Photons, Eye Structure & Visible Light

• Radiant energy has wavelength (distance between two successive peaks) and frequency (cycles/second).
• Radiant energy can be described as a stream of photons (massless particles with wavelike movement).
• Visible light is a small portion of electromagnetic radiation that stimulates photoreceptors, with a wavelength of 400-750 nanometers.
• Different wavelengths of visible light are perceived as different colors.

The Eye: Structure

• The eye is a three-layered, fluid-filled ball.
• The outer layer is the sclera, a tough fibrous white capsule where eye muscles attach.
• The middle layer is the choroid, a dark unreflective layer with a rich blood supply, preventing light reflection inside the eye.
• The inner layer is the retina, the light-sensitive part containing photoreceptors.
• The lens divides the eye into two chambers: the anterior chamber with aqueous humor and the posterior chamber with vitreous humor.
• The cornea is the clear part of the sclera.
• The iris is a ring-like structure surrounding the pupil.
• The pupil is the opening that lets light into the eye.
• The lens is adjustable and focuses the object image on the retina.
• The optic nerve (cranial nerve II) carries information from retinal ganglion cells to the brain.

Optics of the Eye

• The cornea and lens have convex surfaces that helps focus light onto the retina.
• Optics cause an up-down and left-right reversal of images.
• Accommodation adjusts the lens curvature: contraction of the ciliary muscle makes the lens rounder for near vision; relaxation makes it flatter for far vision.
• Myopia (nearsightedness) occurs when light rays focus in front of the retina.
• Hyperopia (farsightedness) occurs when light rays focus behind the retina.
• Glasses correct these focus issues.

Structure of the Retina

• The retina has five major cell types: photoreceptors, bipolar cells, ganglion cells, horizontal cells, and amacrine cells.
• It contains five prominent layers: outer nuclear layer, inner nuclear layer, ganglion cell layer, outer plexiform layer, and inner plexiform layer.

Circuitry of the Retina

• Retinal layers and cells include:
  • Photoreceptors (rods and cones) for phototransduction
  • Bipolar cells receiving input from photoreceptors and synapsing with ganglion and amacrine cells
  • Ganglion cells transmitting output signals to the optic nerve
  • Horizontal cells transmitting signals horizontally between rods and cones (lateral inhibition)
  • Amacrine cells transmitting signals directly to ganglion cells or horizontally.
• Visual acuity decreases as light passes through these layers, except at the fovea, where layers are pulled aside for increased acuity.

Photoreceptors

• Rods detect dim light and are for vision in the dark.
• Cones are for color vision and operate under brighter conditions.
• Major functional segments:
  • Outer segment: contains disks housing photosensitive pigments
  • Inner segment: contains cytoplasm and cytoplasmic organelles
  • Synaptic terminals: connect with horizontal and bipolar cells
• Photoreceptors transmit information by graded potentials, while ganglion cells use action potentials to transmit the final retinal signal.
• Photoreceptors hyperpolarize in response to light.
• In the dark, high cGMP levels keep Na+ permeable channels open (depolarization).
• In the light, cGMP levels decrease, closing the channels and causing hyperpolarization.

Visual Signal Transmission

• Rods and cones release neurotransmitters to stimulate other retinal cells.
• Information travels by direct flow of electric current (electrotonic conduction).
• Only ganglion cells use action potentials.
• Phototransduction leads to hyperpolarization in photoreceptors.

Photoreceptors: Color Perception

• Color pigments are proteins that change structure when exposed to light.
• Rods have rhodopsin (505 nm).
• Cones:
  • Blue sensitive (445 nm)
  • Green sensitive (535 nm)
  • Red sensitive (570 nm)
• Each cone contains only one type of color pigment.
• The nervous system senses different colors because each type of cone has only one type of color pigment.

Color Blindness

• Deuteranopia: loss of green cones, individuals cannot distinguish reds or greens.
• Ishihara Color Test: tests for red-green color deficiencies.

Fovea

• Visual acuity decreases with light passing through retinal layers, but at the fovea these layers are pulled aside to increase acuity.
• The fovea is less than 0.5 mm in diameter.
• Maximum visual acuity occurs in less than 2 degrees of the visual field.
• It contains the highest concentration of cones and the lowest concentration of rods.
• Outside the fovea, visual acuity is poorer because more rods and cones connect to each optic nerve fiber.
• The optic nerve exits the eye without rods or cones, creating a blind spot.

Peripheral Retina

• There is convergence of information; multiple cones and rods feed into a single ganglion cell, creating larger receptive fields in the periphery.

