Visual System

Two different types of photoreceptor cells

Rods and cones

Rods specialize in ____-intensity light

low

Cones specialize in _____-intensity light

high

Range where rod and cone sensitivities overlap

Mesoscopic Range

Where are most of the rods and cones respectively?

Rods are mostly concentrated in the peripheral retina (outer areas of the retina); cones are mostly concentrated in the fovea centralis

Dynamic range

the range/ration between the smallest and largest values that a system can represent

Adaptation

Shifting dynamic range to match the current conditions

Light adaptation

photoreceptors become less sensitive to the same intensity of stimulation when the background illumination is higher

Adaptation expands the _____ ______ of light intensity within which ___ can distinguish the intensity differences between background and an objects

dynamic range, rods

Weber’s Law

States that the smallest noticeable difference between two stimuli (called the “just noticeable difference”, or JND) is proportional to the intensity of the original stimulus.

Brightness Constancy

The tendency to perceive an object as having the same brightness even when the lighting conditions change.

Because photoreceptors (rods and cones) hyperpolarize when exposed to light, what does this indicate about their permeability?

The photoreceptors are more permeable to Na+ at rest

What is Rhodopsin?

A light-sensitive visual pigment found in the outer segment of the rods within the retina. It allows the eye to detect light, especially in dim-light conditions. Opsin + 11-cis retinal = Rhodopsin

What is the structure of Rhodopsin?

Seven transmembrane receptor, internal binding site for 11-cis retinal, opsin, and retinal (inside the opsin)

What is special about 11-cis retinal? (Cis —> Trans)

When it is illuminated by light, there is a double bond that breaks and reforms to cause the molecule to undergo an irreversible conformational change, becoming a trans double bond

Photoisomerization

The process by which light causes a change in the shape (configuration) of a light-sensitive molecule from 11-cis retinal to all-trans-retinal

What is transducin; when is it released and what does it do?

A heterotrimeric G-protein. It is a molecule that is released from its binding site when light activates rhodopsin. Transducin then exchanges GDP for GTP and stimulates PDE (phosphodiesterase). PDE breaks down cGMP into GMP and decreases its concentration.

Light activates ________ to hydrolyze _______; therefore, light _____ cGMP production

PDE, cGMP; reduces/decreases

cGMP directly activates the ______, a cation channel

CNG (cyclic nucleotide-gated channel

What is the structure of CNG channels?

__ transmembrane domains

__ subunits

intracellular cGMP binding domain

calmodulin binding site

6, 4,

What is the relationship between permeability of the cell and light/dark conditions

When exposed to light, the cGMP concentration decreases because the channel is closed. This allows leak K+ channels to dominate and hyperpolarize the cell; in dark conditions, the cGMP concentration dominates and the CNG channels remain consistently open, and keeping the cells in a consistently depolarized state

Recovery (return to dark state) _____ —> ______ —> ______—> ______—>_______ (Overview)

Close CNGs —> decrease in Ca2+ —> GCAP —> guanylate cyclase —> increase in cGMP

The retinoid cycle

is the process by which the eye regenerates the light-sensitive pigment in photoreceptors so they can continue detecting light.

Perception depends on what factors?

Sensory, cognitive, physiological, and environmental

What are the three types of Visual functions (dark environment —> light environment)?

Scotopic, Mesopic, Photopic

Photoreceptors become ____ sensitive to the same intensity if stimulation when the background illumination is higher

less

Photoreceptors become less sensitive to the same intensity if stimulation when the background illumination is _____.

higher

Why does closing cGMP channels produce MORE cGMP during Adaptation? (two different feedback cycles)

1) When exposed to light cGMP channels close, which reduces amount of Ca2+. Ca2+ inhibits guanylate cyclase, so removing Ca2+ produces more guanylate cyclase, producing MORE cGMP. It is a negative feedback loop.

2) Low Ca2+ Rhodopsin phosphorylation increases, increasing Arestin binding to rhodopsin to inactivate Rhodopsin, which stops the light signals

Cones are concentrated in the _____ for ____ - _____ vision

fovea, high-acuity

Cones are ____ sensitive and much _____ than rods

less, faster

Dark current

The current that is constantly entering both a rod or cone in the dark; when the rod or cone is exposed to light, those channels close and the current goes away

Why do cones have a higher spatial-acuity than rods? (Two reasons) What is the trade off?

