Lecture 6, 7

Question 1

What are the bipolar receptive fields?

2 types?

Answer 1

Area of the retina that (when stimmed with light) changes the cells membrane potential

Receptive field centre- circular area of retina providing direct photoreceptor input to bipolar cell

Receptive field surround- surrounding area of retina providing indirect photoreceptor input to bipolar cell via horizontal cells

Slide 2 lecture 6

Question 2

How does a bipolar cell membrane respond in the receptive field compared to surround?

Answer 2

The response of a bipolar cells membrane potential to light in the receptive field centre is opposite to that of light

If light hits surround, it will cause opposite affect of light hitting center

Slide 3 lecture 6

Question 3

Study ON and OFF bipolar cells and light and dark spots in center slide 4-5 lecture 6

Answer 3

Ok

Question 4

How do centre-surround ganglion cell receptive fields work?

Answer 4

The response to stimulation of the centre is cancelled by the response to stimulation of the surround (for both ON centre and OFF centre ganglion cells)

Ganglion cells are sensitive to difference in illumination that occurs within their receptive fields and not to changes in illumination that include both the receptive field centre and surround

Slide 6 lecture 6
Slide 9 lecture 6

Question 5

What is the most effective way to maximize firing of an ON centre or OFF centre?

Answer 5

Best way to maximize firing of ON or OFF centre is to completely illuminate either the ON or OFF area of its receptive field

If both areas of a cells receptive field are illuminated together, there is little reaction from the cell

Slide 7 lecture 6

Question 6

What are negative after images?

Answer 6

When ganglion cells are excited or inhibited for a prolonged period of time, they later show a rebound effect, firing faster or slower than normal

The most important cause of a negative afterimages is adaptation in the rare of firing of retinal ganglion cells

Slide 8 lecture 6

Question 7

What response happens when a light-dark edge crosses an OFF centre ganglion cell receptive field?

Answer 7

Dark centre -> depolarize
Dark surround -> hyperpolarize

This organization leads to a neural response that emphasizes the contrast at light-dark edges

The opponent centre surround arrangement of receptive fields causes the enhancement of contrast

Slide 10-12 lecture 6

Question 8

What is the neural basis of enhancement of contrast?

Answer 8

Slide 11-12 lecture 6

Only ON cells are shown; OFF cells are responsible for the darker appearance of the left side of the darker square

Portion of the inhibitory surrounds of the ON cells near the border receives less illumination, thus these cells have the highest rate of firing

Question 9

What are P type retinal ganglion cells?

Answer 9

90% of population
Small receptive fields
Slow axon conduction velocities 
Sustained firing to the presentation of visual stimuli 
Can transmit information about colour 

Important for high spatial resolution vision
Detailed analysis of shape, size, and colour of objects

Slide 13 lecture 6

Question 10

What are M type retinal ganglion cells?

Answer 10

5% population 
Larger receptive fields
Slow axon conduction velocities
Sustained firing to the presentation of visual stimuli
Can transit info about colour 

High temporal resolution
Evaluating speed, location, and direction of a rapidly moving object

Slide 13 lecture 6

Question 11

What is the retinotopic representation of the visual field?

Answer 11

The 2 retinal foveas are normally aligned in a single target in visual space, causing the visual fields of both eyes to over lap extensively

This binocular field of view consists of 2 symmetrical visual hemifields (left and right)

Slide 1 lecture 7

Question 12

What is the left and right visual hemifield?

Answer 12

Left visual hemifield -> nasal retina left eye and temporal retinal right eye -> right optic tract

Right visual hemifield -> nasal retina right eye and temporal retina left eye -> left optic tract

Slide 2 lecture 6

Question 13

What is the pathway of 4 things that mediated conscious visual perception?

Answer 13

Central projections of retinal ganglion cells

1. Optic tract
2. Lateral geniculate nucleus of the thalamus
3. Optic radiation
4. Primary visual cortex

Slide 3 lecture 7

Question 14

What are the other nonthalamic targets of the optic tract?

Answer 14

Hypothalamic innervation important for synchronizing sleep and wake fullness, with daily dark-light cycle

10% of ganglion cells project to the superior colliculus involved in generating saccadic eye movements

Motor neurons in brain stem, orient the eyes in response to new stimuli in the visual periphery to maximize high-acuity foveal vision

Slide 4 lecture 7

Question 15

What is the pupillary light reflex pathway?

