Past Papers Flashcards
Name five conditions that can induce a spontaneous Pulfrich effect. For each
condition, explain why the effect occurs. Example for illustrative purposes:
Reduced colour perception in a patient with ARMD – occurs due to the loss of
foveal cone function. 10 marks
- Multiple sclerosis – the optic nerve may be damaged which can cause a delay in signal transmission to the brain. The difference in timing of the signal transmissions from each eye can cause spontaneous Pulfrich effect.
- Cataract – the opacities in the lens will absorb light which will cause the stimuli reaching the retina in the affected eye to be reduced compared to the fellow eye.
- Corneal opacity – The light entering the eye will be unequal at the retina due to increased scatter and possible absorption by the corneal opacities causing the signal reaching the retina to be unequal.
- Anisocoria – The difference in pupil size will mean that the intensity of the stimuli reaching the retina will be unequal.
- Central serous retinopathy – fluid build-up behind the retina will cause the neural signals in the affected eye to be delayed
Identify and discuss problems and impairments that may be associated with neurological loss which make communication more difficult with this population. 5 marks
• Patients may have other neurological conditions such as dementia which can limit their ability to understand the findings of an eye exam
• Patients could have neurological impairment due to traumatic brain injury which can cause memory impairment meaning they are unable to retain the information given to them or limit their ability to understand this information
• If patients have suffered recent vision loss due to neurological problems they may be unable to accept their condition (they may be following the loss model and be in stage 1-4 which can affect their ability to deal with their vision loss) making it more difficult for the practitioner to communicate with these patients
- inability to read
- patients may be unable to speak however they can still understand and retain information
ii. Describe 5 adjustments/modifications you could make to your assessment routine to help achieve better communication. (5 marks)
• If the patient has a carer then ensure they are present during the examination.
• While it is important to ensure you are speaking to the patient, the carer may be able to provide important information that the patient cannot
• Keep language simple and speak slowly, ensure that the carer (if present) understands the information being given and invite any questions they may have
• Ensure the priorities are known for this patient
• They may be unable to read so assessing critical reading speed would be unnecessary however, the patient may enjoy other near tasks like drawing which would require an assessment of their best corrected near vision
• It may be appropriate to provide information in writing for these patients, if the patient is able to comprehend this then ensure print size is adequate for their viewing, if not then offer this written information to the carer
• When performing tasks such as subjective refraction it is important to ensure the patient understands the task
- might be easiest to do tests which don’t require the px to speak as much such as retinoscopy
Features of congenital vs aquired CVD 6 marks
Type and severity
- aquired CVD may change in severity over time
- congenital is stable
Other visual functions
- aquired may present with other visual abnormalities
- congenital normal
Type
- congenital usually protean/duetan
- aquired Tritan
Male/female
- congenital usually male
- aquired even between both
Diagnosis
- aquired more difficult to classify and gives mixed responses
- congenital easy to diagnose
Same in both eyes
- congenital identical between eyes
- aquired asymmetry
Disadvantages of croma gen lenses 5 marks
Disadvantages and problems associated with the use of CromaGen contact lenses for
‘treatment’ of CVD are:
Cosmetically poor; monocular filter or 2 different filters placed in front of each eye They do not provide normal colour perception
They dim retinal illumination which can lead to the pulfrich effect
They can impede binocular fusion and stereopsis
They are not always permitted in certain jobs
The contact lenses are hydrogels to accommodate the tint which increases the chances of the lens drying out along with the associated problems such as dry eye, and hypoxia
ii) Briefly, explain why a CromaGen™ contact lens may enable a colour deficient patient to see an isochromatic plate
They reduce transmission of certain wavelengths of light
This in turn introduces a perceived brightness difference between wavelengths of light which did not exist before
I) Define the term “minimum angle of detection (MAD)”.
ii) How would you design an experimental stimulus to measure the MAD? (5 marks)
MAD is forced choice of being able to detect the presence of a stimulus, but with no discrimination eg being able to detect a black and white grating against a grey background, the minimum angle subtended can be recognised is the MAD
MAD is the visual angle subtended by the smallest detectable letter on the test chart. It can be measured using a detection acuity chart.
ii
A standard test for MAD would consist of striped patterns matched with uniformly grey targets of the same mean luminance. The px is asked ‘are the stripes on the R or L’ which is v similar to preferential looking tests.
