Visual Impairment

More evidence for early changes in the eye in Alzheimer’s disease comes from a study involving both rats and postmortem human retinas. Changes were found in the retinal pigment epithelial layer (which harbors the supportive cells located in the back of the eye) and in the thickness of the choroidal layer that has blood vessels providing nutrients to the retina.

The finding is consistent with growing evidence that glaucoma is a neurodegenerative disorder similar to Alzheimer’s.

http://www.eurekalert.org/pub_releases/2014-03/cmc-vie031714.php

http://www.eurekalert.org/pub_releases/2013-11/gumc-cte110113.php

[3613] Tsai Y, Lu B, Ljubimov AV, Girman S, Ross-Cisneros FN, Sadun AA, Svendsen CN, Cohen RM, Wang S. Ocular Changes in TgF344-AD Rat Model of Alzheimer's Disease. Investigative Ophthalmology & Visual Science [Internet]. 2014 ;55(1):523 - 534. Available from: http://www.iovs.org/content/55/1/523

A new study has found that errors in perceptual decisions occurred only when there was confused sensory input, not because of any ‘noise’ or randomness in the cognitive processing. The finding, if replicated across broader contexts, will change some of our fundamental assumptions about how the brain works.

The study unusually involved both humans and rats — four young adults and 19 rats — who listened to streams of randomly timed clicks coming into both the left ear and the right ear. After listening to a stream, the subjects had to choose the side from which more clicks originated.

The errors made, by both humans and rats, were invariably when two clicks overlapped. In other words, and against previous assumptions, the errors did not occur because of any ‘noise’ in the brain processing, but only when noise occurred in the sensory input.

The researchers supposedly ruled out alternative sources of confusion, such as “noise associated with holding the stimulus in mind, or memory noise, and noise associated with a bias toward one alternative or the other.”

However, before concluding that the noise which is the major source of variability and errors in more conceptual decision-making likewise stems only from noise in the incoming input (in this case external information), I would like to see the research replicated in a broader range of scenarios. Nevertheless, it’s an intriguing finding, and if indeed, as the researchers say, “the internal mental process was perfectly noiseless. All of the imperfections came from noise in the sensory processes”, then the ramifications are quite extensive.

The findings do add weight to recent evidence that a significant cause of age-related cognitive decline is sensory loss.

http://www.futurity.org/science-technology/dont-blame-your-brain-for-that-bad-decision/

[3376] Brunton BW, Botvinick MM, Brody CD. Rats and Humans Can Optimally Accumulate Evidence for Decision-Making. Science [Internet]. 2013 ;340(6128):95 - 98. Available from: http://www.sciencemag.org/content/340/6128/95

Damage to the retina (retinopathy) doesn’t produce noticeable symptoms in the early stages, but a new study indicates it may be a symptom of more widespread damage. In the ten-year study, involving 511 older women (average age 69), 7.6% (39) were found to have retinopathy. These women tended to have lower cognitive performance, and brain scans revealed that they had more areas of small vascular damage within the brain — 47% more overall, and 68% more in the parietal lobe specifically. They also had more white matter damage. They did not have any more brain atrophy.

These correlations remained after high blood pressure and diabetes (the two major risk factors for retinopathy) were taken into account. It’s estimated that 40-45% of those with diabetes have retinopathy.

Those with retinopathy performed similarly to those without on a visual acuity test. However, testing for retinopathy is a simple test that should routinely be carried out by an optometrist in older adults, or those with diabetes or hypertension.

The findings suggest that eye screening could identify developing vascular damage in the brain, enabling lifestyle or drug interventions to begin earlier, when they could do most good. The findings also add to the reasons why you shouldn’t ignore pre-hypertensive and pre-diabetic conditions.

A ten-year study involving 7,239 older adults (65+) has found that each common health complaint increased dementia risk by an average of about 3%, and that these individual risks compounded. Thus, while a healthy older adult had about an 18% chance of developing dementia after 10 years, those with a dozen of these health complaints had, on average, closer to a 40% chance.

It’s important to note that these complaints were not for serious disorders that have been implicated in Alzheimer’s. The researchers constructed a ‘frailty’ index, involving 19 different health and wellbeing factors: overall health, eyesight, hearing, denture fit, arthritis/rheumatism, eye trouble, ear trouble, stomach trouble, kidney trouble, bladder control, bowel control, feet/ankle trouble, stuffy nose/sneezing, bone fractures, chest problems, cough, skin problems, dental problems, other problems.

Not all complaints are created equal. The most common complaint — arthritis/rheumatism —was only slightly higher among those with dementia. Two of the largest differences were poor eyesight (3% of the non-demented group vs 9% of those with dementia) and poor hearing (3% and 6%).

At the end of the study, 4,324 (60%) were still alive, and of these, 416 (9.6%) had Alzheimer's disease, 191 (4.4%) had another sort of dementia and 677 (15.7%) had other cognitive problems (but note that 1,023 were of uncertain cognitive ability).

While these results need to be confirmed in other research — the study used data from broader health surveys that weren’t specifically designed for this purpose, and many of those who died during the study will have probably had dementia — they do suggest the importance of maintaining good general health.

