Can adult amblyopia be cured? Flashcards
What is Amblyopia?
Reduced acuity in an otherwise normal eye
Occurs as a result of abnormal visual experience in childhood e.g. Strabismus or Anisometropia
Standard Treatment
Patching the dominant eye to improve monocular function
75% of young patched children show acuity improvement (Repka, 2003)
Decreased efficacy in older children
Attributed to decreased plasticity in mature brain as critical period ends
So, can adult amblyopia be cured?
He et al
(2007)
Investigated which conditions could restore plasticity in adult rats who underwent chronic MD
And whether this increased plasticity could result in improved acuity
Results
Reverse occlusion alone not effective
RO + DE = plasticity restored [VEP] = increased acuity [VEP +Behavioural task]
THEREFORE
DE in adulthood may enhance ocular dominance plasticity by lowering the threshold for the induction of LTP/LTD
However, impractical in humans…
Thompson et al
(2008)
Found that 10z of TMS, when applied to the PVC of adult amblyopes, caused sig. improvements in the ability to distinguish high spatial frequency gratings 30 minutes later
While this method does not lead to long-term improvements in acuity, it does highlight that visual plasticity can be restored in adults under conditions which alter the normal functioning of the visual system.
Binocular function/summation
Normal adults = 1.6
Adult amblyopes = 1
This suggests that mechanisms for BV are damaged
Supported by Hubel & Weisel (1965) who found a lack of binocularly responding cells in the V1 of amblyopic cats
However…
Baker et al
(2007)
Revealed that when the signal attenuation of the amblyopic eye is accounted for, normal BSRs were achieved.
Suggests that reduced BSRs are due to imbalance in monocular signals before the point of summation
THEREFORE
Mechanisms = intact but suppressed
As such, research has begun to explore ways in which this suppression can be prevented
Hess et al
(2010)
3 adult strabismic PS
Demonstrated that info can be combined normally under viewing conditions where suppression (from the good eye) is artificially reduced
Suppression was reduced by making the contrast in the amblyopic eye higher relative to the good eye
Under such conditions, completed six weeks training
Results
Strengthening of BV - after training stimuli could be at the same contrast in each eye to achieve binocular percept
Supported by a) snellen letter chart improvements
b) stereoscopic vision in randot test
THEREFORE
Training which involves systematic reduction in the suppression of amblyopic eye can allow for BS under natural viewing conditions after training = stereoscopic vision
HOWEVER
training paradigm = complex = not practical
To et al
(2011)
Developed training paradigm based on Tetris using lenticular array surface (iPod)
Calculated a balance point of contrast in which amblyopic eye was contributing to the percept as well as the good eye
Daily experimenters adjusted the balance point with the aim of having equal contrast between eyes
Results
The contrast between two eyes was sig. reduced
- 6/9 PS could tolerate same contrast
Sig. improvements in acuity and depth perception
Vetencourt et al
(2008)
Antidepressants = promote neurogenesis by increasing neurotrophins, whose levels have been correlated with increased plasticity
Treated adult amblyopic rats with AD fluoxetine who underwent RO
Results
AD treated rats had recovery of acuity and binocularity
(unlike controls)
Also showed:
- Lower concentration of GABA (inhibitory)
- Higher concentration of BDNF
- Increased white matter LTP
THEREFORE
Reduced GABA and increased BDNF may increase plasticity
Conclusion
Growing body of evidence that adult amblyopia can be treated by
a) behavioural therapies to promote BF
- which = increased acuity and stereopsis
b) AD treatment to promote plasticity
Dichoptic training basis and general procedure
Dichoptic training originates from the idea that amblyopia is an inherently binocular disorder rooted in interocular suppression, and that an effective treatment should engage both eyes.
In dichoptic training, participants with amblyopia are trained on tasks in which stimuli are presented dichoptically with the contrast of the image to the fellow eye attenuated in order to encourage binocular combination of the two inputs.
Perceptual learning and Levi and Li (?)
During perceptual learning using only their amblyopic eye, patients are required to practice a variety of visual tasks.
A review of the extant studies (almost 200 amblyopic subjects distributed over 14 papers) reveals that such practice results in a long-lasting improvement in performance in amblyopic eyes (Levi and Li, 2009).
Huang (?)
It is generally strongest for the trained eye, task, stimulus, and orientation, but appears to improve over a broader spatial frequency bandwidth than in normal vision, indicating some level of transfer (Huang et al., 2008).
Mechanisms of perceptual learning?
Who said it?
Yet, the mechanisms by which it operates—a reduction of internal neural noise and/or more efficient use of the stimulus information by re-tuning the weighting of the information—are central to changing information processing in the visual cortex ( Levi and Li, 2009)
Video gaming: who??
Li et al (2010)
Proposed the use of video games to induce plasticity and improve visual function in adult amblyopia.
They showed that playing either action (Medal of Honor) or nonaction (SimCity) video games with the fellow eye patched improved a wide range of fundamental visual functions, from low-level to high-level, including visual acuity, positional acuity, visual attention and stereopsis.
