Your eyes betray the timing of your decisions

WHEN it comes to making decisions, timing can be everything, but it is often beneficial to conceal the decision that has been made. Take a game of poker, for instance: during each round, the player has to decide whether to bet, raise the stakes, or fold, depending on the hand they have been dealt. A good player will have perfected his "poker face", the blank expression which conceals the emotions he feels and the decisions he makes from the other players sitting at the table.

Increasing numbers of researchers are using brain scanning techniques to perform what is commonly referred to as "mind reading". Their efforts have had limited success, however, and our decisions are still private mental events which are inaccessible to others. Until now, it was unclear whether there are outwardly visible physiological responses associated with the decision-making process. But a new study now shows that dilation of the pupils can predict when a decision has been made before an individual voluntarily reports making the decision.

Wolfgang Einhauser of the Philipps-Universitat Marburg in Germany and his colleagues recruited 20 participants and asked them to perform three simple decision-making tasks. In the "immediate overt response" task, they were asked to sit in front of a computer screen and fixate on a blue dot which turne red for a period of ten seconds. They were required, over 90 trials each, to spontaneously press a button once at any time while the red dot was displayed, and told that they could win a small financial reward if the button was pressed during a "lucky" one second interval. 

The second task (the "covert digit choice") involved a slightly more complex cognitive task requiring working memory. Five digits were displayed on the screen, one after the other, for two seconds each. The participants were asked to secretly choose one of the digits and to indicate their choice by pressing a key afterwards. The participants were again told that they would win a small reward if they chose the "lucky" digit. Another variation of this was almost identical, except that there was no reward and the participants were not given any feedback at the end of each trial. The third experiment was identical to the second, with the exception that the participants were not free to choose a digit. Instead, one of the five presented digits was underlined, and they were required to indicate which one.

The researchers used an infrared eye-tracking device to measure the diameter of the participants' left pupils up to 2,000 times per second while they performed the tasks. They found that pupil dilation was tightly coupled with the time at which the decisions were made, and betrayed the participants' decisions before they were openly revealed. In the first experiment, maximum pupil dilation was observed during the 2-second interval in which the button was pressed. In the second, where there was a delay between the choice and the participants' report of which number they had chosen, their pupils were maximally dilated during the interval at which they chose the digit. During task three, maximum dilation was again observed during the time at which the participants "chose" the underlined number.

The size of the response was smaller for the covert than for the open tasks, but nevertheless was observed across all trials, regardless of whether the choice was related to the voluntary execution of a movement or the selection of a number from the series of digits. Furthermore, by relating pupil diameter to the 2-second intervals during which each digit was displayed, the researchers could successfully predict when the participants made their decisions in the first two experiments and, therefore, which digit they chose within each trial.

Pupil dilation is mediated by the neurotransmitter noradrenaline (norepinephrine), which is released by a brain stem nucleus called the locus coeruleus and other nuclei associated with it. This transmitter is implicated in cognitive functions associated with memory and attention, and is also closely linked to arousal levels, and it is known that the expectation of a reward leads to increased arousal. It is therefore possible that the pupil dilation here was due to the participants' anticipation of the small financial reward, but this can be ruled out because the same effect was observed in the variation of experiment two and also in experiment three, both of which did not involve rewards. This study therefore supports the hypothesis that noradrenaline plays a critical role in the rapid consolidation of behavioural decisions.

Thus, pupil dilation, which is an external physiological response visible to the naked eye, can provide clues about strategic decision making processes, and can be used to predict decisions before they are revealed by the individual. Interestingly, poker players may have known this all along - it may be the reason that many of them like to wear sunglasses while playing. The findings have another important implication, for patients who are in a persistent vegetative state or are suffering from "locked-in" syndrome, and are apparently oblivious to the outside world. The pupil dilation response could possibly be used along with functional neuroimaging to help clinicians assess mental function in such patients.

Related:

• Eye movements reveal unconscious memory retrieval
• Vegetative and minimally conscious patients can learn
  
• The neurological basis of intuition
  

Einhauser, W., et al (2010). Pupil dilation betrays the timing of decisions. Front. Hum. Neurosci. DOI: 10.3389/fnhum.2010.00018

Aston-Jones, D. & Cohen, J. D. (2005). An Integrative Theory of Locus Coeruleus-Norepinephrine Function: Adaptive Gain and Optimal Performance. Annu. Rev. Neurosci. 28: 403-450 [PDF]

Berridge, C. W. & Waterhouse, B. D. (2003). The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes. Brain Res. Rev. 42: 33-84 [PDF]