Chapter 3: The Eye and the Retina

Question 1

Based on the perceptual process, what are the first four steps in visual processing?

Answer 1

1. A distal stimulus occurs
2. Light is reflected off the distal stimuli to create an image on the retina
3. Receptors in the retina (photoreceptors) transform the light into electrical signals
4. Neural Processing: signals travel through the various areas of the brain associated with visual processing.

Question 2

How do we describe light?

Answer 2

Light is described by it’s wavelength.
The smallest known wavelength is gamma rays. (10^-12, one ten billionth of a metre)
The largest known wavelength is radio waves (10^4, 10,000 metres)
Visible light is the light human’s can perceive, it ranges in wavelength from 400-700 nanometres.

Question 3

How does light get from outside the body to the retina?

Answer 3

Light enters the eye through the pupil, and is focused by the cornea and lens to form a sharp image on the retina.

Question 4

How is light focused onto the retina?

Answer 4

The Cornea focuses light first, it does 80% of the focusing but is fixed, so it can’t make adjustments.

The lens does the remaining focusing; cilliary muscles attached to the lens contract and expand the lens to change it’s shape so that it can adjust how the light is bent. This process is called accommodation and ensures that the image on the retina is sharp not blurry.

Question 5

What are the different photoreceptors and how are they distributed on the retina?

Answer 5

Rods and cones are the different photoreceptors, they contain visual pigments which are light sensitive chemicals that react to light and trigger electrical signals.

Cones are concentrated in the fovea (the centre of your visual field. There are only cones here.

Rods are concentrated in the peripheral retina. (the area of the retina besides the fovea. There are also cones in the peripheral retina but rods outnumber them significantly.

Overall: there are ~12 million rods in the retinal and ~6 million cones.

Question 6

What causes the blind spot?

Answer 6

The blind spot is where the optic nerve fibres leave the eye. There are no photoreceptors here which is why we are “blind” there. We don’t really notice our blind spot because the two eyes compensate for each other.

Question 7

What is Macular degeneration and Retinitis Pigmentosa?

Answer 7

Macular Degeneration: a condition that destroys the fovea and surrounding area. Causes blindness in the centre of the field of vision.

Retinitis Pigmentosa: a condition that destroys the peripheral rods first, moving inwards in the field of vision over time. Causes tunnel vision.

Question 8

What are 4 different types of refractive errors?

Answer 8

Refractive Errors: errors in accommodation that result in unclear focusing.

1. Presbyopia: the lens hardens and cilliary muscle weaken so that close up objects cannot be brought into focus.
2. Refractive Myopia: Causes near-sighted-ness. The cornea bends light too much so the light is brought into focus before it reaches the retina.
3. Axial Myopia: Causes near-sighted-ness. The eyeball is too long so the light is brought into focus before it can reach the retina.
4. Hyperopia: Far-sighted-ness, the focal point for the light is beyond the retina making far off objects blurry.

Question 9

How do visual pigments create electrical signals?

Answer 9

Visual pigments have two parts:

1. Opsin: a long protein
2. Retinal: a small light sensitive component attached to the opsin.

When light hits the retina, the visual pigment absorbs it which triggers the retinal to change shape (from bent to straight) until it detaches. Isomerization is when the change in retinal shape triggers thousands of charged molecules to create an electrical signal in the receptor.

Question 10

What is Dark Adaption?

Answer 10

Dark adaption is the process of increasing light sensitivity in dark environments. We measure dark adaption using a dark adaption curve.

Dark sensitivity increases rapidly in the first 3-4 minutes, levels off until 7-10 minutes where it increases again until 20-30 minutes. The first increase in sensitivity is caused by cones, the second by rods

Question 11

What is visual pigment bleaching and visual pigment regeneration. How do these processes work together?

Answer 11

Visual pigment bleaching: when the retinal separates from the opsin due to light exposure. When the pigment is bleached, it is no longer useful for vision.

Visual pigment regeneration: when the retinal reattaches and returns to it’s bent shape on the opsin.

In normal light conditions, our photoreceptors are in constant, varying cycles of bleaching and regeneration. They occur at varying times so that we are not blind during the period of regeneration.

Question 12

How long do photoreceptors take to regenerate. What does this have to do with dark adaption?

Answer 12

Cones take ~6 minutes to regenerate, this is why we experience an initial increase in sensitivity during dark adaption.

Rods take ~30 minutes to regenerate, this is why we have a period where are sensitivity stagnates and then increases again during dark adaption.

Question 13

What is a detached retina?

Answer 13

A detached retina is when the retina detaches from the pigment epithelium, a layer of cells that contain enzymes necessary for pigment regeneration.

Question 14

What is the difference in wavelength sensitivity between rods and cones. How does this affect colour vision during dark adaption?

Answer 14

Rods are more sensitive to short wave light with peak sensitivity at 500nm (blue/green)

Cones are more sensitive to longer waves with peak sensitivity to light at 560nm (yellow/red)

Because we use our rods more than cones during dark adaption, we are more sensitive to short wave light (blue/green) than we are to long wave length (yellow/red). This shift to enhance short wave length perception is called the Purkinje Shift

Question 15

What is the absorption spectrum?

