WASHINGTON: Scientists have decoded the human brain's decision-making algorithms , deciphering precisely what happens in the organ when we make choices.When faced with a choice, the brain retrieves specific traces of memories, rather than a generalised overview of past experiences, from its mental Rolodex, according to new brain-imaging research from the University of Texas at Austin.Led by Michael Mack, a postdoctoral researcher in the departments of psychology and neuroscience, the study is the first to combine computer simulations with brain-imaging data to compare two different types of decision-making models.In one model - exemplar - a decision is framed around concrete traces of memories, while in the other model - prototype - the decision is based on a generalised overview of all memories lumped into a specific category.According to the findings, the exemplar model is more consistent with decision-making behaviour.Researchers asked 20 respondents to sort various shapes into two categories.During the task their brain activity was observed using functional magnetic resonance imaging (fMRI), allowing researchers to see how the respondents associate shapes with past memories.With brain-imaging analysis, researchers found that the exemplar model accounted for the majority of participants' decisions.The results show three different regions associated with the exemplar model were activated during the learning task: occipital (visual perception), parietal (sensory) and frontal cortex (attention).While processing new information, the brain stores concrete traces of experiences, allowing it to make different kinds of decisions, such as categorisation information (is that a dog?), identification (is that John's dog?) and recall (when did I last see John's dog?)."Imagine having a conversation with a friend about buying a new car. When you think of the category "car," you're likely to think of an abstract concept of a car, but not specific details. However, abstract categories are composed of memories from individual experiences. So when you imagine "car," the abstract mental picture is actually derived from experiences, such as your friend's white sedan or the red sports car you saw on the morning commute," Mack said."We flexibly memorise our experiences, and this allows us to use these memories for different kinds of decisions. By storing concrete traces of our experiences, we can make decisions about different types of cars and even specific past experiences in our life with the same memories," Mack said.The study was published in the journal Current Biology.