Vision (neuro)

Question 1

Parts of the eye: retina

Answer 1

• temporal retina: the outside part of the eye looking towards the nasal (inside) area
• nasal retina: the inside part of the eye looking towards the temporal (outside) area
• retina is made up of neural retina, retinal ganglion cell, optic nerve and retinal pigment epithelium

Question 2

Parts of the eye: layers

Answer 2

• outer fibrous layer contains cornea and sclera

- inside layer contains vitreous humour and aqueous humour

Question 3

Parts of the eye: optics

Answer 3

• cornea: primarily responsible for focussing light rays
• iris: controls how much light enters the eye via the pupil
• ciliary body
• suspensory ligaments
• lens: provides additional, variable ‘fine’ focus

Question 4

Primary visual pathway

Answer 4

• retina
• optic nerve
• optic chiasm
• optic tract
• optic radiation
• lateral geniculate nucleus (LGN)
• primary visual cortex (area 17)
• brainstem nuclei involved in eye movements etc

Question 5

Phototransduction

Answer 5

• rods = photoreceptors for night vision

- cones = photoreceptors for day vision

Question 6

Cone photoreceptor

Answer 6

from top to bottom:

• membrane discs
• nucleus
• axon
• synaptic terminal
• resting membrane potential is -45mV to -60mV
• opposite in response to decreased light

Question 7

Transduction

Answer 7

• initiation of the light response
• amplifying biochemical cascade (supports a very rapid response to changes in illumination)
• termination of the response

Question 8

cross section through peripheral retina

Answer 8

• pigment epithelium
• photoreceptors (big gaps in sampling array, allows convergence to increase pixel size)
• interneurons
• retinal tissue (image blurs as it passes through)
• ganglion cells

Question 9

Central vision

Answer 9

• fovea centralis
• central retina
• optic nerve head
• retinal blood vessels
• the fovea is specialised for high resolution:
• good focus - overlying laters are absent
• only cone photoreceptors, primarily red and green
• which are narrow and closely packed
• the signals from the photoreceptors are kept separate throughout the primary visual pathway

Question 10

Peripheral vision

Answer 10

• majority of the retina serves only coarse vision
• the visual image is optically blurred
• the cone photoreceptors are large and widely spaced (separately by larger number of rods)
• the signals from many cones converge onto single ganglion cells

Question 11

Primary visual pathway 2

Answer 11

• image is inverted by the optics: left side of the image to right side of both retinae
• image is mapped on to LGN and cortex
• axons from nasal retina swap sides: right side of both retinae to right side of brain
• with expanded central region
• axons from a ‘retinotopic map’ in LGN and cortex with the maps for the 2 eyes in register

Question 12

Visual receptive fields

Answer 12

• photoreceptors report changes in illumination from one moment to another
• retinal ganglion cells report changes in illumination from one location to another

Question 13

Ganglion cells response to brightness

Answer 13

• half of all retinal ganglion cells respond to increases in brightness
• ‘off’ centre - central photoreceptor depolarised (red) by decreased illumination, bipolar and ganglion cells depolarised by excitatory synapses
• ‘on’ centre - central photoreceptor hyperpolarised (blue) by increased illumination, bipolar cell depolarised by inverting synapse, excites ganglion cell

Question 14

Retinal ganglion cells classes

Answer 14

• retinal ganglion cells can be divided into different classes
• two ganglion cells from the same location in the retina
• parvocellular:
  • small field with strong surround
  • fine resolution
  • accurately follows changes in light
  • needs stable image
• magnocellular:
  • large field with weak surround
  • coarse resolution
  • transient responses to change
  • responds well to fast movement

Question 15

Wavelength selection of retinal ganglion cells

Answer 15

• Some retinal ganglion cells are wavelength selective
• parvocellular:
  • selective inputs from “red” or
  “green” photoreceptors
  • by comparing these responses
  they can encode wavelength
  • RED vs GREEN
• bistratified:
  • selective inputs from “blue” or
  “red+green” photoreceptors
  • by comparing these responses
  they can encode wavelength
  • BLUE vs YELLOW

Question 16

Different cells in the eye

Answer 16

• lateral geniculate cells are pretty faithful relay cells - their receptive fields look like those of their retinal inputs
• primary visual cortical cells have quite different properties
• higher visual cortisol areas have different roles and send it to different parts of the brain
• there is a cortical area that processes colour
• inferotemporal visual areas encode information about object identity