2.0 Visual System Flashcards Preview

MedST IB: Neurobiology and Human Behaviour (NHB) > 2.0 Visual System > Flashcards

Flashcards in 2.0 Visual System Deck (117)
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1

What is the wavelength of visible light?

390nm → 700nm
(violet) → (far red)

2

What are the 4 variables that the visual system can analyse?

1) Intensity
2) Wavelength
3) Space
4) Time

3

Illuminance vs Luminance

Illuminance = Quantification of light from light source (lux)

Luminance = Quantification of light reflected from objects (cd/m²)

4

Define reflectance:

A property of a material that dictates the proportion of reflected light from an illuminating source

5

Define constrast

Contrast is defined as the ratio of incremental light intensity to the mean background intensity (∆I/I)

6

What is pointspread function?

Pointspread function = blurred image of a point source on the retina

Diameter of pointspread function = wavelength of light/diameter of aperture

7

4 factors which may limit the optical quality/visual resolution:

1) Diffraction
2) Chromatic aberration
3) Spherical aberration
4) Glare

8

Define diffraction

Spreading of waves as they pass through an apperture.
Diffraction causes pointspread function

9

Define Spherical aberration

In a spherical surface, rays towards the edge are refracted more

10

Define Chromatic aberration

Different colours are refracted by different amounts (thus focused at different depths)
Human eye = well focused for green. Poor focus for blue

11

Define Glare

Small particles in optical media scatter light in all directions → ↓ contrast of image

12

Define Emmetropic

Object at infinity is sharply focused

13

Define Ametropic

Inability to sharply focus at an object at distance infinity

14

Define Myopia

Short sight (inability to focus on distant objects)
Light is focused at a point before the retina → ↑ pointspread function

15

Define Hypermyopia

Long sighted. Inability to focus on nearby objects

16

Define Presbyopia

A progressive ↓ in the ability of the eye to focus on near objects

Caused by ↓ accomodation because of ↓ lens elasticity

17

Define Grain (receptor spacing)

To see two points as separate points, receptor spacing needs to be at least 1/2 the width of the pointspread function

(this is usually achieved in the fovea)

18

What is the receptor spacing in fovea?

0.5 arc mins apart

19

What is cataracts?

Clouding of the lens

20

What is the power of a lens?

The strength of the lens

Expressed in dioptres

21

How are the lens and cornea supplied with metabolites?

They are avascular. Supply is via the aqueous humour

(secreted by ciliary body and drains in the canal of schlemm + trabecular meshwork)

22

What is glaucoma?

A reduction in the outflow of aqueous humour → ↑ intraocular pressure

23

What is accommodation?

Lens changes its focal length to focus on objects at different distances

Due to combination of:
1) Radial elastic ligaments (suspensory ligaments)
2) Circular ciliary muscle

Near reflex accomodation requires:
1) Constriction of pupils
2) Convergence of the two eyes (to fixate on new target)

24

What are the two muscles that control pupillary size?
What is their innervation?

1) Sphincter pupillae (PSNS)

2) Dilator pupillae (SNS)

25

What is the control pathway from the retinal to the pupillary sphincter (light reflex)?

Retina → Pretectum (midbrain) → Bilateral Edinger-Westphal nucleus → CN III → Ciliary ganglion → Pupillary sphincter

26

What is Argyll-Roberston pupil?

Characteristic of neurosyphilis
Pupil does not react to light but does react to accomodation

27

What are Muller cells?

Retinal glial cells that act as optical wave guides.

28

How wide is the fovea?

1.5mm
5 degrees

29

Features of foveola:

Middle (260um/1 degree) of fovea
Avascular
No rods
Centre of foveola has a cone spacing of 0.5min arc

30

Features of parafoveal region:

20 degrees either side of fovea
Peak rod density
Most sensitive to mesopic + scoptic vision