1 Neuroscience C 3045; Eye and Brain Professor M. Glickstein ...

Neuroscience C 3045; Eye and Brain
Professor M. Glickstein
January 2012
The general problem of vision; what sorts of things can you do with your eyes? The
eye, like other excellent optical devices forms an image of the world in front of it. Visual
direction is probably innate. In addition there is vision for:
Fine detail: Detection of a small object against a background (visual acuity). Clinical
measures 20/20 [6/6] you see @ 20 feet what a normal eye sees at 20 feet. Acuity
falls off in peripheral retina.
Sensitivity: How much light do you need to see? Each rod can be activated by a single
photon.
Visual Depth and Distance Perception: How far away is an object? Which of two objects is
closer?
Monocular Mechanisms (e.g. Motion parallax; relative angular movement of 2 objects
at different distances).
Binocular Stereopsis Fusing of 2 disparate images to form a depth cue.
Colour Vision: Identifying, naming, discriminating colours. How is colour processed in the
brain?
Form vision: Recognition of your friend's car in front of his house; Recognition of your
friend's face.
Movement Detection:
Is an object stationary or moving?
Visual guidance of your own movement.
Eye movements (saccadic, pursuit; vergence).
Accommodation
Fingers and limbs (grasping an object in front of you)
Whole body; walking along a crowded street: climbing a tree or the north fact of the
Eiger.
Related problem of distinguishing self-movement from movement of something in our
field of view.
The constancies: Why does brightness remain constant over a broad range of illumination
intensities? Lump of coal in sunlight.
When we consider the anatomy and physiology of the visual pathways, remember that it is
these phenomena of vision that direct and focus research. How does the brain do it?
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Comparative anatomy of the vertebrate Eye
Discuss the classical light-microscopic view of the structure of the vertebrate eye with some
general principles that apply to all vertebrates and examples of specialised structures that
have evolved in one or another form.
Similarity. To a great degree, all vertebrate eyes are similar. They share an almost identical
imaging system (modified in special environments, such as under water).
Compare with compound eye.
Basic Cellular Plan The basic structural elements of the retina are similar in all vertebrates.
Three nuclear layers with five or six cell-types involved in visual transduction
and coding
Two synaptic (plexiform) layers
Evolution; Darwin’s difficulty
“….if numerous gradations from a perfect and complex eye to one very
imperfect and simple, each grade being useful to its possessor can be show to
exist…then the difficulty… can hardly be considered real” See article by Lamb
et.al. in reference list.
Normal Human Eye
3 "Tunics" of eye
A: Sclera
B: Choroid and iris muscles
C: Retina (and pigment epithelium)
Note also focusing apparatus; cornea and lens and the optic nerve
Differences. Where vertebrate eyes differ it is often related to the light environment in which
the animal lives.
Range of Light Intensity. Range of Physical Intensity approaches 10 12 . No sense organ
could work effectively over such a range (10 3 /second action potentials).
How to Code? Scale compression (Weber's law). We are sensitive to relative, not absolute
light levels.
How to deal with great range in light intensities
1. Photomechanical Changes
Physically restrict the amount of light which is allowed to reach receptors
2. Parallel set of receptors (rods and cones) with different threshold and different
dynamic range
3. "Tuning" ganglion cells so that they report differences in light intensities within
receptive field (lump of coal in sunlight looks black)
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Mechanical Regulation of Light at the Receptor
Pupils
Tiger
What's wrong with the picture?
Association of slits with nocturnal life style (esp. Snakes)
Round vs. slit pupils. Nocturnal retina requires exclusion of light in daylight "basking"
Pigments and stray light. Stray light degrades the image, it is absorbed in cameras and in the
eye by pigment layers.
Monkey
Pigment epithelium and choroid
Pigment epithelium plays an especially important role in light and dark adaptation of lower
vertebrates. Processes interdigitate with tips of receptors.
Pigment epithelium (Hedgehog)
Pigment epithelium is a channel for (choroidal) vascular supply for receptors. Bites off
growing rod tips.
