Week 4 Light & Photoreceptors

Question 1

What are differences between the optic disk and fovea?

Answer 1

Optic disk/blind spot = optic nerve, where out is located, where rods and cones are not produced, causing blindspot

Fovea = central focus of eye and contains only cones, greatest spatial acuity (fine detail), and greatest density of photoreceptors (R, G, B)

Question 2

Differences between rods and cones?

Answer 2

Rods are more sensitive, and take longer to function - operate in scotopic and mesopic light levels

Cones are quick to function, much less sensitive and operate in photopic and mesopic light levels

Question 3

What is eccentricity in vision?

Answer 3

The angle from line of sight (fovea / direction of fixation) and how far the image produced lies from the fovea

Question 4

How many metres away is optical infinity and what does it mean?

Answer 4

Optical infinity occurs at 6m away
when the rays of light from that object are coming in parallel

Implication is that there is no different in spatial acuity at 7m or 20m because rays of light are coming in at same parallel angle

Question 5

What is automatic accommodation?

Answer 5

As the object moves further away from the eye, the focused image moves closer to the lens, therefore to view at different distances the eye must engage in a constant automatic process of shifting the focused image

Question 6

What does accommodation use in vision to adapt to different viewing differences?

Answer 6

The focal length is the distance from the eyes to the focused image, which uses refractive power to bend the light of the image

Question 7

How do eyes accommodate different viewing distances at short and long distances?

Answer 7

Short viewing distances = optics need to increase the refractive power, shape of lens to make fatter/circular

Near point = closest distance to accommodate something

Longer viewing distance = optic needs to reduce the refractive power gets flatter

Question 8

What 2 things needs to happen to the refractive power to be developed and occur?

Answer 8

1. Change the shape of the lens to change refractive power (accommodation process, highly automatic) - mostly done by the cornea and the lens
2. Change the length of the eyeball - from birth the eye grows into the refractive power in a critical window

Question 9

What is refractive power?

Answer 9

The ability of the eye is the ability to bend light
Occurs when light goes from one optical media to another, air to lens - light gets bended
As the object moves further away from the lens, the rays of light come in at a more shallow angle

Question 10

Why does environment induced myopia occurs highly in Singapore?

Answer 10

Thought to occur in highly urbanised areas with high educational levels, as children exposed to predominantly shorter viewing lengths from books/ipads/TV/phones at a young age disproportionately excludes them from being exposed to more natural longer viewing distances, eg. outside seeing the horizon

Not getting a full spectrum of sunlight from being inside too much is also thought to contribute

Question 11

Why is spatial acuity worse at night?

Answer 11

Since there are no rods in the fovea, there is a lowered spatial acuity when only the rods are operating in scotopic conditions with low levels of light.

Question 12

Why is spatial acuity worse in peripheral vision?

Answer 12

In periphery, the ratio of cones to RG is hundreds to 1, so several hundred cones are feeding into 1 RG cell - which increases the receptive field

A bigger RF lowers the visual acuity
So further away from the fovea results in decreased visual acuity

1. We need to make eye movements in the periphery of our vision

Question 13

What’s the best way to view very dim objects at night?

Answer 13

To look slightly away so image falls off the fovea where the rods are located so you can resolve an image

Question 14

How long do rods and cones take to become most sensitive?

Answer 14

Cones take 10 minutes to change sensitivity from one extreme to the other
Rods take 30-40 minutes to change sensitivity from one extreme to the other

Question 15

How does vision sensitivity change with light and dark light levels?

Answer 15

1. Dark adaption = as darkness increases, vision becomes more sensitive (gain increased)
2. Light adaptation = As light increases, vision becomes less sensitive (gain is decreased)

Question 16

Why do we perceive blinding light after being in dark light levels?

Answer 16

1. We perceive a burst of blinding light due to the long inhibition of excitatory ON-centre cells due to a phenomenon known as release from inhibition and a reduction in sensitivity
2. After dark to light conditions, all of the cones activate at maximum brightness

Question 17

In dark conditions, Why does sensitivity increase more for the big amount of light?

