The other day, I reset my password for a social media site. When I went to login today, I inadvertently entered the old password. When that happened, I was using my automatic, “stimulus-response” memory, a rigid, habit-like memory. When I then remembered I had changed my password, I tapped into a different type of memory, a context-based one that required me to go back to the moment I made that change. A new study separates out these memory systems to understand the effects of stress on learning — finding that post-learning stress impairs context memory and favors stimulus-response memory instead.
“Studying these memory systems has made me aware of how frequently we find ourselves in situations where, even though both context and stimulus-response memories could be useful, we can only retrieve and use one of them,” says Elizabeth Goldfarb of NYU, who led the new study published in the Journal of Cognitive Neuroscience. She and colleagues developed a new memory-guided search task to isolate the effects of stress on the different types of memory.
Goldfarb spoke with CNS about this new task, as well as how she became interested in the topic, prior work on stress and memory systems in rodents and humans, and next steps for the work.
CNS: How did you become personally interested in this research area?
Goldfarb: When I was 18, I worked as a research assistant in a clinic for patients with posttraumatic stress disorder (PTSD). One part of the disorder that stood out to me was the importance of the patient’s memory for the traumatic event, but it’s puzzling: memory is both much stronger (e.g., intrusive memories, flashbacks) and much weaker (e.g., poor memory for details) than for everyday events. One way to make sense of this is to consider that we have different memory systems, optimized for different types of information, and these can be unevenly influenced by stress. This made me want to understand how stress, even the mild forms of stress that we experience all the time, could be modulating these different types of memory.
CNS: What’s a good example of the difference between context and stimulus-response memory?
Goldfarb: Imagine that you’re an American vacationing in England and are approaching a crosswalk. Do you look left first (because that’s what you usually do at a crosswalk) or look right (remembering that you are in a different context)? Broadly speaking, stimulus-response memories are rigid, habit-like, and can be performed automatically, whereas context memories are flexible, involve associations between different parts of an event, and are characteristic of episodic memory.
CNS: What have we known previously about stress and memory?
Goldfarb: There’s been a lot of fascinating work in this area, but I’ll try to be brief. There is robust evidence in rodents that, when only one type of memory can be used, acute stress will cause a bias toward using stimulus-response rather than context memory. This happens when stress occurs before learning, and it also happens when stress occurs after learning or even just before a memory test. Even though it always looks like the same shift, it may occur for different reasons depending on when stress happens.
Many studies have shown that, for both rodents and humans, changing when stress occurs can have opposite effects on context memory. For example, stress right after learning can make context memory better, but stress before a memory test can make it worse. There have only been a few studies about stress and stimulus-response memory, but it looks like these effects also change based on stress timing. So, how does stress at all of these time-points consistently make rodents use stimulus-response rather than context memory? In humans, we know that stress before learning also causes a bias toward using stimulus-response rather than context memory, but we don’t know about other time-points, much less how these memories are individually affected by stress to cause such biases.
There are a lot of challenges when studying how stress influences memory —most importantly, you hope that any effects you find are the result of stress impacting memory, not stress impacting some other cognitive process that could also change behavior.
CNS: How did you separately measure the stimulus-response memory and the context memory?
Goldfarb: I’m glad you asked about this, because the task we developed to measure these memory systems is a contribution that I’m excited about. We developed a memory-guided search task to try to pinpoint stress effects on these different types of memory. Participants think they are just looking for a target, but we’ve hidden context and stimulus-response cues in certain trials. On context trials, the spatial pattern of the images give participants a clue about where to look for the target (this has been shown many times and is known as the “contextual cueing effect”), and on our new stimulus-response trials, the color of the images (most of the time) helps them know where to look and which button to press. If they learn and remember what the cues mean, they can use their memories to find the target faster.
There are a lot of challenges when studying how stress influences memory — most importantly, you hope that any effects you find are the result of stress impacting memory, not stress impacting some other cognitive process that could also change behavior. One cool part of this task is that, even if it is clear that they remember the cues, people are not consciously aware that it is a memory task. This helps us bypass a number of cognitive processes — like using a strategy to remember better — that could have been influenced by stress.
CNS: What were your most excited to find?
Goldfarb: We found that stress after learning and before a memory test both made context memory worse, but with important differences. Stress after learning only made context memory worse among people who had a high arousal response during learning, and these people showed that same bias toward SR memory that has been shown in rodents. Although we had expected to see this bias, we were surprised about how it happened: We thought stress would have made context memory better and stimulus-response memory better still. Instead, we saw that context memory got worse and stimulus-response memory wasn’t changed.
Although stress before a memory test also made context memory worse, these effects were transient. When we gave people time to practice, they started using context memory again. So, by the time we tested whether participants would use stimulus-response rather than context memory, there was no bias. Also, just like stress after learning, stress before the memory test did not change stimulus-response memory, emphasizing that context and stimulus-response memory were differentially influenced by stress.
CNS: What do you most want people to understand about this work?
Goldfarb: Most generally, we found that people (just like rodents) could switch to using stimulus-response rather than context memory if they encountered stress after learning. By asking where this bias came from, we discovered that stress impaired context memory, and this was enough to cause a shift toward stimulus-response memory. We also showed that there are important differences in how these different types of memory are influenced by acute stress, with stimulus-response memory being more robust than context memory. Finally, we added to a growing literature about stress effects varying over time by showing that stress after learning and before a memory test had distinct effects on context memory.
CNS: What’s next for this line of work?
Goldfarb: In our search task, participants are looking for a letter “T” among lots of “L” shapes, which is pretty neutral. Now that we’ve shown that stress can influence neutral SR and context memories, we want to see what stress does when these different types of memory contain emotional information. Coming back to my original example of PTSD, many real-world scenarios involve stress modulating memory for information that is itself highly emotional. We know that, at least for recognizing words and pictures, stress has different effects on memory for information that is neutral compared to emotional, and we’re interested in whether this interaction extends to other types of memory.
-Lisa M.P. Munoz