Memory: It’s Complicated

As with so many things involving the human brain, memory is complicated. There’s long-term memory and short-term; there’s skill memory, language-based memory and visuospatial memory. But the overarching issues of memory are storage and retrieval, and each can be affected by stroke.

It would be so convenient if there were a lobe in our brains for each of those, but there’s not — memory is diffuse. One brain component involved is the hippocampus, which is behind our ears in the region of the temporal lobe, deep within the center of the brain. “Think of that as a file drawer through which memories enter the memory system,” said clinical neuropsychologist Karen Postal, who teaches at Harvard Medical School and is president of the American Academy of Clinical Neuropsychology.

As anybody who has used a file cabinet knows, putting things in is easy, organizing them so they are easily retrievable is a different story. That job is accomplished by what is called the frontal executive network. “It’s not just the frontal lobe itself but how that lobe connects with other parts of the brain,” Postal said. For the purposes of understanding memory, think of this network as an executive secretary. A good executive secretary puts information into the file cabinet in an organized way.

If this executive network is affected by a stroke, memories may be getting in through the hippocampus but they are haphazardly organized. “Like kids just stuffing things in files,” Postal said. “The work of the executive system is so critical because if information is organized going into that file cabinet, then it’s going to be a lot easier to retrieve that information.”

“Memories are stored in multiple places in the brain,” said Alex Dromerick, a neurologist and rehabilitation physician at MedStar National Rehabilitation Network. “One of ways we think memories are stored is by strengthening and weakening the connections between neurons. The term for that is ‘long-term potentiation’ or LTP. LTP is a modulation of the connections in the brain, an upregulation or a downregulation.”

If that sounds complicated, it is, but that is only the start — “multiply that by tens of billions of neurons, each one of which has tens of thousands of connections with other cells,” Dromerick said. “Where do you begin to pull that apart and understand how it works?”


Short v Long

Dr. Karen Postal


Most of us know the labels short-term memory and long-term memory, and commonly think of short-term as from now to a week or two ago; long-term memory describes the past beyond that. Neuroscientists have a different concept of short-term, more like a half an hour. “Once something is in your brain for half an hour, it really has been encoded in a way that it’s likely to stay and be there days or weeks later,” Postal said.

“Short-term memory is something that’s over minutes to hours,” Dromerick said. “It’s remembering at 1 o’clock, what you had for lunch at noon. Long-term memory is things like remembering your birthday, what you did on the day you graduated from high school, your wedding day, those kinds of things. Both kinds of memory can be affected together or separately depending on the stroke. There is also prospective memory, where we have to remember something in the future, like remembering to go to a party later in the day at a particular time.”

“When neuropsychologists talk with patients about short-term memory, what we mean is ‘are you able to create new memories, or has that function gone away? Can you tell me what you had for breakfast this morning or dinner last night? That’s what clinicians mean with short-term memory — are you able to lay down new memories?” Postal said.

Dr. Alex Dromerick

It’s safe to say that all long-term memories were short-term memories first, but not all short-term memories turn into long-term memories. “You probably don’t remember what you had for lunch three months ago,” Dromerick said. “But if it’s a memory of an emotion, like anger or fear, that tends to go to an area of the brain called the amygdala. If it’s a memory of a motor skill — hitting a tennis ball, riding a bicycle — that tends to go into the basal ganglia.”

“In the clinical sense, what we mean by long-term memory, is the survivor able to access information from before the stroke?” Postal said. “In almost every case, a long-term memory problem is a problem with retrieval. It’s rare that long-term memories will genuinely be destroyed. For example, with Alzheimer’s disease, it’s very far into the dementia process where people start to lose those long-term memories. They are able to tell you about their work life and their family life and their childhood for years into the dementia process. When there’s devastation throughout the brain, eventually that goes away. But in stroke survivors that long-term memory problem is almost always a retrieval issue.”

Postal suggests that people often imagine our memory system as a video recorder where we just press record and then later press play, but that’s not how it works. Memories are not stored in discrete segments and neat paragraphs or even sequentially. “What happens when we need to retrieve a memory is that we reconstruct it, and we reconstruct it through association,” Postal said. “For example, you may not have thought about high school chemistry in decades, but if you see someone from that class at a reunion and start to talk about it, you will begin the process of reconstructing a more vivid memory. We know from courtroom research that these reconstructions can seem to be accurate but may not be. But we know that our memories are not stored as a single sequential video. We have to reconstruct memories.”

Memories are encoded in the brain and perhaps also the brainstem and spinal cord. The term ‘muscle memory’ is inaccurate as muscles don’t have the neurons that encode memories. “Skills that we learn through repetition, highly learned motor skills, once they become automatic, much of that seems to be in the basal ganglia,” Dromerick said.

