Cultic Studies Review, Vol. 5, No. 6, 2006, Page 7
The Hippocampus
Heightened levels of sustained cortisol also inhibit the normal functioning of another brain
structure, the hippocampus. The hippocampus functions much like a filer, putting
information into its proper time and space folders. The hippocampus is also implicated in
long-term memory consolidation. Thus, if your hippocampus is functioning normally, it will
tell you that you are reading this article in this journal in this particular year (where and
when). The hippocampus, however, is especially sensitive to stress. During trauma it gets
flooded with cortisol, the stress hormone, and when that happens, it goes offline. Therefore,
the traumatic memory does not get placed into its rightful chronological and spatial folders.
Subsequently, when a stimulus reminiscent of the original trauma triggers the memory, the
person responds as if the event were happening here and now instead of there and then.
The ubiquitous phenomena of flashbacks and intrusive thoughts characteristic of PTSD are
the result. To further elucidate this process I turn now to a discussion of how memories are
encoded in the brain.
Implicit and Explicit Memory
Discussion of the psychobiology of trauma would be incomplete without a discussion of
different types of memories and how traumatic memories get reactivated. Implicit
memory, also known as nondeclarative and procedural memory, characterizes the right
brain (where the limbic system primarily resides). Since the right brain is dominant for the
first three years of life, memories of our earliest experiences are laid down in the right
brain, in implicit or procedural memory. These early bedrock imprints exert a powerful
influence over us in subsequent years.
Around four years of age, our left brain becomes dominant and, along with it, explicit
memory, which begins to develop approximately two years earlier. Explicit memory, also
called declarative memory, becomes possible because language is more developed by this
time and memories are laid down in the symbols of the culture—i.e., language. Explicit
memory is also conscious memory.
As noted, the hippocampus is a major structure that functions to consolidate long-term
explicit memories. Therefore, when the hippocampus is inhibited from functioning, as in
traumatic situations, explicit memory cannot occur. Although the amygdala registers the
trauma, the hippocampal ―secretary‖ has not filed it properly. The trauma is ―remembered‖
in implicit memory in perceptual, behavioral, and emotional ways only (Applegate &
Shapiro, 2005).i Later on, other perceptual, behavioral, and emotional stimuli reminiscent
of the traumatic event will ―trigger‖ these unprocessed implicit memories, and the
amygdalic urge to fight or flee will often be the result.
For example, a woman molested as a child reacts with disgust and withdraws when her
husband initiates sex. Her response is automatic, driven by her amygdala. Both she and her
husband are confused by this reaction because she truly loves him and wants to be close.
However, the roots of her reaction have been dissociated from consciousness and her
conditioned reaction to the molestation is encoded in her implicit memory’s neural
circuits. The hippocampus has not registered important information (time and context) of
the earlier event, so the woman‘s amygdala is in charge of the present interaction.
Information Processing
Another piece of this puzzle has to do with other aspects of information processing, which
warrant brief discussion. LeDoux (1996) has pointed out that there are two information
processing pathways—the primitive ―low road‖ pathway that we inherited from our reptilian
ancestors and a more evolved ―high road‖ pathway via the cortex. When information comes
in to the thalamus from the somatosensory cortex where sound, sight, touch, and taste are
received (smell goes directly into the limbic system), that information is relayed to both the
The Hippocampus
Heightened levels of sustained cortisol also inhibit the normal functioning of another brain
structure, the hippocampus. The hippocampus functions much like a filer, putting
information into its proper time and space folders. The hippocampus is also implicated in
long-term memory consolidation. Thus, if your hippocampus is functioning normally, it will
tell you that you are reading this article in this journal in this particular year (where and
when). The hippocampus, however, is especially sensitive to stress. During trauma it gets
flooded with cortisol, the stress hormone, and when that happens, it goes offline. Therefore,
the traumatic memory does not get placed into its rightful chronological and spatial folders.
Subsequently, when a stimulus reminiscent of the original trauma triggers the memory, the
person responds as if the event were happening here and now instead of there and then.
The ubiquitous phenomena of flashbacks and intrusive thoughts characteristic of PTSD are
the result. To further elucidate this process I turn now to a discussion of how memories are
encoded in the brain.
Implicit and Explicit Memory
Discussion of the psychobiology of trauma would be incomplete without a discussion of
different types of memories and how traumatic memories get reactivated. Implicit
memory, also known as nondeclarative and procedural memory, characterizes the right
brain (where the limbic system primarily resides). Since the right brain is dominant for the
first three years of life, memories of our earliest experiences are laid down in the right
brain, in implicit or procedural memory. These early bedrock imprints exert a powerful
influence over us in subsequent years.
Around four years of age, our left brain becomes dominant and, along with it, explicit
memory, which begins to develop approximately two years earlier. Explicit memory, also
called declarative memory, becomes possible because language is more developed by this
time and memories are laid down in the symbols of the culture—i.e., language. Explicit
memory is also conscious memory.
As noted, the hippocampus is a major structure that functions to consolidate long-term
explicit memories. Therefore, when the hippocampus is inhibited from functioning, as in
traumatic situations, explicit memory cannot occur. Although the amygdala registers the
trauma, the hippocampal ―secretary‖ has not filed it properly. The trauma is ―remembered‖
in implicit memory in perceptual, behavioral, and emotional ways only (Applegate &
Shapiro, 2005).i Later on, other perceptual, behavioral, and emotional stimuli reminiscent
of the traumatic event will ―trigger‖ these unprocessed implicit memories, and the
amygdalic urge to fight or flee will often be the result.
For example, a woman molested as a child reacts with disgust and withdraws when her
husband initiates sex. Her response is automatic, driven by her amygdala. Both she and her
husband are confused by this reaction because she truly loves him and wants to be close.
However, the roots of her reaction have been dissociated from consciousness and her
conditioned reaction to the molestation is encoded in her implicit memory’s neural
circuits. The hippocampus has not registered important information (time and context) of
the earlier event, so the woman‘s amygdala is in charge of the present interaction.
Information Processing
Another piece of this puzzle has to do with other aspects of information processing, which
warrant brief discussion. LeDoux (1996) has pointed out that there are two information
processing pathways—the primitive ―low road‖ pathway that we inherited from our reptilian
ancestors and a more evolved ―high road‖ pathway via the cortex. When information comes
in to the thalamus from the somatosensory cortex where sound, sight, touch, and taste are
received (smell goes directly into the limbic system), that information is relayed to both the