Common irregular heartbeat raises risk of dementia

In another study, which ran from 1994 to 2008 and followed 3,045 older adults (mean age 74 at study start), those with atrial fibrillation were found to have a significantly greater risk of developing Alzheimer’s.

At the beginning of the study, 4.3% of the participants had atrial fibrillation (the most common kind of chronically irregular heartbeat); a further 12.2% developed it during the study. Participants were followed for an average of seven years. Over this time, those with atrial fibrillation had a 40-50% higher risk of developing dementia of any type, including probable Alzheimer's disease. Overall, 18.8% of the participants developed some type of dementia during the course of the study.

While atrial fibrillation is associated with other cardiovascular risk factors and disease, this study shows that atrial fibrillation increases dementia risk more than just through this association. Possible mechanisms for this increased risk include:

  • weakening the heart's pumping ability, leading to less oxygen going to the brain;
  • increasing the chance of tiny blood clots going to the brain, causing small, clinically undetected strokes;
  • a combination of these plus other factors that contribute to dementia such as inflammation.

The next step is to see whether any treatments for atrial fibrillation reduce the risk of developing dementia.

Stress may increase risk for Alzheimer's disease

And a rat study has shown that increased release of stress hormones leads to cognitive impairment and that characteristic of Alzheimer’s disease, tau tangles. The rats were subjected to stress for an hour every day for a month, by such means as overcrowding or being placed on a vibrating platform. These rats developed increased hyperphosphorylation of tau protein in the hippocampus and prefrontal cortex, and these changes were associated with memory deficits and impaired behavioral flexibility.

Previous research has shown that stress leads to that other characteristic of Alzheimer’s disease: the formation of beta-amyloid.

In a study in which 14 volunteers were trained to recognize a faint pattern of bars on a computer screen that continuously decreased in faintness, the volunteers became able to recognize fainter and fainter patterns over some 24 days of training, and this correlated with stronger EEG signals from their brains as soon as the pattern flashed on the screen. The findings indicate that learning modified the very earliest stage of visual processing.

The findings could help shape training programs for people who must learn to detect subtle patterns quickly, such as doctors reading X-rays or air traffic controllers monitoring radars, and may also help improve training for adults with visual deficits such as lazy eye.

The findings are also noteworthy for showing that learning is not confined to ‘higher-order’ processes, but can occur at even the most basic, unconscious and automatic, level of processing.

An experiment with congenitally deaf cats has revealed how deaf or blind people might acquire other enhanced senses. The deaf cats showed only two specific enhanced visual abilities: visual localization in the peripheral field and visual motion detection. This was associated with the parts of the auditory cortex that would normally be used to pick up peripheral and moving sound (posterior auditory cortex for localization; dorsal auditory cortex for motion detection) being switched to processing this information for vision.

This suggests that only those abilities that have a counterpart in the unused part of the brain (auditory cortex for the deaf; visual cortex for the blind) can be enhanced. The findings also point to the plasticity of the brain. (As a side-note, did you know that apparently cats are the only animal besides humans that can be born deaf?)

The findings (and their broader implications) receive support from an imaging study involving 12 blind and 12 sighted people, who carried out an auditory localization task and a tactile localization task (reporting which finger was being gently stimulated). While the visual cortex was mostly inactive when the sighted people performed these tasks, parts of the visual cortex were strongly activated in the blind. Moreover, the accuracy of the blind participants directly correlated to the strength of the activation in the spatial-processing region of the visual cortex (right middle occipital gyrus). This region was also activated in the sighted for spatial visual tasks.

We can see shapes and we can feel them, but we can’t hear a shape. However, in a dramatic demonstration of just how flexible our brain is, researchers have devised a way of coding spatial relations in terms of sound properties such as frequency, and trained blindfolded people to recognize shapes by their sounds. They could then match what they heard to shapes they felt. Furthermore, they were able to generalize from their training to novel shapes.

The findings not only offer new possibilities for helping blind people, but also emphasize that sensory representations simply require systematic coding of some kind. This provides more evidence for the hypothesis that our perception of a coherent object ultimately occurs at an abstract level beyond the sensory input modes in which it is presented.

[1921] Kim J-K, Zatorre RJ. Can you hear shapes you touch?. Experimental Brain Research [Internet]. 2010 ;202(4):747 - 754. Available from: http://www.springerlink.com/content/41gq1u30671q3737/

Data from 625 elderly Americans, followed for an average of 8.5 years, has revealed that those with very good or excellent vision at the beginning of the study had a 63% reduced risk of dementia over the study period. Those with poorer vision who did not visit an ophthalmologist had a 9.5-fold increased risk of Alzheimer disease and a 5-fold increased risk of cognitively impaired but no dementia. For the very-old (90+), 78% who maintained normal cognition had received at least one previous eye procedure compared with 51.7% of those with Alzheimer disease. The findings point to the need for older adults to seek treatment for their eye problems. The study raises the possibility that poor vision is not simply a symptom of developing dementia, but a contributing factor — possibly through its effect on curtailing activities which would help prevent it.

