Impaired vision is common in old age and even more so in Alzheimer’s disease, and this results not only from damage in the association areas of the brain but also from problems in lower-level areas. A major factor in whether visual impairment impacts everyday function is contrast sensitivity.
Contrast sensitivity not only slows down your perceiving and encoding, it also interacts with higher-order processing, such as decision-making. These effects may be behind the established interactions between age, perceptual ability, and cognitive ability. Such interactions are not restricted to sight — they’ve been reported for several senses.
In fact, it’s been suggested that much of what we regard as ‘normal’ cognitive decline in aging is simply a consequence of having senses that don’t work as well as they used to.
The effects in Alzheimer’s disease are, I think, particularly interesting, because we tend to regard any cognitive impairment here as inevitable and a product of pathological brain damage we can’t do anything much about. But what if some of the cognitive impairment could be removed, simply by improving the perceptual input?
That’s what some recent studies have shown, and I think it’s noteworthy not only because of what it means for those with Alzheimer’s and mild cognitive impairment, but also because of the implications for any normally aging person.
So let’s look at some of this research.
Let’s start with the connection between visual and cognitive impairment.
Analysis of data from the Health and Retirement Study and Medicare files, involving 625 older adults, found that those with very good or excellent vision at baseline had a 63% reduced risk of developing dementia over a mean follow-up period of 8.5 years. Those with poorer vision who didn’t visit an ophthalmologist had a 9.5-fold increased risk of Alzheimer disease and a 5-fold increased risk of mild cognitive impairment. Poorer vision without a previous eye procedure increased the risk of Alzheimer’s 5-fold. For Americans aged 90 years or older, 78% who kept their cognitive skills had received at least one previous eye procedure compared with 52% of those with Alzheimer’s disease.
In other words, if you leave poor vision untreated, you greatly increase your risk of cognitive impairment and dementia.
Similarly, cognitive testing of nearly 3000 older adults with age-related macular degeneration found that cognitive function declined with increased macular abnormalities and reduced visual acuity. This remained true after factors such as age, education, smoking status, diabetes, hypertension, and depression, were accounted for.
And a study comparing the performance of 135 patients with probable Alzheimer’s and 97 matched normal controls on a test of perceptual organization ability (Hooper Visual Organization Test) found that the VOT was sensitive to severity of dementia in the Alzheimer’s patients.
So let’s move on to what we can do about it. Treatment for impaired vision is of course one necessary aspect, but there is also the matter of trying to improve the perceptual environment. Let’s look at this research in a bit more detail.
A 2007 study compared the performance of 35 older adults with probable Alzheimer’s, 35 healthy older adults, and 58 young adults. They were all screened to exclude those with visual disorders, such as cataracts, glaucoma, or macular degeneration. There were significant visual acuity differences between all 3 groups (median scores: 20/16 for young adults; 20/25 for healthy older adults; 20/32 for Alzheimer’s patients).
Contrast sensitivity was also significantly different between the groups, although this was moderated by spatial frequency (normal contrast sensitivity varies according to spatial frequency, so this is not unexpected). Also unsurprisingly, the young adults outperformed both older groups at every spatial frequency, except at the lowest, where it was matched by that of healthy older adults. Similarly, healthy older adults outperformed Alzheimer’s patients at every frequency bar one — the highest frequency.
For Alzheimer’s patients, there was a significant correlation between contrast sensitivity and their cognitive (MMSE) score (except at the lowest frequency of course).
Participants carried out a number of cognitive/perceptual tasks: letter identification; word reading; unfamiliar-face matching; picture naming; pattern completion. Stimuli varied in their perceptual strength (contrast with background).
Letter reading: there were no significant differences between groups in terms of accuracy, but stimulus strength affected reaction time for all participants, and this was different for the groups. In particular, older adults benefited most from having the greatest contrast, with the Alzheimer’s group benefiting more than the healthy older group. Moreover, Alzheimer’s patients seeing the letters at medium strength were not significantly different from healthy older adults seeing the letters at low strength.
Word reading: here there were significant differences between all groups in accuracy as well as reaction time. There was also a significant effect of stimulus strength, which again interacted with group. While young adults’ accuracy wasn’t affected by stimulus strength, both older groups were. Again, there were no differences between the Alzheimer’s group and healthy older adults when the former group was at high stimulus strength and the latter at medium, or at medium vs low. That was true for both accuracy and reaction time.
Picture naming: By and large all groups, even the Alzheimer’s one, found this task easy. Nevertheless, there were effects of stimulus strength, and once again, the performance of the Alzheimer’s group when the stimuli were at medium strength matched that of healthy older adults with low strength stimuli.
Raven’s Matrices and Benton Faces: Here the differences between all groups could not in general be ameliorated by manipulating stimulus strength. The exception was with the Benton Faces, where Alzheimer’s patients seeing the medium strength stimuli matched the performance of healthy older adults seeing low strength stimuli.
In summary, then, for letter reading (reaction time), word reading (identification accuracy and reaction time), picture naming, and face discrimination, manipulating stimulus strength in terms of contrast was sufficient to bring the performance of individuals with Alzheimer’s to a level equal to that of their healthy age-matched counterparts.
It may be that the failure of this manipulation to affect performance on the Raven’s Matrices reflects the greater complexity of these stimuli or the greater demands of the task. However, the success of the manipulation in the case of the Benton Faces — a similar task with stimuli of apparently similar complexity — contradicts this. It may that the stimulus manipulation simply requires some more appropriate tweaking to be effective.
