The third stress response that does not get as much recognition is the freeze response.   In proper context, it is the "fight, flight or freeze response".

In his book Waking the Tiger, Peter Levine describes this phenomenon in detail based on anecdotal experience and animal research.  He reports that the fight or flight or freeze stress response are survival mechanisms emanating from the instinctual, reptilian brain, one of three parts of the human brain also called the reticular formation and brain stem. He argues in animals, only the reptilian brain and the mammalian brain (limbic) exist.  Reactions to threats in the environment therefore, are purely instinctual, that is, without rational thought (a function of the third part of the human brain, the neo-cortex or frontal lobe). What Levine reports is that in animals it has been witnessed that when threatened, they harness energy to fight or run or if those two options do not bring safety, they freeze.  Freezing becomes a mechanism of survival by entering into an altered state of consciousness when death appears imminent. On the outside it may look like the animal has collapsed, but on the inside there is a build up of energy, which when the threat dissipates, the energy is unleashed into the fight or flight sequence and the stress response cycle has been completed.  Levine argues that if this cycle does not play out, the animal remains in a frozen state, traumatized.  Treatment of this traumatized person thus focuses on completing the cycle.

Humans, however, have developed a third region of the brain that is capable of rational thought, the neo-cortex.  When confronted with a life threatening situation, our rational brains may become confused and over-ride our instinctual impulses. Confusion can lead us to being frozen in fear leading to the creation of traumatic symptoms. Symptoms that originate from the amassing of unleashed energy stored in our nervous systems. When we are not able to unleash and liberate these powerful forces, we become victims of trauma.

This presents one interesting model to understand trauma through an adaptive process and the treatment implications for which Levine outlines in his book. Consistent with this training, however, the "freeze" can also be understood as an over-stimulation of the sympathetic and neuromuscular pathways or over-stimulation of the parasympathetic pathways and loss of tone in the neuromuscular (Everly, 2003). Likewise, Aphrodite Matsakis in her book "Post Traumatic Stress Disorder, A Complete Treatment Guide", writes that when in a dangerous situation, adrenal glands pump either adrenaline or noradrenalin into the body. Adrenaline causes the state of hyper-alertness in which blood pressure, heart rate, muscle tension all increase.  Pupils dilate and blood flow to the extremities decrease, while the flow to the head and trunk increase so that the individual can think and move better and more quickly. Alternatively when noradrenaline is pumped into the system, a freezing reaction can take place. Moving and acting becomes difficult as if moving in slow motion. It is also believed that while this freezing reaction is happening the symptoms of hyperarousal are also evident (Matsakis,1994).

Of importance is assessing and tracking the impact of the stress response when treating trauma. What was the individual's experience to the stressor? Many individuals will have a fight or flight response and still have stress related symptoms that are relevant when doing the debriefing work. However, those who did respond with either fight or fight and secured safety are less likely to develop PTSD then those who experienced the freeze response.