Lecture 12

Vision: Acquisition & Circuit Pathway

• Visual Field:
  • Left and right visual fields.
  • Binocular field: The area seen by both eyes.
  • Visual field of each eye.
• Neural Pathway:
  • Optic nerves transmit information from the eyes.
  • Optic chiasm: Where optic nerves cross.
  • Optic tract carries visual information after the chiasm.
  • Lateral geniculate nucleus (LGN) of the thalamus processes visual information.
  • Suprachiasmatic nucleus of the hypothalamus receives visual input.
  • Visual cortex (left and right) in the brain processes visual information.
• The signal from the right visual field hits the left side of the retina and vice versa.

Vision - The Eye

• Initial visual analysis occurs in the retina.
• Key Processes:
  • Refraction of light onto the retina.
  • Focusing of light in the fovea.
  • High concentration of photoreceptors in the fovea.
• Eye structures:
  • Cornea: Clear outer layer.
  • Lens: Focuses light.
  • Fovea: Primary focal point with best vision, mostly cones.
  • Retina: Contains photoreceptors.
  • Optic disc: Exit point for optic nerve.
  • Pigment epithelium: Layer behind the photoreceptors.
  • Sclera: The white part of the eye, no nerves.
  • Pupil: Hole that controls opening.
  • Iris: Controls the size of the pupil.
  • Ciliary muscles
• Cell Types:
  • Ganglion cell
  • Bipolar cell
  • Photoreceptor
• Pigment epithelium
• Ganglion cells leave retina

Vision - The Retina

• Photoreceptor layer samples the visual image.
• Blind Spot Demonstration:
  • Closing one eye and focusing on a dot reveals a blind spot where a line disappears.
• Five Cell Types, Three Cellular Layers, Two Synaptic Layers
• Cellular Layers:
  • Outer nuclear layer: Contains photoreceptor cell bodies.
  • Inner nuclear layer: Contains bipolar, horizontal, and amacrine cell bodies.
  • Ganglion cell layer: Contains ganglion cells.
• Synaptic Layers:
  • Outer plexiform layer.
  • Inner plexiform layer.
• Neurons in the Retina:
  • Photoreceptors: Rods and cones.
  • Bipolar cells: Rod bipolar and diffuse bipolar.
  • Horizontal cells.
  • Amacrine cells: Communicate between adjacent ganglion cells.
  • Ganglion cells: M ganglion and P ganglion.

Vision - The Retina: Retinal Circuitry

• Cone Signal Circuitry:
  • Photoreceptors
  • Horizontal cells involved in lateral inhibition.
  • Bipolar cells.
  • Amacrine cells.
  • Ganglion cells: Excited in ON light.
• Rod Signal Circuitry:
  • Rod bipolar cells.
  • AII amacrine cells.
  • Ganglion cells: OFF cells excited in the dark.
• Rods only detect light/dark.
• Rods enhance acuity
• Lateral Inhibition

The Retina Photosensitive

• Rods & Cones Have Same Four Functional Regions

  • Outer segment: Contains light-sensitive photopigments.
  • Inner segment: Site of energy and protein synthesis.
  • Cell body: Contains the nucleus.
  • Axon and synaptic terminal: Transmits signals to other neurons.

• Morphology of Photoreceptors:

  • Rod: Rod-shaped.
  • Cone: Conical shape.

• Outer Segment of Photoreceptors:

  • Rod: Discs containing rhodopsin.
  • Cone: Folding of the outer cell membrane.
    Rod vs Cones

• Cones

  • Conical shape
  • Focused in the fovea
  • Color, high order vision

• Rods

  • Rod shape
  • Peripheral vision

• Outer segment: Phododetecting segment

• Inner segment: Energy & protein synthesis

The Retina – Photoreceptors: Wavelength Sensitivity

• Three Cones, One Rod: Wavelength Sensitivity
• Log relative sensitivity of rods, S cones, M cones, and L cones across the wavelength spectrum.
• The sensitivity curves peak at different wavelengths.
• Wavelength (nm)
  • $400$
  • $500$
  • $600$
  • $700$
  • $800$
  • $900$
• 3 cones to detect all colors
  • S (short)
  • M (medium)
  • L (long)
  • Based on wavelength

