Posture and pupil — eye measures of emotions that predict choice
For a long time, we have had measures of emotional arousal — the sweat of the palms of our hands, measured by Galvanic Skin Response; increases in respiration and pulse; and pupil dilation. However, as all researchers of emotions know, arousal is only one of two dimensions of emotional response. Arousal is basically the intensity of the emotion, and it can be high for both negative and positive events. That is, just based on the arousal response, we cannot tell if the person is in love or scared to death.
That is why we need an extra dimension of emotions: the direction of the emotion, so to speak. One way to speak of this is the valence of an emotion — is the response positive, neutral or negative? Other studies have focused on the motivation aspect of a response. As in the ancient greek word, emovere means “to move.” In this sense, emotional response drive changes in behaviour.
One such motivational response is measured directly from the brain, known as the prefrontal asymmetry response. In right-handers, stronger activity in the left than the right frontal parts of the brain are related to approach behaviours. Conversely (yet less robustly studied) greater activity in right than left is associated with avoidance behaviours. This phenomenon, annoyingly simple as it may sound, has a long track record of scientific studies, ranging from psychology, psychiatry and neurology, and from studies of brain lesions, brain stimulation, and neuroimaging (fMRI and EEG alike). But do you have to measure the brain responses to gain insights into the valence or motivation of an emotion? Sure, with EEG you will have superior details and understanding of such responses, but many may not have access to this.
In a recent article, we asked this very question: are there other bodily changes that can indicate changes in valence/motivation? Anecdotally, we know that postural changes may indeed indicate such emotional responses. So the question was whether we could measure this. Fortunately, high-resolution eye-tracking equipment allows a triangulation of pupil distance to measure the distance a person has to the screen. While we normally ask people to keep a 60 cm distance to the screen during stationary eye-tracking, our thinking was that fluctuations in such distance could indicate emotional valence/motivation responses.
We now have a scientific paper that was recently accepted in the APA journal JNPE, and is currently in press (a pre-publication draft can be found here). The title of the article is “Predictive value of body posture and pupil dilation in assessing consumer preference and choice.” Here, we report the results of two studies where we do indeed find that pupil dilation and distance can operate as measures of arousal and valence/motivation, respectively. As we note in the abstract:
In neuroeconomics and neuromarketing, the assessment of arousal has become one of the key measures in our effort to understand the basic mechanisms of value-based choice. While neurophysiological responses such as pupil dilation and galvanic skin response (GSR) have provided a significant explanatory value in the mechanisms of decision-making, other, less known physiological responses indicators such as body posture, may provide an additional valuable insight into decision-making processes. Here, we report the results from two separate high-resolution eye-tracking studies in which pupil dilation and body posture provide both independent and interacting contributions in predicting preference judgments and choice. These results suggest an improved assessment and prediction of choice by using a combination of pupil dilation and posture, relative to only employing one of the measures. However, the use of this combined measure needs to be employed with care, as the dynamic relationship between pupil size and posture is affected by different categories of stimuli, in particular, fashion brands, and wine brand logos. We discuss these findings in light of the academic and commercial call for neuroimaging and physiology measures that can predict and explain the mechanisms underlying preference formation and value-based choice.