Pigment migration in lower vertebrate. The processes of pigment epithelial cells may extend
the length of the receptors.
Image 2 hairbrushes (receptors)
Melanin granules are actively migrating:
Migrate outward in light back (towards cell body) in dark
Dark-adapted fish eye
Light adapted fish eye
Augmentation of Sensitivity. Animals that live in dim light-environment sacrifice precision
of an image for increased sensitivity "snapshots vs. time exposures"
Suppose light goes through receptor layer without ever being absorbed. If it strikes a
reflective surface the receptors have a second chance
Eye shine Leopard
Tapetum Lucidum
(Note Absence of melanin pigment in pigment epithelium)
Toxicity of Excessive Light
Albinos are especially at risk. Four weeks of continuous moderate illumination; albino rat
causes degeneration of receptors
Normal rat retina
Constant light; retinal degeneration
Retinal structure and the accuracy of ophthalmic instruments. To a reasonable approximation
the retina of all mammals (vertebrates) is of constant thickness.
Rat
Whale
Receptors and inner nuclear cells are roughly equivalent in size and number; related to the
fact that these retinal elements do not conduct action potentials.
Receptors and inner nuclear cells convey information over a short length by graded potentials
(receptors, bipolar and horizontals some amacrine)
Ganglion cell carries impulses to brain. The distance varies from a few millimetres to many
centimetres. Ganglion cell sizes vary greatly. There is also variability within a single retina:
Size of cell body is related to fibre diameter.
Consequences of finite retinal thickness for ophthalmoscope and retinoscope. Shine light in;
observe reflected shadow
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Suppose light is reflected from layer other than rods and cones. For thin lens in air
D = f -- 1 by definition
- 2
dD = -f
df or error (if constant) should be a function of the inverse square
of the focal length
Raw data; Eye size and refraction
Log plot
Conclude: retinoscope uses a specular reflection from vitreous-retinal surface
Similar factor applies to opthalmoscope
Growth and Plasticity of Eye Size; Emmetropization
The normal newborn human infant eye measures 17 mm. from cornea to retina; the adult eye
24 mm.
The (strange but true ) evidence that eye growth is under local visual control.
Different focal planes in the bird eye
Fundamental Dichotomy of Rods and Cones
Schultze's dichotomy: Differences in receptor types correlated with the life of the animal:
"pure rod" and "pure cone" retinas.
Retina is mixed rod/cone
Monkey retina 3 layers
Rod/cone (nuclei). Note Difference in nuclei of rods and cones.
Higher power; conical outer segments
Variability in cone density in human (an monkey) retina; rods are relatively constant
Peripheral retina
10 0 OUT
Follow colour of nuclei
Edge of fovea
Central fovea
Other Plans; The Retinal Streak
Lindsay Johnson reindeer
Note 1) Pigment distribution
2) Vascular differences
Receptors and Visual Pigments
Wavelength sensitivity of Receptors.
Outer segments of rods and cones contain photopigments.
1) Absorb light 2) Transduce physical signal into an electrical one:
Wavelength and Absorption
Rods peak absorption at 500 nm
2) Absorption spectra of pigment in cones. In human and monkey. 3 classes by
microspectrophotometry; intracellular recording.
Requirements for Colour Vision
Colour vision requires at least 2 classes of receptor with different wavelength subdivisions.
In principle you could do it either with pigments or with filters.
Oil droplets
Variability in Receptor Morphology Cones and Rods may be Thin or Fat
Frog; Rods and cones
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Note massive rod (also "green" rods; double cones)
Now cut parallel to the inner segments
Retinal mosaic in Gecko.
Gecko retina
Synaptic connections of the retina
Structure of the eye; Cajal 1892
Cajal; Mammalian retina
elements: receptors contact both horizontal and bipolar cells
bipolar cells contact amacrine and ganglion cells
amacrine cells contact bipolars and ganglion cells
ganglion cells as final common path
There are two synaptic layers in the eye. Outer plexiform layer containing cone pedicles, rod
spherules; bipolar and horizontal cell processes
Much more complex inner plexiform layer. Many cell types in the ganglion cell layer.
