Visible Light and the Eyes

Visible Light

• It is the part of the electromagnetic spectrum that stimulates the eye.
• It produces visual sensations.
• Hue is the color of light and is determined by its wavelength.
• Cosmic rays have wavelengths of only a few trillionths of an inch long.
• Radio waves have signals that extend for miles.
• Roses are red, violets are blue; these are examples of visible light.
• Violet light is the shortest, about 400 billionths of a meter in length.
• Red light is the longest, about 700 billionths of a meter.

Parts of the Eyes

• Cornea: Protects eye and bends light toward the lens.
• Lens: Focuses on objects by changing shape.
• Iris: Controls the amount of light that enters the eye.
• Pupil: Widens or dilates to allow more light in.
• Retina: Neural tissue lining the back of the eyeball, contains vision receptors.
• Rods: Visual receptors that respond to dim light.
• Cones: Visual receptors involved in color vision.
• Most humans have three types of cones.
• Bipolar cells: Neurons that conduct neural impulses from rods and cones to ganglion cells.
• Ganglion cells: Neurons whose axons form the optic nerve.
• Optic nerve: Transmits sensory information from the eye to the brain.
• Fovea: An area near the center of the retina.
• Fovea is dense with cones and vision is most acute there.
• Blind spot: Area of the retina where axons from ganglion cells meet to form the optic nerve.

Near and Far Sightedness

• Issues arise from curvature of the cornea or lens, affecting image focus on the retina.
• Near-sightedness (myopia): Excessive curvature, nearby objects seen more clearly.
• Far-sightedness (hyperopia): Insufficient curvature, distant objects seen more clearly.
• Astigmatism: Irregularity in the shape of the cornea or lens distorts and blurs the image.
• Presbyopia: Lenses grow brittle, affecting focus, typically starting in late 30s to mid-40s.
• The term comes from the Greek words for "old man" and "eyes."
• It makes it difficult to perceive nearby visual stimuli.

Transmitting Images

• Neurons transmit images.
• Ganglion cells form the optic nerve.
• The blind spot is where the optic nerve exits the retina so it does not contain rods or cones.
• Feature detectors: Neurons that distinguish contours, orientation, and basic shapes.
• Feature detectors can be fooled by optical illusions.

Light and Dark Adaptation

• Light adaptation: Occurs when moving from dark to bright light and the sensitivity of the receptors is set to dim light.
• Intense light dazzles due to stimulation of rods and cones.
• Photopigment breaks down, producing glare.
• Dark adaptation: Occurs when moving from light to dark.
• Initial blackness is due to cones ceasing function in low light and rod pigments are bleached.
• Rhodopsin regenerates, increasing retinal sensitivity over time (about an hour).
• Pupil size changes reflexively.

Color Vision

• Hue: Determined by wavelength of light.
• Value: Degree of brightness or darkness.
• Saturation: How intense a color appears.
• Trichromatic theory: Proposed by Young (1802) & von Helmholtz (1852).
• The eye detects three primary colors: red, blue, & green.
• All other colors are derived by combining these three.
• Opponent Process Theory: Proposed by Ewald Herring.
• Color perception is controlled by three receptor complexes with opposing actions.
• The three complexes are: red-green, blue-yellow, black-white.
• The mind registers only one color of a pair at a time because the colors oppose one another.
• Explains why one cannot see yellowish-blue or reddish-green.

Color Vision Systems

• Primate retinas contain three types of photoreceptors.
• Each cone uses a different opsin sensitive to a particular wavelength (blue, red, green).
• It supports the trichromatic theory.
• Trichromat: A person with normal color vision.
• Monochromat: Sensitive to black and white only, hence color-blind.
• Dichromat: Sensitive to black-white and either red-green or blue-yellow, hence partially color-blind.
• Retinex Theory: Proposed by Edwin Land to account for color and brightness constancy.
• The cortex compares information from the retina to determine the brightness and color for each area.

