Could complete darkness be the cure for amblyopia?
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Could complete darkness be the cure for amblyopia?
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[post_date] => 2024-12-25 13:34:28
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Amblyopia, or lazy eye, is a common condition of visual impairment caused by unbalanced neural transmission. The lazy eye itself functions normally, but the optical neurons connected to that eye fail to transmit visual information to the brain. Treatment for amblyopia consists of placing a patch over the healthy eye to prevent light from entering, a procedure known as monocular deprivation (MD). The darkness causes photopigments retinal and opsin to combine into rhodopsin. More importantly, amblyopia is cured through induced neural modification. In other words, MD causes the brain to re-route most of the nerve connections with the good eye to the lazy one, improving the vision of the lazy eye. Both eyes contribute visual input to each cerebral hemisphere, facilitating this process. MD must be done in early development, when neurons have higher plasticity, or the ability to form new connections. With aging, neuroplasticity is brought to a halt, resulting in permanent vision loss of the lazy eye for amblyopia patients.
Plasticity in kittens is reduced to almost zero at postnatal day 90 (P90), and, past this point, amblyopia treatment is ineffective. Scientists performed two experiments to test the effects of darkness on amblyopia.
Experiment 1: Kittens with normal vision that undergo MD become amblyopic. Scientists induced amblyopia in kittens by suturing one eye shut for 7 days (P30 to P37). At P38, the kittens were amblyopic, and their visual acuity was then continuously measured for 3 months (Figure 1). At P93, scientists placed these kittens in total darkness for 10 days. Afterward, they continued to measure the visual acuity of both eyes (Figure 1).
Experiment 2: One of the mechanisms through which darkness leads to amblyopia recovery involves specifically decreasing levels of molecules that constrain neuronal plasticity, such as the neurofilament protein NF-L. Up to P40, NF-L proteins are low, but gradually rise to 60% of adult values by P90. To verify the effect of dark rearing on plasticity, scientists measured NF-L levels on animals that were raised with normal vision until P30, at which point they were reared in darkness for either 5, 10, or 15 days. (Figure 2).
Figure 1. Amblyopia was induced by a 7 day monocular lid suture in kittens aged 30 days. After re-opening the eye at P37, visual acuity (y axis) was assessed daily for the non-deprived eye (dotted curve) and deprived eye (solid line). After 90 days, kittens were placed again in a completely dark room for a period of 10 days, after which their visual acuity was measured again. A visual acuity score of 6.7 is considered normal vision.
Figure 2. The effects of dark rearing (DR) on NF-L levels of normal-vision kittens placed in a dark room for 5, 10, or 15 days after P40. Treatments 10 day DR and 15 day DR are significantly different from P40 normal.
Adapted from: Kevin R. Duffy, Donald E. Mitchell. 2013. Darkness alters maturation of visual cortex and promotes fast recovery from monocular deprivation. Current Biology, 23, pp. 382–386
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[question] => What could be expected of the photopigment levels in P110 to P120 of Experiment 1?
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[answer] => 3
[description] => Reason for the Correct Answer:
Darkness causes opsin and retinal to combine and form rhodopsin.
Therefore, in darkness, opsin and retinal levels decrease, while rhodopsin levels increase.
However, P110-P120 is a period where the cats are exposed to light. When exposed to light, the reverse relationship for photopigments is true: Rhodopsin breaks down into opsin and retinal.
So, when exposed to light, opsin levels will increase.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Increased levels of rhodopsin
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[1] => Array
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[each_answer] => B. Increased levels of intracellular sodium
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[each_answer] => C. Increased levels of opsin
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[each_answer] => D. Decreased levels of retinal
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[quiz_unique_key] => 3873426850
[question] => In scientific research the results of one experiment may lead to additional questions and further experimentation. What is the relationship of the results of experiment 2 with experiment 1?
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[answer] => 1
[description] => Reason for the Correct Answer:
Visual acuity for both eyes returned to normal in experiment 1. Thus, 15 days of dark rearing may not lead to better vision than normal.
After 15 days of dark rearing, levels of NF-L are greatly reduced, thus increasing plasticity.
Although 15 days of dark rearing leads to increased neuronal plasticity, the 10 days of darkness in experiment 1 was enough to recover vision to normal levels.
The correlation between NF-L and neuronal plasticity indicates that darkness resetting neuroplasticity is the mechanism that explains the results of experiment 1.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Experiment 2 provides evidence that visual nerves can be rerouted, explaining the results of experiment 1
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[each_answer] => B. Experiment 2 contradicts the results of experiment 1 because neurons became more plastic after 15 days of dark rearing
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[each_answer] => C. Experiment 2 is in agreement with the results of of experiment 1 because photopigment levels decreased after 15 days of dark rearing
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[each_answer] => D. Experiment 2 provides evidence that 15 days of dark rearing may lead to better vision that normal
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Assume that the kitten’s sutured eye depicted in Figure 1 is in anatomical position. What can be concluded about left and right cerebral hemisphere visual information input between P40 and P90?
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[answer] => 3
[description] => Reason for the Correct Answer:
The overall visual input is reduced in both eyes.
Both right and left eyes send information to both cerebral hemispheres
Thus, the right cerebral hemisphere visual input is the same as the left cerebral hemisphere, even though the overall visual input is reduced.
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[0] => Array
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[each_answer] => A. Left cerebral hemisphere visual input is consistently higher than right cerebral hemisphere.
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[each_answer] => B. Right cerebral hemisphere visual input is consistently higher than the left cerebral hemisphere.
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[2] => Array
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[each_answer] => C. Right cerebral hemisphere visual input is the same as the left cerebral hemisphere.
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[each_answer] => D. Right cerebral hemisphere experiences a greater gain in visual input than the left cerebral hemisphere.
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[quiz_unique_key] => 2261298308
[question] => If experiment 1 was repeated in humans and yielded similar results, why might it have clinical applications for people with amblyopia?
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[answer] => 1
[description] => Reason for the Correct Answer:
MD can induce amblyopia in a healthy eye, but this effect is cured when the subject is exposed to complete darkness. In addition, it was demonstrated that MD improves vision in the amblyopic eye, so it would be a useful treatment.
The results demonstrated that MD and darkness exposure allowed for a full recovery of vision.
The result of experiment 1 determined that neural plasticity could be influenced to improve vision of the amblyopic eye, and that darkness exposure allows for both eyes to return to normal acuity levels. So, neural plasticity resetting and dark rearing can be a treatment for amblyopia.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Because the results indicate that neuronal plasticity can be reset and dark rearing may be a treatment
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[each_answer] => B. Because the results indicate that MD leads to amblyopia and it should be avoided as a treatment
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[each_answer] => C. Because the results indicate that MD and darkness exposure alone does not lead to full recovery of acuity
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[each_answer] => D. Because the results show that humans can be cured of amblyopia if diagnosed before 90 days
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[quiz_unique_key] => 574431310
[question] => Which period has the lowest amount of plasticity?
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[answer] => 2
[description] => Reason for the Correct Answer:
The passage clearly states that NF-L decreases plasticity.
Therefore, the period with the highest amounts of NF-L would have the lowest plasticity.
5 day DR has the highest amounts of NF-L. So, it would be the period with the lowest amount of plasticity.
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[0] => Array
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[each_answer] => A. Around P40
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[each_answer] => B. Around 5 day DR
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[each_answer] => C. Around 10 day DR
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[each_answer] => D. Around 15 day DR
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