Jul 08 2008
I have two daughters, about to turn nine and six. They are, in my completely subjective and biased assessment, the most adorable things in the universe. They evoke in me a powerful and complex set of emotions – an experience that every parent understands and no non-parent can truly appreciate.
Despite concerns about the testability of evolutionary psychological explanations, it seems obvious that such a reaction in a parent would be favored by natural selection, as would be any features in a child that provoked such a response from their parents. This in turn suggests that much of the response of a parent to their child is hard-wired in the brain and genetically determined. This doesn’t rule out cultural and learned influences, it merely suggest that a strong parenting tendency will be coded in the genes.
A recent bit of research supporting this notion was published in the latest issue of Pediatrics: What’s in a smile? Maternal brain responses to infant facial cues. The study uses fMRI, which images blood flow to the brain from which the relative activity of various brain regions can be inferred, to measure the reaction of mothers to various pictures – their child happy, neutral, and sad and another child happy, neutral, and sad. The results:
Key dopamine-associated reward-processing regions of the brain were activated when mothers viewed their own infant’s face compared with an unknown infant’s face. These included the ventral tegmental area/substantia nigra regions, the striatum, and frontal lobe regions involved in (1) emotion processing (medial prefrontal, anterior cingulate, and insula cortex), (2) cognition (dorsolateral prefrontal cortex), and (3) motor/behavioral outputs (primary motor area). Happy, but not neutral or sad own-infant faces, activated nigrostriatal brain regions interconnected by dopaminergic neurons, including the substantia nigra and dorsal putamen. A region-of-interest analysis revealed that activation in these regions was related to positive infant affect (happy > neutral > sad) for each own-unknown infant-face contrast.
In order to interpret what this study tells us I must first back up and discuss some basic principles.
I have written a great deal about fMRI studies. This is the latest state-of-the-art neuroscience research tool. But in order to interpret studies like this it is important to understand the limitations of this research tool. First, as I stated above, fMRI measures blood flow to the brain. Brain tissue that is more active because the neurons are firing will require more blood flow, and therefore blood flow can be used to infer relative brain activity. The theory is that when a subject is performing some task the fMRI will show which parts of the brain are involved in that task.
The primary limitation of this use of an fMRI is that it is impossible to know what is going on inside someone’s mind. We cannot know what a subject is actually thinking, feeling, remembering, or attending to. We can only make a rough estimate based upon one of two basic strategies: we can expose the subject to some stimuli and then image their response to that stimuli, or we can make them perform a task. This is a reasonable approach – if someone is looking at a picture, regardless of what else might be going on in their mind, at least we can know they are looking at the picture.
But all the uncontrolled bits of mental activity will act like noise concealing the signal of interest. To control for this researchers generally look at multiple trials of multiple individuals and then use statistical analysis to pick the signal out of the noise – what brain region activity do the subjects have in common? Again, this is a reasonable approach, but it is important to understand how tricky and difficult using fMRI is. Without a solid protocol and careful analysis the blotches of computer enhanced colors produced by the fMRI are little more than a Rorchach test – researchers can see whatever they want in them. It also means that results should be reproducible before we put too much faith in them. Having said that, I think fMRI can be a powerful tool when used properly to help understand how the different parts of the brain are hooked up and interact with each other.
Brain Modules vs Neural Networks
The results of fMRI studies are only meaningful when they are interpreted within a paradigm of brain function and organization. Right now neuroscientists are working to form a consistent and predictive model of brain organization. At one end of the spectrum there are those who advocate what some call the modular brain – specific anatomical parts of the brain serve specific functions. They are like modules – the fear module causes fear when activated, the anxiety module produces anxiety, and the subtraction module performs subtraction calculations, etc.
The neural networks paradigm, on the other hand, emphasizes the network of connections between different parts of the brain as primarily important to function. In this model there is no anxiety module, but rather a certain pattern of networks in a specific individual will produce anxiety. But some of the same parts of the brain active during anxiety may also take part in other networks that will produce other emotions, like happiness.
