Neuroscience of Vision: Cone Responses

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.

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.