Previous research has found that words are identified most quickly when the eyes are near their center (the Optimal Viewing Position effect). A study was conducted to determine whether this same phenomenon is observed during reading, as revealed by a relationship between fixation position in a word and the duration of the fixation. An analysis of three large existing corpora of eye movement data, two from adults and one from children, showed a surprising inverted Optimal Viewing Position curve: mean fixation duration is greatest, rather than lowest, when the eyes were at the centers of words. From this phenomenon, we suggest an alternative explanation to the fixation duration trade-off effect in word refixations [O'Regan & Lévy-Schoen, Attention and performance XII: the psychology of reading (1987)]; the phenomenon also contradicts expectations of both oculomotor and cognitive theories of eye movement control. Attempts to test alternative explanations led to the discovery of another phenomenon, the Saccade Distance effect: mean fixation durations vary with the distance of the prior fixation from the currently-fixated word, being longer with greater distances. The durations of fixations in reading are complexly determined, with influences both from language and perceptual/oculomotor levels.
The pupillary light response is often assumed to be a reflex that is not susceptible to cognitive influences. In line with recent converging evidence, we show that this reflexive view is incomplete, and that the pupillary light response is modulated by covert visual attention: Covertly attending to a bright area causes a pupillary constriction, relative to attending to a dark area under identical visual input. This attention-related modulation of the pupillary light response predicts cuing effects in behavior, and can be used as an index of how strongly participants attend to a particular location. Therefore, we suggest that pupil size may offer a new way to continuously track the focus of covert visual attention, without requiring a manual response from the participant. The theoretical implication of this finding is that the pupillary light response is neither fully reflexive, nor under complete voluntary control, but is instead best characterized as a stereotyped response to a voluntarily selected target. In this sense, the pupillary light response is similar to saccadic and smooth pursuit eye movements. Together, eye movements and the pupillary light response maximize visual acuity, stabilize visual input, and selectively filter visual information as it enters the eye.
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