Visible Light and Eye Focus Mechanisms

Visible Light

• Visible light is a small portion of the electromagnetic spectrum detectable by the human eye, ranging from 360 to 780 nm
• Different wavelengths of visible light are perceived as different colors
• Blue light has a shorter wavelength, between 450-495 nanometres
• Red light has a longer wavelength, between 620-750 nanometres

Focus Mechanisms of the Eye

• Lenses form images through refraction
• Convex lenses converge light rays to a single focal point
• Concave lenses diverge or spread light rays
• Both the cornea and lens act as converging lenses
• The cornea is a hard, transparent surface that provides the greatest optical power
• Most of the light refraction occurs as light enters the cornea and aqueous fluid, due to corneal curvature and refractive index change
• The lens, a flexible and adjustable convex structure, fine-tunes focus via ciliary muscle action, attached by ligament fibres
• Ciliary muscle contraction causes the lens to become more rounded for near objects
• Ciliary muscle relaxation flattens the lens for distant objects
• Accommodation is the lens's ability to adjust shape for focusing on near or far objects
• Short-sightedness (myopia) occurs when the lens is too curved or the eyeball is too long causing too much light refraction
• Light over-refracts causing the uncorrected image to fall in front of the retina
• Long-sightedness (hyperopia) occurs when the lens is less curved or the eyeball is too short which results in insufficient light refraction
• Light under-refracts such that the uncorrected image falls behind the retina

Embryonic Development of the Eye

• Eye development starts around the third week of gestation
• The optic vesicles from the diencephalon infold to create the optic cup
• The lens of the eye starts from an area of thickening called the lens placode
• The lens vesicle forms via gradual transformation of the lens placode, creating a lens pit
• The lens vesicle then matures into the lens
• The optic stalk connects the optic cup to the diencephalon and becomes the optic nerve

The Retina

• After light passes through the lens, it goes through the vitreous humour and then to the retina
• The retina is a light-sensitive, layered 2D tissue at the back of the eye
• The fovea is a small, central area, with the highest visual acuity due to densely packed photoreceptors
• When eyes focus on something, light is directed onto the fovea and signals are sent through the optic nerve
• The blind spot is where the optic nerve exits
• The blind spot has no photoreceptors, creating a gap in the visual field
• Retina layers are structured back to front, with photoreceptors located at the back
• Light has to travel through the entire depth of the retina
• Light may be scattered or absorbed in non-photosensitive regions
• Neural processing kicks in, once light reaches the outer parts of the receptors

Photoreceptor Cell Layer

• Photoreceptors are light-sensitive cells which turn light energy into electrical signals
• There's two types: rods and cones
• Rod cells: 120 million cells, achromatic, responsible for night vision (scotopic vision)
• Rods are extremely sensitive under low light conditions and can respond to individual photons
• Rhodopsin, a photopigment most sensitive to blue/green wavelengths, is located in the outer portion
• Located mainly in the periphery
• Lack of presence in fovea contributes to poor visual acuity
• Dark adaptation involves a slow recovery of visual sensitivity, around 30 minutes, after moving from bright to dim environments
• Retinas adjust to lower levels of illumination, thus switching from cone activity in light to rod activity in darkness (change in sensitivity)
• Bright light bleaches the rods, resulting in overstimulation/saturation
• Sensitivity recovery is assessed via a briefly flashed test probe in complete darkness after a bleaching light
• The recovery plot is often biphasic, involving both cone- and rod-mediated sections
• During cone-mediated section, dark adaptation represents recovery of cone system sensitivity at a quicker pace than recovery of rod system
• The transition is termed the rod-cone break
• During the rod-mediated phase, rhodopsin replenishes/rods become more sensitive

Cone Cells

• Cone cells consist of 6 million cells
• Cone cells enable chromatic day vision otherwise know as photopic vision
• The trichromatic arrangement is the typical version for human perception of specific light wavelengths
• Long-wavelength (L) "red" cones are ~560 nm
• Middle-wavelength (M) "green" cones are ~530 nm
• Short-wavelength (S) "blue" cones are ~420 nm
• Each photopigment in cone cells has a sensitivity function, typically bell-shaped
• Cones' spectral sensitivities aren't static but overlapping, allowing response to a broad range of the visible spectrum
• The brain identifies colours through 3 opposing systems
• Cones have photopigment, opsin, dependent on the type of cone cell
• High cone density in the fovea is associated with visual acuity, despite rods outnumbering them elsewhere in the retina