Laminar structure of inner plexiform layer
Ground squirrel
Differences in ganglion cell size
Cat retina
Functional differences between ganglion cells of different morphology.
1. Large somas have largest-calibre axons (m cells: transient properties)
2. Smaller sized (p cells)
On vs. off centre ganglion cells are associated with different planes within the inner
plexiform layer
The conduction pathway within the eye. Axons take a varied course, some sweeping away
from the fovea enroute to optic nerve (myelinated only when they reach the disc hence non
opaque)
Note Blue Arcs in demonstrations
Monkey whole mount
Convergence & Divergence within the Retina
Receptors as "points"
Ganglion cells as transmitters
Great variability in the complexity of inner nuclear layer
Convergence is generally an adaptation for sensitivity
Hamster
But many animals (especially birds!) have massive inner nuclear layer: an active filter
Tree shrew
Humans and monkeys have highly convergent peripheral retina roughly equal number
of receptors/inner nuclear cells/ganglion cells near fovea
Monkey fofea
Why is fovea shaped as it is?
Owl fovea
Chameleon (Anolis)
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Two aims;
Central visual pathways and visuo-motor links
`Introduce the targets of the eye, and particularly the primary visual cortex
Discuss the connections from visual areas of the cortex to motor areas.
I will talk about the connections from the eye, the nature of the crossing at the optic chiasma,
and the primary and secondary targets of the optic tracts. I will introduce these topics in a
historical context; how did we learn about the primary visual cortical area?
I will review the evidence that led to our recognition of a primary motor area of the cerebral
cortex, and early ideas about how the visual input might be linked to the motor output for
visual guidance of the arm, the legs and the fingers. There are three major ways in which the
visual areas might be linked to motor areas;
1) Via a series of cortico-cortical links
2) By way of basal ganglia
3) A major pathway from cortex to the cerebellum by way of the pontine nuclei
THE EYE AND THE CENTRAL VISUAL PATHWAYS
. Much of the visual system can be thought of as in a series-like organization.
Slide 1: Rhesus monkey retina ; low power to show series like arrangement of
layers
Receptors: (Rod/cones) Inner nuclear cells: (Horizontal, bipolar, amacrine) to ganglion
cells.
Visual information is imaged on the photoreceptors and must be relayed via inner-nuclear
layer to ganglion cells for transmission to the brain.
Centre-surround organization of retinal ganglion cells and scaling of relative intensity.
Within this series-like arrangement, there are at every level, parallel processing mechanisms.
Easiest to see at the level of the photoreceptors themselves.
Rods and cones share the image plane: process visual information in parallel.
Slide 2: High power to show rod and cone nuclei
At the retinal ganglion-cell level. There are ganglion cells with grossly different structure,
connections and function.
Ratio of receptor to ganglion cells varies greatly
Slide 3: Cat retina (note tepetum) Note differences in ganglion cell size
Galgion cells axons project to brain
Note large and smaller ganglion cells (m and p cells). Correlated with fibre-diameter.
These different sized cells have different receptive field properties, different targets in the
brain and different functions.
6

Large cells (m cells); respond transiently to changes of illumination in their receptive field;
and there are relatively more of them in peripheral retina.
Smaller sized (p cells); respond in a sustained manner to appropriate illumination of their
receptive field, have small receptive fields and are more heavily concentrated near fovea.
Much smaller cells “k” cells (konio; dust) and light detection
There is a smaller class of cells, the so-called “k” cells which have different receptive field
properties and different structures. K cells only account for an estimated 10% of all retinal
ganglion cells, but they seem to have important functions as light detectors.
An interesting class of ganglion cell has been discovered relatively recently; the melanopsin
containing ganglion cells. These cells may receive an input from photoreceptors, but they
also have a photosensitive pigment within them. Melanopsin ganglion cells respond to a short
wavelength of 484 nm peak sensitivity.
