Chapter 6 Vision - Notes

Connecting to Research, Objectives, and Outline

• Connecting to Research: Hubel and Wiesel map the visual cortex.
• Behavioral Neuroscience Goes to Work: 3-D Animation.
• Thinking Ethically: Inclusive Web Design.
• Neuroscience in Everyday Life: Are You a Super-Recognizer?
• Learning Objectives:
  • L01: Distinguish between sensation and perception.
  • L02: Discuss major features of visible light as a stimulus.
  • L03: Explain major features and functions of the eye, retina, and photoreceptors.
  • L04: Identify pathways of information from photoreceptors to the secondary visual cortex.
  • L05: Summarize processes responsible for visual object perception, depth perception, and color vision.
  • L06: Describe developmental changes in the visual system over the life span.
  • L07: Differentiate between major disorders that affect human vision.
• Chapter Outline:
  • From Sensation to Perception
  • The Visual Stimulus: Light
  • The Structure and Functions of the Visual System
  • Visual Perception
  • The Life-Span Development of the Visual System
  • Disorders of the Visual System

From Sensation to Perception

• Objective physical reality exists, but our understanding relies on sensory systems.
• Sensory systems transduce (translate) information into action potentials.
• Each organism has sensory capacities tailored to its survival.
• Sensation brings information to the central nervous system (CNS).
• Perception interprets sensory signals; attention is a key gateway.
• Attention focuses consciousness, often automatically directed toward unfamiliar, changing, or high-intensity stimuli.
• Perception is a two-way street:
  • Bottom-up processing: combines simpler meanings to construct more complex ones.
  • Top-down processing: uses knowledge and expectations to interpret meanings.

The Visual Stimulus: Light

• Vision is a primary sensory system in humans, with about 50% of cerebral cortex neurons responding to visual information.
• Vision starts with light energy reflected from objects.
• Visible light is a form of electromagnetic energy.
• Transduction: The transformation of sensory information into neural signals.
• Sensation: Obtaining environmental information and transmitting it to the brain for processing.
• Perception: Interpreting sensory signals sent to the brain.
• Attention: A narrow focus of consciousness.
• Bottom-up Processing: Combining simpler meanings to construct more complex meanings.
• Top-down Processing: Using knowledge and expectation to interpret meanings.
• Electromagnetic Energy: Light energy emitted by stars and artificial sources.

Features of Light

• Wavelength: Distance between successive peaks of waves, determines color. Encoded by the visual system either as color or shades of gray.
• Amplitude: Height of each wave, translated as brightness. Large-amplitude waves are perceived as bright and low-amplitude waves are perceived as dim.

Advantages of Light as a Stimulus

1. Abundant in the universe
2. Travels quickly
3. Travels in straight lines, minimizing distortion

The Electromagnetic Spectrum

• Visible light occupies a small part of the electromagnetic spectrum (400-700 nm).
• Shorter wavelengths (around 400 nm) are perceived as violet and blue.
• Longer wavelengths (around 700 nm) are perceived as red.
• Other forms of energy (gamma rays, X-rays, ultraviolet rays, etc.) are outside human visible range.
• Nanometers: A unit of measurement equaling 10^{-9} m used to measure light wave frequency.

Light Interaction With Objects

• Absorption: Retaining light energy.
• Reflection: Bending back of light toward its source.
• Refraction: Deflection or changing direction of light.
• The color of an object results from the wavelengths of light that are selectively absorbed and reflected.
• Light-colored clothing reflects more electromagnetic energy, while dark clothing absorbs more.
• Eyes are adapted to function in specific environments (air or water).
• Diving birds use special eyelids to maintain clear underwater vision.

The Structure and Functions of the Visual System

Eye Placement

• Frontal eye placement is advantageous for hunting, provides superior depth perception.
• Lateral eye placement is found in prey species, allows scanning of large areas for predators.

