10/30 Visual System - Dr. Fischer

Question 1

Objectives for Visual Lecture

Answer 1

1. Describe the structure of the retina including layers, location of photorecetors, bipolar cells, and ganglion cells.
2. Describe the basic circuitry of the retina: photoreceptor to bipolar cell to ganglion cell to lateral geniculate nucleus to visual cortex.
3. Describe differences in activation of on-center and off-center bipolar cells.
4. Describe the significance of the fovea.
5. Define basic differences between the properties of rod and cone photoreceptors.
6. Differentiate P and M ganglion cells.
7. Describe the pathway from the retina to the lateral geniculate to the visual cortex including laterality of visual field.
8. Differentiate simple and complex cells in the primary visual cortex.
9. Differentiate and define ocular dominance column, orientation columns, blobs
10. Differentiate between the “what” and the “where” pathways from the primary visual cortex.

Question 2

The Retina is Layered

Answer 2

PHOTORECEPTORS:
RODS & CONES

> > > BIPOLAR CELLS

> > > GANGLION CELLS

> > > LATERAL GENICULATE
NUCLEUS

> > > CEREBRAL CORTEX

Question 3

Light passes through layers to get to the photoreceptors.

Answer 3

The types of neurons in the retina are very diverse

Question 4

Circuitry responsible for generating receptive field center responses of ______ .

Answer 4

Circuitry responsible for generating receptive field center responses of retinal ganglion cells.

Question 5

In the dark, _____ have resting membrane potentials ~__mV;

Answer 5

In the dark, cones have resting membrane potentials ~-40mV;

Question 6

When cones are depolarized, they release _____.

Answer 6

When cones are depolarized, they release glutamate.

Question 7

Glutamate excites _____ cells and inhibits ____ cells.

Answer 7

Glutamate excites off-center bipolar cells and inhibits

on-center bipolar cells.

Question 8

Light ______ cones resulting in ______ release of glutamate.

Answer 8

Light hyperpolarizes cones resulting in decreased release of glutamate.

Question 9

With less glutamate, ____ cells are disinhibited (released from inhibition) and they depolarize (sign inverting).

Answer 9

With less glutamate, on-center bipolar cells are disinhibited (released from inhibition) and they depolarize (sign inverting).

Question 10

Depolarization of the on-center bipolar cell causes ____ of glutamate at their terminals which _____ ganglion cells (sign conserving).

Answer 10

Depolarization of the on-center bipolar cell causes release of glutamate at their terminals which excites ganglion cells (sign conserving).

Question 11

Light hyperpolarizes cones resulting in _____ release of glutamate.

Answer 11

Light hyperpolarizes cones resulting in decreased release of glutamate.

Question 12

With ___ glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they
hyperpolarize.

Answer 12

With less glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they hyperpolarize.

Question 13

Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.

Answer 13

Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.

Question 14

Light hyperpolarizes cones resulting in decreased release of glutamate.

With less glutamate, on-center bipolar cells are disinhibited (released from inhibition) and they depolarize (sign inverting).

Depolarization of the on-center bipolar cell causes release of glutamate at their terminals which excites ganglion cells (sign conserving)

Answer 14

Light hyperpolarizes cones resulting in decreased release of glutamate.

With less glutamate, off-center bipolar cells are disfacilitated (less excitatory input) and they
hyperpolarize.

Hyperpolarization of off-center bipolar cells results in less glutamate release at their terminals and ganglion cells decrease their firing rate.

Question 15

Receptive Field

Answer 15

the particular region from which a light stimulus will trigger the firing of a photoreceptor.

Question 16

Simple cells

Answer 16

Neurons in Primary visual cortex that are orientation selective.

Input is from neurons in the lateral geniculate nucleus.

Represents input from multiple retinal ganglion cells and LGN cells that have particular alignment when put together.

Question 17

Complex cells

Answer 17

Neurons in Primary visual cortex that respond primarily to direction of movement across the receptive field and increase or decrease in contrast.

Input is from multiple simple cells.

Question 18

Orientation Columns

Answer 18

Organized regions of neurons that are excited by visual line stimuli of varying angles.

These columns are located in the primary visual cortex and span multiple cortical layers.

Question 19

Blobs

Answer 19

Clusters of neurons interspersed between orientation columns.

Mostly concerned with color.

Question 20

Ocular Dominance Columns

Answer 20

Stripes of neurons in the visual cortex that respond
preferentially to input from one eye or the other.

Alternate in visual cortex.

Primarily found in layer IV where LGN terminates.

