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VISION
The Eye
Sclera, Cornea, Lens, Iris, Vitreous, Aqueous,
Retina
Fovea (in high-acuity animals)
Optic disk
Physiological optics (eye as camera)
Projection of visual world on retina
Point-to-point
Inversion of image
Emmetropic (normal) eye at rest
Object at infinity focussed
Lens power is mainly in cornea (biggest change in refractive index is at
air/cornea interface)
Focussing on closer objects
Lens (dynamic element)
Refractive errors
Myopia (image formed in front of retina; usually, eyeball too long)
Hyperopia (image behind retina; eyeball too short)
Pupil
Another element in focussing (reduction of "blur circle")
Control of light level secondary
Retinal anatomy
Retina as brain
Cross section: layers
Backwards light path (passes through inner retina before hitting photoreceptors)
Receptors
Rods vs. Cones
Membranous disks of outer segments
Photopigment
Hyperpolarizing light response
Bipolars
Connect outer to inner retina
Ganglion cells
Output cells; have axons and generate action potentials
Amacrine, Horizontal cells (interneurons)
Functional Anatomy:
How do ganglion cells get their properties?
Retina as great model system for neural processing
Ganglion-cell Receptive Fields (review)
Define Receptive field
Relate visual space to retinal space
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Wide variation in receptive-field structure
Dimensions
ON- vs. OFF-center
Surround antagonism (most, not all cells have it)
Functional subtypes (classes orthogonal to ON vs. OFF distinction; X vs.
Y in cats; M vs. P in primates)
What is anatomical basis of these specializations?
Receptive-field DIMENSIONS
Related to dendritic field size (sampling bipolar input)
Determinants of dendritic-field size
Eccentricity
Relation to ganglion-cell density distribution
Like Somatic-sensory: yoking of innervation density and receptivefield size
Class specificity
Beta (X), alpha (Y) in cats
Midget (P) vs. parasol (M) in primates
Extreme case of minimum convergence: foveal midget cells get
input (via bipolars) from single cone
ON/OFF channels:
Bipolars:
Two flavors physiologically (depolarizing, hyperpolarizing)
In outer plexiform layer
Different anatomy of two bipolar types
Different membrane receptors on bipolars for the transmitter
(glutamate) released by the photoreceptors
ON bipolars: transmitter hyperpolarizes the bipolar, so
synapse is sign inverting
OFF bipolars: transmitter depolarizes, so synapse is
sign preserving
In inner plexiform layer
ON bipolars synapse in the inner half of the layer, OFF
bipolars in the outer half
Ganglion cells:
Dendritic stratification in inner plexiform layer determines physiological
center type
Outer stratification = OFF center
Inner stratification = ON center
Bistratification (inner and outer) = ON/OFF center
CENTER-SURROUND ANTAGONISM:
Seen already in bipolar
Implies synthesis by outer plexiform circuitry
Horizontal cells and lateral inhibition
Contrast enhancement
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Functional subtypes of ganglion cells
Examples
Cat:
X (tonic; small receptive fields; linear summation)
Y (phasic; large receptive fields; non-linear)
W (various "weird" cells)
Monkey:
X-like ("P" or color-opponent) vs.
Y-like ("M" or broad-band)
Specialized receptive fields
Direction-selective
Suppressed-by-contrast
Orientation-selective
Basis for specialized properties: Not well worked out but related to...
Ganglion-cell morphology
Amacrine vs. Bipolar inputs
Comparative evidence. Species with lots of "special" cells (frogs,
rabbits) have
Thicker inner plexiform layers
More amacrine than bipolar inputs to ganglion cells
Within-species evidence (e.g., cat)
X-like cells are bipolar dominated
Complex types are amacrine dominated
Direction-selective model (role of amacrines)
Asymmetric amacrine inputs
Amacrine anatomy
Extremely varied
Neurotransmitter types (every one seen)
Morphology (e.g., wide vs. narrow field, IPL stratification)
Tremendous opportunities for specialized computations
Area of active research
Ability of visual system (ganglion cells) to respond over wide luminance range
Role of pupil? Not much
Adaptation (partly within receptor, partly neural (poorly understood feedback
from inner to outer retina)
Duplex retina
Rod vs. cones (different response ranges)
Comparative: rod/cone balance related to ecological niche (as is
development of color vision)
Separate channels up to level of ganglion cell
Rod bipolars are distinct from cone bipolars
Depolarizing light responses
Termination in special sublayer of inner plexiform layer
A-II amacrine cells
Obligatory link in rod-to-ganglion cell path
Rod bipolar input to A-II
Output from A-II to cone bipolar terminals
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Inhibitory chemical synapse (glycine) onto OFF cone bipolar
terminals
Excitatory electrical synapse (gap junction) onto ON cone
bipolar terminal
Thus ganglion cells get convergent rod/cone input (and can operate in
both scotopic and photopic ranges)
Visual Fields
Review central visual pathway (nerve, chiasm, tract; LGN and other subcort targets)
Hemifield representation
As in Somatic and motor systems: a CROSSED representation, but not of
eyes; instead of visual hemifields
Goldfish: complete decussation (opposite eye OR field)
Frontal eyes
Advantage: binocular vision  stereopsis (depth)
Question: How arrange...