The Human Eye

• The human eye is roughly spherical, with a diameter of about 24 mm.
• The sclera is the tough, white outer covering that maintains eyeball shape.
• Cornea: The transparent outer layer at the front of the eye.
  • The curved surface directs light to form an image.
  • It is avascular and obtains nutrients from aqueous humor.
• Aqueous humor: The fluid located in the anterior chamber that nourishes the cornea and lens.
• Pupil: The opening in iris adjusted for light levels and emotional state.
  • Diameter adjusted by the iris via the autonomic nervous system.
• Iris: The circular muscle in the front of the eye that controls the opening of the pupil.
  • Color influenced by melanin pigment.
• Lens: Focuses light on the retina, with muscles adjusting focus for near and far objects (accommodation).
  • Transparent due to fiber organization and lack of blood supply.
  • Accommodation: The ability of the lens to change shape to adjust to the distance of the visual stimulus.
• Vitreous chamber: The large inner cavity of the eyeball.
• Vitreous humor: A jellylike substance in the vitreous chamber.
• Retina: The elaborate network of photoreceptors and interneurons at the back of the eye that is responsible for sensing light.
  • The projected image is upside down and mirrored.

Landmarks of the Retina

• The retina is part of the diencephalon that migrates outward during embryonic development
• Optic disk: The exit point for blood vessels and optic nerve axons, creating a blind spot.
• Macula: The area responsible for central vision, lacking large blood vessels.
  • Central vision: The ability to perceive visual stimuli focused on the macula of the retina.
  • Peripheral vision: The ability to perceive visual stimuli that are off to the side while looking straight ahead.
• Fovea: pit in the macula specialized for detailed vision, containing only cones in humans.
• Epithelium: Pigmented cell layer supporting photoreceptors, absorbing random light.

Layered Organization of the Retina

• The retina is 0.3 mm thick, contains multiple layers of neurons and connections:
  • Ganglion cell layer: Contains ganglion cells whose axons form the optic nerve.
    • Ganglion cell: Retinal cell in the ganglion cell layer whose axon leaves the eye as part of the optic nerve.
  • Inner plexiform layer: Contains connections between ganglion, bipolar, and amacrine cells.
    • Inner plexiform layer: The location in the retina containing axons and dendrites that connect the ganglion, bipolar, and amacrine cells.
    • Amacrine cell: Retinal interneuron integrating signals across adjacent retina segments.
  • Inner nuclear layer: Contains bipolar, amacrine, and horizontal cell bodies.
    • Inner nuclear layer: The layer of retinal interneurons containing amacrine, bipolar, and horizontal cells.
    • Bipolar cell: Cell forming straight pathway between photoreceptors and ganglion cells.
  • Outer plexiform layer: Contains connections between bipolar cells, horizontal cells, and photoreceptors.
    • Outer plexiform layer: The retinal layer containing axons and dendrites forming connections between bipolar cells, horizontal cells, and the photoreceptors.
    • Horizontal cell: Retinal interneuron integrating signals across the retinal surface.
  • Outer nuclear layer: Contains photoreceptor cell bodies.
    • Outer nuclear area: The location in the retina containing the cell bodies of the photoreceptors.
  • Nuclear layers contain cell bodies; plexiform layers contain axons and dendrites.

The Photoreceptors

• Rods and cones are named after the shape of their outer segments.
  • Photoreceptor: Specialized sensory cell in the retina that responds to light.
  • Outer segment: The portion of a photoreceptor containing photopigments.
  • Photopigment: A pigment contained in the photoreceptors of the eye that absorbs light.
• Rods: 120 million in each eye, responsible for sensing movement and scotopic vision (dim light).
  • Cylinder-shaped outer segments containing rhodopsin.
  • Rhodopsin: The photopigment found in rods.
  • Scotopic vision: The ability to perceive visual stimuli in near darkness due to the activity of rods.
• Cones: 6 million in each eye, responsible for photopic vision (bright light), sensitive to color and provide images with excellent clarity.
  • Photopic vision: The ability to perceive visual stimuli under bright light conditions due to the activity of cones.
  • Shorter, pointed outer segments contain one of three photopigments.
• Three classes of cones:
  • Blue/short-wavelength cones (cyanolabe): respond maximally to 420 nm (violet).
  • Green/middle-wavelength cones (chlorolabe): peak responses at 534 nm (green).
  • Red/long-wavelength cones (erythrolabe): peak at 564 nm (yellow).
• Rods: Respond maximally to wavelengths of 498 nm (bluish-green).
• Photopigment response initiates with changes in light absorption.
• Rods are more sensitive to low light, while cones are active in daylight.
• Rods increase in concentration away from fovea toward the periphery, cones decrease.

