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Claudiu Simion, Shinsuke Shimojo; How Early Does the Brain “Know” What It Likes? Evidence from Pupilometry. Journal of Vision 2004;4(8):103. doi: 10.1167/4.8.103.
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It has been reported that pupil size is positively correlated with stimulus attractiveness. While it is a well-known effect which makes some professional poker players wear eye shades when they play, little is known about the temporal dynamics of this reflexive reaction. We analyzed the pupil size of observers while they were inspecting pair of stimuli for the purpose of making two-alternative forced-choice decisions about them. They had to indicate which face was more attractive (face-like), which face was rounder (face-round), which face was less attractive (face-dislike) or which of two Fourier-descriptor generated shapes was more attractive (Fourier-like). The trials were terminated by the observers' choice. Meanwhile, we measured the pupil size using the Eyelink2 system. We compared the change in pupil area (averaged over fixations) when observers moved their gaze from their subsequent choice to the non-choice stimulus, when they switched from non-choice to choice, and when they made adjacent fixation on the same stimulus. In the face-like and Fourier-like conditions, the pupil size significantly increased from non-choice to choice, while there was no change in size in the opposite direction. In the face-round task we found no significant change in any direction, while in the face-dislike task we found the opposite result, a size increase only when switching from choice (disliked) to non-choice (liked). We have shown recently that the brain needs the assist of the orienting system to make a decision of preference (gaze cascade effect, Shimojo, Simion et al 2003). There, an increasing gaze bias towards choice was revealed as early as 1s before decision in all conditions, reflecting the contribution of the orienting behavior to the decision making process. However, pupilary responses may be an even better indicator of preference, because unlike the gaze bias, they are correlated with attractiveness regardless of task.
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