Are déjà vu and involuntary autobiographical memories (IAM) based on the same continuum of retrieval processes? This is the core hypothesis proposed by the intellectually stimulating paper by Barzykowski and Moulin (B&M) which we will discuss specifically within the spectrum of déjà vu and IAM induced by intracranial electrical brain stimulation (iEBS) in epileptic patients.

There is an undeniable proximity between déjà vu and IAM with strong arguments presented in B&M's proposal. However, conceptual proximity does not mean mechanistic similarity. For instance, the authors emphasize that IAM are frequent, contrary to déjà vu, which are much rarer. One may wonder why phenomena sharing common mechanisms do not have a correlated prevalence. How this is accounted for is not yet clear. A better definition of IAM could clarify this issue. In fact, there are also inconsistencies in the reported frequency of IAM across studies. While some studies report high frequency, of up to 20 per day (Rasmussen & Berntsen, Reference Rasmussen and Berntsen2011), others report only one per week or per month (Moulin et al., Reference Moulin, Souchay, Bradley, Buchanan, Karadoller, Akan, Schwartz and Brown2014). Do such discrepancies mean that there are different types of IAM which are poorly differentiated in the literature at present?

B&M focus mainly on IAM that occur several times a day during mind wandering due to the associative nature of memory processes (Plimpton, Patel, & Kvavilashvili, Reference Plimpton, Patel and Kvavilashvili2015). But how can IAM such as the memory of Tante Leonie's home in Combray which popped up in Marcel Proust's mind thanks to a madeleine be explained? Proust's IAM was sufficiently intrusive, unusual and disturbing for him to write 2,400 pages largely inspired by this phenomenon. And what about memories that are just as intense and intrusive with no associative trigger in the environment? We contend that such “pop-up” IAM are rare and do not correspond to “mind-wandering” IAM. These “pop-up” IAM seem to have a similar prevalence to that of the déjà vu experience reported in the healthy population. In fact, pop-up IAM not triggered by an environmental or goal-directed memory cue are the minority (approximately 15%) of the IAM experienced in the everyday life of healthy subjects (Ball & Little, Reference Ball and Little2006).

Distinguishing between “pop-up” and “mind-wandering” IAM (this terminology may need to be revised) may be all the more relevant as they do not seem to serve the same purpose. While reminiscing on autobiographical memory during mind-wandering episodes is assumed to play a fundamental role in memory consolidation (Wamsley, Reference Wamsley2019) and possibly in personality construction, whether or not “pop-up” IAM play an adaptive or ecological role is unclear. Again, in terms of functionality, this role seems closer to déjà vu which B&M describe as the result of a metamemory error. “Pop-up” IAM may also lead to some level of surprise similar to déjà vu.

Déjà vu and experiential phenomena in epileptic patients, whether occurring spontaneously during seizures or after iEBS, offer informative snapshots into these highly subjective memory phenomena which can be phenomenologically very rich (Curot et al., Reference Curot, Busigny, Valton, Denuelle, Vignal, Maillard and Barbeau2017). They have an important role as complements in cognitive psychology experiments, in discussing the reality of theoretical models, and in obtaining insights into the physiological mechanisms underlying such phenomena.

Like the memory of Tante Leonie's home for Proust, epileptic subjects can also experience “pop-up” IAM after an EBS and get literally invaded by such memories, unexpectedly and involuntarily (for verbatim details, see Curot et al. [Reference Curot, Busigny, Valton, Denuelle, Vignal, Maillard and Barbeau2017] and corresponding online database https://figshare.com/s/923f93555a0ce51426e4 of >370 experiential phenomena reported in >110 patients; Bartolomei, Lagarde, Médina Villalon, McGonigal, & Benar, Reference Bartolomei, Lagarde, Médina Villalon, McGonigal and Benar2016). Their attention is suddenly projected toward their internal word, which could be interpreted as mental diplopia or a dissociation process in these patients. Patients often spontaneously describe a feeling of temporal and/or spatial projection into their own past. During these “pop-up” IAM, the intention to remember is absent, which may explain why it is such a surprise: There is no “madeleine” or environmental trigger. Interestingly, B&M mention that one of the limitations of the literature is that déjà vu and IAM are studied separately, while the EBS-induced déjà vu and IAM model allows studying these phenomena together in the same patients. Therefore, we propose using this approach to investigate a possible continuum between déjà vu and IAM. Based on the observations made in epileptic patients after iEBS, we suggest that there may be different types of IAM, including “pop-up” IAM, which may be closer to déjà vu than to mind-wandering IAM. We agree that déjà vu and IAM could be part of a continuum involving retrieval processes, but that this could be the case for a certain type of IAM only, those that pop up in the mind without any obvious memory cue.

To examine this hypothesis, we could: (1) Calculate the intra-individual correlation between the number of IAM and déjà vu induced by iEBS in the same brain region; (2) investigate the correlation between the prevalence of these “pop-up” IAM and experiential phenomena such as déjà vu which occur during spontaneous seizures, individually and in a group. If “pop-up” IAM and déjà vu experiences rely on shared underlying mechanisms, we expect a significant positive correlation between their prevalence, but no correlation with mind-wandering IAM; (3) iEBS are performed during intracranial EEG recordings, enabling trapping of the neural correlates of the fleeting moments that correspond to IAM and déjà vu (Barbeau et al., Reference Barbeau, Wendling, Régis, Duncan, Poncet, Chauvel and Bartolomei2005; Bartolomei et al., Reference Bartolomei, Barbeau, Nguyen, McGonigal, Régis, Chauvel and Wendling2012). Functional connectivity networks during each phenomenon could be compared to identify commonality and differences in connectivity patterns. Each network could also be compared to those of episodic memory and mind wandering to assess the specificity of “pop-up” IAM. (4) Finally, in 2017, we proposed a classification of the different types of memories induced by EBS according to their content and the current main concepts of long-term memory (Curot et al., Reference Curot, Busigny, Valton, Denuelle, Vignal, Maillard and Barbeau2017, see online database). This classification could be reviewed in light of the suggestions made by B&M to verify how they fit with the verbatim report of patients.