The Dark Current

• Opsin: A protein found in photopigments.
• Retinal: A chemical contained in rhodopsin that interacts with absorbed light.
• Carrots, which are rich sources of vitamin A, can truly improve your night vision since Vitamin A is converted into retinal.
• Photoreceptors transduce light energy into electrical signals.
• Photoreceptors resting potential in darkness is about -30mV.
• Dark Current: The steady depolarization maintained by photoreceptors when no light is present.
• Sodium channels in rods are kept open by cyclic guanosine monophosphate (cGMP).
  • Cyclic guanosine monophosphate (cGMP): A second messenger within photoreceptors that is responsible for maintaining the dark current by opening sodium channels.
• When rhodopsin absorbs light, enzymes are released that break down cGMP -> fewer sodium channels remain open -> the photoreceptor becomes hyperpolarized.
• Rhodopsin regenerates in about 30 minutes.
• Photoreceptors produce graded potentials rather than action potentials; bright light leads to greater hyperpolarization, while dim light leads to less hyperpolarization.
• Photoreceptors release glutamate when depolarized; largest amounts released in the dark.
• Hyperpolarization reduces glutamate release.

Processing by Retinal Interneurons

• Bipolar and ganglion cells provide direct pathway from photoreceptors to the brain.
• Horizontal and amacrine cells integrate information across the retina.
• Photoreceptor-bipolar-ganglion connections run perpendicular to the back of the eye, horizontal and amacrine connections run parallel to the back of the eye.

Horizontal Cells

• Receive input from photoreceptors, provide output to bipolar cells.
• Combine information from nearby photoreceptors, locate edges and borders.
• Communicate through graded potentials.

Bipolar Cells

• Receive input from photoreceptors and horizontal cells, communicate with amacrine and ganglion cells.
• Produce graded potentials.
• Come in multiple types supporting different levels of color and detail:
  • Rod bipolar cells: contact 15-56 rods each, useful in detecting dim light but unable to support finely detailed vision.
  • Midget bipolar cells: contact only one cone, process information from a tiny portion of the retina, allowing it support finely detailed vision.
  • Blue cone bipolar cells: process information from blue (short wave) cones.
  • Diffuse and giant bipolar cells: contact larger numbers of cones.
• Rather than total light, bipolar cells compare the amount of light falling on different parts of the retina. This comparisons is what allows us to see lines, borders, edges, and patterns.
• Receptive Field: A location on the retina at which light affects the activity of a particular visual neuron.
• Retina as an overlapping mosaic of receptive fields, each associated with a bipolar cell.
• Receptive field organization characterizes processing at many levels of the visual system; more complex at each stage.
• On-center bipolar cells depolarize when light hits the center, hyperpolarize when light hits the surround.
• Off-center bipolar cells hyperpolarize when light hits the center, depolarize when light hits the surround.
• About half the bipolar cells in the human retina are on-center, and the other half are off-center.

Amacrine Cells

• Form connections with bipolar cells, ganglion cells, and other amacrine cells.
• Exact functions are largely mysterious.
• Come in as many as 60 different shapes, releasing variety of neurochemicals.
• Reach across the retina, process changes in light as a function of time, possibly contributing to our understanding of visual movement.

Ganglion Cells

• Output cells of the retina, receive input from bipolar and amacrine cells.
• Axons of ganglion cells leave the eye to form the optic nerve, traveling to higher levels of the brain.
• Ganglion cells form conventional action potentials rather than graded potentials.
• Never completely silent, fire approximately once per second in absence of stimulation.
• The human eye has approximately 1.25 million ganglion cells that are responsible for integrating and communicating input from from approximately 126 million photoreceptors.

