Medical Neuroscience Module -- VISION PRACTICAL
Tony Gardner-Medwin, Dept. Physiology, UCL, ucgbarg@ucl.ac.uk
Objectives
At the end of the practical and demonstrations you should be able to:
Distinguish and describe many of the separate functions that are integrated as ‘vision’
Demonstrate or measure phenomena and disorders related to these functions
Explain the significance for a patient of loss of these aspects of visual function
Many pieces of apparatus are set out. Phenomena marked HOME don’t need special apparatus. Others
can be demonstrated by computer, using web sites or programs available via www.ucl.ac.uk/lapt/med.
Everyone has visual problems at some time in life. Amongst family and friends you will encounter
several issues to learn from. Suggested Book: RHS Carpenter, Neurophysiology (Arnold, 2002)
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Work in pairs and rotate around the topics in the 90 min. Tick items below when completed.
Use the brief notes here plus common sense to figure out how to use the apparatus.
Raise issues with demonstrators, but don’t wait for them to introduce each apparatus
Answer the questions - write in the notes here, or on separate sheets.
Use the revision guide at the end to rehearse and organise what you have learned.
Access the web discussion forum (at www.ucl.ac.uk/lapt/med) and enter queries and comments
about issues without inhibition. This is an experimental approach to helping you with your
learning, so try to make it work well. Both students and staff can help to answer queries, and staff
will try to keep things organised and correct any mistakes. Enter points when they arise. There are
no set times for use of this system as a chat room, but your entries should appear straight away.
Problems: email to one of the course teachers listed on the web site at www.ucl.ac.uk/lapt/med.
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Focus
In the normal relaxed eye parallel rays of light (from objects far away) are brought exactly to focus on
the retina. The far point1 is therefore at infinity. The converging power of the corneal surface is about
42 dioptres2, and the (variable) power of the lens about 20-30 dioptres in a normal 20-yr old.
Seeing close things – Myopia and accommodation
Use the apparatus (or HOME ) to measure your near point1 and far point1………
What is your accommodative power3?…………
For a subject with myopia (short- or near-sight) describe the symptoms (evident to the patient),
the signs (results of tests above) and the anatomical cause (indicated in the diagram above).…….
Would you need a converging or diverging lens to correct myopia?…..
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The near and far points are the smallest and greatest distances at which things can be seen in focus.
Lens power in dioptres =
1
=
100
(+ for converging , - for diverging)
focal length in m
focal length in cm
NB powers add when you put lenses together, so you can measure a lens power by cancelling it with one of known power.
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Accommodative power is the increase of power your lens can achieve: 1/nearpoint –1/farpoint (both in metres)
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1
Identify a spectacle lens to correct myopia. Which of the 4 lens shapes above is it?…..
If a myopic’s far point1 is at 33cm, what power of lens2 would correct the problem?…..
Presbyopia (loss of accommodation or focussing ability) with age
Age doesn't mean old age! Your accommodative power has fallen markedly since infancy and will fall
to near zero around 60. The lens stiffens so the ciliary muscle can no longer alter it.
Identify reading glasses, bifocals and varifocals suitable for dealing with the problem. (HOME)
What are the problems and hazards of walking round with bifocals or varifocals?……
In later life myopics are better off in some respects than normals. How?...
Astigmatism - uneven curvature of the cornea (or lens).
The corrective lens needs a cylindrical component - different power for lines of different orientation.
Identify an astigmatic lens (rotate it as you look through, looking for changes in power). ( HOME)
With this lens look at letters and the astigmatic fan to see the world as astigmatics see it.
Note that astigmatic corrections may be combined with correction for other errors (short/long sight).
Contact lenses
These may be soft or hard, disposable or long-lasting. They need to be relatively oxygen permeable
because the cornea has no blood supply (why not?). Patients sometimes can’t tolerate contact lenses
because of hypoxia, dryness or susceptibility to infection. Surprisingly, bifocal contacts do exist4.
Talk to someone with contacts about their experience, and about any problems.... (HOME)
Cloudy vision (cataract - lens or corneal opacity)
This often develops in later life. The effect is like looking through a dirty windscreen.
Use the graded cataract simulator to decide at what point you would opt for surgical treatment.
See how vision with a cataract is much more impaired with a bright light in front of you (glare).
Acuity testing
6/12 vision (20/40 in old units) means that at 6m (20 ft) you can just read letters that a person with
‘normal’ vision can read at 12m (40ft). The lines on the Snellen Chart are labelled with the distance at
which they can be read by a ‘normal’. The letters are then 1/12 deg high5..
Measure your acuity (without glasses if you wear them). [Don’t remove contacts just for this.]
Pupil Control
The pupil dilates in the dark (raising sensitivity) and with excitement (sympathetic arousal – though
it’s unclear what benefit this gives). Constriction in moderate light improves depth of focus and acuity.
Simulate a refractive error with a lens, and note how a pinhole improves your acuity. ( HOME)
Demonstrate ipsilateral and crossed pupil reflexes in another student, using a small torch.
