Retinal Stimulation Factors

• Retinal stimulation involves a macroscopic understanding of the retina, an organ used in vision.

Factors Affecting Retinal Stimulation:

• Duration of exposure and light intensity have an inverse relationship.
• Less light intensity requires longer exposure time to generate a nerve impulse.
• Shorter exposure time requires high light intensity.
• Small pupils need higher light intensity thresholds due to the limited light entering.
• Large pupils need lower light intensity thresholds due to a bigger opening.
• Small retinal areas have higher light intensity thresholds; big areas have lower thresholds.
• The central retina which utilizes cones has a higher light intensity threshold.
• The peripheral area which utilizes rods has a lower light intensity threshold and generates a lower nerve impulse.
• For light-adapted eyes, yellow has the lowest threshold; for dark-adapted eyes, green has the lowest threshold.
• During daytime vision, yellow is the most sensitive color.
• Green is easily seen at nighttime.
• In photopic vision, the light intensity threshold is higher; in scotopic vision, it is lower.
• Monocular vision requires a stronger stimulus.
• Binocular vision needs lower light intensity to generate a nerve impulse.
• Retinal implants are electronic devices that electrically stimulate surviving retinal cells to create image perception.
• Electrode arrays are distributed in the epiretinal, subretinal, or suprachoroidal areas of the retina.
• Disconnection leads to blindness.
• If someone is blind at birth, a retinal implant will not work.
• Epiretinal arrays are placed between the vitreous humor and retina.
• Subretinal arrays are placed within the layers of the retina.
• Suprachoroidal arrays are placed between the retina and the choroid.
• In retinal detachment, the retina is not detached from the choroid, but rather between its layers.
• Pigmented epithelium is closely attached to the choroid

Rods vs Cones

• Rods have a long, slow response time, bleach, and demonstrate saturation by 10%.
• Cones have a short, and fast response time, do not bleach, and do not demonstrate saturation.
• Adaptation time is directly proportional to the regeneration/resynthesis of the visual purple and is longer in rods because they undergo bleaching, while cones do not.
• Strong stimulus = fast response, and weak stimulus = slow response.
• Rods have high light sensitivity, movement sensitivity, achromatic color sensitivity, and are sensitive to blue-green light (507 nm).
• Cones have low light sensitivity, color sensitivity, chromatic color sensitivity, and are sensitive to yellow-green light (555 nm).
• Rods are located in the peripheral retina, are cylindrical in shape, and contain rhodopsin.
• Cones are located in the central retina, are conical in shape, and contain photopsin/iodopsin.
• Rod pathways are highly convergent (30:1), while cone pathways are less convergent (1:10).
• States that there are functionally two distinct ways that our eyes work.
• The photopic system is associated with cones, and the scotopic system is associated with rods.
• Daytime vision is photopic, nighttime vision is scotopic.
• Rods and cones independently contribute to different aspects of visual perception.
• Duplex retina refers to the existence of two classes of photoreceptors that operate under different lighting conditions.
• Scotopic vision uses rods, photopic vision uses cones, and mesopic vision uses both.
• Candela per sq. meter relates to luminance.
• Retinal illuminance is the amount of light received by the retina.
• The amount of light outside is not equal to the amount of light inside the eye
• Spatial summation relates to rods
• Rods are good at summation because their convergence in the retinal pathway is 30:1, requiring them to add up to produce a stimulus.
• Spatial resolution and temporal resolution are functions of cones.
• Resolution is individualistic.
• Spatial resolution, also known as visual acuity, is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc.
• Light and dark adaptation refers to the ability of the visual system to adjust to prevailing light conditions, allowing the recovery of sensitivity, characterized by photochemical, neural, and photomechanical processes.
• Photochemical adaptation involves the resynthesis of visual pigment/rhodopsin.
• Neural adaptation involves the recovery of active membranes and related processes, changes of the membrane of photoreceptors
• Photomechanical adaptation involves morphological changes in the receptor cells.
• After bleaching, the retinal chromophore becomes waste and is engulfed by RPE.
• Low light intensity causes rods to generate rhodopsin → dark adaptation.
• High intensity bleaches rhodopsin, making rods nonfunctional, shifting dominance to cones → light adaptation.
• Dark adaptation refers to how the eye recovers its sensitivity in the dark after exposure to bright lights.
• Initial blackness is seen because cones cease to function in low light intensity and rods have been bleached out, making them initially nonfunctional.
• Rhodopsin regenerates in the dark, increasing retinal sensitivity over time.
• The slow increase in light sensitivity is related to the rate of regeneration of photopigments and the intracellular concentration of calcium.
• Light adaptation refers to how the eye recovers its sensitivity in the light after exposure to dim illumination.
• Bleaching of rods causes them to stop working, and cones take over.
• Immediate blinding sensation when moving from dark into bright light is caused by massive photopigment breakdown.
• Glare adaptation takes shorter than adapting to dark illumination Occurs by decreasing retinal sensitivity
• After-image is a visual effect that persists after the stimulation has ceased.
• Causes continuous generation of electrical current

Types of After Images:

• Positive: The afterimage appears in the same color as the object seen.
• Negative: The afterimage appears in the complementary color of the object seen.
• Light adaptation is a process by which neurons adjusts to illumination designed to distinguish objects in the background.
• Contrast adaptation is a process by which neurons adjusts to slowly changing or static contrasts levels designed for information maximization
• Perceptual adaptation is a post sensory/decision making process by which visual system alters its operating properties in response to changes in the environment

Muscular Considerations

• Accommodation or focusing ability
• Eye teaming skill or Binocularity
• Eye movement skill or Tracking with fixation, pursuits and saccades