The Retina – Photoreceptors

• Photoreceptors can't fire APS
• Signal Transduction Processes
• Phototransduction and Neural Signaling
  • In the dark: Rhodopsin inactive, $Na^+$ channels open, cell depolarized, glutamate released to bipolar cell.
  • In the light: Rhodopsin active, $Na^+$ channels closed, cell hyperpolarized, glutamate release reduced.
• Molecular Processes in Phototransduction
  • Dark: GC makes cGMP, which activates $Na^+$ channels.
  • Light: Rhodopsin activates PDE, which breaks down cGMP.
• Voltage Response to Light
  • Rod: Significant hyperpolarization in response to light.
  • L cone: Similar hyperpolarization response.
• Reaction Network in Phototransduction
  • GC: Guanylate cyclase Catalyzing GTP to cGMP.
  • PDE: Phosphodiesterase.
  • Increases activity or concentration
  • Decreases activity or concentration
• Lights off (LO)
• Lights on (LON)
• Light energy to chem. signal
• Dark = special ion channels open
  • Cation channels
  • CGMP, made by GC and GTP activates channels
  • $Na^+$ enter excitatory
  • $Ca^{2+}$
• Change & rate of change
• Vision relies on change in stimulus
  • Everything is a stimulus
• GC is active
• Depolarization causes glutamate release
• PDE = Phosphodiesterase
• Light hits pigmented protein & causes conformational change
  • Only light-dependent rxn
• Activates set of proteins that release active subunits
• Activate PDE that breaks down CGMP into GMP
• CGMP channels start closing (not all will close)
• Less cations = less excitation
• Always have ratio of CGMP à GMP
• $Ca^{2+}$ enhances signals that will prevent overfiring

The Retina – Photoreceptors: Light Activation

• Light Activates Pigment Molecules in the Photoreceptors
• Visual Pigment in Rods
  • Visual pigment (rhodopsin) consists of retinal and opsin.
  • Retinal: 11-cis retinal (M = 268)
  • Opsin: Protein component.
• Visual Pigment Amino Acid Sequences
  • Comparison of amino acid sequences in M vs rhodopsin, M vs S, and L vs M.
  • Location of retinal binding
• Lisin that binds to retinal

Ganglion Cells Transmit Neural Images to the Brain

• Over 20 Types: ON vs OFF Cells
• On center cell
• Off center cell
• Light stimulates center region
  • On center cell: Increased firing rate
  • Off center cell: Decreased firing rate
• Light stimulates surround region
  • On center cell: Decreased firing rate
  • Off center cell: Increased firing rate
• No light stimulation
  • Baseline firing rate
  • Baseline firing rate
• Light stimulates both center and surround regions
  • Inhibit each other: Weak response
  • Baseline firing rate
• ON/OFF cells
• Basal firing

The Retina – Ganglion: Sustained vs Transient

• Over 20 Different Kinds: Sustained vs Transient

• Stimulus pattern

  • On area
  • Off area

• ON cells

• OFF cells

• Fire consistently → sustained

  • When stimulus is present
  • Important for identification of obejets

• Only for change transient

  • (beginning & end)
  • Important for locating obejets

• Ganglion Cells Transmit Neural Images to the Brain

• Spatial Filtering is Accomplished by Lateral Inhibition via Horizontals and Amacrine.

• Closer to fovea = smaller receptive fields

• Farther from " = bigger "

  • More general info.
  • Minor details

• Less info. in periphery

  • → more space in cortex

The Retina Color Vision

• Color Vision Begins in Cone-Selective Circuits.

Colorblindness

• Two Common Forms, Protanomaly and Deuteranomaly, Together Affect About 7% of Males
• Retinal Response (percent maximum)
  • Blue
  • Green
  • Red
  • Cones
  • Rods
  • Cones
  • Cones
  • Values:
    • $25$
    • $50$
    • $75$
    • $100$
• Wavelength of Light (nm)
  • $380 nm$
  • $450 nm$
  • $500 nm$
  • $550 nm$
  • $600 nm$
  • $650 nm$
  • $700 nm$
  • $750 nm$
• L-pigment
• M-pigment
• X-linked associated disorder
• I and M-pigments very clustered together
  • Which causes a
    • -deletion or
    • -hybridization

The Constructive Nature of Visual Processing

• Vision is Not Like Operation of a Camera
• Doesn't require processing
• Object
• Light rays
• Lens
• Film (Retina)
• British Empiricist Early Thinking of Vision as an Additive Process
  • color + shape + brightness + etc = perception of final image
• Modern View: Perception is an Active and Creative Process That Involves More Than Just the Information Provided to the Retina.
• Emotions are also involved
• Vision is
  • Memory
  • Highly regulated/Dependent
  • Subjective
  • On
  • Personal experience

The Constructive Nature of Visual Processing - Gestalt Model

• Laws of Perception Support the Gestalt Model
  • Similarity
  • Proximity
  • Good continuation
  • Contour saliency
    • Inconsistent w/ background
• More Evidence: Object Recognition Depends on Segmentation of a Scene into Foreground and Background.