Sensory System and Visual System

• The brain detects external events and directs muscle contractions.
• Afferent neurons carry sensory messages to the brain.
• Efferent neurons carry motor messages to the muscles.
• Stimulus: Any energy capable of exciting a receptor.
• Types of stimuli: mechanical, chemical, thermal, photic.
• Sensory energies are measurable.
• Sensory receptors are specialized nerve cells that transduce energy into neural signals.
• Receptors lack axons and form synapses with dendrites of other sensory neurons.
• Receptors are "mode" specific.
• "Law of Specific Nerve Energies": sensory messages on separate channels go to different brain areas.
• Receptors detect a small range of energy levels: Eye (400-700 nM), Ear (20-20,000 Hz).
• Taste buds detect specific chemicals.
• Visual system function: To detect electromagnetic radiation (EMR) emitted by objects.
• Humans detect light with a wavelength between 400-700 nM.
• Perceived color is related to wavelength of light.
• Brightness is related to the intensity of radiation.
• Functions of vision: To discriminate figure from background, detect movement, and detect color.

Eye Composition

• Aperture (pupil) admits light.
• Lens focuses light.
• Photoreceptive elements (retina) transduce the light stimulus.
• Light passes through the pupil and is focused by the lens onto the retina.
• Retina consists of three layers of cells: ganglion cell layer, bipolar layer, photoreceptor layer.
• The ganglion cell layer is the outermost, and the photoreceptor layer is the innermost.
• Light passes through the outer two layers to reach the photoreceptor layer.
• Two types of photoreceptors: rods and cones.
• Rods: 120 million, light sensitive (not color), are located in periphery of retina, have low activation threshold
• Cones: 6 million, color sensitive, are located mostly in fovea.
• Outer segments (O.S.) of rods or cones contain photopigments that react to light.

Visual Transduction Process

• Photopigments are in the membrane of the outer segment of rods and cones.
• Each pigment has an opsin (protein) and retinal (lipid).
• In the dark, membrane NA+ channels are open so glutamate is released, depolarizing the membrane.
• Light splits the opsin and retinal apart.
• The net effect of light is to hyperpolarize the retinal receptor and reduce glutamate release.

Visual Pathways

• Signals from ganglion cells of the retina are sent to the thalamus via the optic nerve/tract.
• Dorsal lateral thalamic nucleus (LGN) has 6 layers and each of them receives input from one eye.
• Inner 2 layers contain large cells (magnocellular).
• Outer 4 layers contain small cells (parvocellular).
• Koniocellular sublayers are ventral to each of the 6 layers.
• Neurons of the LGN project through optic radiations to the primary visual cortex (striate).
• Information from each visual field crosses over at the optic chiasm and projects to the opposite side of the primary visual cortex.
• Objects in the left visual field are captured by the right side of the eye's retina, and vice versa
• Visual Perceptual Pathway: Retina-Geniculate-Striate Pathway
• Eye lenses -> Retina -> Lateral Geniculate body (thalamus).
• Parvocellular system: Receives fine details of visual stimuli (color,shape).
• Magnocellular system: Receives information about depth perception, location, and movement.
• Visual impulses are projected from the thalamus to the primary visual cortex at the occipital lobe's posterior end.
• Further processing occurs in the Secondary Visual Cortex (V2).

Dorsal and Ventral Streams

• The pathway diverges into two streams: dorsal and ventral.
• Dorsal Stream (Occipito-Parietal Pathway): Processes depth perception and location of objects.
• Dorsal Stream: Secondary Visual Cortex -> V5/MT -> Posterior Parietal Cortex.
• Impulses pass through V5/medial temporal gyrus and reach Posterior Parietal Cortex.
• Posterior Parietal Cortex integrates vision with other sensory modalities.
• Ventral Stream (Occipito-Temporal Pathway): Processes finer details, color, and geometric shapes.
• Visual information is transmitted through two streams to the extrastriate cortex (visual association cortex).
• Dorsal stream processes "where" an object is.
• It receives mostly magnocellular input.
• It projects to post parietal association cortex.
• Ventral stream processes "what" an object is.
• It receives an equal mix of magnocellular and parvocellular input.
• It projects to extrastriate cortex (V2, V3, V4, V5) and to inferior temporal cortex (TEO, TE, STS).

Agnosia

• Agnosia is a failure to perceive or identify a stimulus by means of a sensory modality.
• Apperceptive visual agnosia is a failure in higher level perception.
• Affected person has normal visual acuity but cannot recognize objects based on their shape.
• Prosopagnosia is a form of apperceptive visual agnosia.
• Affected person cannot recognize faces visually but can by hearing their voice.
• Associative visual agnosia is a disconnection between perceptions and verbal systems.
• Affected person cannot name what they see.