An intriguing set of experiments showing how you can improve perception by manipulating mindset found significantly improved vision when:

  • an eye chart was arranged in reverse order (the letters getting progressively larger rather than smaller);
  • participants were given eye exercises and told their eyes would improve with practice;
  • participants were told athletes have better vision, and then told to perform jumping jacks or skipping (seen as less athletic);
  • participants flew a flight simulator, compared to pretending to fly a supposedly broken simulator (pilots are believed to have good vision).

[158] Langer E, Djikic M, Pirson M, Madenci A, Donohue R. Believing Is Seeing. Psychological Science [Internet]. 2010 ;21(5):661 - 666. Available from: http://pss.sagepub.com/content/21/5/661.abstract

Older news items (pre-2010) brought over from the old website

Age-related eye disease associated with cognitive impairment

Age-related macular degeneration (AMD) is the leading cause of visual impairment in industrialized nations, and like Alzheimer's disease, involves the buildup of beta-amyloid peptides in the brain, as well as sharing similar vascular risk factors. A study of over 2000 older adults (69-97) has revealed an association between early-stage AMD and cognitive impairment, as assessed by the Digit Symbol Substitution Test (a test of attention and processing speed). There was no association with performance on the Modified Mini-Mental State Examination (used to assess dementia).
It’s worth noting that in the same journal two studies into the association between dietary fat intake and AMD appeared. The first, four-year, study involved over 6700 older adults and found that higher trans-unsaturated fat intake was associated with a higher incidence of AMD, while higher omega-3 fatty acid and higher olive oil intake were each associated with a lower incidence. The second, ten-year, study involving nearly 2500 older adults, found regular consumption of fish, greater intake of omega-3 fatty acids, and low intake of linoleic acid (perhaps because a higher intake implies a lower intake of omega-3 oils? linoleic acid is an omega-6 fatty acid), were all associated with a lower incidence of AMD. Fish and omega-3 oils have of course been similarly associated with lower rates of dementia and age-related cognitive impairment.

[447] Baker ML, Wang JJ, Rogers S, Klein R, Kuller LH, Larsen EK, Wong TY. Early age-related macular degeneration, cognitive function, and dementia: the Cardiovascular Health Study. Archives of Ophthalmology [Internet]. 2009 ;127(5):667 - 673. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19433718

[754] Chong EW-T, Robman LD, Simpson JA, Hodge AM, Aung KZ, Dolphin TK, English DR, Giles GG, Guymer RH. Fat consumption and its association with age-related macular degeneration. Archives of Ophthalmology [Internet]. 2009 ;127(5):674 - 680. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19433719

[413] Tan JSL, Wang JJ, Flood V, Mitchell P. Dietary fatty acids and the 10-year incidence of age-related macular degeneration: the Blue Mountains Eye Study. Archives of Ophthalmology [Internet]. 2009 ;127(5):656 - 665. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19433717

http://www.eurekalert.org/pub_releases/2009-05/jaaj-aed050709.php

Age-related vision problems may be associated with cognitive impairment

Age-related macular degeneration (AMD) develops when the macula, the portion of the eye that allows people to see in detail, deteriorates. An investigation into the relationship between vision problems and cognitive impairment in 2,946 patients has been carried out by The Age-Related Eye Disease Study (AREDS) Research Group. Tests were carried out every year for four years. Those who had more severe AMD had poorer average scores on cognitive tests, an association that remained even after researchers considered other factors, including age, sex, race, education, smoking, diabetes, use of cholesterol-lowering medications and high blood pressure. Average scores also decreased as vision decreased. It’s possible that there is a biological reason for the association; it is also possible that visual impairment reduces a person’s capacity to develop and maintain relationships and to participate in stimulating activities.

Chaves, P.H.M. et al. 2006. Association Between Mild Age-Related Eye Disease Study Research Group. 2006. Cognitive Impairment in the Age-Related Eye Disease Study: AREDS Report No. 16. Archives of Ophthalmology,124, 537-543.

http://www.eurekalert.org/pub_releases/2006-04/jaaj-avp040606.php

The reorganization of the visual cortex in congenitally blind people

Studies indicate that congenitally blind people have superior verbal memory abilities than the sighted. A new study helps us understand why this is so. Some 25% of the human brain is devoted to vision. Until now it was assumed that loss of vision rendered these regions useless. Now it appears that in those blind from birth, the part of the occipital cortex usually involved in vision is utilized for other purposes. Extensive regions in the occipital cortex, in particular the primary visual cortex, are activated not only during Braille reading, but also during performances of verbal memory tasks, such as recalling a list of abstract words. No such activation was found in a sighted control group. It also appears that the greater the occipital activation, the higher the scores in the verbal memory tests.

[944] Amedi A, Raz N, Pianka P, Malach R, Zohary E. Early /`visual/' cortex activation correlates with superior verbal memory performance in the blind. Nat Neurosci [Internet]. 2003 ;6(7):758 - 766. Available from: http://dx.doi.org/10.1038/nn1072

http://www.eurekalert.org/pub_releases/2003-06/huoj-hur061703.php