PB - vision: information processing

Question 1

What is the function of horizontal cells?

Answer 1

Lateral inhibition and contrast enhancement

Question 2

What protein is present in cones?

Answer 2

opsonin

Question 3

What protein is present in rods?

Answer 3

Rhodopsin

Question 4

What is the mechanism of phototransduction?

Answer 4

In the dark, cis-retinal is bound to opsonin, which inactivates it.
A light stimulus causes cis-retinal -> trans-retinal. This no longer fits in the receptor on opsonin. The receptor is activated.
Transducin dissociates into a and bg subunits.
Activates phosphodiesterase.
Hydrolysis of cGMP.
Closure of Na+ channels.
Hyperpolarisation.
Less transmitter is released (glutamate)

Question 5

Where is trans-retinal converted back to cis-retinal?

Answer 5

Pigmented epithelial cells, not the photoreceptors

Question 6

What type of vision is scotopic vision?

Answer 6

Rod-mediated low-light vision

Question 7

How are rods sensitive to low light?

Answer 7

Basal conversion of cis-retinal -> trans-retinal.
Dim light needs to overcome this to be detected.
This is enhanced by convergence of rods by bipolar and amacrine cells, onto the retinal ganglion cells.

Question 8

What is the function of the tapetum lucidum in nocturnal animals?

Answer 8

Reflective layer in the choroid.
Increases the chance of photons being captured by photoreceptors.
This is at the cost of visual acuity.

Question 9

What is the function of the dark melanin pigment in the choroid in day animals?

Answer 9

Absorbs stray photons that have not been captured by photoreceptors.
This prevents scattering at the expense of acuity.

Question 10

What is the function of scotopic vision?

Answer 10

Cones -> bright light vision.
Less easily saturated and not as much amplification.
Higher in the fovea, which is the site of highest visual acuity.
Little convergence.

Enables greater spatial acuity and higher temporal acuity to rapidly changing stimuli and movement.

Question 11

What is mesopic vision?

Answer 11

Combination between cone threshold and rod saturation.
Lateral inhibition between the cones by the horizontal cells.
Rod signal is fed into the horizontal cell network.
This combination enhances the contrast and spatial acuity.

Question 12

How can animals see colour?

Answer 12

Each cone has 1 type of opsonin. This absorbs light at a particular frequency.
Most mammals are insensitive to red wavelengths.

Question 13

What is the process of light adaptation?

Answer 13

cGMP channels in the outer segment allows for the influx of Na+ and Ca2+.
Ca2+ inhibits guanyl cyclase activity.

Light causes hyperpolarisation and so a decreased influx of Ca2+. This relieves the inhibition of guanyl cyclase activity.
So increases cGMP produced and this opposes the effect of phosphodiesterase. Thus reducing the hyperpolarising response.

Sensitivity is being controlled by level of guanyl cyclase, which in turn are being controlled by absolute light levels.

Question 14

What is the overall function of the light adaptation?

Answer 14

Shifts the sensitivity of photoreceptors as light levels change throughout the day. As well as the pupillary reflex.

Prevents saturation of the photoreceptor response.

Question 15

What is the process of dark adaptation?

Answer 15

Not as rapid.
Change in sensitivity is first determined by the cones, then a break, and then the threshold for the rods change.

Bright light causes cis-retinal -> trans-retinal.
Trans-retinal dissociates from opsonin, so is no longer sensitive to light.
Takes time for trans-retinal -> cis-retinal.

Dark adaptation threshold for rods and cones reflects the recovery in the proportion of photopigment cis-retinal.

Question 16

Up to ganglion cells, how is the signal modified?

Answer 16

Short distance.
Information is conveyed via graded changes in the membrane potential, without action potentials.
Converted to an AP if information needs to travel a long distance.

Question 17

What is the role of glutamate in light detection?

Answer 17

Off-cone bipolar cells - respond when the light is off. Excited by glutamate and responds to light by hyperpolarisation.

On-cone bipolar cells - respond when the light if on.
Inhibited by glutamate and responds by depolarisation.

Allows the visual system to see small spots of light and dark.

Question 18

What is the function of lateral inhibition?

Answer 18

Processes information as a pattern of activity across the sensory surface of neural map in the brain.
Increases contrast by enhancing spatial differences in photoreceptor activity across the retina.
Concentrates information by enhancing edges in the retinal output.

Question 19

How does convergence allow for an increase in sensitivity of scotopic vision?

Answer 19

Horizontal cell network is bypassed, so there is no lateral inhibition to reduce sensitivity.

Question 20

What is the receptive field of on-centre bipolar cells?

Answer 20

Central region: responds to light by increasing the firing rate.
Periphery: responds to light by decreasing the firing rate.

Question 21

What is the receptive field of off-centre bipolar cells?

Answer 21

Central region: responds to light by decreasing the firing rate.
Periphery: responds to light by increasing the firing rate

Question 22

What do the differing receptive fields of on-centre and off-centre bipolar cells allow for?

Answer 22

Edge detection

Question 23

What is the role of simple cells?

Answer 23

Cortical feature extraction.
Receives input from retinal ganglion cells and the lateral geniculate nucleus.
Simple cells have a receptive field that responds to a boundary of particular orientation (rectangular).

Question 24

Where do 80% of the projections of the retinal ganglion cells go to?

Answer 24

Lateral geniculate nucleus

Question 25

Where do the other 20% of the projections of the retinal ganglion cells go to?

Answer 25

Superior colliculus of the midbrain.
Pretectal structures of the midbrain.
Hypothalamus.

Question 26

What is the reason for projections to the superior colliculus of the midbrain?

Answer 26

Maps the visual field with the auditory field.
Integration with other sensory inputs.
Saccadic eye movements move the point of fixation.
Tectal projections allows mammals to respond to sudden changes in the environment.

Question 27

What is the reason for projections to the pretectal structures of the midbrain?

Answer 27

Bilateral input to the Erdinger-Westphal nucleus provides parasympathetic input to the pupil.

PS = pupil constriction

Question 28

What is the reason for the projections to the hypothalamus?

Answer 28

Synchronises intrinsic rhythm to the photoperiod of the day. Day length affects breeding cycle.