visual pathways Flashcards
what is vision?
detecting and interpreting patterns of electromagnetic radiation to make sense of the world
differences in intensity and wavelength (colours)
how did eyes evolve?
sequence of improvements for detecting directions and forming an image
how do the eyes respond to darkness?
SCN instructs production of melatonin to promote sleep
keep circadian clock accurately timed
what are M1 and M2?
what do they do?
what do their axons form?
photosensitive retinal ganglion cells which detect light so affect sleep-wake cycle
send action potentials to the brain
axons form optic nerves
what does exposure to blue light do?
increases alterness and stimulation of cognitive functions
e.g from a phone screen
what is melanopsin?
light-sensitive receptor molecule
what does vision require?
forming and processing an image in the eye
what do rod and cone cells form in the retina?
an array
steps of image processing?
light passes through cornea, acqueous humor, lens and vitreous humor on the way to retina
refracted at each boundary
accommodation of lens to focus image
role of lens?
focus image
role of iris?
aperture to control light entering
role of photoreceptors?
pixels to register image
role of glass body, macula and pigment?
filtering media
role of cornea?
filter to protect lens
role of eye lid?
lens cover for when not in use
role of tears?
cleaning mechanism
role of retinal interneurons?
processing algorithms
relationship between closeness of object and lens shape?
what is the process of changing strength of lens?
nearer the object, the stronger the lens needed to form a focussed image so becomes larger
accomodation
alternative names for near and far sightedness?
near: myopia
far: hyperopia
what does the fovea do and what is it ?
collects info about the visual scene
central portion of retina with most photreceptors and so center of our gaze
eye movements when making a cup of tea?
saccades in anticipation of next movement
moved to particular locations when eyes engaged in visual search and more precise if know where objects were
eyes disengaged from fixating before an action was completed
what can movement be described as?
combination of 3 directions of translation and 3 directions of rotation
yaw - rotation around z axis
roll - x axis
pitch - y axis
what are saccades?
move eye very quickly to new position between periods of gaze stabilisation (fixation) to scan scene across whole field of view
direct fovea to collect info about the visual scene
what are smooth pursuit movements?
slower and keeps a moving stimulus on fovea
what is Optokynetic nystagmus?
brings eye back from a peripheral to more central position after it has followed a large-scale moving stimulus
what is
Vestibulo-ocular movements?
compensate for the movement of the head by moving the eye the same distance but in the opposite direction in order to maintain a constant field of view
where does conscious and automatic control of eye movements come from?
conscious - cortical frontal eye fields (FEF)
automatic - superior colliculus
what are 3 similarities between rod and cone receptors?
both detect light in basically the same way
both recover rapidly from change
have graded responses
difference between rods and cones?
cones
- specialised for vision during the day
- smaller than rod cells and contain less photopigment per cell
- situated in fovea
rod cells
- specialised for vision during the night as have high sensensitivity to dim light
- larger than cone cells and contain large amounts of photopigment
- don’t use central fovea (no rods in fovea only periphery) as acuity (clarity) sacrificed for high sensitivity
what does light cause in receptor membrane?
hyperpolarisation (decrease in membrane potential) of receptor membrane (not depolarisation)
what is opsin?
light-sensitive protein in membrane of photoreceptors (photopigment)
difference between cones and rods in terms of opsins?
cone opsins differ in wavelength-specific affinity to light (as 3 classes of cones and opsin) so only 1 opsin type expressed per cone
all rods express the same type of opsin
what activates G protein?
conformational change in rhodopsin
what does activated rhodopsin cause?
activates messenger (transducin)
causes Na channels to close
membrane therefore becomes more polarised
what are the major 2 pathways for vision?
what are they?
- Geniculate-striate pathway - 90% of retinal projections
- Extrageniculate pathway
projections from retina to other brain areas
what is the role of the extrageniculate pathway?
via superior collilculus
important for eye movements and visual attention
what does the high density of cones in V1 tell us?
that more processing power is required for info originating from the fovea
requires higher acuity
what does damage to V1 cause?
cortical blindness
what is visual transduction?
process by which light is converted into electrical signals in rod, cone and photosensitive genglion cells in the retina of the eye