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

Tracking explicit and implicit long-lasting traces of fearful memories
in humans
Pau Alexander Packard a,b,⇑
, Antoni Rodríguez-Fornells a,b,c
, Lilian Milnitsky Stein d
, Berta Nicolás a
,
Lluís Fuentemilla a,b
a Cognition and Brain Plasticity Group, Bellvitge Biomedical Research Institute (IDIBELL), Barcelona, Spain
bDepartment of Basic Psychology, University of Barcelona, Barcelona, Spain
c Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
d Pontifical Catholic University of Rio Grande do Sul, Porto Alegre, Brazil
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Article history:
Received 17 April 2014
Revised 12 September 2014
Accepted 15 September 2014
Available online 26 September 2014
Keywords:
Explicit memory
Implicit memory
Psychophysiology
Posttraumatic Stress Disorder
Contextual fear
Gist-based memory
abstract
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 gistbased
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., 30 min) or after a delay (i.e., 1 or 2 weeks) 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 2 weeks, 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 30 min or 2 weeks 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 implications
towards the understanding of how the processing of fearful memories could lead to PTSD.