This is different from other eye movements, for which the function is generally clearer for example, saccadic and smooth pursuit eye movements serve to stabilize the retinal image, and to bring relevant objects into foveal vision ( Land, 1999 Walls, 1962)-that is fairly clear. But we do not fully understand why the pupil responds in the way it does. We know what kind of stimuli trigger pupil responses, and we know, more or less, which neural pathways underly these responses. One aim of this review is to discuss how the different kinds of pupil responses are modulated by high-level cognition.Ĭurrent understanding of pupil responses is largely descriptive. This is similar to other eye movements, such as saccades and smooth pursuit, which also have properties of both reflexive and voluntary action (e.g. But pupil responses are also partly voluntary, in the sense that they are modulated by high-level cognition: when you choose to attend to a light in peripheral vision, your pupils constrict more than when you choose to ignore this light (e.g. Pupil responses are partly reflexive, in the sense that the same stimulus always leads to a qualitatively similar response: pupils always constrict, and never dilate, in response to light. The pupil changes its size in response to three distinct kinds of stimuli: it constricts in response to brightness (the pupil light response, or PLR) and near fixation (the pupil near response, or PNR) and it dilates in response to increased cognitive activity, such as increased levels of arousal or mental effort (the psychosensory pupil response, or PPR). In this review, I will focus on this last type of eye movement: pupil responses. Once gaze has been directed at an object of interest, our eyes continue to move to provide our brain with the best possible image: the curvature of the lens changes ( accomodates) to control focus and our pupils enlarge ( dilate) or shrink ( constrict) to control how much of the lens’s surface is exposed, and consequently how much light enters the eye. But eye movements do far more than direct gaze. The saccadic and smooth-pursuit eye movements that control gaze direction have been extensively studied (e.g.
We do not passively let visual information fall onto our retina, but actively seek out objects of interest by moving our body, head, and eyes. In many ways, pupil responses are similar to other eye movements, such as saccades and smooth pursuit: like these other eye movements, pupil responses have properties of both reflexive and voluntary action, and are part of active visual exploration. Although pupil responses likely serve many functions, not all of which are fully understood, one important function is to optimize vision either for acuity (small pupils see sharper) and depth of field (small pupils see sharply at a wider range of distances), or for sensitivity (large pupils are better able to detect faint stimuli) that is, pupils change their size to optimize vision for a particular situation. I also discuss the functional relevance of pupil responses, that is, how pupil responses help us to better see the world. In this review, I describe these three pupil responses, how they are related to high-level cognition, and the neural pathways that control them. Pupils respond to three distinct kinds of stimuli: they constrict in response to brightness (the pupil light response), constrict in response to near fixation (the pupil near response), and dilate in response to increases in arousal and mental effort, either triggered by an external stimulus or spontaneously.
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