Ophthalmoscopy
The pupil is black because the part of the retina you see through someone’s pupil is not illuminated (it
receives an image of your own pupil!). The ophthalmoscope shines a light along the same line of sight,
illuminating this part of the fundus (back of the eye). With experience you can tell a lot about a
person’s health by looking at the condition of the retinal blood vessels – though GPs have not always
developed this skill. You will spend more time on this in the clinical skills classes in Phase 3 of the
medical course. Get the basics now.
Turn the lens disk in the opthalmoscope to 0 dioptres (no lens) and look at a subject’s right eye with
your right eye, from 5-10 cm. Ask the subject to scan distant objects beyond your right ear. If you
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These have concentric rings with different powers. They superimpose focussed images for several different
distances (along with blurred images). I don’t know how well they work but would be interested to hear.
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For comparison, the sun and the moon are both about ½ deg in diameter.
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both have normal vision and you both relax your accommodation you should see the fundus in
focus. Otherwise a lens may be needed for correction.
Note the colour of the retina, due to the visual pigment (mainly rhodopsin in rods)…..
Find the optic disk (much paler) with the blood vessels radiating from it.
Sketch the optic disk. NB
it is hard to do this from life. If
you prefer, try sketching the
fundus picture in an artificial
eye:.......
Visual Field Testing
Field defects can have many causes (e.g. haemorrhage, retinal detachment, glaucoma, macular
degeneration, retinitis pigmentosa, etc.). In addition, though normally unaware of it, you are
completely blind for images that fall on the optic nerve head (optic disk6).
For the left eye, is the blindspot due to the optic disk in the left or right visual field?……..
Use one of these techniques to map the blindspot in one eye and plot it below:
(i) With the perimeter apparatus set for appropriate orientations, find regions where the coloured
spot on a wand is not seen, noting both the range of angular eccentricities and orientations.
(ii) With the computer field test, record the positions where you cannot (from a distance D=25-30cm,
with one eye) detect a randomly presented stimulus. NB Stimulus times are randomised. Why?
(iii) A simple version of (ii) is to draw X on a piece of paper and fixate it from 30cm while moving a
pencil around. marking places where you cannot see the pencil point. (HOME)
This shows typical normal visual
fields (dotted). Mark on the
appropriate diagram your
measured blindspot. [From (i) or
(ii) calculate the approximate
eccentricity as 1 radian*  S/D,
where S is the distance on the
screen or paper, from the fixation
point to the blindspot. [If you are
skilled with your calculator, the
accurate formula is arctan (S/D).]
* 1 radian = 57 deg
Distribution of retinal blood vessels
Touch a pencil torch lightly against the skin beyond the lateral corner of your eye (taking care not to
touch the eye) and keep the light constantly moving round a bit. This illuminates the nasal retina from
the side, so that the moving shadow of the blood vessels can be seen. (HOME)
Sketch the appearance of the pattern of
vessels, noting the branching (away from
the optic disk)………
Repeat after fixating the torch bulb at
arm’s length for a few sec to generate a
foveal afterimage. This enables you to
identify how the vessels are related to the
fovea. Mark this on your sketch……….
Suggest an explanation why the the vessels fade when you stop moving the torch………
Pharmacology of Pupil Control and Accommodation
Thorough ophthalmoscopic examination requires dilation of the pupil (not done in the practical class).
Pupil constriction (and ciliary muscle contraction) are controlled by parasympathetic nerves and pupil
dilation by sympathetic. Atropine or its analogues (e.g. tropicamide, which is shorter acting) are used
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The optic nerve and optic disk are on the nasal side of the eye relative to the fovea (centre of vision)
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(as eyedrops) for dilation. The drug takes 20min to penetrate to the iris and ciliary muscle.
What is the pharmacological action of atropine and its analogues?………
During the period of persistence (2-3 hours) what disturbance would the subject experience?
Vergence and squint (strabismus)
Optimal vision of objects at a particular distance requires both appropriate accommodation and
convergence. The two are linked by reflexes. Convergence will normally achieve fusion of images in
both eyes, if there are enough features in common. Poor vergence control (heterophoria) may cause
double vision (diplopia), suppression of images from one eye, and poor depth perception (stereopsis).
Test a subject for heterophoria. Have the subject view a far object binocularly, then watch one
eye as you block and unblock its view with your hand, a few cm away. The blocked eye may move.
Try to demonstrate the accommodation-convergence reflex. Make the subject accommodate
strongly to see a close object with one eye, while watching the other eye but blocking its view.
Use a Maddox Rod (making a distant lightbulb appear as a line) to detect small degrees of
horizontal or vertical heterophoria by ascertaining how the line in one eye relates to the spot seen
directly with the other eye. NB this may not work with subjects who suppress one image.
Strabismus (squint) is a heterophoria that is not overcome with fused stimuli. Heterophoria in
childhood (sometimes due to refractive errors or muscle imbalance) must be corrected, or overcome
with exercises, to ensure that the CNS develops correctly with normal combinations of visual input.
Test the limits of convergence in a subject. Ask them to look at your finger as you move it
from 30cm closer to their nose, while watching their eyes until fusion breaks down. This is the basis
for exercises to train convergence in children with a tendency to strabismus.
