These are my notes from reading Lise Elliot’s What’s Going On in There?.
“Why Babies Love to Be Bounced: The Precocious Sense of Balance and Motion”.
This chapter talks about the vestibular sense, i.e. the sense of balance and movement.
A few interesting facts I learned:
Already at 10 weeks, a fetus reacts to movement.
Vestibular stimulation (chair spinning) can help the development of babies’ motor skills and reflexes
Children love motion from the moment of birth: rocking, jiggling, bouncing etc, and later on being spun, swung or flipped upside down. This is because they have a highly developed vestibular system – the sense for balance and the body’s movement.
The vestibular system is essential for maintaining head and body posture and for accurately moving most parts of the body, especially the eyes. It is what allows us to go joggin without feeling the world bounce up and down: the vestibular system compensates for the vertical movement of the body by moving the eyes up and down, so the picture stays constant.
The vestibular system is part of the inner ear. It is located in a hollow opening in the skull (the vestibule). There are two kinds of vestibular organs. The three semicircular canals detect head turns: each one is oriented along a different plane so they can detect rotation in all directions. The otolith organs are of two kinds. One detects linear movements (side to side or up and down) and the other detects movement with respect to gravity, such as tilting the head or lying down.
These organs all work fundamentally similarly. They have thousands of hair cells, covered with microscopic hairs or cilia. In the semicircular canals the hairs sit in a fluid, while in the otolith organs they are surrounded by a gel that contains tiny crystals. Either way, when the body moves, the cilia bend and send off a signal to the vestibular nerve. In the brain stem this signal is routed to the eyes (so that they can compensate for the motion), to motor neurons (to control the body) and to the cerebellum (where it is integrated with vision and touch signals to get an even more coordinated sense of balance).
Most of this happens below the level of consciousness, but some vestibular fibers go from the brain stem to the cerebral cortex as well.
Development of the vestibular system
The vestibular sense is very old and it therefore emerges early on in the embryo’s development. By seven weeks of gestation the three semicircular canals have formed, and between seven and fourteen weeks all the hair cells are formed. These immediately start attracting neurons, which also start growing in the opposite direction, towards the brain stem. The vestibular nerve is the first one in the entire brain to start myelinating. By five months the vestibular apparatus has reached its full size and shape, vestibular nerves have started myelinating and the entire system is quite mature. But other parts of the vestibular system do not finish myelinating until puberty.
Because these organs develop so rapidly and so early on, they are very vulnerable during gestation. An entire class of antibiotics (including streptomycin) is known to damage hair cells in both the vestibular system and the hearing part of the inner ear: early exposure can cause deafness as well as permanent damage to the balance sense. Any prenatal influence that can cause deafness will also threaten the vestibular sense, because the organs are structurally very similar. Such threats include the already-mentioned antibiotics, maternal infections (rubella and cytomegalovirus), hereditary factors and hypothyroidism.
By the 12th week of pregnancy the fetus will reflexively move its eyes when the position of its head changes. In the 8th month, sudden changes in position will activate the baby’s Moro reflex – flinging outward with arms and legs. The vestibular system also allows the baby to turn head-down in preparation for birth. Babies with defects in their vestibular system are much more likely to be born breech.
The vestibular function is central for several of the reflexes that are tested when assessing newborns’ neurological health. One is the asummetrical neck response. When a baby’s head is turned to the right (activating the vestibular sense) he extends his right arm and leg, and flexes the left arm and leg. If the baby was standing, this reflex would help maintain balance.
Another reflex is the traction response: when a baby is pulled from lying to sitting, he tries to hold his head up (even though he won’t succeed). The vestibular system senses that the head is moving forward, and tries to lift it to a vertical position, even though the neck muscles are not strong enough. There is also the doll’s eye reflex: when you turn the baby’s head to one side, the eyes will remain facing you, because the vestibular system helps stabilize the gaze.
A third reflex is also related to vision – the nystagmus response. Spin the baby in circles (on a chair, for example). When she stops, her eyes will move back and forth – fast in one direction and slowly in the other – because they continue to compensate for the motion even after it has stopped. This eye movement is faster in babies than in older children or adults. The vestibular system as a whole is over-responsive in babies, reaching a peak between six to twelve months and then declining. This is one of the reasons why toddlers are so wobbly. The vestibular system also needs time to mature: its contribution to maintaining balance keeps improving at least until seven years of age, possibly until puberty.
Vestibular development and the rest of the brain
The vestibular sense is strongly linked to general mental development, and deficiencies in this sense are frequently found among children with emotional problems, attention deficit, language disorders and autism. Mental development is cumulative, and since the vestibular sense is one of the earliest ones to develop, it provides a large share of early sensory inputs. These probably play a large role in the development of other motor and sensory abilities, which in turn play a role in the development of higher cognitive functions.
The opposite also seems to be true: there is evidence that vestibular stimulation can improve the baby’s brain. In one study, babies of various ages (ranging from 3 to 13 months) were exposed to chair spinning while sitting on a researcher’s lap, four times a week for four weeks. The babies loved this, and they also showed more advanced motor skills and reflexes than control groups who didn’t get any chair spinning.
Vestibular stimulation is also good for very young babies. It helps calm them when they cry: rocking, carrying and jiggling is more effective than simply holding the baby. In fact rocking without physical contact (e.g. in a baby seat) was more effective than physical contact without rocking.
Premature babies also benefit from vestibular stimulation: they gain weight faster, are less irritable, breathe better, and sleep better.