Vision and Light Interaction

Vision and Light Interaction

• Light interaction with the eyes enables vision, providing information about the surroundings.
• Light can behave as a wave or a particle referred to as a photon.
• Visible light's electromagnetic radiation ranges from 400 to 700 nanometers in wavelength.
• Wavelength dictates color, while intensity determines brightness.
• Light enters the eye through the pupil.
• The iris controls the amount of light that passes.
• The lens focuses light onto the retina.
• Binocular disparity is when the difference in image position on the two retinas gives depth perception.

Retinal Structure and Function

• The retina consists of five layers of neurons: receptor cells, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
• Photoreceptors referred to as rods and cones, are located at the back of the eyeball.
• Signals propagate to the retinal ganglion cells, and their axons leaving the eye create a blind spot.
• Rods and cones control different kinds of vision based on light availability.
• Cones mediate photopic vision, which occurs with high detail in bright light.
• Rods mediate scotopic vision in dim conditions, lacking fine details.
• Cones have low convergence, enabling high detail vision.
• Rods have high convergence, trading acuity for sensitivity in dim light.
• The fovea, located in the center of the retina, contains only cones and specializes in high-acuity vision.
• Rods reach maximum density 20 degrees away from the fovea.
• Cones (photopic vision) are most sensitive to ~560 nm (yellow), while rods (scotopic vision) are most sensitive to ~500 nm (bluish-green).

Color and Perception

• The Purkinje effect describes the shift in brightness perception as light dims, altering sensitivity.
• Trichromatic color vision relies on three types of cones: red, green, blue, or L, M, S.
• Differential activation of cones allows humans to perceive a wide range of colors.
• Opponent process theory explains color perception through excitatory and inhibitory responses
• Negative afterimages occur due to cone fatigue and the firing of opposing cells.
• When looking at specific colour causes other cones not to fire at all, shifting to a blank surface leads to just the opposing cells firing

Visual Transduction

• Rhodopsin, a G-protein-coupled receptor in rods, absorbs light and informs light detection.
• In darkness, inactive rhodopsin keeps sodium channels open, depolarizing the rod and releasing glutamate, which inhibits bipolar cells, preventing neurotransmitter release.
• Light bleaches rhodopsin, closing sodium channels, hyperpolarizing the rod, minimizing glutamate release, and enabling bipolar cells to depolarize and release neurotransmitters.
• Rods contain one pigment, rhodopsin, whereas cones contain three, facilitating color perception.

Visual Pathways and Cortical Processing

• Action potentials from retinal ganglion cells travel through retina-geniculate-striate pathways.
• The optic nerve carries signals to the lateral geniculate nuclei in the thalamus, then to the primary visual cortex.
• Axons from ganglion cells form the optic nerve.
• The left visual field's inputs go to the right primary visual cortex, while the right visual field's inputs go to the left primary visual cortex.
• Parvocellular layers (P layers) respond to color, fine details, and slow-moving objects.
• Magnocellular layers (M layers) respond to moving objects.
• Vertical columns in the visual cortex correspond to specific retinal areas from each eye.
• The primary visual cortex receives input from the lateral geniculate nuclei.
• The secondary visual cortex receives input from the primary visual cortex.
• The visual association cortex receives input from the secondary visual cortex and other brain areas.
• The primary visual cortex is located in the posterior occipital lobes.
• The secondary visual cortex is located in the prestriate cortex and inferotemporal cortex.
• The visual association cortex is located in the posterior parietal cortex and other cerebral areas.
• Visual information flows from the primary cortex to the secondary to the association cortex.
• The dorsal stream interprets spatial information, while the ventral stream interprets object characteristics.
• The dorsal stream flows from the primary visual cortex to the dorsal prestriate cortex to the posterior parietal cortex.
• The ventral stream flows from the primary visual cortex to the ventral prestriate cortex to the inferotemporal cortex.
• The secondary and association cortices interpret information brought to the primary cortex.

Exploration of light's role in vision, covering its wave-particle duality and the visible spectrum. Focus on retinal structure, photoreceptors (rods and cones), and neural pathways. Also includes binocular disparity.