Salivating for stocking stuffers

In the spirit of this season of holy consumption, I thought it appropriate to write about an article released earlier this year in the Journal of Consumer Research on salivating over material goods. Author David Gal, an economist at Northwestern University, proposes that when we covet an item, be it ice cream or an iPhone, we literally drool over it. He hypothesizes that this response mechanism is caused by our reward system, with desired material items stimulating the same pathways and neural regions that hunger for food or other natural reinforcers do. This includes the striatum, amygdala, and hypothalamus–areas involved in reward responses and homeostatic mechanisms such as hunger and satiety. Activation of these areas in response to salient stimuli signals that these items are rewarding and could be important for survival. Supporting this claim, in previous research, the mesolimbic dopamine reward pathway is seen to light up in a similar manner for luxury items and sports cars, which are secondary learned reinforcers, as they do for natural incentives such as food and drugs.

Taking this a step further, the physical outputs of this heightened reward arousal state can include the secretion of saliva, triggered by the sympathetic and parasympathetic nervous systems. Salivation occurs in response to cues for food or water as part of the natural metabolic system, preparing us for mastication and digestion. However, Gal claims that it is also a byproduct of the autonomal arousal system controlled via the hypothalamus, and salivation can indicate any desired or salient stimulus, whether it be naturally rewarding, such as a member of the opposite sex, or a secondary conditioned reinforcer, like money or material goods.

Gal investigated this theory by presenting 169 undergraduate students with images of either money or mundane items, such as office supplies. While viewing the stimuli, participants were asked to keep cotton dental rolls in their cheeks and under their tongue to measure their saliva output. The weights of these cotton swabs were then compared to baseline measurements taken before the experiment to assess the increase in salivation due to the images presented. A second condition involved priming participants with feelings of either efficacy or helplessness by asking them to recall a time when they had felt either powerful or powerless. Gal hypothesized that money, symbolizing economic control, would be more coveted by those who felt they had little power, making it more desirable and rewarding than to those who deemed that they had greater power at the time of the experiment. Supporting this notion, only participants induced with feelings of powerlessness had significantly increased saliva output in response to the monetary cues. Individuals who felt powerful had no difference in salivatory rates when viewing the money images, nor was there a difference in saliva outputs in either power condition among participants in the control office supply group.

In a second follow-up study, Gal repeated the experiment using coveted luxury items in the place of raw currency. Gal exposed young men to images of sports cars, while also inducing in them the goal of winning a potential mate. He achieved this by presenting some participants with images of attractive women with whom they were to imagine going on a date with, while those in the control condition were to imagine having their hair cut. Men who viewed the sports cars as opposed to the mundane images had greater salivation rates compared to baseline ratings, but only when they had been primed with the goal of mating. The mating prime had no effect on saliva output in the control condition, and viewing the sports cars without the salient goal did not increase salivation rates on its own.

Importantly, increases in saliva production seem to be contingent upon the immediate rewarding value of the goods, only enhancing salivation rates when the presented stimuli were seen to help achieve a recently primed goal. This suggests that the triggering of salivation by reward cues is dependent upon the present desire or need for the item, much like the more visceral feeling of hunger in the presence of food.

So as you are finishing your Christmas wish-list this year, dreaming of drool-worthy duds and mouth-watering machines, perhaps rank your heart’s desire on how quickly they’ll come in handy and how moist your mouth feels afterward. You’ll be sure to find them more rewarding.

Happy holidays everyone!

(Thanks to Emily Barnet for this article.)

The brain’s social network

Neuroscientists often attempt to attribute various behaviors and traits to certain regions of the brain. These findings make for neat science and great headlines, and while some of these results are little better than phrenology claims, many are highly reliable. The good ones are confirmed and replicated by multiple labs and substantiated using a variety of different methods, such as lesioning or animal and human imaging models. For example, we know with relative certainty that much of the occipital lobe is in charge of processing visual information and that the hippocampus is heavily involved in transitioning from short-term to long-term memory. However, there is much in our behavior and our brains that we still do not understand, and it is highly tempting to simply assign certain sections of the brain to different traits when in fact the underlying mechanisms are much more complicated. This tendency has become increasingly easy in the past decade with the rise of functional neuroimaging studies, where a region of the brain is seen to “light up” with activity when performing certain types of tasks. Voxel-based morphometry (VBM) studies take these investigations a step further, looking at how gray matter volume in our brains correlates to different traits and behaviors. Two recent examples of VBM studies have investigated the neural correlates of social networking and extroversion, finding connections between amygdala size (among other regions) and social tendencies.

The first study, out of University College London and published in the Proceedings of the Royal Society Biological Sciencesfound that people with more Facebook friends had increased gray matter volume in certain regions of the brain associated with social interactions. The authors of the study hypothesized that the number of one’s online friends could predict the relative brain size of regions important for social networking, particularly those involved in social cognition and mentalizing (the ability to recognize social cues and take another’s perspective). These areas include the fronto-parietal cortical circuit, medial prefrontal cortex, and amygdala. However, these frontal cortical regions were not identified in the study, and instead the researchers discovered greater volume in the left middle temporal gyrus, right entorhinal cortex, and right posterior superior temporal sulcus, as well as the amygdala to a lesser extent. These areas are implicated in social cognition, perception of movement and intention (both physical and social), and autobiographical and associative memory. Based on these findings, the authors speculate that individuals with greater brain volume in these regions are more adept at the skills needed to maintain online socio-personal connections, such as enhanced memory of face-name combinations and awareness of movement of individuals in social circles. However, of these regions only the amygdala was correlated with real life social interactions, and none of the other originally proposed areas were found to correlate with social network size.