Assumption is that if px can answer correctly, then the visual system must be able to detect the spatial contrast present in the pattern. The stripes get finer towards the bottom of the chart, and the with of the finest bars that can be distinguished is a measurement of detection acuity
i) Define the term “minimum angle of resolution (MAR)”.
ii) How would you design an experimental stimulus to measure the MAR? (5 marks)
The MAR is the smallest visual angle that a stimulus can subtend, and still be resolved by the observer. It is typically measured using a high contrast detailed stimulus (eg. A small letter on a letter chart). The observer is required to recognise a feature of the stimulus (eg. Orientation or letter identification)
ii
This could be done through. The use of a resolution chart where the horizontal and vertical stripes are presented in gratings (Thibos & Bradley, 1993). The patient is asked to report whether the horizontal stripes are on the left or right (the stripes on either side are of the same spatial frequency and contrast). If the patient can answer the question correctly it can be assumed that the visual system has sufficiently resolved the pattern into its element bars, in order to determine their orientation. The spatial frequency of the gratings increases towards the bottom of the chart (lines get narrower) and the width of the finest bar for which the patient correctly performs the task can be used to calculate the MAR. the use of staircase method to bracket the MAR threshold should be used until an endpoint is reached (e.g. a certain number or reversals). Requirements for memory/ experience are limited through the use of two target alternatives: resolution acuity chart and detection acuity
I) Describe the phenomenon of “aliasing” in human vision.
ii) How do the measurements of MAD and MAR allow us to detect aliasing in human vision? What does the presence or absence of aliasing tell us about visual function?
(3 marks)
(Total 20 marks)
Aliasing is a phenomenon that results in ambiguous perception, as the stimulus is misrepresented by under sampling of the nervous system. It occurs as patterns beyond the resolution limit are detectable, but are seen incorrectly. The stimulus is too fine to be resolved by the sampling mosaic, so it becomes misrepresented as an alias of different frequency and orientation
ii
If MAD and MAR are the same value, then no aliasing is present. If MAD is more than MAR, then aliasing is present. The absence of aliasing tells us that there is an occurrence of optical filtering (optical problem causing reduced vision). If there is a presence of aliasing it means that there is neural under sampling (neural problem causing reduced vision). This is particularly important to find out in a px with cataract who may have other pathology eg amd as need to find out prior to surgery which is causing the low vision
Give 2 examples of eye conditions that can cause an aquired colour vision deficiency. And what deficiency is associated
- AMD can cause a tritan defect
- optic neuritis or retrobulbar neuritis can cause red green deficiencies
What is the stiles Crawford effect
Rays entering through the centre of the pupil are more likely to be absorbed by the retinal photoreceptors than peripheral rays
3 aspects of perception and cognition that may be assessed using. Describe how they are tested
Visual neglect
- inability to orientate, report or respond to sensory stimuli in a region of space contralateral to a cerebral lesion
- tested using cancellation testing and line bisection test
Visual perception
- brains ability to recognise, integrate and interpret information detected by different sensory stimuli
- tested using motor free vision perception to test for visual discrimination and spatial awareness
- ox-vps test can test for object recognition
Eye movement disorders
- maintaining primary position
- nystagmus
- phoria/tropia
- cover test
- fixation disparity
- motility
- saccades
Visual field loss
- standard perimetry
Theories of myopic growth mechanism
passive emmetropisation
- natural eye growth
- axial length grows other components grow accordingly
- produces emmetropic eye
- people with flatter corneas have longer eyes
Active
- eye grows towards an emmetropic state
- negative feedback
- hyperopia refocus
- axial length growth controlled by optics and environment
Refraction
- SV correction leaves hyperopia defocus
Choroid
- thinning causes reduced barrier to growth
Sclera
- decreased collagen synthesis
- decreased strength
Name and describe 5 different mono chromatic aberrations
Spherical aberration
- optical power of a lens is not constant across aperture
- positive lenses cause rays to be focused in front of the paraxial ray
Coma
- results from off axis objects
- rays divert differently across lens
- produces a comment shape image, either towards or away from the optical axis
- the more off axis the bigger the comet
Oblique astigmatism
- occurs due to difference in power between the tangential and saggital plane
- similar to on axis astigmatism but differs as the orientation of the focal lines are dependent on the location of the object
Field curvature
- as an object moves away from the axis on the object plane, the distance to the first principle point increases, and image distance changes
- eye compensates
Distortion
- occurs due to different amounts of magnification present across the lens
- pincushion distortion if the magnification increases at edge of the lens
- barrel distortion if magnification decreases
When can aliasing occur in normal vision, and abnormal conditions
Normal
- when spatial frequency is finer than 60 c/degree
Abnormal
- amblyopia
- amblyopic eye nearly as sensitive to contrast but at higher frequencies it is distorted
- amblyopia may be due to aliasing at some stage