The Constructive Nature of Visual Processing - Brain Analysis

• The Brain Analyzes a Visual Scene at Three Levels
  • Low-level processing
    • Orientation
    • Color
    • Contrast
    • Disparity
    • Movement direction
  • Intermediate-level processing
    • Contour integration
    • Surface properties
    • Shape discrimination
    • Surface depth
    • Surface segmentation
  • High-level processing
    • Object motion/
    • Shape from kinematic cues
    • Temporal lobe
      • -Uses memory
    • Object identification
    • ganglion to cortical region

Visual Processing - Geniculostriate Pathway

• Visual Processing is Mediated by the Geniculostriate Pathway
  • Binocular zone
  • Monocular crescent
  • Monocular crescent
  • Left/Right visual field
  • Neural Components:
    • Optic nerve
    • Optic chiasm
    • Optic tract
    • Lateral geniculate body
    • Optic radiation
  • Temporal hemiretina
  • Nasal hemi-
  • retina-
  • Information from eyes
  • LGN layers
  • Primary visual cortex
    • Layers 3 through 6: the parvocellular pathway.
    • Layers 1 and 2: the magnocellular pathway.
  • 12 Magnocellular
    • Contrast & location
    • Light laark
  • Rest Panocellular
    • Color, info., etc.
    • Surface, contour

Three Main Pathways from the Retina

• Not Only Visual Processing
  • Visual processing
  • Pupillary reflex and accommodation
    • Accommodation
    • Ciliary ganglion
    • Pupillary reflex
    • Helps control iris
    • Pretectum
    • Accessory oculomotor nucleus
    • towards location
    • Parietal cortex
    • Thalamic nuclei
      • Dorsal pathway
      • Pulvinar
      • Lateral geniculate nucleus to SC
      • Inferotemporal cortex
      • Ventral pathway
      • Primary visual cortex to temporal lobe
  • Eye movement (horizontal)
    • Posterior parietal
      • runs primarily through midbrain
    • FEF
    • Caudate nucleus
    • Substantia nigra
    • LGN
    • Abducens nerve
    • cortex
    • SC
    • PPRF
    • Abducens nucleus
    • Primary visual cortex

The Receptive Fields of Neurons

• The Receptive Fields of Neurons at Successive Relays in the Visual Pathway Provide Clues to How the Brain Analyzes Visual Form.
• Differential signaling
  • LGN + Cortex
• The Visual Cortex is Organized into Columns of Specialized Neurons
  • Visuotopic map
  • Stimulus
  • Different eyes
  • Individual
  • Ocular dominance columns
    • Not exclusively, preferentially to an angle
    • V2
    • V1
    • Left eye column
    • Right eye column
  • Orientation columns
    • Blobs: color
    • Left
    • Right
    • Orientation preference
  • Blobs, interblobs (V1), and stripes (V2)
    • Stripes
    • Pattern of excitation in response to striped stimulus
    • Thin stripe
    • Thick stripe
    • V2
    • V1

The Dorsal and Ventral Pathways

• What vs Where
• Based on personal experience
  • Dorsal (parietal) pathway
  • Ventral (temporal) pathway
  • Color
  • Depth
  • Direction
  • Orientation
  • Complex form
  • Parvocellular
  • Magnocellular
  • Koniocellular
  • Midget ganglion cells
    • Parvo pathway
  • Bistratified ganglion
    • Konio pathway
    • cells
  • Parasol
    • Magno ganglion cells
      • pathway
  • Propagate primarily through blobs
  • Thick stripe
  • Thin stripe
  • Interstripe
  • From blobs
  • From interblobs
  • V4
  • MT
  • IVB
  • IVCa
  • IVCB
  • V
  • VI

Visual Information Representation

• Visual Information Represented by Variety of Neural Codes
• Population Coding
  • Orientation tuning (spikes/s)
  • Orientation preference
  • Response
  • Strongly activated cell
  • Stimulus
  • Vector components
  • Vector
  • Perceived average orientation
• Fire at certain rate given their preference
• Final output = average of all signals