It might be thought that these effects are a simple product of making stimuli easier to see, but the findings are a little more complex than I’ve rendered them. The precise effect of the manipulation varied depending on the type of stimuli. For example, in some cases there was no difference between low and medium stimuli, in others no difference between medium and high; in some, the low contrast stimuli were the most difficult, in others the low and medium strength stimuli were equally difficult, and on one occasion high strength stimuli were more difficult than medium.
The finding that Alzheimer’s individuals can perform as well as healthy older adults on letter and word reading tasks when the contrast is raised suggests that the reading difficulties that are common in Alzheimer’s are not solely due to cognitive impairment, but are partly perceptual. Similarly, naming errors may not be solely due to semantic processing problems, but also to perceptual problems.
Alzheimer’s individuals have been shown to do better recognizing stimuli the closer the representation is to the real-world object. Perhaps it is this that underlies the effect of stimulus strength — the representation of the stimulus when presented at a lower strength is too weak for the compromised Alzheimer’s visual system.
All this is not to say that there are not very real semantic and cognitive problems! But they are not the sole issue.
I said before that for Alzheimer’s patients there was a significant correlation between contrast sensitivity and their MMSE score. This is consistent with several studies, which have found that dementia severity is correlated with contrast sensitivity at some spatial frequencies. This, and these experimental findings, suggests that contrast sensitivity is in itself an important variable in cognitive performance, and contrast sensitivity and dementia severity have a common substrate.
It’s also important to note that the manipulations of contrast were standard across the group. It may well be that individualized manipulations would have even greater benefits.
Another recent study comparing the performance of healthy older and younger adults and individuals with Alzheimer's disease and Parkinson's disease on the digit cancellation test (a visual search task used in the diagnosis of Alzheimer’s), found that increased contrast brought the healthy older adults and those with Parkinson’s up to the level of the younger adults, and significantly benefited Alzheimer’s individuals — without, however, overcoming all their impairment.
There were two healthy older adults control groups: one age-matched to the Alzheimer’s group, and one age-matched to the Parkinson’s group. The former were some 10.5 years older to the latter. Interestingly, the younger control group (average age 64) performed at the same level as the young adults (average age 20), while the older old control group performed significantly worse. As expected, both the Parkinson’s group and the Alzheimer’s group performed worse than their age-matched controls.
However, when contrast was individually tailored at the level at which the person correctly identified a digit appearing for 35.5 ms 80% of the time, there were no significant performance differences between any of the three control groups or the Parkinson’s group. Only the Alzheimer’s group still showed impaired performance.
The idea of this “critical contrast” comparison was to produce stimuli that would be equally challenging for all participants. It was not about finding the optimal level for each individual (and indeed, young controls and the younger old controls both performed better at higher contrast levels). The findings indicate that poorer performance by older adults and those with Parkinson’s is due largely to their weaker contrast sensitivity, but those with Alzheimer’s are also hampered by their impaired ability to conduct a visual search.
The same researchers demonstrated this in a real-world setting, using Bingo cards. Bingo is a popular activity in nursing homes, senior centers and assisted-living facilities, and has both social and cognitive benefits.
Varying cards in terms of contrast, size, and visual complexity found that all groups benefited from increasing stimulus size and decreasing complexity. Those with mild Alzheimer’s were able to perform at levels comparable to their healthy peers, although those with more severe dementia gained little benefit.
Contrast boosting has also been shown to work in everyday environments: people with dementia can navigate more safely around their homes when objects in it have more contrast (e.g. a black sofa in a white room), and eat more if they use a white plate and tableware on a dark tablecloth or are served food that contrasts the color of the plate.
There’s a third possible approach that might also be employed to some benefit, although this is more speculative. A study recently reported at the American Association for the Advancement of Science annual conference revealed that visual deficits found in individuals born with cataracts in both eyes who have had their vision corrected can be overcome through video game playing.
After playing an action video game for just 40 hours over four weeks, the patients were better at seeing small print, the direction of moving dots, and the identity of faces.
The small study (this is not, after all, a common condition) involved six people aged 19 to 31 who were born with dense cataracts in each eye. Despite these cataracts being removed early in life, such individuals still grow up with poorer vision, because normal development of the visual cortex has been disrupted.
The game required players to respond to action directly ahead of them and in the periphery of their vision, and to track objects that are sometimes faint and moving in different directions. Best results were achieved when players were engaged at the highest skill level they could manage.
Now this is quite a different circumstance to that of individuals whose visual system developed normally but is now degrading. However, if vision worsens for some time before being corrected, or if relevant activities/stimulation have been allowed to decline, it may be that some of the deficit is not due to damage as such, but more malleable effects. In the same way that we now say that cognitive abilities need to be kept in use if they are not to be lost, perceptual abilities (to the extent that they are cognitive, which is a great extent) may benefit from active use and training.
In other words, if you have perceptual deficits, whether in sight, hearing, smell, or taste, you should give some thought to dealing with them. While I don’t know of any research to do with taste, I have reported on several studies associating hearing loss with age-related cognitive impairment or dementia, and similarly olfactory impairment. Of particular interest is the research on reviving a failing sense of smell through training, which suggested that one road to olfactory impairment is through neglect, and that this could be restored through training (in an animal model). Similarly, I have reported, more than once, on the evidence that music training can help protect against hearing loss in old age. (You can find more research on perception, training, and old age, on the Perception aggregated news page.)
For more on the:
Bingo study: https://www.eurekalert.org/pub_releases/2012-01/cwru-gh010312.php
Video game study: