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.

Brain development and decay: More cannibal neurons

The brain is a plastic organ constantly changing and adapting, creating new connections with the inclusion of novel thoughts and memories, and losing others as we age and decay. Two recent discoveries have further illuminated how and when these processes occur, challenging current theories on the processes of neurogenesis, cellular myelination, and neuronal pruning. The majority of these neuro-developmental process takes place when we are young, the brain going through immense cortical growth in different regions of the brain as we mature and learn and require new skills and knowledge. This process occurs in waves throughout the brain, starting in the occipital and parietal lobes during childhood as we perfect sensory abilities, fine motor movement, coordination, and spatial awareness. Next the temporal lobe matures, improving our memory and language abilities. And finally the frontal cortex, responsible for our abilities to inhibit, control, plan, pay attention, and perform demanding cognitive tasks, becomes fully functional during adolescence. However more cells and cell connections are created than are needed, and closely following this neurogenesis comes periods of pruning, when un-used synapses or connections are weeded out and destroyed. This makes the brain more efficient, conserving space and cellular energy, and streamlining neural processes so only the essential and most important regions are utilized.

Until recently, the process by which this pruning occurred was a relative mystery. However, a study published last month in Science  revealed that microglia, specific non-neuronal cells in the brain similar to those in the immune system that identify and destroy invading microbes, may be involved in this process. Microglia travel throughout the brain converging on areas of brain damage to clean up and dispose of leftover dead neurons and cell debris. They accomplish this via phagocytosis, the cellular process of engulfing solid particles, similar to the process of autophagy I wrote about last week.

Scientists at the European Molecular Biology Laboratory in Monterotondo, Italy discovered that microglia cells also monitor synapses in the brain in a similar manner, consuming unused and un-needed connections during periods of brain maturation. Researchers used an electron microscope to look at microglia cells located near synapses in the brains of mice. These cells contained traces of both SNAP25, a presynaptic protein, as well as PSD95, a marker of postsynaptic excitatory activity. Both are an indication of cell interactions, suggesting that these microglia had consumed both presynaptic and postsynaptic parts of these connections. It is still unknown how these cells identify the proper synapses to engulf, though scientists believe it has to do with fractalkine, a large protein involved in the communication between neurons and microglia.

It was initially thought that this process of cortical maturation ended after adolescence, however new evidence suggests that neural development, particularly in the prefrontal cortex, occurs well into one's 30s. Studies published this year in PNAS and the Journal of Neuroscience indicate that both the addition of new white matter (myelinated connective neurons) as well as a decrease in gray matter (synaptic connections eliminated through pruning) continue into early adulthood. The implications of this discovery has to do with the timing of the onset of several psychiatric disorders, including schizophrenia and drug abuse, which commonly arise in the early to mid-20s. If the brain is still developing through this time, it suggests that we may be more vulnerable to environmental influences longer than suspected, or that problems with this period of final development could be at play in these disorders.