1) The fovea does not have neural tissue in the way

2) Less convergence; each cone sends information through relatively separate pathways

The trade-off is lower light sensitivity

The ratio between cones and bipolar cells are usually ____, and thereby produces less convergence in the fovea

1:1

Convergence in rods is _____, reducing _____-_____, and increasing ______ ______

high, spatial acuity, light sensitivity

Cones provide _____ vision

color

What are the three main types of light-absorbing cones in vertebrate retinas? (What colors/lengths do they absorb? What kinds of opsins do they produce?)

S cones: Blue, short, s-opsin

M: Green, medium, m-opsin

L: Red, long, l-opsin

Different types of cones produce different types of _____, which are all copies of some ______ opsin, derived from genetic mutation

opsin, ancestral

Abnormalities of color vision: dichromatic vision

you lack either M or L cones

Benefits of Trichromatic Vision

1.) helps primates find fruit

The Receptive Field

The region in visual space that a neuron responds to

The “receptive field” of a neuron in the visual system refers to the area of _____ field (or corresponding area of the _____) from which activity of a neuron can be ______ by a visual stimuli

visual, retina, influenced

Receptive fields of visual neurons get _____ in size and more _____in property along the visual pathway

larger, complex

Rods and cones form synapses with two types of cells the _______ and ________ cells; the former forms the vertical pathway (synapses with _____); the latter forms the horizontal pathway, which connects across multiple rods and cones in the ____ _____ layer; after these layers comes another horizontal layer made of _____ cells, which form their synapses in the ____ ______ layer.

bipolar, horizontal; RGCs; outer plexiform; amacrine, inner plexiform

What is the singular neuron that produces action potentials for visual processing? Why? What potentials do other cells produce in these layers?

RGCs; because they need to send information across a long distance; other cells produce graded potentials

Retinal ganglion cells use (on/off) ____-____ receptive fields to analyze local contrast

center, surround

An on-center ganglion cell when stimulated by light in its center will ______ its firing rate; an off-center ganglion when stimulated in its center, its firing rate _____, and when the light turns off the firing rate _____ back to baseline

increase, decreases, increases

Rods and cones are always ___-center; they turn off when you shine a light on them

off

On/Off RGCs

on and off retinal ganglion cells are neurons that respond differently depending on whether light increases or decreases in the center of their receptive fields

What cells between the photoreceptors and RGCs determine ON/OFF RGCs?

on/off bipolar cells

Bipolar cells are either ______ or _______ by light based on the glutamate receptors they express

depolarized, hyperpolarized

Off Bipolar cells will have ________ glutamate receptors and the cell will ________; ______ cations enter bipolar cell.

ionotropic, hyperpolarize; fewer

On Bipolar cells will have _______ glutamate receptors and the cell will _________; ______ G-proteins are ____ active, and cation channels are ____ inhibited; the On-pathway is ______.

metabotropic, depolarize; less, less; slower

Lateral inhibition form ______ ___ constructs center-surround receptive fields

horizontal cells

What is important about horizontal cells and producing contrast?

Horizontal cells create lateral inhibition, which enhances contrast and contributes to center-surround organization in retinal bipolar and ganglion cell pathways.

David Marr’s Hypothesis

Three levels of analysis: computational, algorithmic, implemental

Computational: What and why

Algorithmic: How?

Implemental: Where with what?

Utilize Marr’s Hypothesis to explain lateral inhibition

Computational: detect contrast

Algorithmic: lateral inhibition

Implemental: horizontal cell connections

Rods have bipolar cells, but they have their _____ population of bipolar cells and just one type of bipolar cell, _____. All of these cells are ___ and connect into the ___ ______, to form synapses with _____ _______

own, RBC, on, cone pathway, amacrine cells.

Color is sensed by comparing signals from cones with different ______ ________

spectral sensitivities

If you have a blue cone (absorbs blue light), and shine blue light on it, what happens? What about the compliment?

the blue cone hyperpolarizes, but the red and green cones would not absorb the light. If you shine yellow, the compliment, the blue cone will not absorb anything, but now the red and green cones will hyperpolarize.

Color opponency is further enhanced by unput from OFF bipolar cells, which carry mostly additional yellow signals from M-/L cones

Midget bipolar cells

A subtype of bipolar cell that has a smaller cell-body, and higher visual acuity

Color opponency

The central idea is that certain colors are processed in opposing channels, where activation by one color suppresses the response to its opposing color.

Color perception is influenced by _________ assumptions

illumination

What are the center-surround receptive fields of the RGCs?

Parallel pathways (on-off cells) and antagonistic organization (lateral inhibition)

Diverse retinal cell types and their precise connections enable _______ ______ _______

parallel information processing

What is direction selectivity in the retina?