2 types of response?

Answer 15

The pupils of both eyes respond identically normally, regardless of which eye is stimulated
Stimulated eye = direct response
Unstimulated eye = consensual response

Pupil response to light is mediated by parasympathetic innervation of the iris

Question 16

How are the muscle in the iris controlled to adjust the pupils?

Answer 16

Retinal ganglion cells send their axons through optic tract to placate pretectal nucleus (junction of midbrain and thalamus)
Neurons in this nucleus project to parasympathetic preganglionic neurons in the Edinger-Westphal nucleus
The axons if they preganglionic cells exit with the oculomotor nerve and contact the ciliary ganglion cells, which control the pupilloconstrictor muscle in the iris

Slide 5 lecture 7

Question 17

What is an afferent pupillary defect?

Answer 17

Failure to elicit a response (either direct or indirect) to stimulation of the right eye of both eyes respond normally to stimulation of the left eye suggests damage to the sensory input from the right eye, possibly to the right retina or optic nerve

Slide 6 lecture 7

Question 18

What is an efferent pupillary defect?

Answer 18

A direct response in the left eye without a consensual response in the right eye suggests a problem with the visceral motor outflow to the right eye, possibly as a result of damage to the oculomotor nerve or Edinger-Westphal nucleus in the brain stem

Slide 6 lecture 7

Question 19

What are the 6 major cell layers of the lateral geniculate nucleus?

Answer 19

2 central magnocellular layers

4 dorsal parvocellular layers

P-type ganglion cells project exclusively to the parvocellular LGN
M-type ganglion cells project exclusively to the magnocellular LGN

slide 7 lecture 7

Question 20

How is the lateral geniculate nucleus organized?

Answer 20

Received input from both eyes from the contralateral visual field

Each layer receives projections exclusively from either the ipsilateral or contralteral retina so that individual geniculate neurons are monocular drive by either left or right eye but not both

Slide 8 lecture 7

Question 21

What is each layer of the lateral geniculate nucleus (1-6) and it’s ganglion cell type and which eye?

Answer 21

1. M-type ganglion cell, contralateral eye
2. M-type ganglion cell, ipsilateral eye
3. P-type ganglion cell, ipsilateral eye
4. P-type ganglion cell, contralateral eye
5. P-type ganglion cell, ipsilateral eye
6. P-type ganglion cell, contralateral eye

Slide 9 lecture 7

Question 22

What is the retinotopic map in the striate cortex?

Answer 22

Neighbouring locations on the retina project to neighbouring locations in the LGN

Lower portion of V1 represents the top half of visual space
Upper portion of V1 represents the bottom half of visual space
Visual map is distorted, with more tissues devoted to analysis of the central visual field

Slide 10 lecture 7

Question 23

Where do axons carrying info about the superior portion of the visual field travel?

Answer 23

Axons carrying info about the superior portion of the visual field travel in the temporal lobe (Meyer’s loop) before reaching the occipital lobe

Question 24

Where do axons carrying info about the inferior portion of visual field travel?

Answer 24

Axons carrying info about the inferior portion of the visual field travel in the parietal lobe

Slide 11 lecture 7

Question 25

What is scotomas?

What is anopsias?

Answer 25

Scotomas- small visual field deficits (small knicks in retina)

Anopsias- large visual field deficits (prefixes to this indicate the specific region of the visual field from which sight has been lost)
Can happen from mini stroke or big stroke

Slide 12 lecture 7

Question 26

What is bitemporal hemianopsia?

Answer 26

Middle portion optic chiasm damage affects the fibers that are crossing from the nasal retina of each eye, leaving the uncrossed fibers from the temporal retinas intact; visual field deficits from peripheral parts confined to the temporal visual field of each eye (nasal retinas)

Slides 12-13 lecture 7

Question 27

What is homonymous hemianopsia?

Answer 27

Optic tract damage affecting corresponding parts of the visual field in each eye and complete loss of vision in the affected region of the binocular visual field
Right optic tract damage loss of the left visual field (blindness in the temporal visual field of the left eye and the nasal visual field of the right eye)

Slides 12-13 lecture 7