Guest post by Dr. Alan Jern

Netflix’s Altered Carbon depicts a world hundreds of years in the future in which it’s possible to transfer your entire consciousness (via a stack) from body to body (called sleeves). The show raises many questions about identity, the nature of consciousness, and memory – once you can live indefinitely (by jumping from sleeve to sleeve), how will your memory adapt to hundreds of years of experience?

The first episode of the recently released second season highlights one specific feature of memory. In the episode, main character Takeshi Kovacs is attacked and stabbed in the chest, causing him to lose consciousness and forget most of what happened. But later he has an idea: He stabs himself again in the same spot. This activates his “state-dependent memory” and immediately triggers a wave of vivid images of the attack, including the face of his attacker.

Although Altered Carbon is a work of science fiction (a pretty terrific one, if I do say so myself), state-dependent memory is quite real. In this post, I’ll share what psychologists have learned about it.

Context-dependent memory

First, I’ll start with the related and likely more familiar phenomenon of context-dependent memory. If you’ve ever forgotten something, only to be reminded of it when you entered a certain room or arrived at a certain place, you may have experienced this type of memory, defined as memory that is linked to a place, activity, or method of learning.

One of the first, and now classic, demonstrations of context-dependent memory comes from a 1975 experiment by Duncan Godden and Alan Baddeley (1). In the experiment, 16 scuba divers learned a list of words, either on land or underwater, and then were tested on their recall of the words, either on land or underwater. The divers’ recall was best when they took the test under the same conditions in which they had learned the words (land-land or underwater-underwater). On average, the benefit of matching context was about 4 additional words recalled out of 36.

But this is just one study with only 16 participants – not enough to base a theory on. This is where a meta-analysis – a study of studies – is useful. In 2001, Steven Smith and Edward Vela performed a meta-analysis of 75 studies of context-dependence from 41 different articles published through 1997 (2). They found an overall average effect size of 0.28 (using a measure called Cohen’s d). To put this into context, if the diving experiment had an effect size of 0.28, it would have meant that the average benefit of matching context would have been closer to one additional word recalled, instead of four. In other words, not a large effect, but a statistically reliable one.

State-dependent memory

On Altered Carbon, Takeshi doesn’t call his memory context-dependent, he calls it state-dependent, a subtle but accurate distinction. His memory isn’t triggered by returning to the site of the attack, but by the pain from his stab wound – an internal feeling or state.

A more commonly studied source of state-dependent memory is mood: things learned when you are in a good mood tend to be more easily recalled when you are in a good mood. In 1989, Claudia Ucros conducted a meta-analysis of studies of mood state-dependent memory (3). The analysis included 40 studies from 25 articles published between 1975 and 1985. She found an overall average effect size of around 0.44, slightly larger than that of the context-dependent meta-analysis. Ucros notes, however, that this estimate is likely inflated as the analysis only included published articles, which are known to favor results that find effects. This cautionary note applies to most meta-analyses.

What causes these effects?

Although these memory effects are modest (don’t count on state-dependence to help you go from a grade of B to A on a test), the effects are real. But what causes them? One theory is based on the associative nature of memory: when we encode memories, they form associations to related concepts and ideas. Additionally, they may form associations to external (context) and internal (state) cues. As a result, when you are in the same place that you learned something, it’s easier to activate that information because of the association it has to the place. As Steven Smith notes (4), one prediction of this theory is that there should be stronger effects for free recall memory tests than for recognition memory tests (like when you see a bunch of words and you have to pick the ones you saw before). In a free recall test, the associations help you to filter your memories, but in the recognition test, the targets are already presented to you and no filtering is needed. The task becomes more about decision-making than retrieval.

Pain and state-dependent memory

Finally, what about Takeshi’s method of recovering his memory? Can pain actually induce state-dependent memory? There’s not as much research on this question as there is for mood, but one study by Melissa Kuhajda, Beverly Thorn, and Mark Klinger has tested it directly (5).

The researchers ran two experiments. In each experiment, 80 participants learned a list of words while in pain (by placing their hands in freezing water) or not (placing their hands in lukewarm water) and then were tested on their memories of the words while in pain or not. One experiment used a free recall test and the other used a recognition test.

The researchers found no differences in memory in the free recall experiment, but there were differences in the recognition experiment: participants’ memory was worse when they learned the words while in pain or when they were tested while in pain. Specifically, they tended to mistakenly remember 3-4 more words they hadn’t actually seen, on average, when they experienced pain at some point.

Note that none of these results are what the state-dependent memory theory predicts. But the researchers point out that it doesn’t mean that state-dependent effects weren’t present. For example, if the effects of experiencing pain on memory were simply additive – that the more pain you get, the worse your memory – you’d expect memory to be worst in the condition where participants had their hands in freezing water during learning and testing. But that’s not what they found: memory in that condition was about the same as in other conditions with freezing water during learning or testing. The researchers speculated that state-dependent effects in that condition may have counteracted the negative effects of pain on memory, but the pain effects were stronger so they didn’t totally cancel each other out.

Conclusion

Takeshi Kovacs stabbed himself in the chest to help him remember who attacked him. The best you can hope for is that studying for a test in same room where the test will be might make you remember a couple extra answers. Stabbing yourself while you do it probably won’t help. But if you’re going to try, you might want to wait until stacks and sleeves are invented first.

Alan is a cognitive scientist and Associate Professor of Psychology at Rose-Hulman Institute of Technology. He writes about psychology and TV at OverthinkingTV.com.

References

(1) Godden, D. R., & Baddeley, A. D. (1975). Context‐dependent memory in two natural environments: On land and underwater. British Journal of Psychology, 66(3), 325-331.

(2) Smith, S. M., & Vela, E. (2001). Environmental context-dependent memory: A review and meta-analysis. Psychonomic Bulletin & Review, 8(2), 203-220.

(3) Ucros, C. G. (1989). Mood state-dependent memory: A meta-analysis. Cognition and Emotion, 3(2), 139-169.

(4) Smith, S. M. (1994). Theoretical principles of context-dependent memory. Theoretical aspects of memory (2nd ed.), 168-195.

(5) Kuhajda, M. C., Thorn, B. E., & Klinger, M. R. (1998). The effect of pain on memory for affective words. Annals of Behavioral Medicine, 20(1), 31-35.

Images:

1. Pxhere.com. Retrieved from: https://pxhere.com/en/photo/1106175.
2. Wikimedia Commons. Retrieved from: https://commons.wikimedia.org/wiki/File:Hand_in_water_at_the_Blue_Grotto.jpg