Parental experience, as well as changing hormone levels during pregnancy and postpartum, cause changes in the parental brain. Displaying maternal sensitivity towards infant cues, processing those cues and being motivated to engage socially with her infant and attend to the infant's needs in any context could be described as mothering behavior and is regulated by many systems in the maternal brain. Research has shown that hormones such as oxytocin, prolactin, estradiol and progesterone are essential for the onset and the maintenance of maternal behavior in rats, and other mammals as well. Mothering behavior has also been classified within the basic drives (sexual desire, hunger and thirst, fear, power/dominance etc.). Less is known about the paternal brain, but changes in the father's brain occur alongside the mother once the offspring is born.
Maternal hormonal effect
Different hormone levels in the maternal brain and the overall well being of the mother account for 40%–50% of differences in the mother's attachment to her infant. Mothers experience a decrease in estrogen and an increase in oxytocin and prolactin caused by lactation, pregnancy, parturition and interaction with the infant.
The levels of oxytocin in the maternal brain correlate with maternal behaviors such as gazing, vocalization, positive affect, affectionate touch and other similar mother-infant relationship behaviors.
Estradiol and progesterone
In the first few days after giving birth the levels of cortisol are high which correlates with maternal approach behavior and positive maternal attitudes. Mothers with high levels of cortisol were also found to be more vocal towards their children. Mothers who experienced adversity in their own childhood, had higher daily patterns of cortisol levels, and were less maternally sensitive.
Different areas/structures of the brain are associated with different factors which contribute to maternal behavior. One's own infant acts as a special stimulus which triggers activation of different areas of the brain. These brain areas together allow for maternal behavior and related systems.
The Medial Preoptic Area (MPOA) of the hypothalamus contains receptors for estradiol, progesterone, prolactin, oxytocin, vasopressin and opioids. All these hormones are involved in some way in activating maternal behavior in the brain. The following are other behavioral changes necessary for mothering that the MPOA is responsible for:
- mother's affect (changes made by the amygdala, prefrontal cortex)
- stimulus salience (changes made by the amygdala and nucleus accumbens)
- attention (changes made by the nucleus accumbens and medial prefrontal cortex)
- memory (changes made by the nucleus accumbens and medial prefrontal cortex).
The amygdala and medial prefrontal cortex also contain receptors for the hormones which are most likely to be changing behavior at the time of pregnancy, and may be the sites where these changes occur. Increased activity has also been observed in the amygdala as the mother is responding to emotions seen in negative (fearful) faces, positive faces or familiar faces that her baby makes. Primate mothers with damage to the prefrontal cortex have also been associated with disrupted maternal behavior.
The dorsolateral prefrontal cortex (DLPFC) plays a role in the attention, cognitive flexibility and working memory of the mother. It helps the mother identify infant cues. In any environment and efficiently, it allows for the decision-making and action planning process involved in attending to the infant's cues.
In animal mothers
Postpartum, new neuron production is suppressed due to decreased levels of estrogen and increased levels of glucocorticoids mother rats. Mother-infant interaction is also thought to suppress neurogenesis in the hippocampus postpartum in the rat maternal brain. Maternal experience increases neurogenesis in the subventricular zone (SBZ) which is responsible for producing the neurons of the olfactory bulb. Prolactin is the hormone which mediates the increase in neurogenesis in SBZ.
The volume of gray matter increases postpartum in the following brain regions:
These changes in the brain may occur in order to promote appropriate mothering behavior. The mother's positive attitude towards the infant can be used as a predictor for the increase in gray matter in the above stated brain structures.
Also in rats, the increased interaction with pups causes an increase in density in the MPOA. Postpartum increase in gray matter volumes may help the mother activate the motivation to perform maternal behavior in response to cue from their offspring.
In human mothers
Recent research has begun to look at how maternal psychopathology affects the maternal brain in relation to parenting. Daniel Schechter and colleagues have studied specifically interpersonal violence-related posttraumatic stress disorder (PTSD) and comorbid dissociation as associated with specific patterns of maternal neural activation in response to viewing silent video-stimuli of stressful parent-toddler interactions such as separation versus less-stressful ones such as play. Importantly, less medial prefrontal cortex activity and greater limbic system activity (i.e. entorhinal cortex and hippocampus) were found among these post-traumatically stressed mothers of toddlers compared to mothers of toddlers without PTSD in response to stressful parent-child interactions as well as, within a different sample, in response to menacing adult male-female interactions. In the latter study, this pattern of corticolimbic dysregulation was linked to less observed maternal sensitivity during mother-child play. Decreased ventral-medial prefrontal cortex activity in violence-exposed mothers, in response to viewing their own and unfamiliar toddlers in video-clips of separation versus play, has also been associated with increased PTSD symptoms, parenting stress and decreased methylation of the glucocorticoid receptor gene.
Early experiences and shaping
Women who had a positive experience involving their family in their childhood are more likely to be more maternally sensitive and provide that same experience for their own children. Mothers that had negative experiences involving their families undergo neurobiological changes which lead to high stress reactivity and insecure attachment. This causes lower maternal responsiveness to their infant's needs.
Rat mothers provide high levels of maternal care (licking and grooming) to their offspring if they themselves received high maternal care as a pup from their own mothers. Rat mothers who received low levels of maternal care as pups have lower levels of expression of the glucocorticoid receptor gene and lower synaptic density in the hippocampus. In human mothers, lower hippocampal volume has been associated with a lower ability to regulate emotions and stress, which can be linked with decreased maternal sensitivity as a mother. Mothers with insecure attachments to their own mothers display higher amygdala sensitivity to negative emotional stimuli, like hearing their infant cry. Having more difficulty dealing with stress makes mothers less responsive to their infant's cues.
Larger gray matter and increased activations of the following brain areas occur in mothers who had experienced higher quality maternal care as infants:
This allows the mother to be more sensitive to her own infant's needs.
In only 6% of mammalian species, including humans, the father plays a significant role in caring for his young. Similar to the changes that occur in the maternal brain, the same areas of the brain (amygdala, hypothalamus, prefrontal cortex, olfactory bulb etc.) are activated in the father, and hormonal changes occur in the paternal brain to ensure display of parenting behavior.
Paternal hormonal effect
An increase in levels of oxytocin, glucocorticoids, estrogen and prolactin occur in the paternal brain. These hormonal changes occur through the father's interaction with the mother and his offspring. Oxytocin levels are positively correlated with the amount of affection the father displays towards the child. In humans, and in other primate species, lower levels of testosterone have been linked to the display of paternal behavior.
In animal fathers
In father rats, just as in the mother rats, a decrease in neurogenesis in the hippocampus occurs postpartum. Just like in mothers, fathers also have increased levels of glucocorticoids which are thought to suppress the production of new cells in the brain.
Marmoset fathers have enhanced dendritic spine density in the prefrontal cortex. This increase correlates with increase in vasopressin receptors in this area of the paternal brain. With age, this effect is reversed, and is therefore believed to be driven by father-infant interactions.
In human fathers
Being exposed to crying babies activates the prefrontal cortex and the amygdala in both fathers and mothers, but not in non-parents. The level of testosterone in the paternal brain correlates with the effectiveness of the father's response to the baby's cry. Increased levels of prolactin in the paternal brain has also been correlated with a more positive response to the infant's cry.
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