Question 21

Vision facts

Answer 21

(1) Nearly one-half of the Brain is involved in vision
(2) The human visual system recognizes a multitude of objects with ease. This task is still a “challenge” for computerized imaging systems
(3) The impact of vision disorders in the US was estimated at $51.4 billion in 2007 (The Impact of Vision Problems, National Symposium)
(4) Worldwide, about 39 million people are blind, about 230 million have some kind of vision impairment, and as many as 3.2 billion people have refractive error (need glasses)
(5) 80% of vision problems worldwide are avoidable or curable.

Question 22

The retina receives an inverted image

Answer 22

Incident light is “focused” by the cornea, restricted by an aperture (the pupil), and focused again by the lens to project upon the retina

Question 23

Incident light is “focused” by the ______

Answer 23

Incident light is “focused” by the cornea

Question 24

aperture

Answer 24

an opening, hole, or gap.

Question 25

Incident light is restricted by an aperture, the _____

Answer 25

Incident light is restricted by an aperture: the pupil

Question 26

Incident light is focused again by the ____ to project upon the ____ .

Answer 26

Incident light is focused a second time by the lens to project upon the retina

Question 27

Types of neurons can be distinguished based on.....

Answer 27

(1) morphology (2) electrophysiology (3) expression profile (calcium-bufferring proteins, neurotransmitters, neuropeptides, etc.)

Question 28

The purpose of the fovea:

Answer 28

(1) Responsible for central vision and highest visual acuity (2) Contains densely packed cone photoreceptors – but contains NO ROD photoreceptors = high acuity day-time vision (3) Blood vessels excluded and inner retinal neurons are physically skewed to the side, hence “a pit” is formed, and light is not distorted by vessels or inner layers of the retina

Question 29

rods

Answer 29

- high light sensitivity, long integration time - more photopigment - high amplification - low temporal resolution (12 Hz) - more sensitive to scattered light - severely affected by quantum noise

Question 30

cones

Answer 30

- low light sensitivity, short integration time - less photopigment - lower amplification - high temporal resolution (55Hz) - most sensitive to axial rays (+-15 deg) - not affected by quantum noise

Question 31

rod system

Answer 31

- low acuity: not present in the fovea - highly convergent retinal pathways - achromatic = one pigment only

Question 32

cone system

Answer 32

- high acuity: concentrated in the fovea - almost no convergence in the fovea - chromatic, 3 types of cones, each with a distinct pigment that is most sensitive to a different part of the visible spectrum

Question 33

Example of convergence:

Answer 33

The convergence of signals in the rod pathway is very large

Question 34

Color vision deficiencies: | Congenital

Answer 34

affect photopigments

Question 35

Color vision deficiencies: | Acquired:

Answer 35

lesions in visual pathways, usually after stroke

Question 36

Red cone pigment missing

Answer 36

Protanope

Question 37

Green cone pigment missing,

Answer 37

Deuteranope

Question 38

only 2 active photopigments

Answer 38

dichromate

Question 39

no color vision

Answer 39

achromatopsia

Question 40

Blue cone pigment missingents

Answer 40

Tritanope

Question 41

Types of neuronal electrical activity in the retina can be segregated to different cell types

Answer 41

Non-spiking and spiking (action potentials) of retinal neurons in different layers

Question 42

"analog"

Answer 42

more finely graded response, | but becomes "noisy" over distances.

Question 43

"digital"

Answer 43

more "steps", but little or no degradation over long distances.

Question 44

X-chromosomal recessive

Answer 44

Protanopia Deuteranopia Blue Cone Monochromacy

Question 45

chromosome 7 | autosomal dominant

Answer 45

Tritanopia

Question 46

autosomal recessive

Answer 46

Achromatopsia | complete & incomplete

Question 47

Color vision testing

Answer 47

"Ishihara pseudoisochromatic plates"

Question 48

no red opsin

Answer 48

protanope

Question 49

no blue opsin

Answer 49

tritanope

Question 50

no green opsin

Answer 50

deuteranope

Question 51

Separation in ON and OFF channels occurs at the _____ .

Answer 51

Separation in ON and OFF channels occurs at the cone terminals

Question 52

all photoreceptors ____ in response to light, whereas they _____ in response to dark

Answer 52

all photoreceptors hyperpolarize in response to light, whereas they depolarize in response to dark

Question 53

ON

Answer 53

depolarize (get “excited”) in response to light

Question 54

OFF

Answer 54

depolarize (get “excited”) in response to dark

Question 55

OFF CHANNELS

Answer 55

glutamate opens cation (Na+) channels through ionotropic receptors

Question 56

spatial:

Answer 56

homogenous illumination doesn't excite the ganglion cells because of the ON/OFF input to their receptive fields --> redundant information is ignored - saves energy

Question 57

temporal

Answer 57

The dichotomy into ON cells and OFF cells, doubles the dynamic range. - without this the maximal spike frequency must be increased

Question 58

Why have separate ON and OFF channels?

