In new studies, scientists find that the maternal instinct is as much biological as it is social and that early socialization through maternal bonding is critical to offsprings' later adjustment.
Among new findings:
* Motherhood helps learning and memory, which in turn helps mothers better care for their offspring.
* Mothers respond better to cries of their own infants than do fathers.
* The earlier the stress caused by maternal separation, the greater the offspring's later social difficulties.
* Nurturing through touch can lessen some of the negative effects of early stress.
“Understanding the mechanisms at work in parenting and the effects of disruptions to the bonds created between parents and children is vitally important,” says Bruce McEwen, PhD, of Rockefeller University. “It directly influences all individuals, and has an effect on society – interrupted parental bonding can cause behavioral problems in school and the workplace, affecting almost everyone.”
In work with rats, Craig Kinsley, PhD, and his colleagues in the departments of psychology at the University of Richmond and Randolph Macon College, found that the ability of mothers' brains to change in response to the new needs of motherhood enables them to respond with a richer and more enhanced behavioral repertoire.
A first-time mother must engage in many behaviors previously unfamiliar to her. In rats, these behaviors include retrieving offspring who try to wander off, protecting young from predators, and leaving those young to forage for food. Brain changes to accommodate these new behaviors might include enhanced spatial ability to better navigate the environment and reduced fear and anxiety responses to leaving offspring or confronting predators and prey in their environment, Kinsley says.
Kinsley and his colleagues examined predatory behaviors in age-matched female rats kept in a large open enclosure. One group of rats had never borne offspring. The other group was lactating. The rats were deprived of food for a period of time, then given crickets. The rats were required to track, attack, and kill crickets during daily 5-minute observations. In animals, a heightened ability to capture prey means a decreased amount of time the mother spends away from vulnerable offspring. This decreased window of vulnerability means a lower infant mortality rate.
The rats that had never borne offspring took about 290 seconds to catch their prey, whereas the lactating group took only about 70 seconds. In additional experiments, the investigators ruled out changes in auditory ability and proximity to prey as possible explanations for the difference. Further experiments are underway to rule out changes in olfactory ability and to explore the changes in the structure and activity of the hypothalamus and hippocampus as a result of motherhood.
“The mammalian female brain expresses a great deal of plasticity and creativity in service to, and in support of, reproduction,” says Kinsley. “In other words, mothers are made, not born.”
At least some parenting behavior is unique to females, new work finds. Jeffrey Lorberbaum, MD, and his colleagues at the Medical University of South Carolina found that mothers show much greater activity than fathers in widespread brain regions when listening to their own infant cries compared to those of other infants.
Previous studies by Lorberbaum and colleagues had identified brain regions important in maternal behavior and motivation when listening to standard infant cries. The basal forebrain, midbrain, striatum, anterior cingulate, and the prefrontal cortex all constitute a system Lorberbaum's group calls the “maternal circuitry.” These regions are remarkably similar to those found to be involved in rodent behavior studies as well as human studies of addictive behaviors.
The group used functional magnetic resonance imaging to study brain activity in parents. Forty mothers and 10 fathers were exposed to the cries of their own infant and to the cries of an unrelated infant. The mothers experienced more brain activation in response to infants' crying, not just in the maternal circuit, but in other areas of the brain as well. They also experienced more activation in response to their own children than to the unrelated child.
In contrast to the widespread activation in mothers in response to their own infant cries compared to standard infant cries, fathers showed increased activation only in the posterior neocortical and cerebellar regions, which are thought to be more involved in thinking, distinguishing between sensations, and motor planning. Unlike mothers, they did not show increased activation in the limbic and basal forebrain regions, which are important in emotional responses, human addiction studies, and animal studies of maternal behavior. The fathers' brains also appeared to activate in the same way and to the same degree to cries of their own offspring and to cries of an infant unrelated to them.
“Conventional wisdom has long suggested that mothers are more attuned to infants, especially their own, than are fathers,” says Lorberbaum. “Our studies suggest that this may be true. Mothers may be very attuned to their own infant as they activate widespread brain regions including ancient regions believed to be important in rodent maternal behavior. Fathering behavior may be less hardwired and a more recent evolutionary phenomenon as fathers only activate newer regions of the brain involved in sensory discrimination, cognition, and motor planning in response to cries.”
In new work, Judy Cameron, PhD, of the University of Pittsburgh and the Oregon National Primate Research Center at Oregon Health and Science University found that early stress in monkeys influences socialization throughout life and that the timing of stress exposure significantly influences the behavioral effects later in life.
