Neurobiology of Grief, Part 3: What Is Trauma, What Is Healing, and How EMDR Works
So what is trauma, really? What is happening in the brain and nervous system when someone is traumatized? And what does it actually mean—on a biological level—to heal trauma? Why can two people experience the same event, and only one goes on to develop PTSD?
In this post, we’ll pull together the neurobiology from Parts 1 and 2 to help shed light on these questions.
What Is Trauma? A Neurobiological Definition
Trauma occurs when an event (or series of events) is so intensely painful, frightening, or overwhelming that it exceeds the nervous system’s ability to cope. The event “overflows” the system.
When this happens, the autonomic nervous system (ANS) becomes overwhelmed past its ability to regulate. The threat surpasses the person’s limbic capacity—and this can cause lasting changes to brain and body.
This explains why two people can go through the same situation and have very different long-term responses. If one person’s nervous system is, for any number of reasons, able to process the threat and return to regulation, the brain is less likely to store the experience as a traumatic memory.
This is not a measure of strength. Trauma vulnerability is shaped by a long list of factors, many outside a person’s control:
social support (family, friends, community)
emotional resilience skills (coping, problem-solving, self-worth)
meaning-making capacities
safety or danger in the environment
history of past trauma
developmental history
access to mental health care
All these elements influence how a person’s system is able to handle a potentially traumatic event.
Early relational trauma uses the exact same neurobiological pathways but with additional developmental considerations. Those will be discussed in an upcoming series on attachment.
How Trauma Changes the Brain
One of the major neurobiological impacts of trauma occurs in the amygdala—the alarm center of the brain.
Trauma can make the amygdala:
hyperactive,
easily triggered, and
slow to shut off once activated.
This leads to exaggerated physiological fear responses: hypervigilance, startle reactions, scanning the environment, difficulty relaxing. This happens because the communication between the amygdala and the prefrontal cortex (PFC)—the part responsible for logic, time orientation, and regulation—is disrupted.
Early trauma may even increase amygdala size.
Adult trauma often reduces amygdala volume.
Different pathways, same outcome:
a system stuck in threat mode.
These changes explain many PTSD symptoms—because the brain begins responding to reminders of the event as if the threat is still happening.
The traumatic memory becomes “stored” in the amygdala and tightly coupled to the fight-or-flight system. Because the limbic system reacts so quickly, it often suppresses activity in the PFC. This means the rational parts of the brain can’t fully contextualize the event or calm the alarm.
This is where trauma treatment comes in.
What Does It Mean to “Process” Trauma?
In trauma therapy, the goal is simple to describe and complex to do:
Activate the traumatic memory and keep the prefrontal cortex online at the same time.
When both the memory and the regulating system are active, the brain can re-encode the memory into long-term storage in a way that no longer sets off the alarm.
This is what clinicians mean when they talk about “processing trauma.”
The memory shifts from:
living in the amygdala,
triggering survival responses,
to:
being integrated into the PFC,
recognized as something that happened in the past,
and no longer overwhelming.
This process relies on neuroplasticity—the brain’s ability to form new pathways and new meanings.
But how do we activate both systems at once?
Early Trauma Treatments: Prolonged Exposure
One of the earliest widely used treatments was Prolonged Exposure Therapy, developed in the 1980s. The approach involves:
learning basic skills for regulation
then describing the traumatic event in detail
over and over
in a supportive setting
The theory is that repetition gradually reduces fear, allowing the PFC to stay engaged while recounting the memory, and eventually completing the processing.
This method works, but it is often extremely difficult for clients—and draining for clinicians who must hear trauma narratives all day every day. Therapists and clients began searching for approaches that could help the brain heal without requiring endless retelling.
How EMDR Emerged
Enter EMDR (Eye Movement Desensitization and Reprocessing).
EMDR was developed by Francine Shapiro in the late 1980s. The classic story says she noticed that when distressing thoughts came up on a walk, her eyes moved rapidly and the distress decreased. Later, intentionally moving her eyes produced a similar calming effect.
This origin story has been debated—Gerald Rosen’s paper “Revisiting the Origins of EMDR” questions Shapiro’s narrative and draws links to neuro-linguistic programming (NLP), a movement that blurred science with pop-psychology.
Regardless of its debated beginnings, EMDR now has one of the strongest evidence bases of any PTSD treatment.
How EMDR Works: Bilateral Stimulation and Memory Processing
EMDR centers around bilateral stimulation—alternating activation of both sides of the brain, typically through:
guided eye movements
tapping
alternating sounds
The theory is that this bilateral activation helps keep the whole brain “online,” allowing memory networks to communicate more effectively.
Interestingly, some of the best early support for this idea came from sleep science.
During the REM (Rapid Eye Movement") stage of the sleep cycle:
the brain is highly active,
eye movements rapidly dart back and forth,
and memory consolidation is at its peak.
Because REM is where emotional and procedural memories are consolidated, researchers wondered:
If eye movement helps the brain integrate memories during sleep, could it also help the brain integrate traumatic memories when awake?
The answer, supported by decades of research, appears to be yes.
How EMDR Helps Heal Trauma (Neurobiologically Speaking)
If traumatic memories are stored in the amygdala, and traumatic reminders set off the entire threat system, EMDR aims to activate the memory while preventing the brain from entering full survival mode.
By recalling the memory while engaging in bilateral stimulation:
the amygdala activates (memory comes online)
the prefrontal cortex stays active (doesn’t fully shut down)
This keeps the brain out of fight-or-flight long enough for the memory to be reprocessed and re-encoded into long-term storage.
In simple terms:
the memory becomes “unstuck,”
it moves out of the amygdala,
and the whole brain participates in understanding it as something that happened in the past.
After processing, reminders may still bring up emotion—but not the overwhelming physiological survival response.
No panic.
No shutdown.
No dissociation.
No ANS overload.
Just a memory, integrated into the story of your life.
Closing Thoughts
Trauma is not about weakness.
It is not about character.
It is about biology.
When the nervous system is overwhelmed beyond its capacity, the brain adapts in the only way it can to survive. Healing trauma means helping the brain reintegrate the memory with the parts of the mind designed for context, regulation, and meaning-making.
EMDR is one of several pathways to help this process occur. There are many other effective therapeutic modalities that aim at encouraging the same neurobiological process with different techniques and frameworks. If you’re therapist is not trained in EMDR it is no cause for concern as there is a rich tapestry of clinical research and modality development that have created a diverse number of paths to healing and integration.