Visual system

Question 1

What does the macula support?

Answer 1

high acuity site

Question 2

Where is the blind spot in they eye?

Answer 2

at the optic disc

Question 3

How do we percieve the blind spot?

Answer 3

We create an actual neural representation of the surround rather than merely ignoring the absence of information from the blind spot (scotoma)

Question 4

What is the structure of the retina?

Answer 4

A three-neuron chain—photoreceptor, bipolar cell, and ganglion cell—provides the most direct route for transmitting visual information to the brain
Horizontal cells and amacrine cells mediate lateral interactions in the outer and inner plexiform layers

Question 5

What are the characteristics of rods?

Answer 5

greater number of disks and higher photopigment concentration
1000 times more sensitive to light than cones
Produce a reliable response to a single photon of light
~100 million

Question 6

What are the characteristics of cons?

Answer 6

responsible for our ability to see color contain one of 3 different photopigments
Response of an individual cone does not saturate at high levels of steady illumination ~5 million

Question 7

At what lumination do cones start to contribute to perception?

Answer 7

At the lowest levels of illumination, only rods are activated
Cones begin to contribute to perception at about the level of starlight and are the only receptors that function under relatively bright conditions

Question 8

What are characteristics of the peripheral retina?

Answer 8

much higher ratio of rods to cones
higher ratio of photoreceptors to ganglion cells
So peripheral retina much more sensitive to low light levels
many photoreceptors provide input

Question 9

What are characteristics of the central retina?

Answer 9

cones only
low ratio of photoreceptors to ganglion cells
So specialized for high resolution vision
a single photoreceptor feeds information to a ganglion cell

Question 10

What are characteristics of the fovea?

Answer 10

area of retina specialized for high visual acuity in the center of the macula;
Lateral displacement of cells above photoreceptors reduces scattered light so no image blur
high density of cones (less rods -> more thorughout retina)

Question 11

What are the three types of cones when reffering to colour vision?

Answer 11

short- (blue), medium- (green), and long- (red) wavelength cones
The mixing of red, green, and blue light causes equal activation of the three types of cones, and the perception of “white” results

Question 12

What is protanopia?

Answer 12

impairment in perception of long wavelengths

Question 13

What is deuteranopia?

Answer 13

impairment in the perception of medium wavelengths

Question 14

What do both types of colour blindness have in common?

Answer 14

have difficulties with the discrimination of red and green, and for this reason dichromacy is commonly called red–green color blindness

Question 15

What is melonin?

Answer 15

absorbs scattered light

Question 16

How are photoreceptor cells removed by the pigment epithelium?

Answer 16

The tips of the outer segments of photoreceptors are embedded in pigment epithelium. Epithelial cell processes extend down between the outer segments. The disks migrate from the inner to the outer portion of the outer segment over a 12-day period
Expended disks are shed from the outer segment and phagocytosed. Photopigment from the disks enters the pigment epithelium, where it will be biochemically cycled back to “newborn” photoreceptor disks.

Question 17

What is retinitis pigmentosa?

Answer 17

progressive vision loss due to a gradual degeneration of photoreceptors
damage to periphery

Question 18

What is macular degeneration?

Answer 18

progressive loss of central vision
Peripheral vision usually remains normal

Question 19

Explain phototransduction

Answer 19

stimulus - light acts on a receptor to cause a change in protein conformation
G-protein then binds GTP which causes a second messanger to decrease, decreasing Na+ conductance

Question 20

What is the hyperpolarization response to light of photoreceptors?

Answer 20

Photoreceptors are continuously depolarized in the dark because of an inward sodium current, the dark current.
Dark, sodium enters the photoreceptor through a cGMP-gated channel
Light leads to the activation of an enzyme that destroys cGMP, thereby shutting off the Na+ current and hyperpolarizing the cell
dark -> depolarization
light -> hyperpolarization

Question 21

Explain the light-activated biochemical cascade

rhodopsin

Answer 21

The activation of rhodopsin by light
Retinal is inactive in the dark, when it absorbs light it undergoes a conformational change that acitvates the opsin

In the dark, cGMP gates a sodium channel, causing an inward Na+ current and depolarization of the cell
The activation of rhodopsin by light energy causes the G-protein to exchange a molecule, which causes the enzyme phosphodiesterase (PDE) to breaks down cGMP and shuts off the dark current

Question 22

What is the retinoid cycle?