Visual Receptive Field

• The receptive field is the area on the receptor sheet where stimulation increases or decreases neuron activity.
• Receptive field size and complexity increase along the visual pathway.
• The receptive field of a central neuron sums receptive fields of all cells with convergent input.
• Often referred to as hierarchical or serial processing.
• Retinal ganglion cells have center/surround organization, with on-center/off-surround and off-center/on-surround types.
• Photoreceptors release Glutamate, but the transmitter has opposite effects on the on/off-center bipolar cells.
• Off-center bipolar cells have sign-conserving synapses.
• On-center bipolar cells are sign-inverting synapses.
• When glutamate release is low (light is on):
  • Metabotropic receptors on on-center bipolar cells depolarize (removal of inhibition).
  • Ionotropic receptors on off-center bipolar cells hyperpolarize (reduced excitation).
• When glutamate release is high (light is off):
  • Metabotropic receptors on on-center bipolar cells hyperpolarize (increased inhibition).
  • Ionotropic receptors on off-center bipolar cells depolarize.

Lateral Inhibition

• Horizontal cells receive glutamate from photoreceptors; more glutamate in the dark, less in the light.
• Horizontal cells release GABA onto neighboring photoreceptors, modulating neurotransmitter release on bipolar cells.
• GABA release from horizontal cells hyperpolarizes photoreceptors.
• Light in the surround depolarizes photoreceptors in the center (decreased inhibition); dark in the surround hyperpolarizes photoreceptors in the center (increased inhibition).
• The surround mechanism opposes the central photoreceptor’s response when illumination to the center and surround are similar and augments it when they differ.
• In the Hermann Grid Illusion, on-cells whose receptive field center is at an intersection has larger area of inhibitory surround exposed to light, so the brain thinks there is less light there, hence perceived darkness at intersections.

Visual Pathway

• Each optic nerve conveys information about the left and right visual field.
• The lateral portion of the optic nerve carries action potentials from retinal ganglion cells in the temporal retina and conveys information the contralateral (opposite) visual field
• The medial portion of the optic nerve carries action potentials from retinal ganglion cells in the nasal retinal, conveys information from the ipsilateral (same) visual field.
• At the optic chiasm, axons in the medial portion of each optic nerve cross to the opposite side.
• The optic tract contains a representation of the contralateral visual field.
• The majority (~80%) of neurons in the optic tract continue to the lateral geniculate nucleus (LGN) of the thalamus.
• The nasal retina processes information from the contralateral visual field and sends axons to the LGN on the contralateral side of the brain.
• Neurons in the LGN send axons to innervate primary visual cortex (V1).
• Retinal P cells project to the parvocellular layers of the LGN and are responsible for color, fine textures, patterns, longer latency, and fine detail.
• Retinal M cells project to the magnocellular layers of the LGN and are responsible for motion detection, shorter latency, coarser detail.

Visual Pathway: Lateral Geniculate Nucleus

• The LGN contains 6 major cellular layers: Magnocellular (lower two layers) and Parvocellular (upper four layers).
• Each layer has a complete map of the contralateral visual field but receives input from only one eye, either ipsilateral (2, 3, and 5) or contralateral (1, 4, and 6). Segregation of information:
  • Magnocellular: larger receptive fields, better sensitivity, motion processing (course details).
  • Parvocellular: smaller receptive fields, better acuity, color processing (fine details).
• Lesions at various points in the visual pathway will result in different visual deficits.

Primary Visual Cortex (V1)

• Visual information travels from the LGN to the occipital lobe (primary visual cortex).
• The visual field and primary visual cortex is divided into 12 sections.
• Each half of the visual field is represented (mapped) on the opposite side of the cortex.
• There is more cortical representation devoted to the fovea (areas 1-4).
• There is a magnification of the foveal region in the cortical map.
• Visual cortex are similar to somatosensory cortex devoted processing is in proportion to receptor density rather than size.

Primary Visual Cortex (V1): Ocular Dominance

• Retinal ganglion cell inputs from each eye are segregated, projecting to specific layers in the LGN and maintaining segregation up to primary visual cortex.
• Ocular dominance columns: neurons in a column respond mainly to stimulation of one eye.
• Adjacent column neurons respond to stimulation of the other eye.
• Binocular cells at edges of columns respond to stimulation of both eyes and play a role in depth vision.

Simple vs Complex Cells

• Simple cells in the primary visual cortex respond to oriented edges and gratings, they have elongated subregions that respond either to light or dark.
• Complex cells are the most common type in the primary visual cortex and are also orientation selective.
• Unlike simple cells, complex cells do not have subregions selective for light or dark, they respond to both.
• Some simple cells and complex cells are selective for motion in addition to orientation.
• Simple cells are built up from many LGN cells with the same center/surround structure, whose centers are distributed along a line.
• Complex cells are built up from many simple cells with the same preferred orientation and overlapping receptive fields, but different arrangement of subregions.

Orientation Tuning

• Orientation columns: Simple cells within these columns have the same orientation.
• Within each ipsilateral and contralateral ocular dominance column all orientation columns are represented.

Higher Levels of Visual Processing

• Two Streams of Visual Analysis: Dorsal ("Where" pathway for location and action) and Ventral ("What" pathway for object recognition).
• P-cell information goes to the ventral stream, M-cell information goes to the dorsal stream.
• The two pathways are not totally independent.
• The inferotemporal cortex in the ventral pathway responds to complex stimuli like faces.