Convergence of input from two eyes and...
Preservation of CROSSED cortical representation?
Answer: DECUSSATION
Relationship of visual field to each retina
Inversion
Convention: visual world as seen by that eye
Nasal vs. temporal (retina AND field)
Optic disk / blind spot
Superimpose two fields of view at Fixation point
Binocular vs. monocular field (WIGGLE FINGER)
Decussation anatomy
Rule: Nasal fibers cross
Optic nerve carries info from one eye, both hemifields
Optic tract carries info from both eyes, one (opposite) hemifield
Higher centers
Reflect crossed hemifield representation of the optic tract feeding
them
Defer true binocular convergence until cortex
LGN: individual cells (or layers) are monocular
Layer 4: ocular dominance stripes
Extragranular layers of striate cortex: first truly binocular cells
Functional significance:
Fusion of image of two eyes into single, unified
perceptual world
Stereopsis (disparity-tuning of cortical cells)
Field defects
Eye or nerve - monocular (SCOTOMA)
Chiasm - BITEMPORAL HEMIANOPIA
Tract, higher - HOMONYMOUS HEMIANOPIA
Lateral geniculate nucleus
Main relay of retinal info to striate cortex
Layers (e.g., monkey, cat, tree shrew) --- Why?
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Ocularity
Autoradiography
Degeneration after eye removal
Topographic alignment of two eyes (first step)
Cell class
Monkey
Parvo from X-like,
Color sensitive
High acuity
Low sensitivity
Magno from Y-like
Color insensitive
Low acuity
High sensitivity
S-layers
Mysterious
W-like inputs
Cat
A-layers: X plus Y
C-layers : other ("W"), weird RFs
ON/OFF: separate layers in some species
Striate cortex
Topographic order
Magnification (like somatic, highest where...
Receptors in retina are most densely packed
Receptive-fields are smallest
Spatial acuity (resolving power) is highest
Channel segregation
In granular layer inputs (4C)
Magno (4C-alpha) vs.
Parvo (4C-beta)
Beyond the granular layer?
Some mixing
But certain targets get only magno, implies some segregation?
Cytochrome oxidase blobs in striate cortex: twist in story
Discovery and topography
Physiology
Color selective
Non-orientation (punch holes in orientation matrix)
Input?
S-layers, Interlaminar zones (koniocellular layers) of lateral geniculate
Indirect parvo channel input too?
Extrastriate
Huge number, extent
Inputs
Striate or other visual cortex
Subcortical
Higher-order
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Visual agnosias: Mistook Wife for Hat
RF properties
More complex, specialized
Large size
Even across midline (callosum)
Generalizing some operation over space (recognize object
regardless visual-field location)
Hierarchy
Anatomical signature
Current view (Van Essen)
Serial processing? Not really
Simultaneous activity
Parallel organization within...
What vs. Where stream
Temporal: what
Proximity to temporal lobe memory systems (recognition requires
memory)
Parietal: where (3D location, movement of objects: motor [reach, look])
MT as best example
Extrageniculate subcortical
SC and oculomotor Reflex eye movements;
Possible perceptual role
Extralemniscal pathway through pulvinar to cortex;
Blindsight
Accessory optic system [relate to nBOR in frog lab] - optokinetic nystagmus;
Pretectum and pupillary light reflex
Suprachiasmatic nucleus of hypothalamus and circadian entrainment.
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