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Researchers have noticed a decline in reading ability starting in your 40s. And learning more about why this happens might help us tell the difference between healthy aging and Alzheimer’s disease.

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If you’ve reached a certain age, you might notice you’re not plowing through books the way you used to. And like so many things about getting older, that “certain age” comes sooner than you expect. It gets worse in your 60s or 70s, but some researchers have noticed a decline in reading ability starting in your 40s.

So why do people read more slowly as they get older? Well, we’re not totally sure -- the reasons seem to vary. But understanding some of the details might actually help us tell the difference between healthy aging and Alzheimer’s disease.

Most people experience a decline in reading speed as they get older. That’s the case even though other mental factors tend to stay pretty strong, as long as you’re otherwise healthy. Which makes it tough to pin down what’s causing reading to slow down, specifically.

Like, most people on average tend to have their overall mental processing slow a bit as you get older. But your memory for word meaning stays pretty consistent into old age. It’s also possible that declining vision could slow you down.

But studies show that’s a separate factor in other words, people slow down their reading even if their vision is fine. Researchers have also proposed that maybe people slow down on purpose. That even if they don’t realize it, they’re slowing down to be a little more cautious about what they’re taking in -- so as to avoid mistakes.

But studies haven’t borne that out, either. In a 2018 study, researchers had both young and old people read at their own pace. Then, they tested their comprehension after making them double their speed.

If older adults deliberately set a slower pace than the younger people in the study, they should make fewer mistakes when forced to speed up. The older group did pace themselves a little more slowly, and made more mistakes overall, as expected. But doubling their speed made everyone, young and old, equally worse.

So it seems like the mistakes weren’t a result of their self-paced speed. So if we’ve ruled out things like declining vision or going slower on purpose, why does reading speed decrease? It kind of depends on how old you are.

Early on, between young adulthood and middle age, reading slows because all kinds of processing slows. Generally, we get a little bit worse at processing incoming information, and our executive function gets a little worse. If you control for things like working memory and processing speed, middle-aged adults can read just as quickly as younger adults.

One thing in particular that can trip you up a little more is an effect called visual crowding. Most people can identify a letter sitting on its own more quickly than a letter surrounded by other letters. That’s especially true for your peripheral vision.

And reading requires some anticipation of what’s in your peripheral vision, as your eyes move from word to word. A 2017 study published in Scientific Reports found a relationship between age, crowding, and reading speed. The researchers reported that older adults experience a bigger crowding effect.

That means letters don’t need to be as close for people to start making mistakes because of crowding. And this was related to their reading speed. The researchers found that adults over 50 read more slowly than those in their 20s.

And the younger group likewise had a smaller crowding zone. That means older adults might be more distracted by other things on the page skipping ahead to irrelevant things instead of focusing on the part they need to process next. So that may be part of what slows us down.

The shift in reading comprehension with age might also be related to a specific kind of memory called your phonological loop. Imagine a conversation where you’re not really paying close attention, and then you notice the other person is waiting for you to respond. So you quickly think of the last few words they said so you can rephrase it as a question.

That’s the phonological loop in action. It’s like a buffer of a few moments of auditory information your mind keeps ready in case you need to go back, like if you need to repeat a phone number to yourself before you dial. And that’s really useful when you’re reading.

Researchers in the year 2000 found this was part of the decline in reading comprehension with age. Basically, once they controlled for how much people can keep in their phonological loop, readers in their 40s and 50s showed about the same reading comprehension as those over 60. And that suggests the phonological loop is the key factor it stores less as you age.

So at the very least, you can take comfort in the fact that this is a typical part of healthy aging. But another reason why this research is important is because of that key word, “healthy.” Because focusing on the details of your reading ability might also help distinguish typical aging from Alzheimer’s disease. Like we said before, this phenomenon is surprising because most people keep a fairly consistent level of word knowledge and memory as they age.

But that’s not the case for people with Alzheimer’s disease. For them, meanings are impaired, too. They perform worse at naming words that fall in specific categories like things at a grocery store.

A 2016 study identified a few key brain regions that help distinguish those with Alzheimer’s from others, such as the amygdala. In people with Alzheimer’s, their verbal ability was related to how much gray matter they had in those regions — but not the control group. Older studies have had a hard time distinguishing Alzheimer’s patients from controls based on verbal abilities alone.

Understanding the differences, paired with brain scans, may help us understand the unique features of Alzheimer’s. So if your reading is slowing down as you get older, don’t worry. You’re not alone -- and you’ll still enjoy that novel just as much as those kids are enjoying that dance challenge online.

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