Answer 22

Following photoisomerization, retinal is converted into retinol and is transported by the chaperone protein interphotoreceptor retinoid binding protein (IRBP) into the pigment epithelium
There, in a series of steps, it is converted to retinal and transported back to the outer segment (again via IRBP), where it recombines with opsin

Question 23

Explain photoreceptor light adaption

Answer 23

Calcium in the outer segment inhibits the activity of guanylate cyclase and rhodopsin kinase, and reduces the affinity of cGMP-gated channels for cGMP
Light-induced closure of channels in the outer segment membrane leads to a reduction in Ca2+ concentration and a reduction in Ca2+-mediated inhibition of these elements of the cascade As a result, the photoreceptor’s sensitivity to photon capture is reduced

Question 24

What are the two classes of bipolar cells?

Answer 24

ON bipolar cells have G-protein-coupled receptors (mGluR6) and hyperpolarize to glutamate released by photoreceptors. (cascade closes cGMP-gated Na+ channels)
OFF bipolar cells have glutamate-gated cation channels (AMPA, kainate) and depolarize to glutamate release
* OFF and ON refer to whether cells depolarize when the light is OFF (more glutamate) or ON (less glutamate)

Question 25

What is the direct pathway from photoreceptors to bipolar cells?

Answer 25

An ON-center bipolar cell is depolarized by light in the receptive field center via the direct pathway

Question 26

What is the indirect pathway from photoreceptors to bipolar cells?

Answer 26

Light in the receptive field surround hyperpolarizes the ON-center bipolar cell via the indirect pathway bipolar cells recive input from surrounding photoreceptors via horizontal cells * Because of the intervening horizontal cell, the effect of light on the surround photoreceptors is always opposite the effect of light on the center photoreceptors

Question 27

How does the number of photoreceptors contributing to the receptive field of a retinal ganglion cell varies depending on location on the retina?

Answer 27

A cell near the fovea receives input from fewer receptors covering a smaller area, whereas a cell farther from the fovea receives input from many more receptors covering a larger area

Question 28

What are the ON and OFF center ganglion cell responses to stimulation of receptive field centers?

Answer 28

ON and OFF bipolar cells correspond with ON and OFF ganglion cells Light spot in center of receptive field will cause firing of ON center ganglion cells dark spot in center of receptive field will cause firing of OFF center ganglion cells

Question 29

What is the response of bipolar cells and ganglion cells to a light spot in the receptive fiels center?

Answer 29

center cone - hyperpolarize ON center bipolar cells - depolarize OFF center bipolar cells - hyperpolarize ON center ganglion cells - increase firing OFF center ganglion cells - stop firing

Question 30

What is the response of bipolar cells and ganglion cells to a dark spot in the receptive fiels center?

Answer 30

center cone - depolarize ON center bipolar cells - hyperpolarize OFF center bipolar cells - depolarize ON center ganglion cells - stop firing OFF center ganglion cells - increase firing

Question 31

What is the most effect way to maximize the firing of an ON or OFF center cell?

Answer 31

completely illuminate either the "on area" or the "off area" of its receptive field * if both areas of a cells receptive field are illuminated together, there is little reaction from the cell

Question 32

What are mach bands?

Answer 32

illusory bands visible to enhcacne contrast percieved light between colours

Question 33

How does lateral inhibition produce contrast enhancement?

Answer 33

caused by the opponent center-surround arrangement of the receptive fields of the retinal ganglion cells The image of the transition between lighter and darker regions falls across some of these receptive fields The cells whose centers are located in the brighter region but whose surrounds are located at least partially in the darker region will have the highest rate of firing

Question 34

What occurs in the retinal cells with light stimulation of the center + surround?

Answer 34

leads to hyperpolarization of the horizontal cells and a decrease in the hyperpolarizing influence of horizontal cell processes on the photoreceptor terminals. The net effect is to depolarize the center cone terminal, offsetting much of the hyperpolarization induced by the transduction cascade in the center cone’s outer segment -> **decreased firing of ganglion cells and decreased depolarization of bipolar cells**

Question 35

What are the three components of the non-image forming visual system?

Answer 35

hypothalamus - regulation of circadium rythym pretectum - reflec control of pupil and lens superior colliculus - orienting the movements of head and eyes

Question 36

WHat is the retinohypothalamic tract?

Answer 36

Suprachiasmatic nucleus receives a selective input from the retina that is necessary and sufficient for photic entrainment of circadian rhythms

Question 37

What is the role of melanopsin ?

Answer 37

it is expressed in a small population of ‘intrinsically photosensitive’ retinal ganglion cells (ipRGC) that respond directly to light Retinal ganglion cells containing melanopsin innervate the SCN (suprachiasmic nucleus)

Question 38

Are retinal photoreceptors required for visual image formation are necessary for circadian photoreception

Answer 38

No

Question 39

What does a pulse of light porduce in ipRGCs?

Answer 39

a burst of AP

Question 40

What is the pupillary light reflex?

Answer 40

Light shone in either eye elicits pupillary constriction in the same eye (direct response) and in the opposite eye (consensual response) The pupillary response to light is mediated by the parasympathetic innervation of the iris Retinal ganglion cells send their axons through the optic tract to the olivary pretectal nucleus Neurons in this nucleus project to parasympathetic preganglionic neurons in the Edinger-Westphal nucleus The axons of the preganglionic cells exit with the oculomotor nerve and contact the ciliary ganglion cells, which control the pupilloconstrictor muscle in the iris

Question 41

What is the afferent pupillary defect?

Answer 41

Failure to elicit a response (either direct or indirect) to stimulation of the right eye if 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

Question 42

What is the efferent pupillary defect?

Answer 42

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

Question 43

What is the optokinetic response?

Answer 43

a combination of a slow-phase and fast-phase eye movements It consists of initial slow phases in the direction of the stimulus (smooth pursuits), followed by fast, corrective phases (return saccade)

Question 44

What are saccades in the optokinetic response?

Answer 44

fast movements of the eyes that present various parts of the visual scene to the fovea—a process called foveation. When we fix our gaze, small saccades that we are unaware of avert photoreceptor adaptation

Question 45

Explain sensorimotor integration in the superior colliculus

Answer 45

The sensory map of visual space in the superior colliculus is in register with the motor map that generates eye movements Neurons in a particular region of the superior colliculus are activated by visual stimuli in a limited region of visual space This activation leads to the generation of a saccade by activating neighboring upper motor neurons that move the eye by an amount just sufficient to align the foveae with the region of visual space that provided the stimulation

Question 46

What is blindsight?

Answer 46

ability to respond to (behaviour) something outside the visual field

Question 47

What are the characteristics of P-type ganglion cells?

Answer 47

- 90% population - small receptive fields - slow axon conduction velocities - sustained firing to the presentation of visual stimuli - can transmit information about color

Question 48

What are the characteristics of M-type ganglion cells?

Answer 48

- 5% population - large receptive fields - transient burst of APs to the presentation of visual stimuli - fast conduction velocity - more sensitive to low contrast stimulus - cannot transmit information about color

Question 49

What is the function of P-type ganglion cells?

Answer 49

important for high spatial resolution vision the detailed analysis of the shape, size, and color of objects

Question 50

What is the function of M-type ganglion cells?

Answer 50

high temporal resolution, such as evaluating the location, speed and direction of a rapidly moving object

Question 51

How are images displayed on the retinal surface?

Answer 51

inverted and left–right reversed

Question 52

Where do points in the binocular portion of the left visual field fall?

Answer 52

on the nasal retina of the left eye and the temporal retina of the right eye

Question 53

Where do points in the binocular portion of the right visual field fall?

Answer 53

on the nasal retina of the right eye and the temporal retina of the left eye

Question 54

Where to the axons of ganglion cells in nasal and tempoal retina go?

Answer 54

The axons of ganglion cells in the nasal retina cross in the optic chiasm, whereas those from the temporal retina do not As a result, the right optic tract carries information from the left visual field, and the left optic tract carries information from the right visual field

Question 55

What would result in the visula field from a deficit originating from the right optic nerve?

Answer 55

the nasal and temporal hemiretina of the right eye are affected

Question 56

What would result in the visula field from a deficit originating from the right optic chiasm?

Answer 56

the nasal hemiretina of both eyes are affected

Question 57

What would result in the visula field from a deficit originating from the right optic tract?

Answer 57

the nasal hemiretina from the left eye and the temporal hemiretina of the right eye are affected

Question 58

What is anopsia?

Answer 58

Large visual field deficit

Question 59

What is the function of the lateral geniculate nucleus?

Answer 59

send information to the primary visual cortex

Question 60

What is the organization of the lateral geinculate nucleus?

Answer 60

receives inputs from both eyes, but the inputs are segregated in separate layers

Question 61

What are the two layers in the lateral geniculate nucleus?

Answer 61

magnocellular (1-2) - M-type ganglion cells parvocellular layers (3-6) - P-type ganglion cells

Question 62

What visual fields do the right and left LGN project?

Answer 62

right - left visual field: left nasal retina and right temporal retina left - right visual field: right nasal retina and left temporal retina

Question 63

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

Answer 63

temporal lobe

Question 64

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

Answer 64

parietal lobe

Question 65

What does lesion of the optic radiation fibers that curve into the temporal lobe cause to the visual field?

