The principal finding was a relatively brief surge in activity in the medial orbitofrontal cortex at around 130 ms in response to infant faces but not in response to adult faces. In contrast, in the early visual regions including the FFA, face processing of infant and adult faces followed a similar pattern. Thus, the medial orbitofrontal cortex appears to exhibit a very early specific neural signature or specific pattern of activity in response to infant faces. This signature is likely to be directly related to saliency of the structural features of the infant face rather than to other factors, since the infant and adult faces were carefully matched in terms of emotional valence and arousal, and attractiveness (see Methods
). Supporting this, the signature, which is well-characterised both temporally and spatially, is not found in the right fusiform face area and other early visual areas, where both infant and adult faces instead elicit very similar neural responses.
The early specific surge of activity in the medial orbitofrontal cortex to infant faces at around 130 ms in the 10–15 Hz band ( and ) was then followed by an enhanced response at around 165 ms in the FFA in the 20–25 Hz band ( and ). This suggests that the medial orbitofrontal cortex provides a top-down amplification of the activity in the FFA specifically related to infant faces.
This finding extends previous research which has shown that early orbitofrontal cortex activity facilitates visual recognition of masked
line drawings of everyday objects 
. In this task, Bar et al. tested the subjective certainty of visual object recognition of very briefly presented line drawings (63 ms) which were preceded and followed by visual masks. During these brief presentations the authors found early activity in the orbitofrontal cortex, which was not, however, evident when the same line drawings were presented non-masked for a longer stimulus duration (198 ms). The effect would thus seem to be task specific in that it was only present on those trials where participants were instructed to indicate their level of knowledge about the identity of the object. The early activity in medial orbitofrontal cortex is therefore likely to be closely linked to the salience or attentional processing of the masked stimuli. In the context of the present experiment, where we have demonstrated that early activity in the medial orbitofrontal cortex is linked to the presentation of salient non-masked
infant faces but not to adult faces, these findings would suggest that the structural configuration of infant faces might act as a heightened attentional/emotional biasing mechanism, consistent with the recent behavioural findings using infant faces in a dot probe attentional paradigm 
The medial orbitofrontal cortex may thus provide the necessary attentional–and perhaps emotional–tagging of infant faces that predisposes humans to treat infant faces as special and elicits caring, as suggested by Darwin 
and Lorenz 
Since the infant and adult faces used in the present study were carefully matched by independent panels of participants for emotional valence and arousal, and attractiveness (see Methods
), the present findings provide evidence that it is the distinct features of the infant faces compared to adult faces which are important, rather than evaluative subjective processing such as attractiveness or emotional valence.
A number of functional MRI studies have found a correlation between facial attractiveness and changes in the BOLD signal in the medial orbitofrontal cortex 
. This was interpreted as indicating that the medial orbitofrontal cortex has a specific role in attractiveness. However, there are also several fMRI studies, which have found that the BOLD signal in the medial orbitofrontal cortex is correlated with the subjective ratings of many different stimuli in different modalities such as olfaction 
, gustation 
, somatosensory 
and multimodal 
. It has therefore been argued that these findings indicate a role for the medial orbitofrontal cortex in the monitoring, learning and memory of salient reward-related stimuli in the environment–rather than attractiveness per se 
Furthermore, all these fMRI findings are concerned with ongoing evaluative changes in activity in the medial orbitofrontal cortex which occur both later in time and over longer periods, compared to the early transient burst of activity to infant faces at around 130 msec found with MEG in the present study. The present findings are therefore potentially of interest in that they suggest a temporally earlier role than previously thought for the medial orbitofrontal cortex in guiding affective reactions, which may be even non-conscious. This processing could be an important foundation for subsequent integrative and evaluative subjective processing.
The suggested monitoring process of the medial orbitofrontal cortex is consistent with intriguing findings in spontaneously confabulating patients with lesions to the medial orbitofrontal cortex 
. The evidence from human neuroimaging and neuropsychology studies suggests that there are medial-lateral and posterior-anterior distinctions within the human orbitofrontal cortex 
. This meta-analysis of the existing neuroimaging data showed that activity in the medial orbitofrontal cortex is related to the monitoring, learning and memory of the reward value of reinforcers, whereas lateral orbitofrontal cortex activity is related to the evaluation of punishers, which can lead to a change in ongoing behaviour.
Notably, the results did not differ when restricted to the non-parent portion of the sample. However, further investigation is required into a number of areas including the relevance of parental experience, gender, and valence of emotional expression (whether brain responses differ between positive and negative emotions) and whether there are specific brain responses to individual infant features such as large eyes. In addition, given the evidence from various behavioral studies showing that many cute infantile things appear to invoke Lorenz's “innate releasing mechanisms” 
. It might also be of interest to future studies to investigate the brain responses to infants of other species.
There is a potentially important clinical application of the present findings in relation to postnatal depression. Postnatal depression is common, occurring in approximately 13% of mothers after birth 
and often within six weeks 
. Postnatal depression has been associated with a range of adverse child outcomes including attachment, behavioural and emotional disturbances and there is also some evidence for poorer cognitive outcomes. There is increasing evidence that certain features of the behaviour of depressed mothers are associated with adverse outcome, in particular their lack of responsiveness to the infant, the reduced ability to perceive their infant's signals and less mimetic behavior 
with a resultant lack of contingency between the infants actions and the mothers responses 
. Furthermore, it has been shown experimentally that infants respond adversely with distress, crying, increased arousal and then avoidance to an unresponsive maternal face (the still face paradigm) 
. 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. Further research could identify whether these early and specific medial orbitofrontal responses to infant faces (own and others) 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 
To conclude, we found a very specific, rapid neural signature of activity in the medial orbitofrontal cortex in response to infant faces. This provides evidence in humans of a potential brain basis for the “innate releasing mechanisms” described by Lorenz for affection and nurturing of young infants. Although the degree to which these responses are innate rather than learned is unknown; at the very least, the specific responses to unfamiliar infant faces in the medial orbitofrontal cortex occur so quickly that they are almost certainly quicker than anything under conscious control.