Your "Sherlock Holmes" Character: What Pupil Size Says About People Facing a Decision

Credit: © shefkate / Fotolia

The precision with which people make decisions can
be predicted by measuring pupil size before they are
presented with any information about the decision.

You're writing a story in which your character, especially a detective, has extraordinary powers of observation.  Here's a tidbit you can use to demonstrate your character's ability to observe - the size of another character's pupils predicts how well he or she will make a decision and how well the character will perform once the decision is made.  A small point, but successful character development is based on such small things.

Here's the report:

The precision with which people make decisions can be predicted by measuring pupil size before they are presented with any information about the decision, according to a new study published in PLOS Computational Biology this week.

The study, conducted by Peter Murphy and colleagues at Leiden University, shows that spontaneous, moment-to-moment fluctuations in pupil size predict how a selection of participants vary in their successful decision making.

  • A larger pupil size indicates poorer upcoming task performance, due to more variability in the decisions made once the relevant information was presented. 
  • Certain individuals who had the largest pupils overall tend to be the least consistent in their decisions.
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About the study
The results were obtained by measuring pupil size before each segment of the task began and monitoring each participant's subsequent performance in deciding which direction a cloud of dots was moving in. These results were then combined with a simple mathematical model that described how people make decisions.
These findings reveal that a person's state of responsiveness, as measured by pupil size, is a key determinant of the variability of the decisions they make about the world around them. When hyper-responsive, our decision making appears to be less reliable and will more likely lead to undesirable outcomes. 
The results were obtained by measuring the pupil size of 26 volunteers as they performed a visual choice-based task designed to mimic the kinds of challenging perceptual decisions that are frequently encountered in everyday life. Pupil size gives a good indication of how responsive a person is at any given moment, with larger pupils correlating with increased responsiveness, though little was previously understood about how pupil size might relate to our ability to make reliable perceptual judgements.

The results
Dr Murphy comments, "we are constantly required to make decisions about the world we live in.
Researchers have long known that the accuracy and reliability of such everyday decision making can be tremendously variable for different people at different times, but we understand quite little about where this variability comes from. In this study, we show that how precise and reliable a person is in making a straightforward decision about motion can be predicted by simply measuring their pupil size.

This finding suggests that the reliability with which an individual will make an upcoming decision is at least partly determined by pupil-linked 'arousal' or alertness, and furthermore, can potentially be deciphered on the fly."

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Story Source:  Materials provided by PLOS. Peter R. Murphy, Joachim Vandekerckhove, Sander Nieuwenhuis. Pupil-Linked Arousal Determines Variability in Perceptual Decision Making. PLoS Computational Biology, 2014

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