The memory-loss masquerade

Although what appear to be memory deficits are not uncommon after stroke, other stroke deficits can masquerade as memory problems. “For people with stroke, what looks like memory loss can actually be something else,” Dromerick said. “Absolutely, you can have loss in short-term memory after a stroke, but you’ll also get that with aging. You also get it with side effects of blood pressure medicine, sleeping pills and other medicines. People who are hard of hearing or have trouble with vision may appear to have forgotten something when they never heard or saw it to begin with. People with stroke may have problems paying attention, especially early after stroke. It may be their medicines making them drowsy. Sleep apnea may leave them sleep deprived, which causes problems with attention and concentration. If you can’t pay attention, it’s very hard to encode any memories. All those things can look like memory loss, especially to family members, but actually, the underlying process is quite different. The good news is that these causes of apparent memory loss can be treated.”

Family members may mistake a survivor’s inability to recall the names of things as memory loss. “This is called anomic aphasia or anomia, but it isn’t memory loss,” Dromerick said. “It’s a matter of summoning the memory. It’s a problem with language. Forgetting is losing memory, but in the case of anomia, the name is still stored in there. It’s a matter of bringing up that word, but it’s not a memory loss.”

In relation to skill memory, other conditions affecting other systems may be imitating a loss of memory. “Let’s say somebody had a stroke and they can’t play the piano anymore or they have trouble playing,” Dromerick said. “It could be that they have lost the motor control to move their fingers in the right way. Or that they’ve lost the sensation to know where their fingers are and which key it is that their finger is on. It’s possible they have apraxia and can’t organize the movement of their fingers up and down. Maybe they can’t generate the right pattern to play the keys in the right order. Or it could be that they’ve just forgotten the melody. In which case, finally, we’ve gotten to something that relates to memory. Actually, it turns out to be more complicated because there are all these different things going on. As you begin to pull them apart, it may be that memory doesn’t play a role at all. It may have to do with some other cognitive, sensory or motor function or a combination.”

What helps

Let’s get the bad news out of the way first — if the problem really is memory loss, there is currently neither drug nor therapy that will fix that. There are memory meds for Alzheimer’s patients, but they do not appear to be very effective for stroke survivors. Some computer games promote themselves as memory improvers, but so far, studies show that all they seem to do is improve a player’s ability in that game. The improvement does not generalize to other areas of life. Though there is no treatment for memory loss, there are ways to compensate (see “Short-term memory tools”).

There is good news as well. “Aerobic exercise does wonders for the memory system,” Postal said. “When we exercise three things happen that help memory. First, exercise triggers new brain cell growth. Brain cells are born every time we engage in aerobic exercise. They may be born in various parts of the brain, but we know for sure that they’re born in the hippocampus, the very part of the brain that allows new memories to be formed.”

Second, aerobic exercise stimulates our executive secretary. “That frontal lobe and its connections with other parts of the brain starts working faster, better, stronger,” Postal said. “After we exercise we are better able to organize information to store new memories. Plus, the executive system is responsible for focus and concentration and resisting distractions.”

Third, aerobic exercise releases brain-derived neurotrophic factor (BDNF). “BDNF helps repair cells that are damaged and helps strengthen connections between the synapses of the nerve cells,” Postal said. “When you release BDNF, you’re helping with cell repair but you’re also helping to consolidate memories.”

Of course, after you’ve had a stroke, there is a huge emphasis on preventing another one, and that means taking care of your cardiovascular system — eat a heart-healthy diet, get regular aerobic exercise, reduce stress and stop smoking. “For the benefit of your heart, your brain and your memory, the last thing you want is to be sedentary,” Postal said. “I always recommend that my patients work with their physical therapist to figure out a way to get aerobic exercise.”

Dromerick agrees that exercise benefits memory and points to another practice that aids memory — adequate sleep. “There’s something called ‘consolidation,’ where as time passes, or particularly as you sleep, your memories become refined and more long-term,” he said. “Actually, researchers are looking at using sleep to increase the effectiveness of stroke rehabilitation, because sleep and rest periods may be important for people trying to learn new skills.”

In the past decade, there has been growing interest in transcranial direct current stimulation (TDCS) and transcranial magnetic stimulation (TMS), but both experts agree that the jury is still out on their effectiveness for helping memory.

Finally, Dromerick’s insight that not all of what looks like memory loss is memory loss means there is more good news for survivors. “The hardest thing to treat is memory loss, but these other conditions that imitate memory loss, that can be treated,” he said. “So, problems with sleep, medications, depression or with hearing or vision — those things can be treated and improve.”

See our profile on survivor Kelli Smith, who experiences rare challenges with both long- and short-term memory> 

The Stroke Connection team knows that it can sometimes be hard to understand how profoundly post-stroke memory issues may be impacting a survivor. Sometimes, apparent memory problems may be due to something else. We’ve created a quick-reference sheet that you can use to talk with your healthcare provider and others.


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