28 March 2014. Being self-centered may not endear us to our friends, but apparently it is an ingredient for vivid memories. A study published online March 10 in Proceedings of the National Academy of Sciences found that experimentally induced out-of-body experiences in healthy participants degraded the quality of their memories formed at the time. Led by H. Henrik Ehrsson at Karolinska Institutet in Stockholm, Sweden, the study also found that the poorer quality memories were associated with abnormally weak activation of the hippocampus.
The study suggests a direct link between a person’s sense of self and their autobiographical memories. This offers an explanation for the memory impairments in psychiatric disorders marked by dissociative symptoms, including schizophrenia. While people’s center of awareness comes from within their own body, those with schizophrenia, as well as post-traumatic stress disorder, sometimes report feeling detached from themselves, as though this sense of self is located outside of their bodies. The new study directly tests the association between self and memory in healthy participants by inducing an out-of-body experience using a virtual reality set-up.
The findings also link two seemingly disparate roles of the hippocampus, which is essential for recording events to be stored into memory, as well as for representing space. Because our bodies act as a key point of reference as we experience the world around us, memories about the details of our surroundings and our feelings about them are laid down relative to this self-centered reference frame.
"Collectively, these findings show that efficient hippocampus-based episodic-memory encoding requires a first-person perspective of the natural spatial relationship between the body and the world," the authors write.
First author Loretxu Bergouignan and colleagues induced the out-of-body illusion in 32 healthy college students in a room in which they were to undergo a mock oral exam on material they had just spent 10 minutes reading up on. A "professor" (actor) conducted the exam while facing the participant. Through a combination of video cameras, virtual reality goggles and earphones, the students were presented with visual and auditory feedback from the opposite side of the room. This, combined with touch stimuli delivered to the student’s body, created the sensation of being outside of their own bodies.
Each participant underwent four "exams" on four different subjects. For two of these, the out-of-body illusion was induced; for the other two, no illusion was induced, though all the same equipment was used. One week later, the participants returned to answer questions about their experiences during each exam. After a period of free recall, the experimenter asked pointed questions to get at the details of their memory about what had happened, and from this calculated a global remembrance score.
Memories established during the out-of-body condition scored about one-third lower compared to those from the in-body, control condition. These out-of-body memories were foggier on details about where and when things happened. A second experiment done in a new group of 32 healthy participants found the same effect when the out-of-body illusion was produced so that the students felt they were to the side of where they actually sat, yet could still see the face of the professor.
The researchers argue that these memory impairments arose because the out-of-body experience was somehow more distracting: Immediately after their exams, the participants rated their performance and emotions similarly in both the in-body and out-of-body conditions. A supplemental experiment also found that tests of verbal fluency given immediately after the exam gave similar scores across these conditions, suggesting that the strangeness of the illusion did not compromise cognition.
Pondering posterior hippocampus
Though the cortex stores memories, the hippocampus acts as a gateway not only for encoding new memories, but also for retrieving old ones. During retrieval, the hippocampus is strongly activated during initial recall of a life event, but then this becomes progressively weaker with repetition (Svoboda and Levine, 2009). Thus, the researchers predicted that the hippocampus would be weakly activated during the initial retrieval of memory for the out-of-body exams compared to the in-body memories.
Indeed, this is what they found, in a new group of 21 healthy participants. Two weeks after their out-of-body and in-body exam experiences, their brains were scanned by functional magnetic resonance imaging (fMRI) while recalling each exam separately. The researchers found that the left posterior hippocampus had significantly reduced activation during recall of memories encoded during the out-of-body experience compared to those encoded during the in-body condition.
This weak activation may reflect a memory not very well encoded, with details of the experience missing due to the warped, out-of-body frame of reference. Virtual reality paradigms already in use in rodents may eventually probe this link between self and memory in single neurons (e.g., Harvey et al., 2009).
Although the connection to mental illness, and particularly schizophrenia, remains speculative, the findings potentially draw together some of the disparate symptoms of psychotic illness that depend upon normal memory function.—Michele Solis.
Bergouignan L, Nyberg L, Ehrsson HH. Out-of-body-induced hippocampal amnesia. Proc Natl Acad Sci U S A. 2014 Mar 10. Abstract