Primitive reflexes are automatic, involuntary movements originating in the brainstem that act as the foundation for an infant’s survival and early neurological development. These movement patterns are present before thought or intention develops, helping the newborn interact with its environment and begin building muscle tone. The Asymmetrical Tonic Neck Reflex (ATNR) is a foundational reflex and a temporary, normal part of human development that prepares the body for later voluntary movement.
How the ATNR Reflex Works
The ATNR is often called the “fencing reflex” because of the distinct posture the infant assumes when the reflex is triggered. The stimulus that elicits this reflex is a simple turning of the baby’s head to one side while they are lying on their back. This head movement causes an immediate, involuntary response in the limbs on both sides of the body.
When the head is turned, the arm and leg on the side the face is pointing toward automatically extend or straighten out. Simultaneously, the arm and leg on the opposite side, the side facing the back of the head, will flex or bend inward. This symmetrical but opposite action links the visual field with the movement of the limbs, giving the infant early practice in coordinating vision with hand movements. The reflex also provides necessary stimulation for developing muscle tone.
This patterned movement helps infants practice unilateral movement, which is moving one side of the body independently of the other. It strengthens extensor muscles on one side of the body while activating flexor muscles on the other side. By repeatedly exhibiting this reflex, the baby receives visual feedback as the extended arm comes into the line of sight. This is an early step toward developing hand-eye coordination and visual tracking skills.
The Normal Developmental Timeline
The Asymmetrical Tonic Neck Reflex begins developing early, often observed as early as 18 weeks into gestation. It is normally present at birth and is a common part of a newborn’s neurological assessment. However, the continuation of this reflex beyond the first few months can interfere with the progression of motor skills.
For development to proceed normally, the ATNR must “integrate,” meaning the brain’s higher centers inhibit the involuntary movement pattern. This integration allows for more complex, voluntary movements to take over. The ATNR typically integrates between four and six months of age, with seven months being the outer limit.
If the reflex integrates successfully within this window, the infant can then achieve milestones like rolling over, bringing both hands to the midline of the body, and transitioning to bilateral coordination. Integration permits the infant to move their head freely without causing automatic movement in their arms and legs.
Consequences of a Retained Reflex
When the ATNR fails to integrate by the six-month mark, it is considered a retained primitive reflex, and the involuntary movement pattern persists into later childhood. This retention interferes with the development of higher-level motor skills and academic performance because the reflex continues to dominate movement.
One noticeable challenge is difficulty with handwriting, as turning the head to look at a paper can cause the writing arm to involuntarily extend. This makes it difficult to maintain a consistent pencil grip and often results in poor, labored penmanship. The retained reflex also impedes the ability to cross the midline of the body, the imaginary line running vertically down the center.
Tasks requiring bilateral coordination become challenging because the two sides of the body cannot work together effectively. These tasks include:
- Catching a ball
- Tying shoelaces
- Cutting with scissors
- Visual tracking, making it difficult for the eyes to smoothly follow a line of text across a page, which may contribute to reading difficulties.
Poor balance and an uncoordinated gait can also be observed, especially during dynamic activities like running or skipping, as involuntary limb responses destabilize the body. The persistent reflex action forces the child to exert continuous effort to override the pattern, leading to increased muscle tension and poor posture. These persistent, low-level challenges can significantly impact a child’s confidence and ability to participate easily in physical and academic tasks.