The Pupil as Marker of Cognitive Processes
Of all peripheral measures of (neuro-)physiological activity, pupil size is probably the easiest to access. Far beyond its well-known reaction to light incident on the eye, pupil size is a rich marker of many cognitive processes. Since the turn of the mil
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1 The Pupil Is a Readily Accessible Marker of Neural Processes Of all peripheral physiological measures of neural activity, pupil size is arguably the easiest to access. In fact, it is one of the few markers of another’s internal state that is available to the observer’s naked eye. This property renders pupil size a cue that can be exploited in social interactions. This is no news to those renaissance women, who extracted atropine from the deadly nightshade (atropa belladonna) to dilate their pupils in order to increase their perceived attractiveness, eventually leading to the plant’s commonly used name of “belladonna”. Not surprisingly, pupillometry has been used rather widely in the early days of cognitive psychology and flourished in the 1960s. With the advent of other techniques, like EEG and later PET and fMRI, peripheral physiology, and thus pupillometry, became somewhat unfashionable. However, since the turn of the millennium, when video-based oculography started to become widespread, pupil size could be measured as a “by-product” of eye-tracking data. This led pupillometry to have its own renaissance with the number of pupillometry publications increasing each year. The present chapter will deal with those recent developments with a strong focus on the cognitive aspects controlling pupil size.
W. Einhäuser (✉) Institut für Physik, Technische Universität Chemnitz, Chemnitz, Germany e-mail: [email protected] © Springer Science+Business Media Singapore 2017 Q. Zhao (ed.), Computational and Cognitive Neuroscience of Vision, Cognitive Science and Technology, DOI 10.1007/978-981-10-0213-7_7
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2 Modulation of the Pupil’s Response to Light by Cognitive Factors 2.1
Awareness and Imaginary Light Sources Modulate the Pupil Light Reflex
Thinking of changes in pupil size, the pupil light reflex (PLR) readily comes to everyone’s mind: when light levels incident on the eye increase, the pupil constricts. Even this seemingly simple and reflexive behavior has, however, cognitive components attached. Rather than responding to light physically entering the eye, the pupil at least to some extent reacts to the subjective perception of lightness. This somewhat counterintuitive phenomenon becomes particularly evident when two stimuli of different luminance are presented to the two eyes. At each instant, both pupils reflect the luminance of the stimulus that reaches awareness: when the brighter stimulus is consciously perceived, the pupil constricts more than during times when the darker stimulus is perceived (Naber et al. 2011; Fahle et al. 2011). Similarly, when one eye is perceptually suppressed from awareness, flashes presented to this eye elicit less constriction than during periods when the respective eye is dominant (Bárány and Halldén 1948; Lorber et al. 1965). Such cognitive command over the pupil light reflex is not restricted to stimuli that are actually physically present. A substantial body of recent research indicates that the mere indication of the presence of
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