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Questions and Answers
What is the primary role of intrinsically photosensitive ganglion cells (ipGCs)?
Which type of color opponency is associated with the red-green color discrimination?
What structure of the brain is involved in allodynia, which is the perception of non-painful stimuli as painful?
Which of the following structures is responsible for coordinating eye movements?
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What happens when light activates the G-protein coupled receptor in intrinsically photosensitive ganglion cells?
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Which type of response is determined by the opponent cone photoreceptor feeding into horizontal cells?
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What is the function of the optical pretectal nucleus?
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Which protein do intrinsically photosensitive ganglion cells express?
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What effect does attempting to fixate on a static object while moving have on the vestibulo-ocular reflex (VOR) in fish?
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In cats, what impact does fixating on a moving object have on the VOR?
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Which statement best describes how the relative inputs from the retina and cortex function in monkeys?
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What primary roles does the vestibular system serve?
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What type of motion does the vestibular system primarily detect?
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What occurs in the preferred direction of light for ooDSGCs?
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What characterizes the null direction of light for ooDSGCs?
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How do cone responses contribute to color discrimination?
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What role does melanopsin play in response to light stimuli?
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What is observed in blind rd1 mice concerning ipGCs?
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What happens to ooDSGCs when they receive little inhibition?
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What is the significance of the combination of cone responses?
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In the null direction of light, what effect does increased GABA release have on ooDSGCs?
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What is the main function of cones in the visual system?
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How do rods and cones work together in vision?
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What affects the threshold for detecting stimuli according to Weber's Law?
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What happens to rods without light adaptation?
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What is true regarding the sensitivity of cones and rods in different lighting?
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What is one of the mechanisms related to light adaptation in photoreceptors?
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In terms of response kinetics, how do rods and cones differ?
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What happens to the ability to detect stimuli as background light levels increase?
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What condition is characterized by an inability to perceive fluid motion after bilateral damage to area MT or MST?
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Which part of the hierarchical model of motion perception is responsible for performing global motion integration?
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Which cells in V1 encode the direction of stimulus movement?
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What is a key feature of blindsight in patients with damage to V1?
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In the processing of motion in the brain, which area comes after V1 in the hierarchical model?
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What happens when stimuli move in a non-preferred direction in terms of neuronal response latencies?
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Which of the following statements correctly describes the alternative motion processing pathways?
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What is the primary function of the dorsal visual stream?
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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.