“Tiling of retinal ganglion cells” and the extent of ganglion cell dendritic tree
Slide 4: Cajal; Mammalian retina
See reference to Dacey et. al. in reference list
The three cell types cover the entire retina. Since there are about 80% p cells, 10% m cells
and an estimated 10% k cells, it follows that if the entire retina is covered by the dendrites of
these cells, the m and k cells must have larger dendritic territories than the p cell.
Magnocellular/parvocellular division of LGN; Also interstitial cells
Axons of the optic nerve
Axons of the ganglion cell collect at the back of the eye after running along the inner retinal
surface as unmyelinated fibres. (Why are they not yet myelinated?)
Blind spot and exit zone.
Slide 5: Monkey eye to show optic nerve (blue-arcs in demo)
On the course of the optic nerve
Optic nerve fibres proceed backwards and seem to unite in the X-shaped Optic Chiasma
(Greek X = Chi). The re-sorted fibres after the chiasma are called the optic tract.
At the chiasma the optic nerve fibres arising from ganglion cells in the nasal retina cross;
those from the temporal retina remain uncrossed.
The effect of this pattern of crossing is to project the right visual field onto the left
hemisphere.
Slide 6: To show location of chiasma at the ventral surface of the brain
There was no agreement on what happens at the chiasma. Some believed in no
decussation, some total decussation. (Isaac Newton had suggested the correct
arrangement)
There was always the problem of why we see singly with two eyes. Solved in different ways
by different authors.
Slide 7: Des Cartes; Chiasma and pineal
Slide 8: Hemi decussation; John Taylor; Frankfort 1750
Left Visual Field is projected onto the right side of the brain.
On the consequences of lesions 1) before the chiasma 2) of the crossing fibres in the chiasma
3) of the optic tract and beyond.
HEMI-AN-OPIAS
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Homonymous (right or left)
Heteronymous (bitemporal or binasal)
Mnemonic: a person (with optic tract), visual radiations, or complete unilateral cortical
lesion sees only on the side of the lesion.
Partial decussation in dogs, cats and horses. How far lateral is the eye? Principal of Visual
Field Representation in mammals.
Slide 9: Tree shrew to show laterality of the eyes
Slide 10: Tree Shrew; LGN and optic tract
On the targets of the optic tract
The targets in terms of relative numbers of fibres in man are:
Lateral Geniculate Nucleus (LGN). The major target in humans
Superior Colliculus
Pretectal Nucleus; role in pupillary control
Accessory optic system
Hypothalamus particularly the suprachiasmatic nucles. Role in diurnal cycling.
Lateral Geniculate Nucleus in turn relays visual information to the cortex.
The Laminar Organization of the LGN.
Slide 11: Human LGN (Recall centre surround receptive fields of LGN cells
Strict segregation of input, but perefectly aligned. Problem of the blind spot.
Phenomenon of transneuronal atrophy (Minkowski, 1920)
Slide 12: LGN: Normal monkey
Slide 13: Trans-neuronal atrophy
Numbering from ventral to dorsal 1-6
Contralateral eye projects to layers 1, 4 and 6
Ipsilateral eye projects to layers 2, 3 and 5
Note important distinction of
Magnocellular layers (1 and 2) and
Parvocellular layers (3, 4, 5, and 6) laminae
K cell projection to cells within the interlaminar fibre layers.
Receptive fields of cells in the LGN
Magnocellular layers relay m cells; achromatic; transient
Parvocellular layers relay p cells colour-opponent; sustained
K cells as light detectors; other properties
Slide 14: Human colliculus; parasagittal
The superior colliculus
The superior colliculus receives a direct input from the optic tract and from the cerebral
cortex.
Importantly involved in the control of saccadic eye movements.
Lesions of superior colliculus produce a transient deficit in initiating eye movements. Large
lesion of the entire colliculus leads to a transient fixed gaze; which recovers in a few days.
