Physiology of the Visual System

Physiology of the Visual System

• Visible light makes up a small portion of the electromagnetic spectrum, detectable by the human eye within the range of 360-780 nm and is sensitive to the part of the electromagnetic spectrum emitting the most energy.
• Light of varying wavelengths is seen as different colors.
  • Blue light has a shorter wavelength, ranging from 450-495 nanometers
  • Red light has a longer wavelength, between 620-750 nanometers.

Focus Mechanisms of the Eye

• Lenses form images by refracting light.
  • Convex lenses converge light rays to a single point (focus)
  • Concave lenses diverge or spread out light rays.
• The cornea and lens work together as converging lenses to focus light onto the retina.
  • The cornea allows light to enter providing the most optical power.
  • The majority of focusing is done as light enters the cornea and aqueous fluid, due to the curvature of the cornea and refractive index differences.
• The lens is a flexible, adjustable, soft convex lens that fine-tunes focus.
  • The shape of the lens is altered by ciliary muscles attached via ligament fibers.
• When looking at near objects, the lens becomes more rounded due to ciliary muscle contraction; for distant objects, the lens flattens via ciliary muscle relaxation.
• Accommodation refers to the lens's ability to change shape and focus based on the distance of objects.
• Short-sightedness (myopia) means the lens is more curved than normal, or the eyeball is too long.
  • The uncorrected image falls in front of the retina because light is refracted too much.
• Long-sightedness (hyperopia) means the lens is less curved than normal, or the eyeball is too short.
  • The uncorrected image falls behind the retina because light is not refracted enough.

Embryonic Development of the Eye

• Eye development begins around the third week of human gestation.
• Optic vesicles, originating from the diencephalon, infold or invaginate to form the optic cup.
• The lens of the eye develops from a thickening called the lens placode.
• The lens vesicle forms from the gradual transformation of the lens placode into the lens pit.
• The lens vesicle matures into the lens proper.
• The optic stalk connects the optic cup to the diencephalon and becomes the optic nerve.

The Retina

• Light passes through the vitreous humor and onto the retina after passing through the lens.
  • The retina is a light-sensitive, highly organized, layered 2D tissue at the back of the eye.
• The fovea is a small, central region with the highest visual acuity due to a dense concentration of photoreceptors.
• Light from a focused point is directed onto the fovea, sending signals through the optic nerve.
• The blind spot occurs where the optic nerve exits the eye, creating a gap in the retina without photoreceptors, causing a break in the visual field.
• The retina's layers are structured back to front, with photoreceptors at the back, requiring light to travel through its depth, causing scattering and absorption.
• Neural processing begins when light reaches the outer parts of the receptors.

Photoreceptor Cell Layer

• Photoreceptors, classified as rods and cones, are unipolar neurons that engage in phototransduction.
• Rod cells, about 120 million, are achromatic and responsible for night vision (scotopic vision)
  • They are sensitive in low-light conditions and respond to single photons of light, ideal for night vision.
  • Rhodopsin resides in the outer segment, reacting with light and most sensitive to blue and green wavelengths
  • Rods are predominantly located in the periphery of the retina, absent from the fovea, and provide poor visual acuity.
  • Dark adaptation involves a slow recovery of visual sensitivity, about 30 minutes, after exposure to intense light.
• The retina adapts to decreasing light levels, transitioning from light-adapted (photopic) to dark-adapted (scotopic) states, changing light sensitivity from cone to rod activity.
• Bright light can lead to rods entering a state of overstimulation or saturation, reducing sensitivity.
• Recovery after exposure to a bleaching light is assessed using a briefly flashed test probe in darkness.
• The recovery plot has cone and rod-mediated sections and is biphasic.
  • The cone-mediated section represents the initial and quicker portion of dark adaptation due to recovery of the cone system.
  • The transition point between cone-mediated thresholds and rod-mediated thresholds is the rod-cone break.
  • Rod-mediated section replenish rhodopsin for more sensitive sensitivity.
• Cone cells, approximately 6 million, enable chromatic day vision (photopic vision).
  • color vision is possible through having three different types of cone cells, that have photopigment sensitive to a specific light wavelength (trichromatic arrangement)
  • Long-wavelength (L) cones are “red” cones, sensitive to ~560 nm.
  • Middle-wavelength (M) cones are “green” cones, sensitive to ~530 nm.
  • Short-wavelength (S) cones are “blue” cones, sensitive to ~420 nm.
  • a bell-shaped sensitivity function is a defining trait of each photopigment for each type of cone cell
• Spectral sensitivities of cones are not static and differ, resulting in an overlap that is broad enough to enable responsiveness to light throughout much of the visible spectrum.
• Brain processes color throughout three different opposing systems.
• Opsin is a type of photopigment and is contained within cones in the type of cone cell (“red-sensitive”, “green-sensitive” or “blue-sensitive” opsin)
• High desnity of cones in he fovea contribute to visual acuity, even though rod cells outnumber cone cells

