What is the primary reaction of an infant when its foot is stroked?

The primary reaction of an infant when its foot is stroked is to fan out the toes, a response known as the Babinski reflex. This reflex is a normal response seen in infants, typically observed from birth until about 2 years of age. When the lateral aspect of the sole is stroked, the toes will spread apart and the big toe may extend upward. This reaction is an important indicator of neurological function and indicates the presence of healthy spinal cord and brain activity in infants.

As infants grow and their neurological development progresses, this reflex is replaced by the adult (plantar) response, which is curling of the toes when the same area is stimulated. The presence of the Babinski reflex in infants is a natural part of their development, revealing the immature state of their nervous system and its developmental trajectory. This response is generally absent in healthy adults, where the neurological pathways have matured to a different response pattern.