How do we decide and sub-decide?
Disagreement remains over how to model decision-making processes.
Should you go for coffee with Jules, or go to the movie theater with Jim? Both options require you to make additional decisions, for example, which café would you go to, or what movie could you see? Many of our day-to-day decisions have multiple layers of sub-decisions embedded within them that are not necessarily independent. Our opinions of the cafés in town and the movies showing at the theater may influence our decision over whom to spend the afternoon with.
In 2015, researchers at the Netherlands Institute for Neuroscience performed experiments in macaques to try to work out how the brain makes these decisions. The monkeys learned to choose between two visual stimuli (decision 1). The outcome of decision 1 determined whether the animals then had to make decision 2 or decision 3. The results suggested that the monkeys initially made all three comparisons independently and in parallel, before combining the evidence to select their overall strategy. This process is referred to as hierarchical decision-making. In the analogy above, one would compare the relative merits of Jules versus Jim, café A versus café B, and a horror movie versus a comedy at the same time before deciding what to do.
Alexandre Hyafil and Rubén Moreno Bote have now reanalyzed the data published in 2015 using new computer simulations. This second analysis suggests the results are in fact more consistent with an alternative model of decision-making called a flat model, in which the brain compares all of the final options simultaneously (Jules + café A; Jules + café B; Jim + horror movie; Jim + comedy) before choosing between them.
Making decisions by comparing the final outcomes becomes easier as the brain learns through experience to associate stimuli that often occur together. Hyafil and Moreno Bote hypothesize that in response to a new situation, the brain may sometimes start off by using hierarchical decision-making before switching to a more accurate flat model as experience allows.
In response to these findings, Ariel Zylberberg and colleagues — who conducted the work reported in 2015 — reanalyzed the original data and re-ran the simulations. Zylberberg and co-workers argue that the flat model provides a poor fit to the original data. A new experiment with human volunteers suggests that modifying the task by adding even more decisions can lead to the different comparisons being made one after the other (in series) rather than all at the same time (in parallel), before the decision is made. This is difficult to explain with a flat model. Zylberberg and colleagues argue that these findings confirm the original conclusion that the monkeys use a hierarchical strategy. Moreover, the new results expose a previously unknown limit in the number of decisions that the brain can evaluate at any one time. If this limit is exceeded, decision-making becomes serial.
Future studies can build on these conflicting findings by further exploring the limits of parallel decision-making, which may help us to understand how the brain is able to make multiple decisions while keeping the future consequences in mind.
To find out more
Read the eLife research papers on which this eLife digest is based: “Breaking down hierarchies of decision-making in primates” (June 26, 2017) and “Serial, parallel and hierarchical decision making in primates” (June 26, 2017).