The second study, published this week in PLoS ONE, also reports that individuals who are more extroverted show increased volume in the amygdala, as well as in the orbitofrontal cortex (OFC). Researchers from the Netherlands administered the NEO Five Factor personality assessment to 65 individuals to subjectively measure extroversion and neuroticism levels. They also had participants undergo an MRI scan and used VBM analysis to measure the size of certain pre-determined regions of the brain against extroversion scores, including the amygdala, anterior cingulate cortex, and OFC. Controlling for age, sex, and total gray matter volume, researchers discovered that individuals who scored higher on the extroversion scale had significantly larger amygdala and orbitofrontal cortices, as well as finding a significant correlation between total gray matter volume and extroversion scores.

As stated above, the amygdala is one of the brain’s emotional centers and is important in social interactions, both online and offline. It is crucially implicated in recognizing and processing positive and negative emotions, both in oneself and from the facial expressions of others. The OFC is also commonly associated with emotion regulation, as well as reward valuation and decision-making, mainly through its connections to limbic structures such as the amygdala, striatum, and hypothalamus. However, it is not typically linked to social interactions, and the authors speculate that their findings are evidence of the amygdala and OFC’s involvement in a greater sensitivity to positive experiences and social interactions, rather than interpersonal skills themselves.

While the findings from these two studies are intriguing and compliment one another nicely, caution must be taken in the interpretation placed on these results. Correlation analyses state only an association, not a causation, and, as recently brilliantly exhibited by Business Week, these connections can be highly questionable at times. This is particularly true of imaging studies, where investigators can go fishing for regions to attribute their target behaviors to. Interpretations of correlations are quick to come by, and rationales for connections in unexpected areas of the brain can be justified all too easily when a publication is on the line. A priori regions of interest are thus crucially important, providing groundings for current explorations based on previous studies and alternative research methods. I am in no way denouncing VBM studies and their value and viability generally, or these studies in particular, however, I do caution against the interpretations that can be carelessly made with them. Additionally, in studies like these, it is unknown whether the size of the regions predicts the behavior or if the brain adapts and grows to incorporate new connections based on the repetition and reinforcement of certain actions. In regards to the studies at hand, their confirmation of the amygdala’s role in social interactions is highly supported, however, it is unknown whether the increase in brain size is a predictor of social ability and network size, or whether practice of interpersonal skills helps to foster neurogenesis in these regions.

Impaired adolescent decision-making

I am pleased to announce that my first first-author publication has recently been released online by the journal Developmental Psychology.

The article, on decision-making in children and adolescents, looks at the developmental trajectory of affective decision-making abilities using the Iowa Gambling Task (IGT) in children between the ages of 8 and 17. It compares this type of “hot” executive function with more typical “colder” cognitive abilities, such as impulse control and working memory. Contrary to the accepted belief that children improve universally on cognitive tasks as they age, we discovered that early adolescents (ages 11-13) are actually more impaired on this task than some of the younger participants, making riskier decisions and failing to learn from their mistakes.

The IGT requires participants to choose between four decks of cards that give out varying amounts of wins and losses. Two of the decks issue low wins but also low losses, resulting in an overall net gain, whereas the other two decks are riskier options, giving high payoffs but also higher losses, making them ultimately disadvantageous. A net score is calculated by subtracting the total number of disadvantageous choices from the total advantageous decisions. Early adolescents had significantly lower mean net scores on the task than older participants, but did not differ from the younger children in their ability. However, the total trajectory of mean scores across all ages resulted in a significant J-shaped curve, signifying a dip in ability in early adolescence.

We speculate that this curvilinear trajectory is due to the varying developmental schedules of different regions of the brain, particularly the striatum (involved in reward processing) and the prefrontal cortex, which is responsible for more inhibitory control. Structures in the basal ganglia typically develop earlier in adolescence,  whereas the prefrontal cortex is not fully matured until the early 20s. This earlier development of the striatum could lead adolescents to place undue emphasis on the initially high reward, but ultimately disadvantageous options in the IGT. Coupled with the delayed development of the prefrontal cortex, this group could also lack the necessary inhibitory control to offset this reward-driven urge. Supporting this theory, other imaging studies investigating developing cognitive ability have shown adolescents to disproportionately recruit from subcortical regions, particularly the basal ganglia, on tasks involving monetary rewards.

Conversely, younger children performed neither overtly advantageously nor disadvantageously on the task, choosing between the decks more randomly. This could be due to an earlier neurodevelopmental stage, before the striatum and other limbic regions had fully developed, making them less sensitive to the risky high reward options. Also supporting this J-shape trajectory theory, older adolescents performed the most advantageously on the task, improving their performance and successfully inhibiting the urge to make impulsive choices. This improvement presumably correlates with the continued maturation of their prefrontal cortices, as these inhibitory abilities come on-line.

Notably, all other cognitive tasks administered during the course of testing improved linearly across age, demonstrating that affective decision-making is a unique process that taps into the limbic regions, rather than just relying on the cortical cognitive network.

Importantly, these results are not implying that all adolescents are impulsive risk-seekers doomed to make lasting poor decisions. We all go through these stages of neurodevelopment and the vast majority of us emerge from adolescence relatively unscathed. Also, as this was not an imaging study the neural correlates of the abnormal decision-making development is speculative. However, this study does provide an interesting glimpse into how we develop in our affective decision-making tendencies and how they change as we mature.