Sample Q 2018b Flashcards
What are the eye diseases for which retinal implants are being developed?
RETINITIS PIGMENTOSA (RP)
- an inherited visual disorder that results in tunnel vision initially and eventually cause complete blindness
- progressive degeneration of rods (and eventually cones)
AGE-RELATED MACULAR DEGENERATION (AMD)
- damage to photoreceptors in the macula (high resolution central vision)
- we rely on our macula to do most tasks eg. recognise faces, drive, etc
What are the common types of retinal implants (depending upon their placements)?
EPIRETINAL PLACEMENT
- electrodes placed on top of the ganglion cells on the inner surface of the eye
SUBRETINAL PLACEMENT
- electrode placed where the phororeceptors were, on the outer layer of the retina
SUPRACHOROIDAL PLACEMENT
- electrode placed between the sclera (outer protective layer of the eye) and the choroid (vascular layer)
What are the advantages and disadvantages of the different retinal implants?
EPIRETINAL PLACEMENT
Advantages:
• Very close contact between electrodes and retinal
ganglion cells.
• It potentially offers higher visual acuity compared to
suprachoriodal/subretinal placement because the
electrodes are so close to the target tissue.
Disadvantages:
• Surgery is more complex than for suprachoroidal as a
full vitrectomy is required.
• All components are inside the eye, making removal
difficult if necessary.
• Placement has proved so far to be less stable over
time.
SUBRETINAL PLACEMENT
Advantages:
• implants are stable
Disadvantages:
• Retinal ganglion cells are a long way from the electrodes, so spatial resolution is poor.
• Surgery requires a retinal detachment (using air) and re-attachment. Very complex and dangerous.
SUPRACHOROIDAL PLACEMENT
Advantages:
• the surgical procedure is short and placement is stable over time
Disadvantages:
• the retinal ganglion cells are a long way from the electrodes, so spatial resolution is poor.
Describe the electoretinogram and its potential uses.
ELECTRORETINOGRAM (ERG):
- measure entire retina’s electrical response to light
- bright flash ERG can identify diseases affecting the whole retina
How do the electroretinogram and the visual evoked potential differ?
ELECTRORETINOGRAM (ERG):
- measure entire retina’s electrical response to light
ELECTROENCEPHALOGRAM (EEG):
- measure electrical activity of our brains to various types of stimuli
VISUAL EVOKED POTENTIALS:
- the potential measured from EEG that correspond with visual system
- record how well light gets to the ipsilateral and contralateral side of brain (measure integrity of visual pathways up to V1)
Describe similarities between the retina and brain that may make it a useful place to study the central nervous system.
- retina is part of the CNS because it’s an embryological extension of the diencephalon
- retina is highly metabolically active like the brain (photoreceptors are actually the most metabolically active tissue by weight in the body)
- retina contains glia and uses neurotransmitters/ receptors like glutamate, glycine, dopamine, NMDA and GABA
- retina experiences similar neurodegenerative diseases due to aging
Describe the two-stage decision-making model of Carpenter, Reddi &
Anderson (2009) and compare it with single-stage decision making models.
RANDOM WALK:
- detection of stimulus
- accumulation of noisy sensory info
LATER (Linear Approach to Threshold with Ergodic (random) Rate:
- decision based on evidence, which allows it to progress
1. expectation (starting point)
2. rate of information arrival (steepness)
3. Urgency (threshold line) - once the 1st stage finishes, it provides a constant input signal to the 2nd stage
What determines the limits to spatial resolving capacity of the visual system?
SPATIAL RESOLVING CAPACITY:
- resolution limit at the fovea is around 40-60 cycles per degree
- Retinal factor- as you move away from the centre of the eye, this gets exceptionally worse because the photoreceptors are less dense
Defocus:
- Optical factor- the amount of discrepancy there is with regards to light focusing on the retina
Sampling:
- discrete measurement (in space or time) of a continuous function
- increased sampling frequency= more faithful representation of original signal
Describe the relevance of ‘Nyquist limit’ to perception of gratings that are coarser or finer than the resolution limit.
Aliasing:
- results from inadequate sampling of the signal (eg. undersampling)
- signal reconstructed will differ significantly from original, continuous signal
NYQUIST LIMIT:
- aliasing can be avoided by remaining below the Nyquist limit
f=N/2 or N=2f
- Sampling frequency (N) must be at least twice the highest spatial frequency (f) to faithfully represent the signal
Describe the pupillary light reflex pathway and the neurotransmitters involved.
Pupillary constriction
Pupillary dilation
Describe the clinical picture and the basic defect in internuclear ophthalmoplegia.
MEDIAL LONGITUDINAL FOSCICULUS (MLF)
- tract bringing signals from the 6th cranial nerve (abducens) nucleus to the contralateral 3rd cranial nerve (oculomotor) such that both eyes can move in synchrony
INTERNUCLEAR OPHTHALMOPLEGIA (INO)
- when the MLF is lesioned on one side, then he affected individual will have trouble adducting their eyes on that side
- the eye on the other side can still adduct and vergence can still occur in both eyes
What are the different types of eye movements and their functions?
Adduction- towards the nose
Abduction- away from the nose
Intorsion- rotation towards the midline (nose)
Extorsion- rotation away from the midline
Elevation- upwards
Depression- downwards
What are the different types of eye movements and their functions?
- Adduction- towards the nose
- Abduction- away from the nose
- Intorsion- rotation towards the midline (nose)
- Extorsion- rotation away from the midline
- Elevation- upwards
- Depression- downwards
What are the extra-ocular muscles that control each eye movement and what are their respective innervations?
• MEDIAL RECTUS
- adductor
- 3rd cranial nerve
• LATERAL RECTUS
- abductor
- 6th cranial nerve
• SUPERIOR RECTUS
- elevator
- 3rd cranial nerve
• INFERIOR RECTUS
- depressor
- 3rd cranial nerve
• SUPERIOR OBLIQUE
- intorsion
- 4th cranial nerve
• INFERIOR OBLIQUE
- extorsion
- 3rd cranial nerve
Describe the neural control of saccades.
Superior colliculus- initiates saccades and orientates our gaze