Answer 17

Because when the light is a small dot, it is only enough to only be centred on the fovea, which only uses cones,

while with a large dot of light, both cones and rods are used because light hits both fovea and other retinal areas

Question 18

What’s the best colour to use in night vision to read a map? (RED!)

Answer 18

1. Wait 40 minutes so rods adapt to their greatest sensitivity, which are highest after 40 minutes, so do not expose yourself to light, which would degrade the rod sensitivity
2. You need the light to activate the cones to read the map but also need to maintain dark adaptation.
3. Red light (650 nm+) because it is outside of the sensitivity range of the rods, so the rods will be able to maintain their dark adaptation

Question 19

What colour to save battery torch power?

Answer 19

Green at 550 nm uses the least amount of power where colours are most sensitive.

Question 20

What are the 3 ways that the visual system maintains sensitivity to different luminances?

Answer 20

1. Dilating the pupil
2. Rods and cones - photoreceptors
3. Light and dark adaption (to the environment)

Question 21

How do we save visual load and what do we focus on instead?

Answer 21

1. We save on visual load through Troxler fading (of retinal cells and environment, fading percept of non-moving “dormant” stimuli
2. We focus on changes in luminance and colour in contours of shapes - because encoding ‘red-white’ contour of a shape is more efficient than encoding a point-by-point representation of the percept

Question 22

Where do tuning properties in the brain occur?

Answer 22

1. In the eye retinal ganglion (RG)
2. In the subcortical level - lateral geniculate nucleus (LGN)
3. In the Primary Visual Cortex (V1)

Question 23

What are 3 qualities of simple ON and OFF cells?

Answer 23

1. On Centre Positive regions in the middle are excitatory cells (increasing firing rate of a cell)
2. Off Centre negative regions that are inhibitory regions
3. Discharge level - Regardless of light entering the eye, they are firing away at a baseline firing rate, quite low

Question 24

What is the ideal stimulus for an on-center and off-center cell?

Answer 24

On-centre - more light to be falling in the centre than surround (bright dot of light)

Off-centre cells - more light to be falling in the surround than center

Question 25

Why don’t we have a single system where cells could detect both light increments and decrements?

Answer 25

Too much metabolic costs!

1. More conscious work adjusting to light constantly
2. Baseline discharge level would have to be 50% of maximum firing rate to detect excitatory and inhibitory
3. So the transition between on and off cells would take time
4. Decreased luminance levels = If the dynamic range of cell’s ability was decreased, our ability to resolve a wide range of luminance levels would be decreased

Question 26

How to explain the Hermann Hering illusion with ON and OFF cells?

Answer 26

Look at much light is on the excitatory region at the lines and the interactions of the grid.

The lines in the grid have more areas of excitation and will be perceived as brighter than the intersections of the grid

The result is that the intersections of the grid will appear to have small illusory regions of darkness because they appear darker than the lines at a perceptual level

Question 27

What are simple cells in V1 tuned to?

Answer 27

Simple cells are tuned to orientation and width (not length) and luminence

This was detected through microelectrode in animals, recording response of neural spikes to different light and dark stimuli

Question 28

How can you map out cells orientation and tuning bandwidth? (What range of orientations is the cell tuned to?)

Answer 28

Single cell recording in monkeys!
You present someone with a stimulus shape and vary its orientation and measure a specific simple cells firing rate, this cell fires more regularly at vertical orientation, and goes down to baseline (not responding) to a horizontal orientation

Question 29

What are 3 things are complex cells tuned towards? d

Does it dependent whether they are in the receptive field?

Answer 29

Orientation, width and motion!

Not dependent on when a stimulus occurs in a RF

Question 30

What are simple cells and complex cells made up of?

Answer 30

1. Simple cells are driven from combined responses of LGN centre-surround cells
2. Complex cells are made from a combination of simple cells

Question 31

What cells are first sensitive to colour and when do they receive binocular input?

Answer 31

Colour selective centre surround type cells start in RG and LGN

Cells in cortex V1 are first to receive binocular input, left and right eye, but mostly overlap.