Answer 58

spatial : homogenous illumination doesn't excite the ganglion cells because of the ON/OFF input to their receptive fields --> redundant information is ignored - saves energy temporal : The dichotomy into ON cells and OFF cells, doubles the dynamic range. - without this the maximal spike frequency must be increased

Question 59

Action potentials sent by retinal ganglion cells to higher visual centers code for:

Answer 59

(1) Changes in color (2) Changes in brightness/darkness (3) Movement across the visual field (4) Position within the visual field (5) Edges = a changes in brightness/darkness

Question 60

Summary

Answer 60

Information sent from the retina is divided into several parallel channels to segregate different important image features Neuronal signals (Action Potentials) from the retina represented data reduction to convey only relevant information = the retina is a change detector To convey only the important information, there is an enourmous amount of image processing by relatively few retinal cells

Question 61

Information sent from the retina is divided into several parallel channels to ______

Answer 61

Information sent from the retina is divided into several parallel channels to segregate different important image features

Question 62

Neuronal signals from the retina represented data reduction to convey only relevant information = the retina is a ___ detector

Answer 62

Neuronal signals (Action Potentials) from the retina represented data reduction to convey only relevant information = the retina is a change detector

Question 63

To convey only the important information, there is an enourmous amount of image processing by ___ ___ ____ cells.

Answer 63

To convey only the important information, there is an enourmous amount of image processing by relatively few retinal cells.

Question 64

Organization of the | central visual pathways

Answer 64

Most of the projections (axons) from the retina to the brain end in the LGN (lateral geniculate nucleus), which in turn project to the primary visual cortex

Question 65

Visual information from the retina to the cortex is compressed, yet a significant portion of the cortex participates in visual processing

Answer 65

retina to LGN to cortex 0.3% of the cortex (retina: ~100 mg) >>>> 50% of the cortex participates in visual perception

Question 66

There is a concentration of ___ cells in the fovea.

Answer 66

P cells

Question 67

M and P cells

Answer 67

ganglion cells that project into the M or P pathway

Question 68

M cells

Answer 68

NO color contrast higher luminance contrast higher temporal frequency lower spatial frequency

Question 69

P cells

Answer 69

YES color contrast lower luminance contrast lower temporal frequency higher spatial frequency

Question 70

higher spatial frequency?

Answer 70

P cells

Question 71

lower temporal frequency?

Answer 71

P cells

Question 72

higher temporal frequency?

Answer 72

M cells

Question 73

color contrast?

Answer 73

P cells

Question 74

higher temporal frequency

Answer 74

M cells

Question 75

Lateral geniculate nucleus | LGN

Answer 75

90% of ganglion cell axons terminate in the LGN - lesions in the optic tracts cause "blindsight" - some visual sensation is maintained by retinal projections to the Superior Colliculi neurons. Retinotopic representation of contralateral visual field from both ipsilateral (I) and contralateral (C) retina. Strong representation of fovea (50% of all). Segregation of Magno- and Parvocellular pathways M & P pathways carry information of contralateral visual field but inputs from both eyes remain segregated LGN neurons maintain center/surround receptive fields that are similar to the retina's

Question 76

_____ of ganglion cell axons terminate in the LGN

Answer 76

90% of ganglion cell axons terminate in the LGN

Question 77

lesions ____ cause "blindsight"

Answer 77

lesions in the optic tracts cause "blindsight"

Question 78

some visual sensation is maintained by retinal projections to the _____.

Answer 78

some visual sensation is maintained by retinal projections to the Superior Colliculi neurons.

Question 79

The LGN has retinotopic representation of contralateral visual field from both ipsilateral (I) and contralateral (C) retina. Strong representation of fovea (____ of all).

Answer 79

The LGN has retinotopic representation of contralateral visual field from both ipsilateral (I) and contralateral (C) retina. Strong representation of fovea (50% of all).

Question 80

M & P pathways carry information of ____ visual field, and inputs from both eyes ___ ____.

