## Discussion Our study is the first to demonstrate the existence—at least in epileptic patients—of déjà-rêvé as a specific experiential phenomenon reported after EBS. This is a neurological phenomenon that can be induced in epileptic patients like déjà-vu and reminiscence \ [4, 7, 8, [\[10\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib10), [\[11\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib11), [\[12\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib12), [\[13\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib13), [\[15\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib15), 15, 23\]. Déjà-rêvé, like EBS-induced reminiscence, is very rare in the literature and in our database \ [4\]. We identified 7 instances of déjà-rêvé among 45 different types of reminiscence associated with content in our database, which is a higher proportion than in the literature. This discrepancy suggests that the prevalence of déjà-rêvé may have been underestimated overall. Further work needs to be carried out to determine if déjà-rêvé can spontaneously happen during seizures or in other medical conditions like déjà-vu happens \ [3, 14\]. Déjà-rêvé appears to be different from déjà-vu—the two must not be confused. By construction, all déjà-rêvé episodes we collected had content, which is not the case for déjà-vu. Furthermore, déjà-vu and déjà-rêvé never involved the same bipolar contacts in the instances when both could be induced by EBS in the same patient. These results are critical since they support the idea that déjà-rêvé is a true phenomenon, different from déjà-vu. Moreover, déjà-rêvé is a generic term for distinct entities: recollection of a specific and detailed dream similar to an episodic memory, reminiscence of a vague dream or elements of dream(s) which feels familiar, and finally experiences that resemble or feel like what happens during dreams. Explicit content with mental imagery referring to a previous dream exists in the first two types, but the exactness of the details and contextual elements vary. In the third type, the content of the experience is not at the foreground, but the subject has the feeling of a known sensation or a state of consciousness similar to night dreaming. Only the first two types should be called déjà-rêvé, while the last type should be referred to as “dreamy state”. We propose definitions for each type in Fig. 2. It may be disputed that we cannot be sure that déjà-rêvé (episodic-like or familiarity-like) are linked to true previous dreams. EBS could simply produce a random combination of visual and auditory elements, creating phantasmagoric and unrealistic experiences similar to dreams. However, EBS-inducing déjà-rêvé were all located in the temporal lobes, although extratemporal brain areas were commonly stimulated \ [4\]. They were also exclusively induced by medial temporal lobe EBS in our database. Thus, like other subtypes of experiential phenomena, specific locations of EBS seem to correlate with déjà-rêvé subtypes. This suggests it is improbable the déjà-rêvé we reported here are pure hallucinations, otherwise patients would have reported similar phenomena after stimulation of other brain areas. Furthermore, the brain regions implicated in déjà-rêvé include structures involved in long-term memory (especially in episodic memory and familiarity processes) \[ [\[27\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib27), [\[28\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib28), [\[29\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib29)\], in memory formation during sleep \ [29\], in normal dreaming \ [30, 31\] and probably in dream recall \ [32\]. We also found a clear majority of right EBS in our database. We cannot draw conclusions from this because there are too few observations; however, right hemisphere activation during dream recall has already been suggested \[ [\[33\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib33), [\[34\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib34), [\[35\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib35), [\[36\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib36), [\[37\]](https://pmc.ncbi.nlm.nih.gov/articles/PMC6028740/?utm_source=chatgpt.com#bib37)\]. For example, the right hemisphere may be linked to dream materials and the left hemisphere to dream encoding and interpretation \ [34\]. Lower alpha activity of the right temporal area has also been associated with successful recall of a dream \ [37\]. Lastly, patients reporting a reminiscence after EBS are confident that it corresponds to true memories. There is no reason to think this should be different when they report dreams. Thus, it seems likely that déjà-rêvé (“episodic-like”) corresponds to true dreams that are partly remembered after EBS. Following this interpretation, déjà-rêvé is similar to the recall of other reminiscences induced by EBS (detailed in \ [4\]) but refers to the recall of exact content. Whether specific neurophysiological mechanisms underlie déjà-rêvé remains to be investigated. However, stimulation of the same contacts could induce either reminiscence of déjà-rêvé or reminiscence of other types of memories in some of our patients, suggesting a proximity between these types of memories. These results emphasize the importance of detailed and suitable follow-up interviews of the patients to clarify the nature and content (affective, perceptual and mnemonic components) of the experiential phenomenon in all future studies and in routine clinical practice. This may explain in part why many phenomena from the literature were unclassifiable and why déjà-rêvé may have not been identified as such. There also appears to be some discrepancies in the types of classifiable phenomena, since the literature observations formed the bulk of the “dreamy state” subtype. This is likely a bias in our classification due to the simplification of the reports by many authors and the preponderance of Penfield's report in this category (he mainly stimulated the lateral part of the temporal lobes) \ [7, 8\]. In the literature, patient reports are often summed up in a few sentences and most of the time, the patient's follow-up interview has not determined exactly the type of déjà-rêvé. It is therefore possible that more episodic-like or semantic-like déjà-rêvé episodes were induced, but that due to the simplification of the reports, we classified them as “dreamy state”. However, the fact that the dreamy state was the most common subtype in the literature may also not be an artifact: a meaningful difference in the type of the phenomena reported in literature _vs_ our database is supported by the differences of EBS locations, which appear more diffuse and lateral for the dreamy-state subtype. Better identification and definition of déjà-rêvé in epileptic patients may be relevant for clinical practice. It could guide neurologists when exploring temporal lobe dysfunction. EBS can induce déjà-rêvé in non-temporal epilepsy (as in patient 1 whose epileptogenic zone was frontal) but even in this case, EBS had to be in the temporal lobe. Our results thus suggest spontaneous déjà-rêvé may be specific to seizures involving the temporal lobes. Furthermore, the qualitative nature of the déjà-rêvé seems to depend on the stimulation site, like we have previously demonstrated for other memories induced by EBS \ [4\]. Separating déjà-rêvé from dreamy state could thus provide supplementary information as episodic-like déjà-rêvé points mostly to medial, rather than lateral, temporal lobes dysfunction. For instance, the report of an “episodic-like” déjà-rêvé as an aura of seizure should guide the neurologist towards an epileptogenic zone involving the medial temporal lobe. In contrast, a “dreamy state” (as defined in our classification) could point more generally to temporal lobe epilepsy. A majority of right EBS-inducing déjà-rêvé in our database could also suggest a potential lateralizing value of déjà-rêvé. But while this is consistent with fMRI results demonstrating that the recall of dreams and fantasies differentially activated a right hemisphere network \ [36\], more data are needed to assess the strength of this hypothesis. Nonetheless, EBS on the same site does not automatically replicate déjà-rêvé. The low prevalence of EBS inducing those phenomena suggests that a very particular combination of other—poorly understood—factors is required, including the technique used, electrical parameters, and maybe the baseline brain state at the moment EBS is applied. Future work should aim at improving our understanding of these factors. Finally, EBS-induced déjà-rêvé could be an interesting approach to better understanding physiological dreams that cannot be reproduced under laboratory conditions. Understanding why some dreams can be remembered and decoding the mental content of dreams are great challenges. Most studies focus on REM (rapid-eye movements) sleep period \ [39\] and dream reports obtained by awakening a sleeping subject \ [40\]. However, non-REM sleep dreams account for a significant portion of all typical dreams and several factors might render dream reports less trustworthy—especially the sleep stage before awakening—when compared with reports of waking experience \ [40\]. Analyzing intracranial EEG oscillations during induced déjà-rêvé could be an additional way to catch dreams by trapping EEG activity during this sudden, unexpected, and brief cognitive moment.