The Fencer’s Response, formally known as the Asymmetrical Tonic Neck Reflex (ATNR), is an involuntary movement pattern observed in newborns and infants. This primitive reflex originates in the brainstem and represents one of the earliest signs of a developing nervous system. It is foundational to early neurological maturity, helping to establish initial connections between the head, eyes, and limbs. The presence of the ATNR confirms that lower-level brain functions are operating as expected.
Describing the Fencer’s Posture
The Fencer’s Response earns its name from the distinct pose an infant adopts when the reflex is activated. When an infant’s head is gently turned to one side, the arm and leg on the side the infant is facing will straighten or extend. Simultaneously, the arm and leg on the opposite side will bend or flex. This asymmetrical pattern creates a look similar to an “en garde” position in the sport of fencing. This reflex is often tested by medical professionals while the baby is lying on their back. The movement is automatic and not controlled by the baby, as it is a reflex governed by the brainstem. The body’s response is directly triggered by the shift in the head and neck position. This action demonstrates the baby’s inability to move the head independently of the limbs early in life.
The Developmental Purpose of the Reflex
The automatic movement of the ATNR serves several roles in preparing the infant for later, voluntary actions. It helps develop muscle tone on both sides of the body by repeatedly engaging extensor (straightening) and flexor (bending) muscles, building strength for future motor milestones. The reflex also plays a part in the development of hand-eye coordination. When the head is turned, the extended arm is brought into the infant’s line of sight, forcing the eyes to track the moving hand. This visual tracking lays the groundwork for later skills like reaching and grasping objects.
Furthermore, the ATNR assists the fetus during the birthing process by promoting the necessary twisting motion down the birth canal. The reflex also helps prevent the infant from rolling over too soon, before the muscles and balance system are ready. By keeping one side extended and the other flexed, the reflex anchors the baby, which stabilizes the body until more control is achieved. This early separation of movement between the left and right sides is foundational for more complex actions like crawling. The ATNR acts as a temporary neurological bridge connecting head movement to limb movement before voluntary control takes over.
When the Reflex Integrates
The Fencer’s Response is typically present at birth, sometimes emerging as early as 18 weeks into gestation. It is a temporary reflex that must be suppressed by the brain as the nervous system matures. The process where a primitive reflex disappears or is inhibited by higher brain centers is called integration. Integration of the ATNR occurs between four and seven months of age, with the average timeframe being around six months.
Once the reflex integrates, the infant is freed from involuntary limb movements tied to head turning. Its disappearance allows the baby to achieve symmetrical movements, such as bringing both hands together at the midline. This ability is important for tasks like reaching for toys or bringing the hands to the mouth. If the reflex persists strongly beyond this window, it impedes the development of voluntary and coordinated movements.
Recognizing Signs of a Retained Reflex
If the Fencer’s Response does not integrate within the normal timeframe, it is considered a retained ATNR, which interferes with later development. In older children, a retained reflex often manifests as difficulties with bilateral coordination—the ability to use both sides of the body together in a controlled manner. Activities like catching a ball or tying shoelaces may become challenging.
The persistence of the reflex also affects a child’s ability to cross the midline of the body (the imaginary line dividing the left and right halves). Difficulty crossing this line is a common sign of a retained ATNR, impacting reading and writing skills. Handwriting is particularly affected because turning the head to look at the paper may cause the writing arm to involuntarily extend. This involuntary extension can lead to:
- Poor pencil grip.
- Fatigue while writing.
- Difficulties keeping text aligned on the page.
- Balance issues when the head is turned quickly.
Other signs include problems with visual tracking, where the eyes struggle to follow a line of text smoothly. Parents who observe persistent signs of this reflex past six months should consult a pediatrician or developmental specialist.