Answer 15

The absorption spectrum is a plot of the amount of light absorbed versus the wavelength of the light. There are 3 spectra for cones and one for rods.

Rod Pigment (R) absorb best at 500nm (blue green)
Shortwave Pigment (S) absorbs best at 419nm (blue)
Medium wave pigment (M) absorbs best at 531nm (green/yellow)
Long wave pigment (L) absorbs best at 558nm (yellow)

Question 16

What are the different cells that make up the retina and how are they connected?

Answer 16

Photoreceptor cells (rods and cones) connect to bipolar cells which connect to Ganglion cells (whose axons form the optic nerve).

Horizontal cells transmit information between receptor cells.
Amacrine cells transmit information between bipolar and ganglion cells.

Question 17

What is neural convergence and what does it look like with different types of photoreceptors?

Answer 17

Neural convergence is when multiple cells synapse onto a single neuron.

There is a lot of convergence in the retina; 126 million receptors converge onto 1 million ganglion cells.

Rods and cones converge differently. There is MORE convergence for Rods. Cones in the fovea often have a 1 to 1 connection with ganglion cells.

Question 18

What effects do the different rates of convergence have on perception?

Answer 18

Rods: Increased convergence causes rods to be more sensitive than cones. Multiple rods are stimulating single ganglion cells so their signals combine. This means that each rod can produce a smaller signal and still stimulate the ganglion cell.

Cones: Decreased convergence means that cones have better acuity than rods (better ability to see detail). Because there is less convergence, a ganglion cell firing from a cone cell tells the brain MORE information about the specific location of that light and it’s strength.

Question 19

What is a receptive field? What are it’s characteristics?

Answer 19

A receptive field is a region of the retina that must receive illumination in order to stimulate a specific ganglion cell. Every ganglion cell has a receptive field and their is often overlap between ganglion cell’s receptive fields.

Receptive fields are organized in centre surround fields. This means that the centre of the receptive field will respond to light differently (either excitatory or inhibitory than the surrounding area.

Question 20

What is Centre surround antagonism?

Answer 20

It is the idea that, because of the centre-surround organization of the receptive field that increase light coverage in the receptive field only increases the stimulation of the ganglion cell up to the point where it crosses into the surrounding area. (if the cell is on-centre, off-surround).

Essentially:

• The highest response occurs when only the excitatory area is stimulated
• The lowest response occurs when only the inhibitory area is stimulated
• an Intermediate response occurs when both areas are stimulated.

Question 21

What is lateral inhibition?

Answer 21

Lateral inhibition is when horizontal and amacrine cells transmit inhibitory signals laterally across the retina.

Photoreceptors receive stimulation and send signals to bipolar cells. Bipolar cells inhibit signals and send reduced messages to the brain through horizontal cell communication.
When a cell receives more stimulation, it increases the amount of inhibition it directs to it’s neighbouring cells.

What our brain gets (the output) is the photoreceptors initial response MINUS the inhibition signal)

Question 22

What determines our sensitivity to light?

Answer 22

Two things:

• The concentration of photopigments (how many photoreceptors are there (and unbleached)
• The speed of photopigment regeneration (how quickly can a photopigment move from bleached to regenerated)

Question 23

Explain how the Hermann Grid works in the context of lateral inhibition.

Answer 23

In the Hermann Grid, the centres of the cross points of the white sections look darker than the other areas despite being the exact same colour. Why? Because those points are surrounded by more white (bright) areas than any other white areas so there is more inhibition.

For example an area at the cross point on the white is surrounded by white (bright on all four sides). If we assume an inhibition of 10% and that every white point on has a stimulation of 500 than the four points around the centre one would send -50 each to the centre out put that means that the centre output would be: (500 - 50*4) –> (500 - 200 = 300).

Meanwhile if we look at a point in the middle of two black squares we would have a white point with two white areas around it and two black areas. If we again assume an inhibition of 10% and say that every white has a stimulation of 500 and every black point has a stimulation of 200 than the white points would send -50 to the centre but the black points would send -20. That means the centre output would be (500 - 502 - 202) –> (500 - 100 - 40) –> (500-140 = 360).

360 is higher than 300 which is why the area surrounded by more white appears darker than the area surrounded by a less white.

Question 24

What are Mach Bands? How do they work in the context of lateral inhibition?

Answer 24

Mach bands is the illusion that, at the border between a darker area and a light area, the area right on the edge of the lighter area will appear brighter than all the other light areas and the area right on the edge of the dark area will appear darker than all the other dark areas.

Why?
On the shadow side (low intensity) there is additional inhibition –> because it is also being inhibited by the light area This means that the end output of this area is lower than anywhere else in the dark area.

On the light side (high intensity) there is less inhibition –> because it is getting some inhibition from the dark side. This means that the end output of this area is higher than anywhere else in the light area.

Question 25

What is simultaneous contrast?

Answer 25

The illusion of changed brightness in colour due to being surrounded by a different level of brightness (e.g. a grey square on a white background will appear darker than the same grey square on a black background). Why?

Receptors stimulated by bright surrounding areas send more inhibition to the centre than receptors stimulated by dark surrounding areas.