Lesions of Area 8 of the cerebral cortex (the "frontal eye fields") also produce a temporary
deficit in initiating eye movements.
Combined bilateral lesions of both lead to a permanent and severe deficit in initiating eye
movements.
8

Pretectal Nuclei and the pathway for the pupillary reflex
Direct and consensual pupillary reflexes illumination of the eye causes pupillary constriction
Retinal ganglion cells (certain small cells) Pretectal nuclei
Edinger Westphal nucleus in midbrain and thence via NIII to Ciliary ganglion controlling
Radial muscles of the iris.
Melanopsin ganglion cells as another input to pupillary control.
THE GENICULO-CORTICAL PROJECTION
Slide 15: Brodmann, 1905; Monkey cortical areas, lateral
Slide 16: Brodmann, 1905; Medial view
Slide 17: Human Weil stain to show stripe of Gennari
Segregation of the input from the left and right eye in the visual cortex; ocular dominance
columns.
We saw that projections from the left and right eyes are segregated in their connections to
LGN. This segregation is maintained at the first cortical synapse (in lamina 4 of cortex). The
segregation shows up in both histological and physiological experiments.
Inject one eye of a monkey with a labelled amino acid. Taken up and transported to LGN.
Some labelled protein taken up by neurons across the synapse and transported by LGN
neurons to the cortex. Terminals of left & right eye run in parallel stripes of about 0.5 mm
each on the cortex within layer IV.
Slide 17: Ocular dominance columns; Hubel
If one eye is masked in infancy vision is severely impaired in the deprived eye, the ocular
dominance columns become asymmetric; and few cells can be activated by the deprived eye.
"Take over" is failure to retract terminals.
On the normal growth of the eye in development
Slide 18: Human eye; neonate and adult
Slide 19: Wiesel and Raviola ; Normal and form-deprived eye
The form-deprived eyegrows much longer, and is seriously myopic
Cells in more superficial and deep layers of cortex begin the process of fusing the input from
the two eyes;
Cytochrome oxidase "patches" or “blobs”in primary visual cortex
Livingston and Hubel showed that colour information is targeted on the cytochrome oxidase
patches. But nocturnal primates who lack colour vision also have the cytochrome oxidase
patches.
Multiple visual areas
Slide 20: Brodmann; lateral view of monkey brain
Slide 21: Brodmann; Medial view of monkey brain
Is the striate cortex the sole target of LGN axons? Old evidence form the cat, recent
evidence in monkeys. Macular sparing “blindsight” and other odds and ends
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VISUAL GUIDANCE OF MOVEMENT
1. Some general problems: What do you want to move?
Slide 1) YO YO MA PLAYING THE CELLO
Slide 2) Bouree from Bach’s 3d suite for unaccompanied cello
Demo Catching a small object
Slide 3) (MONKEY REACHING FOR A PELLET
Arms, legs etc. (catching a cricket ball/baseball)
Fingers and wrist (threading a needle)
Eyes: On the categories of voluntary eye movements conjugate vs vergence
Smooth pursuit vs saccadic
Why has so much attention been paid to eye movements?
General problem of load compensation in limbs;
Eyes always weigh the same
How are visual areas connected to motor areas of the brain.
Visual cortex to motor cortex; (superior colliculus to red nucleus).
Three possible routes: 1) Cortico-cortical circuits. 2) Basal Ganglia, 3) Cerebellar
Side look into history. The discovery of the motor cortex
Slide 4: FRITSCH AND HITZIG
Electrical stimulation
Slide 5: MOTOR CORTEX
The first attempt to identify the visual area led to an error.
FERRIER: "CAUSING BLINDNESS"
Ferrier's mistake was gently pointed out to him by Hermann Munk
MUNK: OCCIPITAL LESIONS CAUSES HEMIANOPIA
SALOMON HENSCHEN “HEMIANOPISCHE FÄLLE”
“KEINE HEMIANOPSIE”
By the end of the nineteenth century, the identity and location of the primary visual
and motor areas of the human and monkey cerebral cortex were clear. Less was understood
about further processing of the visual signal.