Bipolar Cell Layer

• Horizontal cells are inhibitory interneurons
• Horizontal cells transfer information laterally
• Horizontal cells synapse with photoreceptor and bipolar cells, creating an indirect pathway
• Horizontal cells get chemical synaptic inputs from cones/rods
• Horizontal cells generate feedback signals that modulate neurotransmitter release
• Help integrate/regulate signals by providing feedback/feedforward inhibition to photoreceptors/bipolar cells
• Bipolar cells create main pathways: photoreceptors to ganglion cells
• Interface: Ganglion cells connect with bipolar cells directly or indirectly through amacrine cells
• Amacrine cells are pre- and post-synaptic
• Amacrine cells modulate signal transfer between bipolar and output neurons (ganglionic)

Ganglion Cell Layer

• Retinal ganglion cells are main output neurons, linking retinal input to visual processing centers in the nervous system
• 120 million rods & 6 million cones transmit through 1.5 million retinal ganglion cells
• Ratio: approx. 100 rods & 4 cones per ganglion cell
• Retinal neuronal layers apply signal "pre-processing"
• The input is processed from photoreceptor clusters, which represents a small section of the overall retinal image (neural receptive field)
• Midget cells link to the parvocellular layers of the lateral geniculate nucleus
• Midget cells possess small receptive fields, high spatial resolution (input principally from single cone located centrally), and strong, slow colour selectivity
• Parasol cells project to magnocellular layers in the lateral geniculate nucleus
• Parasol cells have low spatial resolution, larger receptive fields, rapid and transient response with poor colour selectivity

Receptive Fields of Ganglion Cells

• The retinal area will produce a firing change in a ganglion cell
• Every retinal ganglion cell reacts to the light onto particular photoreceptors for input, and neuronal responses only arise in receptive field

Properties of Ganglion Cells

• Retinal ganglion cell receptive field is dependent on position in the retina
• Can be mapped inserting an electrode into the ganglion cell when shining light
• Cells near the fovea possess small receptive fields via input from few photoreceptors, with receptive field increasing with distance from fovea
• Process output: Photoreceptor cluster
• Classified into two main categories, based on stimuli reactions
• Each behaves antagonistically

Ganglion Cell Types

• ON-center/OFF-surround cells depolarize (excited) with greater light exposure, but hyperpolarize (inhibited) when the surrounding field is illuminated
• OFF-center/ON-surround are hyperpolarized when the light hits the middle of the receptive field, however depolarizes when outside of receptive field
• When both regions are stimulated, there is no effect, as the activity stays baseline
• Center-surround fields improve edge details

Edge Detection

• Ganglion cells change during variable light in focus and the surrounding
• Edge detection relies on detecting the point at which one object ends
• Sharp changes mean seeing edges
• Mach bands: uniform bars of light will possess equal light, but show off a gradient; light perception

Properties of Light Analysis

• Cells found in dark regions show decreased reactions
• Conversely, cells within brighter zones will reveal increased activity

Neural Connections

• Retinal ganglion cell axons move through the inner retina to the ocular disk
• Visual signals are delivered to the lateral geniculate nucleus & thalamic structures

Lateral Geniculate Nucleus

• Thalamus structure consisting of six layers that react to input passing through axons
• Ganglion cells connect to first synaptic area
• Optic neurons have a synapse for each neuron layer
• The axons that are on the temporal retina are ipsilateral/terminate on areas 2,3 & 5
• Ganglion axons are contralateral terminate to layers 1,4,6
• Divided within magnocellular & parvocellular areas which correspond to Ganglion cells.

Characteristics of LGN cells

• M cells get feedback passing through retinal nerves
• P cells receive information from bistratified cell(large receptive) found between magnocellular & parvocellular layers in the the blue cones
• Ganglion posses similar focus to retinal ganglion cells
• The neurons in large sections posses a large monochromatic
• Magnocellular cells show increased field size and show increase nerve fiber size
• Colour reaction oppose with spectrally distinct signals
• P Cells have focus reaction to distinct colour combos
• P cells compare long length feedback that releases medium length feedback produces particular colour reaction
• In individual colour cell reaction increases colour and less wavelength release in surround area

Color Opponency

• Instead of monochromatic ganglion cells cells react when light shows in whole area
• The are not optimal for finding edges so cells in the cortex are
• Double colour reacts if there is greater amounts
• Light won't enter in homogeneous light
• Red On or Green Off cells

LGN Maps

• Both layers show separate layers and functionality
• Every layer shows topography mapping of the visial area
• The neurones on the column react to similar area
• Neurons axons also remain in same position
• V1 is related to fovea

Visual Cortex

• Receives input
• Has has 6 laminas
• Specificity
• Simple Cells means show in layers 4 & 6
• Focus will rely with specific type like hate increasing the increasing with hat
• ON / OFF regions