Ganglion Receptive Fields

• Like bipolar receptive fields, ganglion receptive fields also take the shape of doughnuts.
• On-center bipolar cells connect to on-center ganglion cells, and off-center bipolar cells connect to off-center ganglion cells.
• Ganglion cell receptive fields vary in size. Ranging in diameter from 0.01 mm in the macula to 0.5 mm in the periphery of the retina.
  *

Types of Ganglion Cells

• At least 17 different types of ganglion cells are identified, but three types comprise the vast majority: midget cells, parasol cells, and small bistratified cells.

  • Midget ganglion cells: small ganglion cells that Usually, They receive input that originates from one to four red or green cones
  • Parasol ganglion cells: Large ganglion cell that responds to all wavelengths regardless of color, subtle differences in contrast, and stimuli that come and go rapidly.
  • Small bistratified ganglion cells: ganglion cells that processes blue and yellow. That connect to bipolar cells that, in turn, make connections with blue photo-pigment cones.

• Rod bipolar cells feed information into the pathways established by the three main types of ganglion cells, often in complex ways involving amacrine cells and gap junctions.

Visual Pathways

• Optic nerve: Axons from the ganglion cell layer of the retina.
• Each human optic nerve divides in half, with the outer half traveling to the same side of the brain (ipsilaterally) while the inner half crosses to the other side of the brain (contralaterally).
• Partial crossing ensures that information from both eyes regarding the same part of the visual field will be processed in the same places in the brain.
• Rabbit's eyes are placed on the side of the head, so 100 percent of the fibers cross the midline.
• Optic chiasm: The area at the base of the brain where the optic nerves cross to form the optic tracts.
• Optic tracts: The fiber pathways between the optic chiasm and destinations in the forebrain and brainstem.
• Almost 90 percent of the axons in the optic tract proceed to the thalamus (specifically, the lateral geniculate nucleus (LGN)). The last 10% goes to the suprachiasmatic nucleus of the hypothalamus(for light information) and superior colliculus in the midbrain.

The Superior Colliculus

• In many species, including frogs and fish, the superior colliculus is the primary brain structure for processing visual information.
• Superior colliculus: A structure in the tectum of the midbrain that guides movements of the eyes and head toward newly detected objects in the visual field.

The Lateral Geniculate Nucleus (LGN) of the Thalamus

• Nearly 90 percent of optic tract axons form synapses in the lateral geniculate nucleus (LGN), located in the dorsal thalamus.
• Lateral geniculate nucleus (LGN): A nucleus within the thalamus that receives input from the optic tracts.
• Neurons in the LGN possess the same doughnut-shaped, antagonistic center-surround organization of receptive fields that we observed in the retinal bipolar and ganglion cells.
• The LGN keeps information from the two eyes completely separate. Alternating layers of the LGN receive input from the ipsilateral and contralateral eyes.
  • Layers 1 and 2 (the most ventral layers) contain larger neurons. These magnocellular layers receive input from the parasol ganglion cells in the retina.
    • Each of the six layers are very small neurons makes up the koniocellular layers, which receive input from the small bistratified ganglion cells.
    • The other four are referred to as parvocellular layers, which receive input from the midget ganglion cells.
• LGN features six distinct stacked layers, numbered from ventral to dorsal
  • Magnocellular layers: the two ventral layers of the LGN that receive input from parasol ganglion cells in the retina.
  • Parvocellular layers: The four dorsal layers of the LGN that receive input from midget bipolar cells in the retina.
  • Koniocellular layers: Layers of very small neurons between the larger six layers of the lateral geniculate nucleus that receive input from small bistratified ganglion cells in the retina.
• LGN is not a passive relay station in the flow of information to the cortex. It receives much more information from the rest of the brain than it sends to the cortex.

The Striate Cortex

• Primary visual cortex/striate cortex: The location in the occipital lobe for the initial cortical analysis of visual input; also known as V1 (visual area 1).
• It is located in the occipital lobe.

