A specific brain region lights up within a split second after seeing an infant face, a finding researchers say may help explain why we instinctively treat babies as special, protecting them and thus helping them to survive.
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Darwin originally pointed out that there is something about infants which prompts adults to respond to and care for them. This allows our species to survive.
Then, Nobel-Prize-winning zoologist Konrad Lorenz proposed that it is the specific structure of the infant face, including a relatively large head and forehead, large and low lying eyes and bulging cheek region, that serves to elicit these parental responses.
But the biological basis for this has remained elusive.
Signs in the brain
Now, researchers have reported a possible brain basis for this parental instinct.
The authors showed that a region of the human brain called the medial orbitofrontal cortex is highly specifically active within a seventh of a second in response to (unfamiliar) infant faces but not to adult faces.
This finding has potentially important clinical application in relation to postnatal depression, which occurs in approximately 13% of mothers after birth and often within six weeks. The present findings could eventually provide opportunities for early identification of families at risk.
How the study was conducted
The research team used a neuroimaging method called magnetoencephalography (MEG).
Because the researchers were primarily interested in the highly automatised processing of faces, they used an implicit task that required participants to monitor the colour of a small red cross and to press a button as soon as the colour changed. This was interspersed by adult and infant faces that were shown for 300 ms, but which were not important to solve the task.
The authors found a key difference in the early brain activity of normal adults when they viewed infant faces compared to adult faces.
In addition to the well documented brain activity in the visual areas of the brain in response to faces, early activity was found in the medial orbitofrontal cortex to infant faces but not adult faces.
An instinctive response
This wave of activity starts around a seventh of a second after presentation of an infant face. These responses are almost certainly too fast to be consciously controlled and may therefore be instinctive.
The medial orbitofrontal cortex is located in the front of the brain, just over the eyeballs. It is a key region of the emotional brain and appears to be related to the ongoing monitoring of salient reward-related stimuli in the environment.
In the context of the experiment, the medial orbitofrontal cortex may provide the necessary emotional tagging of infant faces that predisposes us to treat infant faces as special and plays a key role in establishing a parental bond.
Link with depression
Also, there is now evidence from deep brain stimulation linking depression to the nearby subgenual cingulate cortex which is strongly connected with the medial orbitofrontal cortex. This lends support to the possibility that changes to activity in the medial orbitofrontal cortex secondary to depression may adversely affect parental responsivity.
Postnatal depression is common and there are some experimental evidence suggesting that mothers with postnatal depression have difficulties in responding to infant cues.
Further research could identify whether the present finding of early and specific medial orbitofrontal responses to infant faces are affected and even suppressed by depression, thereby helping to explain this lack of maternal responsiveness. The present paradigm could eventually provide opportunities for early identification of families at risk.
The study was published in the journal PLoS ONE on February 27. – (EurekAlert)
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