BRODMANN LATERAL VIEW
Note Area 18 and 19; "Association areas"
BRODMANN MEDIAL VIEW
Parietal Lobe Lesions Cause Visuo Motor Impairment
Why did Ferrier make the mistakes that he did? In his earliest experiments, Ferrier operated
in unsterile conditions. Because of the risk of infections, he killed the monkeys three days
after they were operated on. He later joined the faculty of the Medical School at King's and
learned sterile operative technique from his colleague, Joseph Lister.
His animals could now live for weeks, months or years. In the second edition of his book,
Functions of the Brain, he modified his statement
FERRIER: "TEMPORARY COMPLETE ...."
Angular gyrus and parietal lobe
BALINTS CASE
BALINTS DESCRIPTION
What is the Circuit between Visual cortex and Motor Cortex?
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BERNSTEIN 1900
The discovery of the motor cortex. Fritsch & Hitzig's Experiment
The discovery of the visual cortex. Ferrier's Mistake: Munk
The "obvious" nature of cortico-cortical links.
If cortico-cortical circuits are essential, the first link must be by way of the parietal lobe
.
Special problem of integration between the hemispheres (backhanded shot in tennis).
The evidence (or lack of it) for the role of cortico-cortical fibres.
. Section of corpus callosum does not produce obvious deficits in bimanual
coordination or sensory guidance of movement;
Direct cuts of cortico-cortical fibres do not block visual guidance of the hand and arm.
(Myers, Sperry & McCurdy)
MYERS ET AL 1963
But see Kuyper's interpretation
.
The Corticoponto-cerebellar pathway.
Corticopontine (and tectopontine) fibres:
HUMAN BASIS PEDUNCULI
How to trap them
Basis pedunculi - (pyramidal tract + corticocular fibres)
Corticopontine fibres
HUMAN MEDULLA
PONS
MATANO, STEPHAN
How Does the cerebral Cortex Connect to the Pontine Nuclei?
Two related questions;
1) Which Cells
2) Which Areas
Inject pontine nuclei with a retrograde tracer; Identify labelled cortical cells
TREE SHREW
MONKEY: SOURCE OF CORTICO PONTINE VISUAL FIBRES
VAN ESSEN EXTRA-STRIATE VISUAL AREAS
NUMBER OF VISUAL AREAS AS A FUNCTION OF TIME
Distiction of parietal and temporal lobe directed cortical connections
1) Layer V pyramidal cells.
2) The differential distribution from extrastriate visual areas in the monkey.
(Superior Colliculus; Pretectum and LGN)
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Pontocerebellar Pathway
Cerebellar afferent fibres
Mossy vs. climbing fibres
3 peduncles
HUMAN PONS; SAGITTAL SECTION
MIDDLE PEDUNCLE
middle > (inferior and superior)
all (well nearly all) from n. pontis.
Contralateral and ipsilateral projection in the middle cerebellar peduncle: Possible solution to
some old problems.
INFERIOR OLIVE (HUMAN)
Climbing fibre input to the cerebellum. (J. Simpson)
Accessory optic system dorsal cap of the inferior olivary nucleus. Flocculus and adjacent
flocculus.
On the difference between mossy fibre and climbing fibre visual afferents to the cerebellum.
Some behavioural evidence for cerebellar role in visuo motor coordination and plasticity and
some unresolved questions.
1. Plasticity of the VOR
2. Saccadic Adaptation
2. Prism adaptation
3. Nictitating membrane conditioning
References
Eye and Central Visual pathways
Classic work on the comparative anatomy of the eye is:
Walls,G. (1942) The Vertebrate Eye and its Adaptive radiation Bloomfield Hills; Cranbrook
A good general source for sensory function
Barlow, H., and Mollon, J. (1983) The senses Cambridge
A recent massive reference :
Chalupa, L., and Werner, J. (2003) The Visual Neurosciences Cambridge, MIT Press
9:
Dacey DM, Packer OS. (2003), Colour coding in the primate retina: diverse cell types and
cone-specific circuitry. Curr Opin Neurobiol. 2003 Aug;13(4):421-7. Review.