Cortical Mapping of the Visual World

• Allows us to use the location of neural activity to understand the position of an object in the visual field.
• Areas of the cortex that respond to input from the fovea of the retina are much larger than the areas responding to images seen in the periphery.
• Fovea contains 0.01 percent of the retina's total area, but signals from the fovea are processed by 8 to 10 percent of the striate cortex.
• Cortical magnification is another reason why focusing an image onto the fovea provides the greatest amount of fine detail.

Cortical Receptive Fields

• They do not respond to simple dots of light that activate bipolar and ganglion cells in the retina or cells within the LGN.
• Instead, cortical receptive fields are capable of processing much more complex features, such as moving bars of light.
• Simple cortical cell: A cortical interneuron that responds to stimuli in the shape of a bar or edge with a particular slant or orientation in a particular location on the retina.
• Complex cortical cell: A cortical interneuron that has a preferred stimulus size and orientation, and in some cases, direction of movement, but not location within the receptive field.
• Hubel and Wiesel defined simple cortical cells as cells that respond to stimuli shaped like bars or edges with a particular slant or orientation in a particular location on the retina.
• Hubel and Wiesel defined complex cortical cells as cortical cells that share the simple cells’ preference for stimulus size and orientation but without reference to the stimulus’ location, as long as it appears in the receptive field.
• Some complex cortical cells are also known as end-stopped cells

End-Stopped Cells

• End-stopped cells: A cortical interneuron that responds most vigorously to a stimulus that does not extend beyond the boundaries of its receptive field.

Cortical Columns

• Cortical neurons are organized in columns that run perpendicular to the surface of the brain.
• One type of column in the striate cortex is an ocular dominance column, which responds to input from either the right eye or the left eye but not both
• Ocular dominance column: A column of cortex perpendicular to the cortical surface that responds to input from either the right or left eye but not to both.
• Orientation column: A column of primary visual cortex that responds to lines of a single angle.
• Hypercolumn: A complete set of orientation columns that span 180 degrees.
• Cytochrome oxidase blob: Named after an enzyme, cytochrome oxidase. Neurons in areas with high concentrations of cytochrome oxidase appear to process information regarding color.

Cortical Modules

• Cortical neurons respond to line orientation, movement, and color. At some point, our visual system puts these separate characteristics back together to form coherent images.
• Cortical module: A unit of primary visual cortex containing two sets of ocular dominance columns, 16 blobs, and two hypercolumns.
• Contains the neurons it needs to process the shape, color, and movement of an image falling on a specific part of the retina.

Visual Analysis Beyond the Striate Cortex

• At least a dozen additional areas of the human cerebral cortex participate in visual processing. Because these areas are not included in the striate cortex, they are often referred to as extrastriate areas. Areas are also referred to as secondary visual cortex.
• Next to the striate cortex is an area known as V2 (visual area 2).
• Dorsal stream: A pathway leading from the primary visual cortex in a dorsal direction that is thought to participate in the perception of movement and object location.
• The dorsal stream travels from the primary visual cortex toward the parietal lobe and then proceeds to the medial temporal lobe
• Ventral stream: A pathway of information from the primary visual cortex to the inferior temporal lobe that is believed to process object recognition.
• The ventral stream proceeds from the primary visual cortex to the inferior temporal lobe.
• Area MT: An area in the medial temporal lobe believed to participate in motion analysis.
• Area MST: An area in the medial superior temporal lobe believed to participate in large-scale motion analysis.
• Area IT: An area in the inferior temporal lobe believed to participate in object recognition.
• Fusiform face area (FFA): An area in the inferior temporal lobe believed to participate in the recognition of familiar faces, especially in the right hemisphere.

Visual Perception

Bottom-Up or Top-Down?

• Hubel and Wiesel model implies a bottom-up, hierarchical organization in which simple cells contribute input to increasingly complex cells.
• Requires both hierarchical, and top-down processing.

Spatial Frequencies

• Hubel and Wiesel suggest striate cortex may respond to patterns of lines instead of isolated lines and bars.
• Simplest Patterns of lines are gratings.
  • A high-frequency grating has many bars in a given distance (fine detail).
  • A low-frequency grating has relatively few bars.
  • A high-contrast grating has a large amount of difference in intensity between bars.
  • A low-contrast grating has a more subtle difference in intensity between bars
• Contrast sensitivity function (CSF): The mapping of an individual’s thresholds for contrast over a range of frequencies.