The modular paradigm works a bit better for fMRI studies. If a part of the brain lights up during an activity then researchers can conclude that that part of the brain serves the function under study. The neural network paradigm makes fMRI studies more difficult (but not impossible) to interpret – the parts of the brain that light up are involved in the network but it is not clear what they are doing.
Michael Shermer wrote an interesting article for Scientific American called The Brain is Not Modular where he argues that fMRI’s have led some scientists to over-apply the modular metaphor. In the comments section of the article neuroscientist Marco Iacoboni critiques Shermer’s article. The result in good summary of the debate over modularity.
This is a complex and rapidly evolving area of research and I do not pretend to be an expert as it is not my area of research. But here is my best shot and synthesizing what I have read. It seems that in practice most neuroscientists are somewhere in the middle between modules and networks – combining both concepts. I think this is the right approach because I think the brain combines both approaches. There does appear to be modules in the brain – we had evidence for this long before fMRI scans. As a clinical neurologist I can examine a patient with a stroke and in most cases tell you exactly the size and location of the lesion that we will see on the MRI – based solely on the deficits on exam. If a patient has an isolated Wernicke’s aphasia – an inability to understand verbal commands – then they will have a medium-sized lesion in the angular gyrus of the dominant (usually left) hemisphere. That piece of the brain serves a very specific purpose.
But while the more basic or straightforward cortical functions are highly modular, the more complex or sophisticated higher cortical functions are not. There does not appear to be a piece of the brain that provides attention or concentration, or creativity. These seem to follow more of a network model of brain organization.
It is also possible that while there are modules in the brain their function may be more abstract and the specific effect they have varies depending upon what other parts of the brain that connect to them are also active. The trick for neuroscientists may be finding the most accurate way to describe the complex and abstract function of the various brain modules. There may be a layer or two of hidden complexity to what the fMRI studies are actually telling us. It seems that the more we study the brain the more complex a puzzle it becomes, and perhaps we have not yet crossed the threshold where new information isn’t just giving us more questions.
I don’t mean to downplay the vast amount of information we already have about brain function. As a said, I put this into practice almost every day. Rather I think that fMRI scanning has given us a new window into brain function and it is revealing a new layer of depth to its complexity. The modularity debate reflects our current struggle to understand this new depth. As is typical of scientific endeavors, the debate is healthy and is likely to lead to improved models of brain organization and function.
With all this in mind, what does this new study of mothers looking at their smiling infants tell us? The abstract’s conclusion says:
When first-time mothers see their own infant’s face, an extensive brain network seems to be activated, wherein affective and cognitive information may be integrated and directed toward motor/behavioral outputs. Dopaminergic reward-related brain regions are activated specifically in response to happy, but not sad, infant faces. Understanding how a mother responds uniquely to her own infant, when smiling or crying, may be the first step in understanding the neural basis of mother-infant attachment.
It is no surprise that something as complex as a mother’s reaction to their infant is very complex – I think I could have told you that without this study. But it does give us some specific information. There is an emotional component to the reaction – one that involves the reward center of the brain. This means that smiling babies make their mother’s feel good is a way that reinforces the behavior. Some news stories of this study have likened this response to the high addicts get from crack. I think that’s a stretch. The reward system seems to be one of those multi-purpose modules where the significance of its activity can only be understood in the context of the network in which it is firing.
The study also suggests that the emotions generated by seeing one’s smiling infant also is connected to, or triggers, certain thoughts. These emotions and thoughts also connect to specific behaviors. Although the study did not say it, I strongly suspect that one of those behaviors is the smiling, waving, and cooing that most parents will give back to their smiling infants. It likely also connects to more complex behaviors, like the instinct to protect and nurture the adorable little bundles of joy.
Crying babies (as any parent can tell you) elicits a very different emotional response. This is a very negative experience – one is motivated to stop the crying as soon as possible and failure to do so quickly may result in guilt, shame, and feelings of inadequacy. It is no surprise that a crying baby elicits no reward response – must make the baby smile and laugh. Anything to make it smile… Aahhh! It’s smiling – how adorable.
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