The study summarized below was conducted on rats, but parent-child bonding at the physiological level is pretty much the same in all mammals - so this does translate well to humans.

The study found that the presence and nurturing behaviors of the mother (or father, or primary care-giver) toward the newborn directly shapes the wiring and function of the infant's brain. This is the first study to show this process (which is well-known) WHILE it is happening.

Pretty cool. The paper itself, of course, is behind a paywall, so below is the summary from Science Daily, followed by the abstract of the original article.

Measuring nurture: Study shows how 'good mothering' hardwires infant brain

Date: July 17, 2014
Source: NYU Langone Medical Center
 
Summary:
By carefully watching nearly a hundred hours of video showing mother rats protecting, warming, and feeding their young pups, and then matching up what they saw to real-time electrical readings from the pups’ brains, researchers have found that the mother’s presence and social interactions — her nurturing role — directly molds the early neural activity and growth of her offsprings’ brain.

Mother rat carrying her baby in her mouth, 5 days old (stock image). Researchers at NYU Langone Medical Center have found that the mother's presence and nurturing directly molds the early neural activity and growth of her offsprings' brain.

By carefully watching nearly a hundred hours of video showing mother rats protecting, warming, and feeding their young pups, and then matching up what they saw to real-time electrical readings from the pups' brains, researchers at NYU Langone Medical Center have found that the mother's presence and social interactions -- her nurturing role -- directly molds the early neural activity and growth of her offsprings' brain.

Reporting in the July 21 edition of the journal Current Biology, the NYU Langone team showed that the mother's presence in the nest regulated and controlled electrical signaling in the infant pup's brain.

Although scientists have known for decades that maternal-infant bonding affects neural development, the NYU Langone team's latest findings are believed to be the first to show -- as it is happening -- how such natural, early maternal attachment behaviors, including nesting, nursing, and grooming of pups, impact key stages in postnatal brain development.

Researchers say the so-called slow-wave, neural signaling patterns seen during the initial phases of mammalian brain development -- between age 12 and 20 days in rats -- closely resembled the electrical patterns seen in humans for meditation and conscious and unconscious sleep-wake cycles, and during highly focused attention. These early stages are when permanent neural communication pathways are known to form in the infant brain, and when increasing numbers of nerve axons become sheathed, or myelinated, to speed neural signaling.

According to senior study investigator and neurobiologist Regina Sullivan, PhD, whose previous research in animals showed how maternal interactions influenced gene activity in the infant brain, the latest study offers an even more profound perspective on maternal caregiving.

"Our research shows how in mammals the mother's sensory stimulation helps sculpt and mold the infant's growing brain and helps define the role played by 'nurturing' in healthy brain development, and offers overall greater insight into what constitutes good mothering," says Sullivan, a professor at the NYU School of Medicine and its affiliated Nathan S. Kline Institute for Psychiatric Research. "The study also helps explain how differences in the way mothers nurture their young could account, in part, for the wide variation in infant behavior among animals, including people, with similar backgrounds, or in uniform, tightly knit cultures."

"There are so many factors that go into rearing children," says lead study investigator Emma Sarro, PhD, a postdoctoral research fellow at NYU Langone. "Our findings will help scientists and clinicians better understand the whole-brain implications of quality interactions and bonding between mothers and infants so closely after birth, and how these biological attachment behaviors frame the brain's hard wiring."

For the study, a half-dozen rat mothers and their litters, of usually a dozen pups, were watched and videotaped from infancy for preset times during the day as they naturally developed. One pup from each litter was outfitted with a miniature wireless transmitter, invisibly placed under the skin and next to the brain to record its electrical patterns.

Specifically, study results showed that when rat mothers left their pups alone in the nest, infant cortical brain electrical activity, measured as local field potentials, jumped 50 percent to 100 percent, and brain wave patterns became more erratic, or desynchronous. Researchers point out that such periodic desynchronization is key to healthy brain growth and communication across different brain regions.

During nursing, infant rat pups calmed down after attaching themselves to their mother's nipple. Brain activity also slowed and became more synchronous, with clearly identifiable electrical patterns.

Slow-wave infant brain activity increased by 30 percent, while readings of higher brain-wave frequencies decreased by 30 percent. Milk delivery led to intermittent bursts of electrical brain activity that were double or five times higher than before.

Similar spikes in rat brain activity of more than 100 percent were observed when mothers naturally groomed their infant pups.

However, these brain surges progressively declined during weaning, as infant pups gained independence from their mothers, leaving the nest and seeking food on their own as they grew past two weeks of age.

Additional experiments with a neural-signaling blocking agent, propranolol, confirmed that maternal effects were controlled in part by secretion of norepinephrine, a key neurotransmitter and hormone involved in most basic brain and body functions, including regulation of heart rate and cognition. Noradrenergic blocking in infant rats mostly dampened all previously observed effects induced by their mothers.

Sullivan says her team next plans similar experiments to look at how behavioral variations by the mother affect infant rat brain development, with the added goal of mapping any differences in brain development.

Long term, they say, they hope to develop diagnostic tools and therapies for people whose brains may have been impaired or simply underdeveloped during infancy.

Sarro says more research is also under way to investigate what other, nonadrenergic biological mechanisms might also be involved in controlling maternal sensory stimulation of the infant brain.

Story Source:
The above story is based on materials provided by NYU Langone Medical Center. Note: Materials may be edited for content and length.

Journal Reference:
Sarro, EC, Wilson, DA, Sullivan, RM. (2014, Jul 3). Maternal Regulation of Infant Brain State. Current Biology; Epub before print. DOI: 10.1016/j.cub.2014.06.017

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Maternal Regulation of Infant Brain State

Emma C. Sarro, Donald A. Wilson, Regina M. Sullivan
DOI: http://dx.doi.org/10.1016/j.cub.2014.06.017
Publication stage: In Press Corrected Proof

Highlights

• The mother’s presence reduces infant rat cortical desynchronization
• Maternal behaviors (e.g., milk ejection and grooming) increase desynchronization
• Maternal effects on infant cortical activity decline with age
• Norepinephrine receptor blockade reduces impact of dam on infant cortical activity

Summary

Patterns of neural activity are critical for sculpting the immature brain, and disrupting this activity is believed to underlie neurodevelopmental disorders [ 1–3 ]. Neural circuits undergo extensive activity-dependent postnatal structural and functional changes [ 4–6 ]. The different forms of neural plasticity [ 7–9 ] underlying these changes have been linked to specific patterns of spatiotemporal activity. Since maternal behavior is the mammalian infant’s major source of sensory-driven environmental stimulation and the quality of this care can dramatically affect neurobehavioral development [ 10 ], we explored, for the first time, whether infant cortical activity is influenced directly by interactions with the mother within the natural nest environment. We recorded spontaneous neocortical local field potentials in freely behaving infant rats during natural interactions with their mother on postnatal days ∼12–19. We showed that maternal absence from the nest increased cortical desynchrony. Further isolating the pup by removing littermates induced further desynchronization. The mother’s return to the nest reduced this desynchrony, and nipple attachment induced a further reduction but increased slow-wave activity. However, maternal simulation of pups (e.g., grooming and milk ejection) consistently produced rapid, transient cortical desynchrony. The magnitude of these maternal effects decreased with age. Finally, systemic blockade of noradrenergic beta receptors led to reduced maternal regulation of infant cortical activity. Our results demonstrate that during early development, mother-infant interactions can immediately affect infant brain activity, in part via a noradrenergic mechanism, suggesting a powerful influence of the maternal behavior and presence on circuit development.