The Perception of Depth

• The image projected on the retina is two-dimensional, so the visual system uses a number of cues to provide a sense of depth. Several of these cues are monocular, requiring the use of only one eye.
  • Perspective, in which lines we expect to be parallel, such as the edges of a road, are made to converge or come together at the horizon, is a centuries-old artistic device to give the illusion of depth on a flat surface such as a painting.
  • Texture, shading, the occlusion of a more distant object by a nearer one, and a comparison of the size of familiar objects can also provide a realistic impression of depth in two dimensions,
• Retinal disparity: The slightly different views of the visual field provided by the two eyes.
• Binocular cells: respond most vigorously when both eyes are looking at the same stimulus.
• Disparity-selective cell: fire more when the preferred features are seen by different parts of the two eyes.

Coding color

• Three light colors that make all the colors. Red, green, and blue

The Trichromatic Theory

• Trichromatic theory: The theory that suggests human color vision is based on our possession of three different color photopigments.
• Maximal activity of blue, green, and red receptors

Opponent Processes

• An alternate theory of opponent processes based on three types of receptors: a red–green receptor, a blue–yellow receptor, and a black– white receptor.
• Opponent process theory: A theory of human color vision based on three antagonistic color channels: red-green, blue-yellow, and black-white.
• Midget cells possess red–green center-surround organization
• The small bistratified cells possess antagonistic center-surround organizations responding to blue and yellow.

Color Deficiency

• Occasional errors occur in the genes that encode the cone photopigments. As a result, individuals with these genes demonstrate several kinds of atypical responses to color, known as color deficiency.
• Dichromacy: Having eyes that contain two different cone photopigments.
• Monochromacy: The ability to see in black and white only.
• Anomalous trichromacy: A condition characterized by having three cone photopigments that respond to slightly different wavelengths than normal.
• Color contrast: The fact that colors can look different depending on the surrounding colors.
• Color constancy: The concept that an object’s color looks the same regardless of the type of light falling on the object.
• If color contrasts, Edwin Land's, used red and green filters to make photographs look like a wide set of colors.

The Life-Span Development of the Visual System

• Infants prefer to look at patterns rather than at uniform screens.
• Visual acuity is dependent on resolution.
• As we age, predictable changes occur in our vision. Some of these changes are related to the structures of the eye.
  • Presbyopia: The reduced rate and extent of accommodation by the lens that results from aging.
  • Hardening of the lens begins in middle age.
  • Lens and iris changes in color and accommodation, size of retinal and cortex cells.

Disorders of the Visual System

• A variety of conditions can interfere with vision, ranging from the very mild and correctable to a complete loss of vision. Visual deficits occur due to problems in the eye and retina as well as to central problems in the brain.

Amblyopia

• Amblyopia: A condition also known as lazy eye, in which one eye does not track visual stimuli.

Cataracts

• Cataract: Clouding of the lens.

Visual Acuity Problems

• Myopia: An acuity problem resulting from an elongated eyeball; also known as nearsightedness.
• Hyperopia: An acuity problem resulting from a short eyeball; also known as farsightedness.
• Astigmatism: A distortion of vision caused by the shape of the cornea.

Blindness

• Blindness: Defined for legal and medical purposes as having no better than 20/200 vision in a person’s best eye using correction (e.g., glasses or contacts).
• Scotoma: An area in the visual field that can’t be seen, usually due to central damage by stroke or other brain injury.
• Blindsight: An abnormal condition in which parts of the visual field are not consciously perceived but can be subconsciously perceived by non-cortical parts of the visual system.
  retinal prosthetic devices are used like the argus II retinal

Visual Agnosias

• Visual agnosia: A disorder in which a person can see a stimulus but cannot identify what is seen.
• Prosopagnosia: The inability to recognize known faces.
• Jane Goodall type of prosopagnosia is not genetic; not prevented her from making observations of her chimpanzees. Acquired and developmental.