Stereopsis and depth perception
Central comparison of binocular images (stereopsis - identifying small differences due to parallax)
generally provides the must accurate judgement of relative distance. Monocular cues (image size,
perspective, motion, accommodation, etc.) can also be important, and the change of an image with
head movement can to some extent substitute for binocular vision.
View the random dot (Julesz) images so the right eye sees the right image directly and the left
eye sees the coincident reflection of the left image. There are no monocular cues in these images,
but after 1-2 s binocular processing you should see the central region (what shape?) set in front of or
behind the background. The images are identical, except for the central region where they are
displaced a few pixels.
In the depth perception apparatus, try to set the central white rod to the same distance as the
lateral rods with (a) monocular, (b) binocular viewing. Typically the results come out in the way
shown graphically (in 2 different ways) below. What calculation from these data would express and
quantify the main effect of binocular viewing?
setting (mm)
170
20
monocular
180
190
200
210
220
230
binocular
histogram count (in 5mm bins)
10
monocular
0
170
175
180
185
190
195
200
4
205
210
215
220
225
230
binocular
VISION PRACTICALS - REVISION AID
Separate aspects of visual function and some related tests, disorders and treatments are briefly
set out here in 4 groups, to help you organise and relate the different aspects of your
knowledge. Go through the table and consider the relationships and distinctions between
different elements.
To discuss these or other topics, use the discussion forum at www.ucl.ac.uk/lapt/med .
OPTICS & VISUAL FIELD
Issue
Focus
Symptoms/tests
Acuity testing
pinhole tests
Accommodation
near, far points
Cloudy optics
Glare with bright lights
Reduced contrast & acuity
poor night vision if constricted
poor depth of focus if dilated
Perimetry (lo- or hi-tech)
High acuity (in bright light)
High sensitivity, low acuity
Pupil control
Visual Field
...(macula & fovea)
...(periphery)
Common Disorders
Myopia (short sight)
Hypermetropia (long sight) Astigmatism (orientation dependent)
Presbyopia (lens stiffens with age)
receding near point
Cataract (lens or cornea)
floaters (debris near retina)
[reflexes lost with cns damage]
[constricted in glaucoma theraby]
vascular, glaucoma, detachment
Age-related macular degeneration
Tunnel vision (e.g. retinitis pigmentosa)
Treatments
spectacles, contacts
corneal surgery
reading glasses, bifocals
varifocals
lens implant, corneal transplant
[pupil may be constricted
by treatment for glaucoma]
laser surgery
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Common Disorders
Atherosclerosis, haemorrhage,
diabetes, aneurysms, etc.
Glaucoma (raised pressure)
poor drainage or excess fluid)
Distortions cause nerve damage
Treatments
laser surgery
[treat systemic diseases]
Beta-blockers or AcCh-esterase inhibitors) or surgery
[Treat primary disorders]
retinitis pigmentosa (degeneration,
particularly of rods)
Genetic anomalies: variant pigments
(blindness - absence of cones is rare)
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Common Disorders
Nystagmus, attention disorders
Labyrinthitis, ototoxic antibiotics, etc
Dizziness, nystagmus,
Oscillopsia (world seems to move)
Heterophoria, diplopia, suppression
Strabismus (squint)
Amblyopia (poor neural development)
Possible but uncertain role. [Blood
vessel, floater, aftter-images fade]
Treatments
Slow adaptation to lost function
(Total loss of function may be
better than partial, variable loss)
Refractive correction, fusion
exercises, occlusion, surgery.
Important to correct in childhood
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RETINAL & SUPPORTIVE FUNCTION
Issue
Blood supply
Intra-ocular pressure
Optic disk
(nerve head)
Dark Adaptation
After-images
Colour Vision
Symptoms/tests
Ophthalmoscopy
Reveals systemic diseases
Corneal contact / air puff
pressure monitor (tonometer)
Blindspot
Cupped in glaucoma & elevated
with intra-cerebral pressure.
10000-fold increase of sensitivity
Fade, evolve, re-appear with blink.
Discrimination in bright light only
Ishihara Tests
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EYE MOVEMENT & CONTROL
Issue
Fixation
Optokinetic reflex
Vestibular reflexes
lateral, vertical,
& torsional
Convergence
Adaptation to
stable images
Symptoms/tests
Saccades (sudden shifts)
Steady conjugate movement
Rotation nystagmus, caloric test,
test acuity during head motion
Binocular fusion, stereopsis,
wandering eye movement,
Accom.-convergence reflex
Tremor, saccades essential for
vision. Stabilised images fade
CENTRAL PROCESSING
Issue
Field defects
(scotomas)
Amblyopia (poor visual
processing)
Object recognition,
attention, inference
Symptoms/tests
Common Disorders
Treatments
Perimetry [NB scotomas may
migraine (transient), stroke, trauma,
not be obvious to the subject]
degenerative disorders
NB Defects above LGN affect the same (contralateral) visual field in both eyes
Acuity deficits, not due to
Poor optics / fusion in infancy
critical early diagnosis &
refractive or optical causes
O2 trauma at birth. CNS disease.
correction in childhood.
neurocognitive tests, illusions
strokes, local trauma
MRI, functional MRI
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