The Visual System

Question 1

Feature detection

Answer 1

The brain has specific neurons or circuits of neurons specialised for detection of particular features of the sensory world

Question 2

Adaptation

Answer 2

Neural signals in features which remain constant are dampened down, the brain is mostly interested in changes in the environment

Question 3

What is the goal of the visual system?

Answer 3

Build a predictive model of the external world based on incident light

Question 4

_____ radiation is converted into ______ impulses

Answer 4

Electromagnetic, neural

Question 5

What controls the amount of light entering the eye?

Answer 5

The iris and the pupil

Question 6

Pupillary constriction/dilation

Answer 6

• The iris adjusts the size of the pupil

- This is controlled by how much light enters the eye

Question 7

Pupil in the dark

Answer 7

Iris muscle relaxes and pupil dilates

More light enters the eye so sensitivity is improved, but acuity is poorer

Question 8

Pupil in the light

Answer 8

Iris muscle contracts and pupil gets smaller

Less light enters but the image on retina is sharper and acuity is improved

Question 9

Why less acuity with greater light?

Answer 9

When more light enters, each and every point projects to a larger area on the retina, and these areas overlap, creating blurring

Question 10

Cornea

Answer 10

Transparent cover for the front of the eye
Helps to focus incoming light
Fixed focus

Question 11

Lens

Answer 11

Sits behind the pupil

Can change shape through the process of accommodation

Question 12

What holds the lens in place?

Answer 12

Suspensory ligaments called zonules

Question 13

Accommodation in close vision

Answer 13

• Ciliary muscle contracts
• Ciliary body moves closer to the lens
• Tension in zonula fibres is reduced and the lens rounds up
• Reduces the focal distance

Question 14

Accommodation in distant vision

Answer 14

• Ciliary muscle relaxes
• Ciliary body moves away from the lens
• Tension of zonula fibres increases and the lens flattens
• The focal length increases

Question 15

Retina

Answer 15

Thin, light sensitive tissue lining the back of the eye

Contains a layer of photoreceptive cells that convert light into neural signals

Question 16

Macula

Answer 16

Centre of the retina
Contains very high concentration of photoreceptors
Macular degeneration - loss of central vision with age

Question 17

Fovea

Answer 17

Centre of the macula

Site of our sharpest vision

Question 18

Why does red eye occur?

Answer 18

Flash of the camera reflects off the blood in the choroid and back through the pupil

Question 19

What is reflective eye?

Answer 19

Many nocturnal animals have a layer of reflective tissue, called tapetum lucidum, which reflects light back through the retina

Question 20

Sclera

Answer 20

Tough protective layer of connective tissue (white of the eye)

Question 21

Choroid

Answer 21

Layer of tissue between the retina and sclera

Contains many blood vessels and is critical for providing oxygen and glucose for retinal cells

Question 22

Why is the retina inverted?

Answer 22

Theory that it is a space saving solution, eyes would need to be larger to enable light to be focused on photoreceptors

Question 23

Photoreceptors

Answer 23

Convert light into neural signals via visual transduction

Question 24

Types of photoreceptors

Answer 24

Rods and cones

Question 25

Rods

Answer 25

• Scotopic (dim light) vision
• Operate in low lighting
• High convergence - many rods connected to one bipolar cell - poor acuity but high sensitivity
• One type so monochromatic
• Peripheral vision

Question 26

Cones

Answer 26

• Photopic vision (well-lit)
• Only operate in good lighting
• Low convergence - each cone connects to one bipolar cells - good acuity but poor sensitivity
• Three types (red, green, blue) so responsible for colour
• Central vision

Question 27

The vast majority of cones are crammed into the fovea, giving it high ______, _____ vision

Answer 27

Acuity, colour

Question 28

Rods in the dark

Answer 28

• Rhodopsin inactive
• Sodium channels are kept open by cyclic GMP
• Cell depolarised
• Rods continually release glutamate

Question 29

Rods in the light

Answer 29

• Rhodopsin active
• Cyclic GMP broken down and sodium channels close
• Cells hyperpolarised
• Glutamate release is reduced

Question 30

Bipolar cells

Answer 30

• Process input from photoreceptors and output to the retinal ganglion cells
• Allow some low-level signal processing to occur in the retina
• Photoreceptors converge via bipolar cells onto retinal ganglion cells

Question 31

Few-to-one conversion

Answer 31

Maintains excellent resolution (cones)

Question 32

Many-to-one

Answer 32

Maintains excellent sensitivity (rods)

Question 33

Retinal ganglion cells

Answer 33

Wired up to bipolar cells and photoreceptors

Facilitate the detection of the edges in images

Question 34

What cells are present in the fovea?

Answer 34

Only cones, no rods

Question 35

Where are the blood vessels in the fovea?

Answer 35

Displaced to the side along with other cells, minimising the distortion of the light hitting the photoreceptors

Question 36

Around ____ of the nerve fibres in the optic nerve are supplied by the fovea

Answer 36

50%

Question 37

Blind spot

Answer 37

Ganglion cell axons leave the retina in the optic nerve where there are no photoreceptors

Question 38

Completion

Answer 38

The brain fills in the gap where the blind spot is

Question 39

Why does the blind spot appear white?

Answer 39

Axons are covered in a myelin sheath, a white fatty substance

Question 40

Why does the eye move so much?

Answer 40

A brain that provides foveal vision at all locations would weigh 10 tonnes?
Therefore, the eye continually scans the visual field with the fovea
Visual system integrates this information to produce wide-angled, high acuity, coloured perception

Question 41

How many fixations does the eye make per second?

Answer 41

Three