Earlier studies showed that significant stress in very young primates—both in monkeys and humans—caused difficulties in the development of appropriate social skills and behavior that manifested in many ways, including increased anxiety, depression, “acting out,” and substance abuse.
In the current work, young monkeys were exposed to stress by having their birth mothers removed at various ages. The monkeys were then raised in social groups of monkeys varying in gender and age. One group had their mothers removed during their first week of life, before social skills are thought to have developed. The second group had their mothers removed when they were one month of age, during a time when social skills are developing. The third group was removed from their mothers at six months. This group was considered the control group, because six months is the approximate age at which monkeys in the wild start to become independent of their mothers.
Cameron and her team observed the three groups into adulthood and found clear behavioral differences depending on the age at the time of separation. “Monkeys separated at one week were less social when they were young, a characteristic that continued into adulthood,” Cameron says. The monkeys tended to exhibit a higher than normal amount of self-comforting behaviors, such as thumb or toe sucking. In addition, these monkeys tended to have less social dominance in the group setting, perhaps in relation to their reduced social skills.
The group of monkeys separated from their mothers at one month sought more social interaction and were more socially dominant than the group separated at one week. This behavior continued into adulthood.
Later in the study, Cameron and her team exposed the monkeys when they were adults to a new stress by placing them in new social groups with previously unknown monkeys. The monkeys who had been separated from their mothers at one week appeared the most distressed by the change. They were more agitated and aggressive than the other groups. The monkeys separated from their birth mothers at one month of age experienced changes in eating and play behaviors, such as eating more and playing less.
In another separate, but related study, Cameron and her colleagues then examined whether the behavioral consequences of early social bond disruption and stress could be reversed by substituting an alternative social bond, in this case surrogate mothers.
Infant monkeys were separated from their mothers at 1 week of age, and again reared in social groups with three or four other monkeys. When the infants were 25-75 days old, an experienced mother monkey was introduced, either during the second or third month after birth. Meanwhile, a fourth control group of young monkeys were separated from their mothers at 6 months old. Both groups were observed for a year.
Cameron's group again found that the monkeys separated from their mothers at one week tended to interact less with the other monkeys in their group than did the control group. When a surrogate mother was introduced after the first month, however, the normal pattern of development was quickly restored. When a mother was introduced in the second month, the same result was achieved, but at a slower pace. However, young monkeys who were given an adoptive mother in the third month continued with their atypical, solitary behavior patterns, showing a profound decrease in social interactions with other monkeys in the social group.
“This finding suggests that children separated from their mothers at birth should be placed in the care of an alternative mother as soon as possible,” Cameron says. “This differs greatly from the policies in place in most states where a child removed from their family (such as a foster child) is cared for, but not paired with a permanent family—a period that can last months—until a final decision is made as to whether they can be returned to their birth family.”
In other work, Martha Welch, MD, of Columbia University 's division of neuroscience, found that reintroducing nurturing—specifically touch—to children with behavioral problems reduced their severe behavioral symptoms.
A test group of 102 children, ages 5-18, with severe behavioral symptoms, such as defiance, aggression, cruelty, impulsivity, poor motivation, withdrawal, and inability to receive comfort or to be reciprocal, were treated with their families in two consecutive eight-hour days of facilitated Prolonged Parent-Child Embrace (PPCE) therapy, with family PPCE continuing at home.
“PPCE therapy leads to a positive physiological change in the child when parent and child resolve conflicts and reach a state of synchronous attunement,” Welch says. “In PPCE, the child learns to give and receive comfort and to modulate his or her own symptomatic behavior.”
B ehavioral problems of children in the test group were reduced over a one-year period by more than 50 percent. Welch has been using the therapy since the 1970s.
Welch and her colleagues theorize that the behavioral improvements seen with PPCE therapy result from the stimulation of brain–body pathways that control and condition stress responses. “I believe that basic neurobiological mechanisms associated with early maternal-infant interaction play a critical role in a wide range of developmental disorders,” Welch says. The mechanism includes at least two peptides associated with maternal nurturing, secretin and oxytocin, which Welch is currently investigating. These peptides work in the brain and body to influence behavior and stress hormone output.
Welch's laboratory hopes to develop treatments that mimic the actions and interactions of these neuropeptides on the body and brain to reverse behavioral and developmental disorders. This work challenges a widespread assumption that severe behavioral disorders, especially among adolescents, are chronic and incurable.
“I believe that these disorders can be improved or even reversed by the body's natural peptide systems, through family therapy that restores effective maternal nurturing and/or through combined peptide therapy that enhances the mechanisms of maternal nurturing,” Welch says.
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