L18 - Vision Flashcards
(12 cards)
Describe Light - electromagnetic spectrum
Of the vast spectrum of electromagnetic radiation, there is only a narrow band that we call visible light
- visible light = approximately. 400nm to 750nm
- visual system detects visible radiation and uses this information as the basis for formation of visual perceptions of the world
Radiant energy is described by wavelength and frequency
- wavelength (m) is the distance between two successive wave peaks
- frequency (Hz) is the number of cycles per second
Humans have 3 colour receptors ~1 million colours
describe the eye (two basic components of it)
- the organ that detects light
- begins processing visual information
Two basic components:
- optical component: collects light, focus light onto the retina
- neural component: phototransduction (light -> change in membrane potential)
- conduct action potentials to parts of the brain that decode the electrical signals to generate visual perceptions
describe optics - refraction in the eye (diagram on slides)
Refraction: bending of light waves, allows accurate image of objects to be focused onto the retina
- refraction occurs when light passes between different medial with different refractive indices eg. air to water, different viscosity solutions
Refractive endex:
- describes the angle of light bending
- is also related to the curvature of the refractive surface
- refraction by lens changes as it changes shape
describe optics - accomodation
Viewing distant objects:
- ciliary muscle sis relaxed
- zonular fibres are taught
- lens is flattened
Viewing close objects:
- increases parasympathetic signalling to ciliary muscle
- ciliary muscle contracts
- tension removed from zonular fibres
- lens becomes more spherical
- eye’s refractive power increases
Lens loses elasticity with age = less accomodation for near vision = reading glasses for presbyopia
Unaccomodated: low = 60 diopters
Fully accommodated: High = 75 diopters
describe optics - the near response
As you shift your gaze from distant to near objects, three processes occur:
Accommodation:
- contraction of ciliary muscle
- lens becomes more rounded
- increased refraction
Constriction of pupil:
- constriction of pupil
- improved depth of focus
- fewer optical aberrations
Convergence of eyes:
- eyes converge (‘cross eyed’)
- aim to keep object’s refracted light on fovea
describe the neural component of the retina
- retina is part of the brain
- photoreceptors = light sensitive cells (furthest from incoming light)
- information flows between photoreceptors and ganglion cells via interneurons
- ganglion cells send information, through axons that become the optic nerve, to brain for further processing
describe the photoreceptors in the retina
Phototransduction: conversion of light energy into a change in membrane potential
Outer segment:
- machinery to respond to light (stacked layers of membrane called disks)
Inner segment:
- cell organelles and synaptic terminal
Cones:
- 8 lilion per retina
- many fovea
- require relatively high light levels (day and twilight vision)
- 3 types, sensitive to red, green and blue light
Rods:
- 120 million per retina
- few in fovea
- function in low light (night vision)
- do not report information about colour
describe colour vision (how we see colour - rods, cones, etc)
Photoreceptors contain phtotpigments - transmembrane proteins called opsin
Rods:
- rhodopsin (do not detect colour)
Cones:
Contain one of 3 types of photopsin:
- S-photopsin: short wavelengths (blue)
- M-photopsin: medium wavelengths (green)
- L-photopsin: long wavelengths (red)
Light specific wavelengths is reflected from coloured objects
Perception of colour created by relative activation of 3 cone types - humans = ~1 million colours
describe phototransduction
In dark conditions:
- Na+ ion channels are open
- RMP in photoreceptor = -40mV
- neurotransmitter being released onto bipolar cells
Photon absorption by rod or cone:
- causes conformation change in retinal
- opsin interact with transducer ( a G-protein)
- transducer activated cGMP phosphodiesterase
- cGMP decreases
- Na+ ion channels close -> reduces cation influx
- cell becomes hyper polarised to = -70mV
- reduction in neurotransmitter being released onto bipolar cells
Resetting:
- bleaching - retinal and opsin broken down, retinal converted back to cis form
- reassembly - retinal and open reassembled
- takes longer for rhodopsin than for photopsins
describe the visual pathway (the four components)
Retinal ganglion cell axons project to four subcortical visual areas:
1. suprachiasmatic nucleus (hypothalamus)
- controls dinural/circadian rhythms
2. Pretectum (brainstem)
- reflex control fo pupils with light
3. superior colliculus (midbrain)
- match eye movements to head movement
4. lateral geniculate nucleus (thalamus)
- sensation of vision
describe the visual pathway (how it crosses and inputs from both eyes)
- axons from ganglion cells in nasal retina cross at the optic chiasm to project controlaterally
- axons from ganglion cells in temporal retina do not cross, they project ipsilaterally
- therefore, information concerning images from visual space facing on equivalent parts of the two retinae is brought together for central processing
describe the visual pathway - monocular and binocular vision (diagram on slides)
because of the crossing of axons at the optic chasm each hemisphere gets input from each eye
With both eyes open:
- the outer regions of our visual field is perceived by one eye only (monocular vision)
- the central portion the visual fields from the two eyes overlap (binocular vision)
The ability to compare overlapping information from the two eyes allows for depth perception and improves our ability to judge distances
