features, colour sensitive
K type: large receptive field, function unknown
What is the pathway for information to get from the eye to the visual cortex?
Eye -> Optic nerve -> Optic chiasm -> LGN -> Optic radiation -> Visual cortex
What is the purpose of the lamina cribrosa?
Acts as a seal to maintain eye pressure
What fibres do not cross over at the optic chiasm?
Temporal axons from retinal ganglion cells
What does the parietal pathway do?
Determines where things are in space
What is the LGN and what does it do?
The lateral geniculate nucleus acts as a relay centre between the eye and the visual cortex
It adds the signals from the two eyes to make a 3D representation
Describe the three main features of the primary visual cortex
Occular dominance columns (information from the left and right eyes are kept separate) Orientation columns (each column is sensitive to a certain orientation) Blobs (detect colour)
What are saccades?
Voluntary fast eye movements used to bring an image onto the fovea
What is the purpose of fixational eye movements and what are the three features they are composed of?
Keeps the fovea on a fixed target of interest
Made of microsaccades, drifts and tremors
What is the name of the eye movement used to keep track of a moving object?
Smooth pursuit
Uses predictive movement
What is the fastest reflex in the body?
The vestibulo-ocular reflex
Where are action potentials initiated?
Axon initial segment
What are the different types of eye movement?
Sacades Fixations Smooth pursuit Optokinetic nystagmus Vestibulo-occular reflex Vergence movements
In what circumstances are smooth pursuit, optokinetic nystagmus and the vestibulo-occular reflex used?
Smooth pursuit = When a single object is moving
Optokinetic nystagmus = When the whole visual field is moving
Vestibulo-occular reflex = When the head/body is moving
What is a method used to measure eye movement?
Video oculography
Describe the vestibulo-occular reflex
Eye movements that compensate for head and body movements
Fastest reflex in the body
Involuntary, controlled at the level of the brainstem
Rotational VOR is mediated by the semi-circular canals
Translational VOR is mediated by the otolith organ
What is a lack of a vestibulo-occular reflex called and how is it tested for?
Oscillopsia
Tested for using a Barany chair
How do we view two things that are at different distances?
Vergence eye movements
There is no disparity between the eyes for items being focused on but there is for everything outside of the focal plane
Describe infantile nystagmus
An involuntary movement of the eyes, usually horizontal
Often hereditary
Uses a foveation strategy
Can be idiopathic or caused by diseases that cause sensory deficits e.g. albinism
