
==== Front
Proc Natl Acad Sci U S A
Proc Natl Acad Sci U S A
PNAS
Proceedings of the National Academy of Sciences of the United States of America
0027-8424
1091-6490
National Academy of Sciences

38498730
202402870
10.1073/pnas.2402870121
commCommentarypsych-socPsychological and Cognitive SciencesneuroNeuroscience424
431
437
Commentary
Social Sciences
Psychological and Cognitive Sciences
Biological Sciences
Neuroscience
Reminiscing under the radar
Moharramipour Ali alimoharrami1371@gmail.com
a 1
Lau Hakwan a
aRIKEN Center for Brain Science, Wako 351-0106, Japan
1To whom correspondence may be addressed. Email: alimoharrami1371@gmail.com.
18 3 2024
26 3 2024
18 9 2024
121 13 e2402870121Copyright © 2024 the Author(s). Published by PNAS.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

RIKEN Center for Brain Science RIKEN Internal funding Hakwan Lau
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pmcIn George Orwell’s precinct fiction, 1984, Winston Smith apprehensively refrained from recalling some of his memories too often, for fear that the act itself would make them deviate further from reality. Unfortunately, neuroscience has in fact shown that even without the hypothetical intervention by some evil authority, our memories do change as we reactivate them—a phenomenon known as retrieval-induced forgetting (1, 2). This often concerns memories that are not themselves the main targets of recall but are nonetheless related. For example, as we try to vividly remember what happened at a birthday party 14 y ago, we may start to mix up events that happened at other parties around the same time, maybe at the same venue, with some but not all of the same people. We may end up falsely inserting people into the memory of the said birthday party, even though they were not actually there. These kinds of errors are disturbingly common, as human memories are known to be constructive rather than entirely passive in nature (3, 4). Reminiscence, while beneficial for both sentimental and mnemonic purposes, seems to also come with a hidden cost. Like a good oil painting, memories are somewhat best kept without excessive exposure.

This is the context in which Tal et al.’s recent study (5) is so interesting and relevant, as they uncovered a potential mechanism by which one may be able to bypass this challenging problem, so that we can “relive” our memories beneficially without the risk of negative repercussions. The authors experimentally induced memory activation by presenting only a part of a set of content that the human participants had to remember (Fig. 1A). By presenting that partial “cue,” just as expected, memories for the rest of the set improved overall, as if it triggered an active process of memory update and reinforcement. However, some attributes of the memory content also got mixed up, leading to impairments in some specific aspects of memory performance. In contrast, when these partial memory cues were presented nonconsciously, i.e., so weakly that it was below the threshold for conscious perception (Fig. 1B), the benefits of reactivation were overall less strong (as they were only found for memories that were weak to begin with). Most intriguingly, this nonconscious cuing effect did not come with a detrimental influence on other memories attributes (Fig. 1C).

Fig. 1. (A) Illustration of the memory task used by Tal et al. (5). In this task, participants had to remember the combination of a word (e.g., LOOSE), a visual object (e.g., pear), and a spatial location. (B) The paradigm used in Tal et al.’s study (5) to reactivate a memory by visually presenting the word (as a cue). A brief presentation of a target word was sandwiched between a mask word (MWMWM) and a blank screen. It is possible to make the target world invisible (i.e., subliminal) to the individuals by shortening the blank screen duration. (C) A table summarizing the effect of conscious and nonconscious memory reactivation found by Tal et al. (5). Strong memories were those with more accurate location recall before memory reactivation. When the word was presented supraliminally, the participants showed improvement in recalling objects later on. When they were asked to name the memorized objects from the same category (e.g., fruits), they named more of those that were reactivated than non-reactivated. However, this also led to some detrimental effects: They made more errors in remembering the spatial location of related non-reactivated memories (e.g., non-reactivated fruits), but this was only true for strong memories. On the other hand, the nonconscious reactivation did not show any detrimental effects and benefited only weak memories in better recalling their spatial location. (D) Assuming a hypothetical memory task involving different color stimuli, their subjective perceptual similarity can be obtained in individual subjects. On such an empirically measured dimension of similarity (x-axis here), it is possible to examine how conscious reactivation of the color red “spills over” to other perceptually similar colors, boosting the memory of some colors but degrading others. This “spillover” function for nonconscious reactivation may differ from conscious reactivation (solid curve) in different ways, e.g., weaker activation of the same function, as represented by the dashed curve, or activation of a narrower function, as represented by the dotted curve.

