Physiology of vision Flashcards

1
Q

Focus of the eye

A

Carried out by the lens

  • Changes its shape and power to focus an image
  • 1/3 of ray bending

Also by the cornea
- Does 2/3 of ray bending

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2
Q

Function of iris

A

Varies retinal intensity by varying its diameter

- 4x change in diameter= x16 change in retinal intensity

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3
Q

Pigment layer

A

Located behind the retina

Absorbs unwanted light.

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4
Q

Optic disk

A

The region where the optic nerve leaves the eyes.

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5
Q

Fovea

A

Small region in the retina that gives the clearest image
- Densely packed with cones

Appears as a small yellow spot on through an opthalmoscope.

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6
Q

Hypermetropia

  • Cause
  • Treatment
A

Long sightedness

  • Image is focused beyond retina
  • Eyeball too short
  • Lens too weak

Treatment
- Convex corrective lens [spectacle/ contact lens]

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7
Q

Myopia

A

Short sightedness

  • Image is focused before normal plane of focus
  • Eyeball too long
  • Lens too strong

Treatment
- Concave corrective lens

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8
Q

Measurement for refractive power of lens

A

Diopters

- Reciprocal in metres [e.g 2D = 0.5m]

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9
Q

Photoreceptors

A

Rods- dim light
- A lot more present

Cones- bright light and colour

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10
Q

Processing layers of retina

A

3 direct layers:

  • Receptors
  • Bipolars
  • Ganglion cells

2 transverse layers

  • Horizontal
  • Amacrine cells
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11
Q

Rhodopsin

A

G-protein couple receptor found in rods.
- Very sensitive to light, responsible for seeing dim light

Mechanism:

  • Photon from light hits the molecule—> changes from 11-cis to all-trans
  • Triggers intracellular events that hyperpolarise plasma membrane
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12
Q

Peak spectral sensitivities of human cones

  • Red
  • Green
  • Blue

Rods

A

Red cones [the most cones]
- 560nm

Green
- 530 nm

Blue [the least cones]
- 420 nm

Rods
- 500nm

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13
Q

Colour blindness

A

Loss in modification in one or more of the three cone visual pigments.

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14
Q

Reg/green colour blindness

A

X-linked condition

- Mutation in the genes for red and green cone visual pigment/

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15
Q

Blue colour blindess

A

Caused by mutation in the blue cone pigment gene
- On chromosome 7

Since the chromosome is paired, it is unlikely for individuals to have it, compared to red/green.

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16
Q

Central achromatopsia

A

Colour blindness caused by damage to the colour-processing area in the cortex.

17
Q

Scotoma

A

Blind spot in a normal field of vision

Causes:

  • Retinal damage
  • Lesions in visual cortex
  • Tumour restricting optic nerve/ chiasm/ tract/ radiation
18
Q

Homonymous hemianopia

A

Visual field loss of the left OR right side of the vertical midline.

Caused by lesion in the left or right optic tract.

19
Q

Bitemporal hemianopia

A

Visual field loss of the outer half in both right and left fields.

Caused by lesion at optic chiasm.

20
Q

Left-sided blindness

A

Loss of left vision field

- Lesion on left optic nerve

21
Q

Visual pathway

A
  1. Optic nerve
    - From retina
    - Divides in each eye into left and right halves.
  2. Optic chiasm
    - Left and right halves from each eye combines to form optic tract.
  3. Optic tract
    - Some of it goes to superior colliculus
    - Most of it is relayed in the lateral geniculate nucleus [thalamus]
  4. Lateral geniculate
    - Relays information to the striate cortex in occipital lobe
  5. Striate cortex
    - The halves of each combined visual field [from optic tract] is shown as a half.
  6. Information of image is passed to other areas
    - Foveal region is exaggerated
    - Other regions process depth, motion, colour etc
22
Q

Columnar organisation of visual cortex

A

Three overlapping patterns- columns

  1. Ocular dominance
  2. Smaller orientational
    - Orientation of optimal stimuli varies across the surface [L OR R]
  3. Colour blobs
    - Passed to other regions.

Hypercolumn
- One set of all columns

23
Q

Dorsal stream

A

Occipital —->Parietal cortex

Processes:

  • Location
  • Motion
  • Action
24
Q

Ventral stream

A

Occipital —> Temporal

Processes:

  • Object + face identity
  • Conscious perception
25
Visual agnosia
Condition where you can see but not recognise and interpret visual information Due to lesion in temporal lobe
26
Prosopagnosia
The inability to recognise similar faces Due to specific damage of the temporal - Mainly fusiform gyrus
27
Blindsight
Being able to respond to visual stimuli that you cannot consciously see - Due to lesion in striate/ primary visual cortex Mechanism - Some projections from the lateral geniculate nucleus and superior colliculus reach areas in cortex involved with movement perceptions [without passing through V1] - V1 processing only allows conscious processing
28
Vestibulo-ocular reflex
Stabilises the gaze whilst the head moves. - Produces eye movement in opposite direction to head movement. CN3 nucleus---> CN8 nucleus---> semi-circular canals
29
Optokinetic reflex
Stabilises the image of moving object on the retina Oculomotor nuclei----> optic tract nucleus
30
Pupillary reflex
Illumination of one eye causes constriction in both pupils Pretectal and Edinger Westphal nuclei receive signals from both eyes. Damage in one optic nerve - Eye will not constrict - Other eye pupil will Damage to oculomotor nerve - Pupil contraction will not occur when that eye is stimulated - Pupil contract [in damaged nerve eye] occurs when other eye is stimulated