Neuroscience of Vision: Cone Responses
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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?

    <p>Superior colliculus</p> Signup and view all the answers

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

    <p>It activates phospholipase C</p> Signup and view all the answers

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

    <p>Surround response</p> Signup and view all the answers

    What is the function of the optical pretectal nucleus?

    <p>Controlling eye dilation</p> Signup and view all the answers

    Which protein do intrinsically photosensitive ganglion cells express?

    <p>Melanopsin</p> 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?

    <p>It enhances the VOR.</p> Signup and view all the answers

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

    <p>It suppresses the VOR.</p> Signup and view all the answers

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

    <p>Cortical inputs dominate over retinal inputs.</p> Signup and view all the answers

    What primary roles does the vestibular system serve?

    <p>To maintain balance and gaze fixation.</p> Signup and view all the answers

    What type of motion does the vestibular system primarily detect?

    <p>Translational motion along various axes.</p> Signup and view all the answers

    What occurs in the preferred direction of light for ooDSGCs?

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

    What characterizes the null direction of light for ooDSGCs?

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

    How do cone responses contribute to color discrimination?

    <p>The combination of cone responses encodes specific wavelengths.</p> Signup and view all the answers

    What role does melanopsin play in response to light stimuli?

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

    What is observed in blind rd1 mice concerning ipGCs?

    <p>Activity of ipGCs is absent due to degeneration.</p> Signup and view all the answers

    What happens to ooDSGCs when they receive little inhibition?

    <p>They respond rapidly and fire frequently.</p> Signup and view all the answers

    What is the significance of the combination of cone responses?

    <p>Allows the encoding of specific wavelengths for color perception.</p> Signup and view all the answers

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

    <p>It significantly reduces their firing rate.</p> Signup and view all the answers

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

    <p>To respond quickly to a wide range of luminance</p> Signup and view all the answers

    How do rods and cones work together in vision?

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

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

    <p>It increases with background illuminance.</p> Signup and view all the answers

    What happens to rods without light adaptation?

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

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

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

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

    <p>Recovery of photoreceptors from stimulus exposure</p> Signup and view all the answers

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

    <p>Rods have slower response kinetics compared to cones.</p> Signup and view all the answers

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

    <p>It becomes more challenging due to increasing noise.</p> 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?

    <p>Akinetopsia</p> Signup and view all the answers

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

    <p>Area MT</p> Signup and view all the answers

    Which cells in V1 encode the direction of stimulus movement?

    <p>Direction selective cells</p> Signup and view all the answers

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

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

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

    <p>Area MT</p> Signup and view all the answers

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

    <p>Neurons with shorter response latencies activate first.</p> Signup and view all the answers

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

    <p>They bypass V1 to project directly to area MT.</p> Signup and view all the answers

    What is the primary function of the dorsal visual stream?

    <p>Motion perception and spatial awareness.</p> 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.

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