The ability of certain retinal neurons to respond more strongly to movement in one particular direction than to movement in the opposite direction.

Direction Selectivity in the Retina (preferred and null directions

Preferred direction = response; null direction = no respone

Direction Selective Ganglion Cells (DSGCs) detetc _____ _____

motion direction

How to generate direction selectivity (in theory) - Reichardt model

enhancement of the preferred direction is due to the integration of delayed excitation; Their asymmetric GABAergic inhibition helps direction-selective ganglion cells distinguish preferred from null-direction motion.

How to generate direction selectivity (in theory) - Levick model

suppression of the null direction due to asymmetric inhibition

If you want to compute the direction that something is moving, you need to ______ one input relative to the other

delay

What cells are mainly responsible for producing direction selectivity in the retina?

Starburst amacrine cells

Starburst amacrine cells are primarily _______, and release two different types of neuro transmitters, _______ and ____

inhibitory, GABA, ACh

Direction-selectivity of RGCs arises from ______ inhibition by ________ cells

asymmetric, amacrine

DSGCs detect _____ _____ through ______ ________, causing ____-selective _______ release at synapses between ________ and _______

motion direction, asymmetric inhibition, direction, GABA, SACs, DSGCs

ipRGCs, unlike rods and cones, are ______ by light

depolarized

ipRGCs use a photoreceptor color molecule called _______, which has a different absorbance spectrum

melanopsin

What is a theory that has arisen from the discovery of ipRGCs?

Do not look at your phone at night because it will disrupt your circadian rhythm

What are some advantages of graded potentials in many retinal cell types?

Graded potentials are faster

What two brain regions are concerned with conscious visual perception?

The lateral geniculate nucleus and the primary visual cortex

Each eye receives input from _____ sides of the visual field

both

What are the four main quadrants of the visual field? what do they indicate: toward… (upper, lower, nose, ear)

Superior (upper) /Inferior (lower); Nasal (nose) /Temporal (ear)

What projections describe where information travels relative to the brain?

In the visual system, ipsilateral and contralateral projections describe whether information travels to the same side or opposite side of the brain from where it originated.

Ipsilateral v. Contralateral Projections: ________ remain on the same side of the brain; _______ cross over to the opposite side

ipsilateral, contralateral

The temporal retina is _______, and the nasal retina is _________; using the _____ _____, they cross in a region called the ______ _____, and starts the _____ ______.

ipsilateral, contralateral, optic nerve, optic chiasm, optic tract

Damaging the right optic nerve impairs ____ of the right eye visual field

all

Damaging the optic chiasm would result in loss of ______ vision

peripheral

Damaging the right optic tract would result in the loss of the _____ hemifield

left

Damage to the fibers passing back to the visual cortex on the right superior side would result in the loss of vision in a ______ of the ___ ____ side

quarter, top, left

Damage to right optic radiation causes damage in the ____ _______

left hemifield

In binocular vision, even when the signals from each eye are combining in the optic tract, their axons remain ______ and when they arrive at the LGN, they project into different _____ of the LGN

separate, layers

The Dorsal Lateral Geniculate Nucleus is a ______ structure: ________ (layers 1-2) and ______ (3-6).

layered; Magnocellular, Parvocellular; Magnocellular

What layers of the LGN are ipsilateral? What layers are contralateral?

2, 3, and 5; 1, 4, 6

What is each type of layer (Magnocellular, Parvocellular) responsible for?

Magnocellular: motion, movement, changes in brightness

Parvocellular: color and fine detail

Mixing of the pathways from the two eyes first occurs in the _____

cortex

What are ocular dominance columns?

Ocular Dominance Columns are alternating bands of neurons in the primary visual cortex (V1) that respond more strongly to input from one eye than the other.

Determining ocular dominance via experimentation

radioactive amino acid injected into one eye → transsynaptic transport through LGN terminates in layer 4 of orimary visual cortex (V1) → terminations are visible as bands on autoradiogram

What is cortical lamination?

The organization of the cerebral cortex into distinct layers of neurons, with each layer having different cell types, connections, and functions.

If you want to know where primary visual cortex is in the brain, what should you look for?

A thick layer 4

Information flow in V1:

1) Which V1 layer primarily receives input from the LGN?

2) Which layers send information to other cortical visual areas?

3) Which layer sends feedback to the LGN?

4) Which layer sends output to subcortical structures?

1) Layer 4

2) Layer 2/3

3) Layer 6

4) Layer 5