Neuroscience of Vision: Cone Responses

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Questions and Answers

What is the primary role of intrinsically photosensitive ganglion cells (ipGCs)?

To regulate circadian rhythms (correct)

To enhance color discrimination

To transmit visual images to the brain

To coordinate eye movements

Which type of color opponency is associated with the red-green color discrimination?

Red-green opponency (correct)

Yellow-red opponency

Green-blue opponency

Blue-yellow opponency

What structure of the brain is involved in allodynia, which is the perception of non-painful stimuli as painful?

Optical pretectal nucleus

Lateral habenula

Posterior nucleus of the thalamus (correct)

Suprachiasmatic nucleus

Which of the following structures is responsible for coordinating eye movements?

Superior colliculus Signup and view all the answers

What happens when light activates the G-protein coupled receptor in intrinsically photosensitive ganglion cells?

It activates phospholipase C Signup and view all the answers

Which type of response is determined by the opponent cone photoreceptor feeding into horizontal cells?

Surround response Signup and view all the answers

What is the function of the optical pretectal nucleus?

Controlling eye dilation Signup and view all the answers

Which protein do intrinsically photosensitive ganglion cells express?

Melanopsin Signup and view all the answers

What effect does attempting to fixate on a static object while moving have on the vestibulo-ocular reflex (VOR) in fish?

It enhances the VOR. Signup and view all the answers

In cats, what impact does fixating on a moving object have on the VOR?

It suppresses the VOR. Signup and view all the answers

Which statement best describes how the relative inputs from the retina and cortex function in monkeys?

Cortical inputs dominate over retinal inputs. Signup and view all the answers

What primary roles does the vestibular system serve?

To maintain balance and gaze fixation. Signup and view all the answers

What type of motion does the vestibular system primarily detect?

Translational motion along various axes. Signup and view all the answers

What occurs in the preferred direction of light for ooDSGCs?

Light moves from dendrite to soma, causing little GABA release. Signup and view all the answers

What characterizes the null direction of light for ooDSGCs?

Light induces significant GABA release, causing decreased firing rates. Signup and view all the answers

How do cone responses contribute to color discrimination?

The combination of cone responses encodes specific wavelengths. Signup and view all the answers

What role does melanopsin play in response to light stimuli?

Melanopsin's expression alteration affects membrane potential changes. Signup and view all the answers

What is observed in blind rd1 mice concerning ipGCs?

Activity of ipGCs is absent due to degeneration. Signup and view all the answers

What happens to ooDSGCs when they receive little inhibition?

They respond rapidly and fire frequently. Signup and view all the answers

What is the significance of the combination of cone responses?

Allows the encoding of specific wavelengths for color perception. Signup and view all the answers

In the null direction of light, what effect does increased GABA release have on ooDSGCs?

It significantly reduces their firing rate. Signup and view all the answers

What is the main function of cones in the visual system?

To respond quickly to a wide range of luminance Signup and view all the answers

How do rods and cones work together in vision?

They complement each other for optimal visual responses across varying light conditions. Signup and view all the answers

What affects the threshold for detecting stimuli according to Weber's Law?

It increases with background illuminance. Signup and view all the answers

What happens to rods without light adaptation?

They saturate quickly and cannot respond to changes in stimuli. Signup and view all the answers

What is true regarding the sensitivity of cones and rods in different lighting?

Rods are more sensitive initially at lower background illuminance. Signup and view all the answers

What is one of the mechanisms related to light adaptation in photoreceptors?

Recovery of photoreceptors from stimulus exposure Signup and view all the answers

In terms of response kinetics, how do rods and cones differ?

Rods have slower response kinetics compared to cones. Signup and view all the answers

What happens to the ability to detect stimuli as background light levels increase?

It becomes more challenging due to increasing noise. Signup and view all the answers

What condition is characterized by an inability to perceive fluid motion after bilateral damage to area MT or MST?

Akinetopsia Signup and view all the answers

Which part of the hierarchical model of motion perception is responsible for performing global motion integration?

Area MT Signup and view all the answers

Which cells in V1 encode the direction of stimulus movement?

Direction selective cells Signup and view all the answers

What is a key feature of blindsight in patients with damage to V1?

They can navigate through obstacles despite not seeing them. Signup and view all the answers

In the processing of motion in the brain, which area comes after V1 in the hierarchical model?

Area MT Signup and view all the answers

What happens when stimuli move in a non-preferred direction in terms of neuronal response latencies?

Neurons with shorter response latencies activate first. Signup and view all the answers

Which of the following statements correctly describes the alternative motion processing pathways?

They bypass V1 to project directly to area MT. Signup and view all the answers

What is the primary function of the dorsal visual stream?

