From a physiological perspective, "profound alterations in stress hormone secretion and memory processing" are found in people suffering from PTSD. Due to "ascending amine projections," soldiers with PTSD showed the physical symptoms of "flexor changes in posture, hyperkinesis, violently propulsive gait, tremor at rest, masklike faces, cogwheel rigidity, gastric distress, urinary incontinence, mutism, and a violent startle reflex" (Van der Kolk, 1987).

Kolb first proposed that "excessive stimulation of the CNS at the time of the trauma may result in permanent neuronal changes that have a negative effect on learning, habituation, and stimulus discrimination" (Van der Kolk, 1987). Dr. Bessel van der Kolk explains that traumatic memories are stored at the "implicit" memory level which changes a person's "biological stress response" to those memories (Van der Kolk, 1987).

Because these memories are processed outside of the "hippocampally mediated memory system," they are difficult to "extinguish" (Van der Kolk, 1987). Implicit memories also are different from normal memories, called narrative, semantic, or declarative, in that they are not stored verbally.

Because they are so physiologically overwhelming, they bypass verbal encoding and processing and are stored in the more primitive, "somatosensory or iconic," part of the brain as sensations, images, and feelings. These memories are stored as "emotional responses, skills and habits, and sensorimotor sensations related to experience" (Van der Kolk, 1987).

"Speechless terror" can be used to describe a traumatic event in which the "emotional impact...interfere[d] with the capacity to capture the experience in words or symbols" (Van der Kolk, 1987). Conversely, declarative memories are formed by attaching to "existing mental schemata: once an event or a particular bit of information is integrated into existing mental schemes, it will no longer be accessible as a separate, immutable entity," contrasted with traumatic memories that dissociate and remain separate from the central ego (Van der Kolk, 1987).

The two types of memory stimulate different parts of the brain, as observed on a PET scan. When implicit memory is recalled, instead of being evaluated rationally, the memories are more likely to stimulate a fight or flight reaction. People with PTSD have trouble over-riding their physical responses to memory-triggering stimuli because they have no verbal connection to the memories with which to mediate (Adams, 1998).