Answer 80

M & P pathways carry information of contralateral visual field but inputs from both eyes remain segregated

Question 81

LGN neurons maintain center/surround receptive fields are similar to retina

Answer 81

LGN has segregation of Magno- and Parvocellular | pathways

Question 82

Primary visual cortex

Answer 82

V1 or Area 17

Question 83

superior colliculus (SC)

Answer 83

Bilateral structure on the roof of the midbrain; direct projection from retina (map of contralateral visual field = ipsilateral retina) Projection through pulvinar (thalamus) to cortex ("indirect pathway") - mediates "blindsight" Extensive input from cortex (not only visual) Multiple sensory maps superimposed in a topographic fashion (auditory, somatosensory)

Question 84

Visual field representation in the visual cortex

Answer 84

V1 = striated cortex = Area 17 (Brodmann area 17) LGN and SC receive information exclusively from the contralateral visual field

Question 85

striated cortex

Answer 85

Primary visual cortex = V1 = Area 17

Question 86

axons from the LGN all terminate in layer ____ of the cortex

Answer 86

axons from the LGN all terminate in layer 4 of V1 (v1 = primary vision, area 17) of the cortex

Question 87

"simple" cells

Answer 87

orientation selective excitatory & inhibitory zones in Receptive Field

Question 88

"complex" cells

Answer 88

larger Receptive Fields not clearly defined on/off zones movement of bar particularly effective

Question 89

Receptive Fields of | "simple cells" in V1

Answer 89

retina and LGN: circular anuli On/Off center V1: elongated zones with excitatory or inhibitory flanks. Retinotopic and orientation selective

Question 90

V1

Answer 90

elongated zones with excitatory or inhibitory flanks. Retinotopic and orientation selective.

Question 91

Receptive fields of complex cells:

Answer 91

orientation is important, but not really position-dependent within the receptive field no clear inhibitory or excitatory zone input from several simple cells Orientation of an edge, the direction of movement across the receptive field, plus the increase or decrease in contrast is what matters to a complex cell.

Question 92

what matters to a complex cell?

Answer 92

Orientation of an edge, the direction of movement across the receptive field, plus the increase or decrease in contrast is what matters to a complex cell.

Question 93

Types of stimuli that V1 cells have preferential responses to: orientation columns

Answer 93

cells in columns have identical retinal position and axis of orientation (orientation columns) using tangential penetration with microelectrode, an orderly shift in axis from one column to the next was shown - about 3/4 mm for one full rotation

Question 94

Types of stimuli that V1 cells have preferential responses to: Blobs

Answer 94

First identified via cytochrome oxidase staining (measure of metabolic activity: "heightened neural activity") Mostly concerned with color, not orientation

Question 95

orientation columns divided in | ocular dominance columns

Answer 95

unilateral injection of a label into one eye is transported to the cortex to identified regions of layer 4 in V1 that recieve input from 1 eye

Question 96

Normally, most cortical neurons ____ receive input from both eyes

Answer 96

Normally, most cortical neurons above layer IV receive input from both eyes

Question 97

Visual experience is required for the establishment of ocular dominance columns in the cortex

Answer 97

neurons outside of layer IV receive primary input from the left eye

Question 98

hypercolumns

Answer 98

(about 1 mm2) Each hypercolumn recieves and processes information from a defined part of the visual field Each hypercolumn contains - a complete set of orientation columns - one set ocular dominance columns - several blobs (color processing) Regular repetition of these modules across the surface of V1

Question 99

``` retinal projections (ganglion cells axons) connect to the ____ and to the _____ ```

Answer 99

retinal projections (ganglion cells axons) connect to the superior colliculus and to the lateral geniculate nucleus

Question 100

V4

Answer 100

color

Question 101

Dorsal parietal cortex:

Answer 101

motion and depth Where is it located?

Question 102

Inferior temporal cortex:

Answer 102

face and object recognition What is it?

Question 103

Responses of cells in the inferior temporal cortex

Answer 103

shape selective cells color selective cells

Question 104

Responses of a neuron in the inferior temporal cortex to complex stimuli:

Answer 104

example of a face-sensitive neuron

Question 105

RFs no retinotopic organization, sometimes whole visual field,

Answer 105

"position invariance" --> recognition of form anywhere (hands, faces, facial expressions...)

Question 106

Where?

Answer 106

Dorsal Motion, motion direction Where is it located? Dorsal parietal cortex: motion and depth

Question 107

What?

Answer 107

Ventral What is it? Inferior temporal cortex: face & object recognition RFs no retinotopic organization, sometimes whole visual field, "position invariance" --> recognition of form anywhere (hands, faces, facial expressions...)

Question 108

ON channels

Answer 108

glutamate closes cGMP-controlled Cation channels through metabotropic receptors

Question 109

Sensory maps superimposed | in a topographic fashion (auditory, somatosensory)?

Answer 109

superior colliculus (SC)

Question 110

In V1, there is interruption of orientation columns by ____ in the visual cortex.

Answer 110

In V1, there is interruption of orientation columns by "blobs" in the visual cortex.