Coming from a single study, it is unclear yet how robust these effects will turn out to be. However, one benefit of this kind of behavioral research is that, unlike, e.g., neuroimaging, they are easy to replicate. Such replication efforts are probably already ongoing and will certainly be more than worthwhile. This is because, conceptually, these effects are rather unique and more informative than currently standard studies on the same topic.

Like a good oil painting, memories are somewhat best kept without excessive exposure.

In most common studies of the cognitive impact of subliminal stimuli, typically the effects are either null or just some minimal variants of conscious processing (6). That is, under nonconscious presentation, the stimuli may either trigger no effect or something similar to that of conscious processing, only at a much weaker magnitude. This leads to the general worry that such manipulation is not exactly about consciousness per se, but rather just the strength of the perceptual signal. When the perceptual process is weak, naturally the relevant cognitive impact is small, just as expected. One therefore wonders whether the theoretically loaded notion of consciousness is really needed at all for the interpretation of these findings.

Here, in Tal et al.’s study (5), just as in some previous pioneering studies (7), the effects of nonconscious memory reactivation were qualitatively different from that of conscious processing, arguing against the interpretation that the difference here is simply perceptual signal strength. To further verify and clarify this point, future studies can, for example, vary the strength of perceptual processing systematically and see whether the divergence between conscious and nonconscious memory reactivation effects happens exactly at the threshold of conscious perception—an approach that was also employed in Tsushima et al. (7).

Another potentially interesting manipulation here would be to reactivate the same memories repeatedly with nonconscious cues. As long as each cue presentation is nonconscious, future studies can then assess whether such nonconscious effects will accumulate over many cuing repetitions and come to behave like the effects of conscious reactivation. If many repetitions of nonconscious cuing result in significant benefits on reactivated memories and yet without the negative impact on non-reactivated memories, it could be a finding with both great theoretical interest and practical potential.

However, it is not unthinkable that as nonconscious reactivation gets stronger through repetition, it may end up leading to the same detrimental effects as conscious reactivation too. Such an outcome would undermine the current interpretation and suggest that maybe the lack of detrimental impact of reactivation is due to the strength of the cuing signal after all. Tal et al. (5) suggest that this may be unlikely, as the mechanism for these detrimental effects is due to specific inhibitory mechanisms affecting related memories, exclusively triggered by conscious reactivation. This is a reasonable interpretation and is compatible with some previous studies (7). However, it is not impossible that such inhibitory effects are in fact a kind of “spillover” from strong reactivation, regardless of consciousness. To assess whether this negative spillover to related memory representations is really qualitatively different between conscious and nonconscious reactivations (rather than just being less exercised in the latter), one can quantitatively assess the shape of this spread function, using psychophysics methods. For example, future studies can carefully measure the spread of reactivation effects over a set of mnemonic stimuli, according to how subjectively similar they are to each other. Along this dimension of subjective stimulus similarity, which can be assessed by rating or ranking methods, one can then construct a curve representing the spread of reactivation effects (Fig. 1D) and compare them between conscious and nonconscious reactivations accordingly. Using similar approaches, recent research has in fact demonstrated the relevance of subjective similarity in explaining certain aspects of memory performances (8, 9).

In summary, like all great seminal studies, Tal et al. (5) raised more questions than they have answered. The study is particularly interesting because currently popular theories treat consciousness as global, complex, ignited, reflected, predictive, deeper, stabilized, recurrent—or in other words, just strong—information processing. If this common dogma is right, it is not clear what going below the conscious radar would really buy us. Even if such feeble signals were processed at some depth, they are not ever meant to be functionally superior. Tal et al. (5) raised the intriguing possibility that with respect to memory reactivation, there may be qualitatively different mechanisms at work, as we bypass consciousness. If true, this could in turn potentially ease the worries of Winston Smith and ourselves alike: How do we enjoy and rehearse our treasured memories, without ever contaminating them in the process? Perhaps consciousness is part of the Memory Hole that Winston Smith worried about. If a nonconscious reactivation method can help us bypass this worry, understanding the relevant mechanisms may inform both pedagogy in the classroom as well as medical applications in the clinic.

Author contributions

A.M. and H.L. wrote the paper.

Competing interests

The authors declare no competing interest.

See companion article, “The reach of reactivation: Effects of consciously triggered versus unconsciously triggered reactivation of associative memory,” 10.1073/pnas.2313604121.
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