Vision - Physiology of retina and visual cortex.

Question 1

What are saccadic eye movements?

Answer 1

Rapid and jerky (3-5/s) as gaze moves from one object to another.

Question 2

In saccadic eye movements is our visual perception turned off or on?

Answer 2

Visual perception turned off.

Question 3

What is smooth pursuit eye movement?

Answer 3

Tracking moving objects.

Question 4

What is the sensitive part of the retina called?

Answer 4

The fovea.

Question 5

What are the 3 types of photoreceptors and how many of each are there?

Answer 5

Rods (1), cones (3), photoreceptive ganglion cells - the output cells of the retina (1).

Question 6

What do rods and cones contain stacks of? and how are these formed?

Answer 6

Disc membranes, formed by invagination of plasma membrane.

Question 7

Why is the optic disc is referred to as the blind spot?

Answer 7

Because it contains no receptors.

Question 8

Which photoreceptors are involved in night and which are involved in day vision? Which are involved in central and which in peripheral vision?

Answer 8

Night and peripheral = rod. Day and central = cone.

Fovea centralis has a very high cone density.

Question 9

What is the photopigment found in rods?

Answer 9

Rhodopsin.

Question 10

What is scotopic vision?

Answer 10

Rods being very sensitive to low level light.

Question 11

What do rods have none of?

Answer 11

No colour or vision acuity.

Question 12

Cones have 3 opsin photopigments, why?

Answer 12

Because each is sensitive to a different wavelength of light. Red, green and blue. High density in foveal region.

Question 13

When are cones responsive and rods maximised/saturated?

Answer 13

In high illumination levels.

Question 14

Loss of cone photopigment results in what?

Answer 14

Colour blindness = X-chromosome mutation.

Question 15

Transduction of light energy into chemical energy occurs where? and leads to what?

Answer 15

Photopigment molecular complex. Leads to subsequent activation of biochemical second messenger cascade which leads to hyperpolarisation in receptor membrane potential.

Question 16

In light what are the biochemical occurrences?

Answer 16

There is a decrease in levels of cGMP, this closes Na+ ion channels and shuts off the ‘dark current’, which hyperpolarises photoreceptor cell and alters the transmitter release .

Question 17

In the dark what biochemical changes take place?

Answer 17

cGMP levels increase and cGMP binds to the membrane Na+ channel protein, this allows dark current (carried by Na+) to flow into the cell (rod or cone).

Question 18

What are the 4 anatomical targets of central projection of retinal ganglion cell axons? Describe the function of each.

Answer 18

Geniculo-cortical pathway (dorsal lateral geniculate (LGNd) and visual cortex) - visual recognition, cells respond to specialised features of a stimulus.

Superior colliculus - visual localisation, cells respond to existence of of a stimulus at a certain place, play a role in eye and head movements.

Suprachiasmatic nucleus (SCN) in hypothalamus - biological clock, responds to light intensity and and duration.

Pretectal region - visual proprioception, cells respond to movement of the visual field as a whole, sends info to the vestibular system and cerebellum, involved in compensatory postural reactions.

Question 19

How many layers does the LGNd have?

Answer 19

6

Question 20

What layers are involved in the ipsilateral retinal input and which involved in the contralateral retinal input?

Answer 20

Ipsi - 2,3,5.

Contra - 1,4,6.

Question 21

What cells are found in layers one and two?

Answer 21

Magnocellular cells, 20% of population, code brightness and contrast.

Question 22

What cells are found in layers 3-6?

Answer 22

Parvocellular cells. 80% of population, code for colour contrast.

Question 23

Area 17 is known as what?

Answer 23

Brodmann’s area 17, primary visual cortex, V1. (V2-30 found in nearby areas 18 and 19).

Question 24

What is mapping?

Answer 24

When responses recorded with micro-electrodes positioned in visual pathways and RF properties studied by projecting stimuli onto a screen or computing system. Eg: V1 neurne selective for moving edge orientated at 270 degrees.

Question 25

Edge selective cells are orientated how in V1?

Answer 25

Neurons concerned with a specific orientation are organised into vertical ‘orientation’ columns.

Question 26

Many V1 cells receive binocular input, what does this help?

Answer 26

Providing info about 3-D shape of objects, stereopsis, object distance.

Question 27

Binocular-driven cells are organised how?

Answer 27

Into columns called ocular dominance columns, independant of orientation columns.

Question 28

What is stereoblindness?

Answer 28

Inability to see in 3D using stereo vision, resulting in an inability to perceive stereoscopic depth by combining and comparing images from the two eyes.

Question 29

What is retinal disparity?

Answer 29

Visual cortical binocular driven neurons computing the difference of image position in each eye.

Question 30

V1 transmits info to 2 primary pathways, what are they?

Answer 30

Ventral stream. V1 - V2 - V4 - V8 - inferior temporal lobe. What pathway. Associated with object recognition and long-term memory.
Dorsal stream. V1 - V2 - V3 - middle temporal area (V5) - inferior parietal lobe. How pathway. Associated with motion perception and representation of object location. controls eyes and arms - visual information used to guide saccades/reaching.

Question 31

Lesion in V5 leads to what?

Answer 31

Motion blindness, jittery movement.

Question 32

Lesions in visual area V8 leads to what?

Answer 32

Achromatopsia - see in shades of grey only.

Question 33

Parietal area - V7 processes what?

Answer 33

Spatial relationships of objects in visual space.

Question 34

Lesion in V7 leads to what?

Answer 34

Neglect syndrome.

Question 35

Area 21 - inferotemporal area is involved in what?

Answer 35

Facial recognition and interpretation.