Answer 65

loss of vision in the upper quadrant of the contralateral visual hemifield in both eyes

Question 66

What does partial lesions of the visual cortex cause to the visual field?

Answer 66

deficits in portions of the contralateral visual hemifield The central area of the visual field tends to be unaffected by cortical lesions because of the extent of the representation of the fovea (macular sparing)

Question 67

How many layers is the striate cortex divided into?

Answer 67

six principal cellular layers 1, 2/3, 4A, 4B, 4C, 5, 6

Question 68

What is layer 4C of the striate cortex dominated by?

Answer 68

spiny stellate neurons, whose dendrites are confined to this layer

Question 69

What is the most numerous cell typr in the neocortex of the striate cortex?

Answer 69

Pyramidal cells with prominent apical and basilar dendrites are the most numerous cell type in the neocortex; they are located in all layers except 4C

Question 70

Where to LGN axons mostly terminate in the striate cortex?

Answer 70

Lateral geniculate axons terminate most heavily in layers 4C and 4A, with less dense projections to layers 1, 2/3, and 6; the terminations in layer 2/3 are “patchy."

Question 71

What layer gives rise to axons that terminate more superficaly?

Answer 71

Laminar organization of major intracortical connections. Neurons in layer 4C give rise to axons that terminate in more superficial layers

Question 72

Where do the parvocellular and magnocellular layers arrive in the visual cortex?

Answer 72

The parvocellular layers project to layer IVCβ and the magnocellular layers to layer IVCα In addition, the afferents from the ipsilateral and contralateral layers of the lateral geniculate nucleus are segregated into alternating ocular-dominance columns

Question 73

Where does the mixing of pathways from two eyes first occur?

Answer 73

striate cortex Axons project from layer IVC to more superficial layers. Most layer III neurons receive binocular input from both left and right eyes. There are layer III neurons with responses dominated by the right eye, left eye, or roughly equally responsive to input from the two eyes Ocular dominance columns contain neurons with input dominated by one eye, and the columns alternate between left and right eye dominance

Question 74

What it binocular disparity?

Answer 74

Depth is computed from the positions at which images occur in the two eyes The image of an object lying in the plane of fixation falls on corresponding points on the two retinas The images of objects lying in front of the plane of fixation or behind it fall on noncorresponding locations on the two retinas

Question 75

How are visual cortex neurons are selective for particular ranges of disparity?

Answer 75

Some neurons are tuned to a narrow range of disparities and thus have particular disparity preferences (tuned excitatory or tuned inhibitory neurons), whereas others are tuned broadly for objects in front of the fixation plane (near cells) or beyond the plane (far cells)

Question 76

How are visual cells in the LGN different from those in the cerebral cortext?

Answer 76

Visual cells in the lateral geniculate nucleus have concentric receptive fields, like those of retinal bipolar cells and ganglion cells Visual cells in the cerebral cortex are more responsive to bars of light and show orientation specificity

Question 77

What is orientation selectivity?

Answer 77

The responses of an orientation-selective neuron are monitored as visual stimuli are presented in its receptive field The visual stimulus is a bar of light Light bars of various orientations elicit very different responses

Question 78

What are the receptive fields of neurons displaced along the radial axis of the cortex?

Answer 78

are centered on the same region of visual space and exhibit similar orientation preferences

Question 79

What are the receptive fields of neurons displaced along the tangential axis of the cortex?

Answer 79

exhibit an orderly progression of receptive field properties

Question 80

What is direction selectivity?

Answer 80

With a bar stimulus at the optimal orientation, the neuron responds strongly when the bar is swept to the right but weakly when it is swept to the left

Question 81

What is a simple cell receptive field?

Answer 81

The response of a simple cell to optimally oriented bars of light at different locations in the receptive field Notice that the response can be ON or OFF depending on where the bar lies in the receptive field

Question 82

What is a complex cell receptive field?

Answer 82

a complex cell responds best to a bar of light at a particular orientation. However, responses occur to both light ON and light OFF, regardless of position in the receptive field

Question 83

Explain the model proposed for summation of center–surround inputs to generate the receptive field of a simple cell with orientation selectivity

Answer 83

Activity from simple cells summates to generate the receptive field properties of complex cells, the next level of neuronal processing

Question 84

What is the area V5 (MT) responsible for?

Answer 84

responsive to visual motion

Question 85

What are characteristics of neurons beyond V1?

Answer 85

have more complex receptors fields -> repond to different forms of stimulus in the visual field