Vision

Question 1

What is the electromagnetic spectrum visible to humans, and in which way are the frequencies different to soundwaves?

Answer 1

The electromagnetic spectrum visible to humans is 400-700nm.

400 has shorter waves and 700 has longer waves, which is opposite to soundwaves; higher numbers correlate with lower frequencies (and longer waves).

Question 2

When white electromagnetic (light) waves hit a prism, they’re bent.

What is the difference in optical density of the prism which causes this, and how are short and long waves bent?

Answer 2

White light is made of many different waves, and when it hits a prism, the prism has a higher optical density than the air and the waves are bent.

Short waves get the most bent and long waves the least.

Question 3

What are the simplifying assumptions of geometrical optics, based on light being rays that move in straight lines?

Answer 3

1. Light waves travel in straight lines unless they make contact with a different medium
2. When they encounter a different medium they bend or split in two
3. If the refractive index changes in a medium, light waves follow a curved path.
4. When light waves strike objects, they may be absorbed, reflected or transmitted.

Question 4

What’s the difference between specular reflection and diffuse reflection?

Give an example of the outcome of a diffuse reflection in regards to our perception.

Answer 4

Specular reflection is a mirror-like reflection off opaque objects, the light waves bounce in one direction.

Diffuse reflection happens when light waves bounce off a rough surface, they’re split into separate waves which all bounce in different directions.

We perceive the colour of fabrics as a result of diffuse reflectance.

Question 5

What is the job of the choroid in the eye?

Answer 5

The choroid nourishes photo receptors

Question 6

How did scotopic and photopic vision prove the duplicity theory(1896) true?

Answer 6

The duplicity theory states that we need 2 different types of photosensitive receptors in order to operate in different luminance regimes. Hence we have scotopic vision (low-light, rod dominated) and photopic vision (high light levels, con dominated)

Question 7

There are 20 times more rods than cones, and only 1 million retinal ganglion cells.

How do different degrees of convergence facilitate sensitivity of rods over cones, and what is the cost of this system to signals from rods?

Answer 7

When more rods converge onto a single ganglion, they provide a greater chance of detecting light but the resolution is poorer due to the high level of compression, which takes the form of redundancy reduction.

Question 8

What is redundancy reduction in convergence, and at what level of processing does this happen?

Answer 8

Redundancy reduction means getting rid of unnecessary signals. This happens before these signals reach the brain, at the level of the ganglions after convergence.

Question 9

Why is there a higher probability of a photon being absorbed by a rod than a cone?

Answer 9

Rods are bigger than cones, the extra length increases the chance of a photon making contact with them.

Question 10

Give 2 reasons that rods produce lower resolution signals than cones

Answer 10

1. Because so many rods converge on one retinal ganglion cell, the exact location of the stimulus is lost.
2. Redundancy reduction means that not every signal makes it to the brain - there will be missing information which the brain has to imply. I guess that redundancy reduction is not a perfect process!

Question 11

What is the process happening during a dark adaptation curve, and how long does it take to adapt to the lower light?

Answer 11

After being in bright light, all my photoreceptors are desensitised. If I then enter a very dark room, the adaptation process occurs as rods and cones gradually become more sensitised.

Cones reach full sensitivity quite quickly (after about 5 minutes) but rods continue adapting for up to 20 minutes.

Question 12

How much of the electromagnetic spectrum, ranging from wavelengths of about 10-6 to 10-7metre, is visible to humans?

Use a proportion of the total spectrum to answer.

Answer 12

Only about 1/70th of the electromagnetic spectrum is visible to humans

Question 13

When a surface appears black or white to the human eye, what is the interaction of that surface with the incident light?

Answer 13

A surface is pure black because it absorbs all the incident light on the surface; nothing is reflected.
(In reality a good black absorbs most and reflects about 5%)

A surface that is pure white is the opposite; all the visible wavelengths in the incident light are reflected.
(In reality a good white reflects about 90%)

Question 14

Which wavelengths (S,M,L) are reflected and absorbed from surfaces for them to appear red, blue or green?

Answer 14

Surfaces which appear red are reflecting all the long wavelengths and absorbing the short and medium. Blue reflects all the short and absorbs the long and medium and green reflects all the medium.

Question 15

Discounting the illuminant is another term for

Answer 15

Discounting the illuminant is another term for colour constancy

Question 16

What are metamers?

Answer 16

Metamers are combinations of different WLs that will stimulate the same pattern of behaviour in a neurons receptive curve.

eg After calculating weighted sums for 2 different metamers, both will have the same response to these different patterns of WLs.

Question 17

Define incident light

Answer 17

Incident light is the final light amount returned to the eye. This consists of whatever wavelengths made contact with a particular object and, depending on that objects absorbency, the net luminance which is reflected.

Our retina cannot measure which amount came from the original light source and which amount was reflected.

Question 18

What is the difference between the reflectance of an achromatic object compared to a coloured object?

Answer 18

An achromatic object reflects all light waves equally whereas a coloured object reflects all light waves in different ratios

Question 19

Why do we have less blue cones?

Describe chromatic aberration, thinking about the different reflectance properties of different wavelengths (S,M,L).

Answer 19

When light hits the eye, it’s refracted in the same way as when it hits a prism; the wave lengths are bent depending on their length. Short waves (blue) are bent the most (up to 2.5 dioptres), whereas long waves (red) are bent the least.

This bending causes a diffuse, or out of focus effect, meaning that the blue signal is spread out further, therefore we don’t need as many to achieve the same result. This is chromatic aberration.

Question 20

Colour vision percepts can be predicted using weighted sums for photon absorption within which parts of the visual system hierarchy?

Answer 20

Weighted sums can be used to calculate photon absorption for early visual areas until V1