Bipolar Cell Layer

• Horizontal cells are inhibitory interconnecting interneurons that transfer information laterally.
• Horizontal cells synapse with bipolar and photoreceptor cells, forming an indirect pathway to bipolar cells.
  • Horizontal cells receive chemical synaptic inputs from rods and cones producing feedback for neurotransmitter release.
    • The feedback and feedforward inhibition is lateral, helps integrate signals, and regulate signals from photoreceptor cells.
• Bipolar cells provide the main pathways from photoreceptors to ganglion cells, interfacing with ganglion cells directly or via amacrine cells.
• Amacrine cells, being presynaptic to bipolar cells and postsynaptic to retinal ganglion and amacrine cells, modify signal transfer between bipolar cells and the output neurons (ganglionic cells).

Ganglion Cell Layer

• Retinal ganglion cells are the main output neurons of the retina, connecting the retinal input to visual processing centers in the central nervous system.
• The retina has 120 million rods and 6 million cones in the eye, with the output transmitted by ~1.5 million retinal ganglion cells, that means approximately 100 rods and 4 cones per ganglion cell and significant “pre-processing” of the visual signal by retinal neural layers.
• Input comes from clusters of photoreceptors, representing only a part of the total retinal image, or the neural receptive field.
• Midget cells project to the parvocellular layers of the lateral geniculate nucleus.
  • They have small receptive fields, high resolution due to input principally in central fovea, strong colour selectivity from unequal configuration and slow, sustained response.
• Parasol cells project to the magnocellular layers of the lateral geniculate nucleus.
  • They have larger receptive fields, low resolution, colour selectivity, rapid and transient response.

Receptive Fields of Ganglion Cells

• The area of the retina where a small spot of light produces change in ganglion cell firing rate.
• Each retinal ganglion cell responds to light projected onto some photoreceptors from which it receives input, meaning neuronal responses only occur within this receptive field.
• The receptive field of a retinal ganglion cell depends on its position in the retina that is mapped by shining light at different positions and inserting an electrode into the ganglion cell.
  • Cells near the fovea have smaller receptive fields, reflecting input from fewer photoreceptors but have an increase in size as you move from the fovea.
• Input from the process comes from clusters of adjacent photoreceptors.
• The two broad types are dependent to light stimuli, where the centre and surround react antagonistically (centre-surround organisation; spatial opponency) with recordings showing different patterns.
  • The ON-center cells are depolarized (excited) by increased illumination of the receptive field center, and hyperpolarized (inhibited) when illuminating the surround
  • The OFF-center cells are hyperpolarized (inhibited) by increased illumination of the receptive field center and depolarized (excited) when illuminating the surround
• When excitatory and inhibitory regions are activated and stimulate the environment, the resulting activity remains at baseline
  • responses of ganglion cells change with changes in light falling in one region but not in overall luminance (ratios are key)
  • edge detection (process of distinguishing where objects start and end), corresponds to the qualitative content between neighboring location, like physical boundaries or edges
  • a lack of separation can cause the mach bands illusion, which is where bands of equal luminance will appear to have a gradient and where sections are lighter on one side, and darker on the other