Motion perception and spatial awareness. Signup and view all the answers

Study Notes

ooDSGCs

ooDSGCs receive inhibition from SBACs
The direction of light movement influences the amount of GABA released from SBACs and therefore the firing rate of ooDSGCs
When light moves from dendrite to soma of SBACs, there is little GABA release, ooDSGCs receive little inhibition and fire rapidly
When light moves from soma to dendrite of SBACs, there is lots of GABA release, ooDSGCs receive lots of inhibition and their firing rate reduces

Wavelength Detection

All cone types respond differently to specific wavelengths of light
Combined cone response signals specific wavelengths in the visible spectrum
Melanopsin is a G-protein coupled receptor, when exposed to light, its Gq protein dissociates and activates phospholipase C

Colour Discrimination

Each type of cone responds differently to each wavelength of light
Combination of cone responses in the retinal ganglion cells (RGCs) encodes specific colours, which is then transmitted to the brain
RGCs have colour opponent centre-surround receptive fields
Centre is depolarised (excited) by one colour and surround is hyperpolarised (inhibited) by an opponent colour
The central response of RGCs is determined by cone type in the direct pathway
The surround response is determined by opponent cone type in the horizontal cell pathway
Two main forms of colour opponency are red-green and blue-yellow

Non-image Forming Functions

These functions are based on the activity of intrinsically photosensitive ganglion cells (ipGCs)
ipGCs express melanopsin
ipGCs project to several brain regions
ipGCs contribute to synchronizing the circadian rhythm through the suprachiasmatic nucleus
ipGCs regulate mood through the lateral habenula
ipGCs are involved in photophobia, through the posterior nucleus of the thalamus
ipGCs help coordinate eye movements through the superior colliculus
ipGCs are involved in the pupillary light reflex in the optical pretectal nucleus
Axons of ipGCs synapse with neurons in the ipsilateral pretectal nucleus before reaching the lateral geniculate nucleus (LGN)

Light Adaptation

Light adaptation is the process of adjusting to different levels of illumination
It helps maintain optimal vision in different light conditions
Cones do not saturate at high light levels but have poorer sensitivity
Rods are more sensitive at lower light levels and cones are more sensitive at higher light levels
Cones have faster response kinetics and a wider operating range compared to rods
Cones and rods work together to optimise the visual response to the changing illumination levels
Weber’s law describes the relationship between the background intensity and the threshold required to detect a change in stimulus.

Mechanisms of Light Adaptation

Ca^2+^-independent processes
- Pigment depletion: Recovery of rhodopsin in rods
Ca^2+^-dependent processes
- Calcium-activated potassium conductance: This mechanism enhances sensitivity. Ca²⁺ entry through the photoreceptor's outer segment during light stimulation increases K⁺ conductance. This hyperpolarizes the cell, reducing its excitability. In the dark, Ca²⁺ levels decrease, reducing K⁺ conductance and increasing sensitivity.
- Phosphorylation of rhodopsin: This mechanism reduces the light sensitivity of rhodopsin. Light activation of rhodopsin triggers a cascade of events leading to phosphorylation of rhodopsin, making it less responsive to light, leading to reduced sensitivity.
- Transducin deactivation: Transducin, the G-protein in phototransduction, is deactivated in the dark by a process called GTPase activity, which removes the bound GTP and reduces the activity of the transducin molecule, decreasing light sensitivity.

Vestibular and Optokinetic Systems Interaction

The vestibular system maintains balance and gaze fixation during head movements
The vestibular system receives sensory input from the optokinetic pathway
The optokinetic system detects smooth motion, optic flow, and complex biological motion
Optokinetic input can alter the vestibular system response
When the stimulus moves in the preferred direction, neurons with longer response latencies are activated first
When the stimulus moves in the null direction, neurons with shorter response latencies are activated first
The time difference between the two inputs will make it difficult to reach threshold, making it harder to perceive the motion

Motion Perception

The dorsal visual stream is a hierarchical pathway for motion perception
The dorsal visual stream includes V1, MT, MST, and PPC
V1 encodes the direction of stimulus movement
MT performs global motion integration
MST integrates motion over larger areas and is involved in the perception of complex motion patterns

Akinetopsia

Akinetopsia is motion blindness
Bilateral damage to the MT or MST area causes akinetopsia
Akinetopsia makes it difficult to perceive fluid motion, objects appear to appear and disappear

Blindsight

Blindsight is a condition where patients with damage to V1 cannot consciously perceive visual stimuli
Despite their inability to see, patients with blindsight can still respond to visual stimuli
There are alternative motion processing pathways that bypass V1 and project to MT

Alternative Motion Processing Pathway

Motion processing can occur without the involvement of V1.
One such pathway involves the superior colliculus, an area that processes visual information from the retina and sends it directly to MT. This pathway contributes to the perception of motion direction and speed.
Another pathway involves the pulvinar nucleus, a thalamus structure that receives input from V1 and projects to MT. The pulvinar participates in the perception of complex motion patterns, including the motion of multiple objects.

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Description

Explore the intricate mechanisms of how visual signals are processed in the retina. This quiz covers ooDSGCs, wavelength detection through cones, and the principles of color discrimination. Test your understanding of the neural connections and functions involved in vision.