Functions beginning to catch up with Cajal’s anatomy.
Lamb et. A;.(2007), Evolution of the vertebrate eye: opsins , photoreceptors, retibna and eye
cup. Nature Neuroscience Reviews; 8 . 960-975.
Evidenvce of evolution beginning to reassure Darwin
For the central visual pathways, any good neuroscience text.
Eye Movements
Robinson, D.A. (1981) Control of eye movements in (V. Brooks, Ed.). American
Physiological Society Handbook of Physiology Section 1. The Nervous System, Volume II
Motor Cortical, Ch. 28, 1275-1320.
Carpenter, R.H.S. (1988) Movements of the Eyes. London: Pion.
Lee, R.J., and Zee, D.S. (1999) The Neurology of Eye Movements New York; Oxford U
12

ENTOPTIC PHENOMENA, VISUAL PHENOMENA AND EYE ANATOMY
The pupil as a first line of defence against excessive light.
DEMONSTRATION 1. PUPIL SHAPE; SIZE CHANGE
Make a triangular shape with the tips of your fingers; bring the triangle closer to your eye:
What happens to the shape of the triangle?
Now open and close the other eye. Observe your own (consensual) Pupillary Reflex.
The pupillary reflex has been well studied quantitatively. If we arbitrarily increase gain of
the pupillary reflex.
DEMONSTRATION 2. OSCILLATING PUPIL
Shine penlight just on the edge of the pupil. Pupil will oscillate in size.
In general with poor focus as in uncorrected myopia size of blur circles is related to the size
of pupil. Therefore decreasing pupil size causes improved resolution with sufficient ambient
light.
DEMONSTRATION 3. REFRACTIVE DISORDER/PINHOLE
Chose a myopic subject: Find the far point of distinct vision by seeing how far they can see
the tip of a pin clearly. Place text just beyond far point so as to be unreadable. Now view
through narrow pinhole.
ABBERATIONS AND DIFFRACTION
The pupil plays a complex role in relation to image quality and hence to acuity. Three major
physical factors are related to pupil size - can a point of light be imaged as a point on the
retina?
Spherical Aberration
In any simple spherical lens the edges of the lens refract more than the centre. Therefore a
distant point can not be imaged as a point.
(Human cornea is actually somewhat flatter at edge; consider effects of hard contact lenses
with large pupils).
Diffraction
Points of light are actually imaged as concentric bright/dark rings. Diffraction effect is
greater with smaller pupils.
Chromatic Aberration
Short wave length light is refracted more than long wave lengths.
Therefore given a white light, different wavelengths will be imaged at different depths.
DEMONSTRATION 4. BLUE-FREE ZONE IN CENTRAL FOVEA
DIRECTIONALITY OF VISION
Visual world is imaged on the retina as in a camera (left-right; up-down inversion).
Yet we locate directions accurately relative to our position in space.
DEMONSTRATION 5. PRESSURE PHOSPHENES
Press gently with a cotton swab on the cornea of your eye. Can you see a pale flash of light?
Where does it seem to come from?
Schlodtmann's Experiment and the innate nature of visual direction.
(Keep in mind the orderly relationship between retinal position and visual direction: How is
it that we can adapt to optical disruption of direction such as in inverting or lateral-displacing
prisms?)
RETINAL VESSELS AND THE STABILIZED IMAGE
Retinal receptors are supplied by way of the choroidal circulation and the pigment
epithelium.
Inner nuclear and ganglion cells by way of central retinal artery.
Why don't you see these vessels since they lie in the path of the light?
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Probably because their shadows are stabilized on the retina.
Therefore if we illuminate eccentrically, we might see them.