Retinal Ganglion Cells

• Retinal ganglion cells project axons across the retina inner surface towards the optic disk to form the optic nerve.
• Information moves towards the lateral geniculate nucleus and thalamic structures
• The thalamus contains six cellular layers that receive input from the optic tract fibres.
  • The first synaptic site is the retinal ganglion cells and make up the sensory input to the primary visual cortex.
• Fibres from each optic tract form synapses in the layers of the LGN.
  • Ganglion cell axons from the temporal retina remain uncrossed through the chiasm and in layers 2, 3, and 5 will terminate the ipsilateral LGN.
  • Ganglion cell axons from the nasal retina cross in the chiasm and terminate in layers 1, 4, and 6 and form a contralateral lateral geniculate structure.
• Layers are divided into magnocellular (M) and parvocellular (P), correlating with the parasol and midget classes of retinal ganglion to properties of cells layers in LGN.
  • The magnocellular layer receives input from large parasol ganglion cells.
  • The parvocellular layer receives input from small midget ganglion cells.
  • The koniocellular layer received input from bistratified cells and is placed between the parvo- and magno- cellular layers
• Same centre-surround can is had between LGN neurons and retinal ganglion cells
  • The are still 2 states: ON-centre and OFF-surround
• Magnocellular cells are spatially opponent, small and monochromatic but parvocellular are largeer are color opponent.
• Field size increase substantially with retinal eccentricity for both magno- and parvocellular cells.
• Opponent processing of spectrally distinct photoreceptor signals.
  • Parvocellular cells possess centre-surround receptive fields that respond to colours, resulting from approximate one one mapping.
  • The receptive fields of parvocellular the signals cone photoreceptors (both L and M) to produce colour selective responses.
  • This can explain human color perception with the human range of colors
• Single colour-opponent cells, excited (increased firing rate) by stimulus but the color in center is inhibited/surrounded opposite color response

Retinotopic Maps in the Lateral Geniculate Nucleus

• LGN, known to be functionally and spatially segregated and compartmentalised, has each layer with a retinotopic map of one half of the visual field.
• The retinal topography via ganglion cells is maintained, meaning LGN cells have adjacent receptive fields.
• Neurons across layers are sensitive in the same retinal area.
• The retina topography is preserved on its way from the ganglion axons all the way to the to LGN layers for vision to V1.
• The fovea gets greater attention in neuronal devotion (cortical magnification).

Primary Visual Cortex

• It’s the posterior region of the occipital lobe and is responsible for input from the lateral geniculate nucleus and its structure consists of six defined layers
  • Where the LGM receives input and further segregates what comes in with relation to what cell/type (magnocellular/parvocellular/koniocellular).
  • Cell synapse in 4cB + 4A with synapses in 4Ca cells from magnocellular class cells.
• It has a high orientation specificity meaning around 80% of cells are simple cells
  • Are found mainly layers 4 and 6;
  • Respond to tuned stimuli within receptive field with orientations
  • An inhibitory-excitatory arrangement results in Mexican profile hat
  • Have elongated and parallel receptive fields, rather than centre-surround receptive fields or vice versa
• The cells also have edge detectors which are for lines of contrast
  • Some alternating patterns come from combining centre-surround cells in the center of cells
• Many complex cells come from layers (2,3,5);
  • It is where selection that is insensitive based on orientation takes place
• The area has a special phase for insensitivity which means that orientation can exist in all phases that is the receptive field
• Combining receptive fields for receptive fields creates orientation but reduces their sensitivity.
• Some cells can form an end to end hypercomplex connection which will allow the brain to take in optimal information.

Orientation Columns

• The V1 contains a highly structured organisation (which is known as ocular columns) that reflects any preffered
• This alternating pattern results in repeating patterning.
• Some cases can show post-mortem effects to see regions in the brain that are active.
• Some cases after they die they were stained with a chromic structure for better visuals.
• Other layers show an increase or decrease in eccentricity, in this case being dependent on the cortex.

Retinotopic Mapping

• The Retinotopic mapping is responsible for arranging signals in the cortex that are geometric in order to be mapped.
• If not there will be the presence of field as visual field is transformed and the magnified, (center of visual field) there is greater access.