Can words influence pupil dilation?

We know that our pupils dilate (mydriasis) and contract (mycosis) depending on ambient light and our mood. A recent study conducted by researchers at Aix-Marseille University’s laboratories of Cognitive Psychology, and Speech and Language, along with the University of Groningen (Netherlands) shows that the meaning of a word also influences the size of our pupils. How can a word trigger pupil dilation or contraction?

During the 70s, Eckward Hess, a pioneer in pupillometry (and former head of the psychology department at the University of Chicago) observed that, in general, pupil size increases when a person observes something or someone "stimulating." Ads for cosmetics have also clearly capitalized on the fact that the eyes are a key element of seduction. Indeed, in these ads, you'll notice that the faces (both male and female) always have very dilated pupils (photoshop most often does the job of accentuating this trait). The toy industry has also taken advantage of this discovery; just look at the number of dolls and other figurines on the market with over-dilated pupils.

Far from these commercial considerations, the current study is part of a larger quest to understand the incarnation of language. These theories are based on the fact that when a person processes the meaning of a word (which refers to concrete actions or objects), they mentally simulate the associated sensorial input. For example, when we read the word "keyboard," we mentally simulate the action of typing, or for the word “sun,” we imagine a ball of fire shining in the sky. Some of these theories, which are based on a strong embodiment of language, assert that such simulations are necessary for understanding.

In the current research, the aim was to show that if word comprehension activates brain areas known to be involved in processing non-linguistic visual information (sensorial representations), then understanding words that evoke lightness or darkness could trigger pupillary responses. To test this hypothesis, the French and Dutch scientists carried out two studies in which participants read or listened to words evoking brightness (example: day, sun, shine) or darkness (example: night, dark, gloom) or neutral words (example: house, rabbit). The size of their pupils was measured. 30 subjects (21 women, 9 men, 18-54 years of age) participated in the visual experiment, 30 others (19 women, 11 men, 18-31 years of age) participated in the auditory experiment, and 30 observers participated in the control experiment (neutral words). What were the findings?

The researchers observed that pupil size varied according to the sense of brightness or darkness conveyed by the word. Thus, the pupils retract when the participants read or heard words that conveyed brightness, and dilated for words that evoked darkness. The effect isn’t linked to the visual or auditory properties of the stimuli, suggesting that the word’s meaning is sufficient for triggering the pupillary response.

These results open a new avenue for better understanding language processing in the brain. Does word comprehension necessarily involve the creation of mental images or are these image simply the indirect consequence of language processing (a reflex of the nervous system linked to the situation evoked by the word)? In the face of these questions, the researchers are continuing their experiment, testing the hypothesis in other languages.
Source: Sebastiaan Mathôt, Jonathan Grainger, Kristof Strijkers, “Pupillary Responses to Words That Convey a Sense of Brightness or Darkness,” in Psychological Science, 14 June 2017.


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