DEMONSTRATION 6. PURKINJE VESSEL FIGURE
Illuminate eye through the sclera with a penlight. It is now possible to see an important point
about the position of retinal vessels relative to the fovea.
DEMONSTRATION 7. AFTER IMAGE IN FOVEA WITH VESSELS
Rods have lower thresholds and different spectral sensitivity than cones. Purkinje observed
the relative brightness of the rose and its leaves in his garden.
DEMONSTRATION 8. RED AND BLUE: BRIGHTNESS IN DIFFERENT LEVELS
OF ILLUMINATION
SPEED OF RESPONSE
Cones work more rapidly: Activate ganglion cells sooner and the message reaches the brain
sooner. Even within the photopic range, lower illumination levels lead to slower response of
receptors and ganglion cells.
DEMONSTRATION 9. REACTION-TIME UNDER CONDITIONS OF DIM AND
BRIGHT ILLUMINATION
Two illusions induced by differences in latency of response.
DEMONSTRATION 10. FLUTTERING HEARTS
A more subtle effect can be seen binocularly.
DEMONSTRATION 11. PULFRICH EFFECT
Object moves back and forth
Seen as a disparity by the two eyes. Therefore interpreted as depth hence
elliptical orbit.
PERSISTENCE OF ROD AFTER IMAGES
DEMONSTRATION 12. FLASH AFTER 5 MINUTES DARK ADAPTATION
CENTRAL COURSE OF OPTIC FIBRES WITHIN RETINA
Ganglion cells fires and axon conducts centrally. First along
retinal surface and thence, through to the brain.
Transmission via unmyelinated optic nerve fibres.
DEMONSTRATION 13. BLUE ARCS
(1) Why is it shaped as it is?
(2) Why coloured blue?
STABILITY OF THE VISUAL WORLD: EFFERENCE COPY AND THE ILLUSION OF
MOVEMENT
When we sweep our eyes over the scene in front of us it appears to remain stable. Why? Not
necessarily because it is stable.
DEMONSTRATION 14. JIGGLING YOUR EYE WORLD MOVES
Helmholtz Interpretation: Signal from brain nulls apparent movements of the world when we
voluntarily move our eyes.
(Note Not brought into play during vestibular induced movements).
THERE EXIST A PARALLEL SET OF EXPERIMENTS WHICH REINFORCE THE
VIEW
Suppose you have a stable image on your retina and will the eyes to move. What happens?
Conversely, if you jiggle your eye, the after-image remains stable.
DEMONSTRATION 15. ACTIVE AND PASSIVE MOVEMENT OF THE EYE WITH
AN AFTER IMAGE ON FOVEA
EYE MOVEMENTS AND VISUAL CONTROL OF MOVEMENT: CHANGING THE
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IMAGE AND THE MODIFICATION OF VISUALLY INDUCED RESPONSES
The function of vision is to guide movement. Either the whole body, individual limbs, or the
eyes themselves.
Some systematics about movements of the eye.
DEMONSTRATION 16. SMOOTH PURSUIT VS. SACCADIC MOVEMENT
Observe your partner 1) following a smoothly moving target
2) trying to replicate that movement
Only saccade is under voluntary control.
DEMONSTRATION 17. ACCOMMODATION AND VERGENCE MOVEMENT
(pupil size decreases)
Ask subject to view a distant target: observe eyes and pupil as you bring it closer to their
face.
VESTIBULO-OCULAR REFLEX
DEMONSTRATION 18. SHAKE HAND WITH HEAD STILL; SHAKE HEAD WITH
HAND STILL
also READ TEXT WHILE SHAKING HEAD OR BODY
LATERALLY DISPLACING PRISM
DEMONSTRATION 19.
Point at target Adapt: Mispoint when prism removed.
Argument that effect is Not Visual (1) Doesn't transfer between arms
(2) Transfer to pointing to a sound source
Vision in conflict with propioception.
DEMONSTRATION 20. VISUAL CAPTURE
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