Tracking explicit and implicit long-lasting traces of fearful memories in humans

Pau Alexander Packard, Antoni Rodríguez-Fornells, Lilian Milnitsky Stein, Berta Nicolás, Lluís Fuentemilla

Recent accounts of Posttraumatic Stress Disorder (PTSD) suggest that the encoding of an episode within a fearful context generates different implicit and explicit memory representations. Whilst implicit memory traces include the associated emotional states, explicit traces include a recoding into an abstract or gist-based structural context of the episode. Theoretically, the long-term preservation of implicit memory traces may facilitate the often untreatable memory intrusions in PTSD. Here, we tracked in two experiments how implicit and explicit memory traces for fearful episodes dissociate and evolve over time. Subjects (N=86) were presented with semantically-related word-lists in a contextual fear paradigm and tested for explicit memories either immediately (i.e., 30min) or after a delay (i.e., 1 or 2weeks) with a verbal recognition task. Skin Conductance Response (SCR) was used to assess implicit memory responses. Subjects showed high memory accuracy for words when tested immediately after encoding. At test, SCR was higher during the presentation of verbatim but not gist-based words encoded in a fearful context, and remained unchanged after 2weeks, despite subjects being unaware of words' encoding context. We found no clear evidence of accurate explicit memory traces for the fearful or neutral contexts of words presented during encoding, either 30min or 2weeks afterwards. These findings indicate that the implicit, but not the explicit, memory trace of a fearful context of an episode can be detected at long-term through SCR and is dissociated from the gist-based memory. They may have implicationstowards the understanding of how the processing of fearful memoriescould lead to PTSD.