Fixational eye movements are subdivided into tremor, drift, and microsaccades. All three types of miniature eye movements generate small random displacements of the retinal image when viewing a stationary scene. Here we investigate the modulation of microsaccades by shifts of covert attention in a classical spatial cueing paradigm. First, we replicate the suppression of microsaccades with a minimum rate about 150 ms after cue onset. Second, as a new finding we observe microsaccadic enhancement with a maximum rate about 350 ms after presentation of the cue. Third, we find a modulation of the orientation towards the cue direction. These multiple influences of visual attention on microsaccades accentuate their role for visual information processing. Furthermore, our results suggest that microsaccades can be used to map the orientation of visual attention in psychophysical experiments.

Mathematical models have become an important tool for understanding the control of eye movements during reading. Main goals of the development of the SWIFT model (R. Engbert, A. Longtin, & R. Kliegl, 2002) were to investigate the possibility of spatially distributed processing and to implement a general mechanism for all types of eye movements observed in reading experiments. The authors present an advanced version of SWIFT that integrates properties of the oculomotor system and effects of word recognition to explain many of the experimental phenomena faced in reading research. They propose new procedures for the estimation of model parameters and for the test of the model's performance. They also present a mathematical analysis of the dynamics of the SWIFT model. Finally, within this framework, they present an analysis of the transition from parallel to serial processing.In modern society, reading is a central skill, which demonstrates how efficiently a range of different cognitive processes (e.g., visual information processing, word recognition, attention, oculomotor control) can work together to perform a complex everyday task. Consequently, a full account of how we read is among the crucial problems of cognitive research. Here, we focus on the fact that eye movements in reading represent an important example for a coupled cognitive-motor system. Therefore, a detailed analysis of the interface between high-level cognition (word recognition) and eye-movement control (saccade generation) is essential to contribute to our knowledge of reading.The measurement, analysis, and modeling of eye movements is one of the most powerful approaches to studying the way visual information is (a) processed by the human mind and (b) used to guide our actions (Findlay & Gilchrist, 2003). Measurements of fixation durations on words or on regions of text are central for investigating cognitive processes underlying reading (Liversedge & Findlay, 2000;Rayner, 1998). Therefore, it is of central importance to develop a detailed understanding of how the experimental observables are related to the underlying cognitive systems.Over the last decades, there has been a considerable increase of knowledge about eye movements and visual information processing (e.g., Hyönä, Radach, & Deubel, 2003; Radach, Kennedy, & Rayner, 2004;Rayner, 1998). The question of how the contributing cognitive subsystems for a specific task such as reading are coordinated is a research problem representative of questions that we believe cannot be investigated without fully quantitative mathematical models. Although it is still possible to investigate aspects of eye-movement control (e.g., word skipping or programming of refixations) in a nonmathematical way, a fully quantitative approach in which most of the experimental phenomena are integrated is necessary to test the interaction of different theoretical assumptions (e.g., the potential impact of a mechanism for word skipping on refixation behavior). In perspective, computational models can be approximated with ...

Reading requires the orchestration of visual, attentional, language-related, and oculomotor processing constraints. This study replicates previous effects of frequency, predictability, and length of fixated words on fixation durations in natural reading and demonstrates new effects of these variables related to previous and next words. Results are based on fixation durations recorded from 222 persons, each reading 144 sentences. Such evidence for distributed processing of words across fixation durations challenges psycholinguistic immediacy-of-processing and eye-mind assumptions. Most of the time the mind processes several words in parallel at different perceptual and cognitive levels. Eye movements can help to unravel these processes.Keywords: eye movements, fixation duration, gaze, word recognition, reading Reading is a fairly recent cultural invention. The perceptual, attentional, and oculomotor processes enabling this remarkable and complex human skill had been in place for a long time before the first sentence was read. Of course, reading also fundamentally presupposes language, reasoning, and memory processes. If we want to understand how internal processes of the mind and external stimuli play together in the generation of complex action, reading may serve as an ideal sample case, because, despite its complexity, it occurs in settings that are very amenable to experimental control. In addition, the measurement of eye movements yields high-resolution time series that have proven to be very sensitive to factors at all levels of the behavioral and cognitive hierarchy. Most importantly, we already know or can determine basic perceptual, attentional, and oculomotor constraints which any theory of reading and any computational model implementing such a theory at a behavioral microlevel must respect.Looking at the eyes, reading proceeds as an alternating sequence of fixations (lasting 150 to 300 ms) and saccades (30 ms). Information uptake is largely restricted to fixations. For example, fixation durations reliably decrease with the printed frequency of words and with their predictability from prior words of the sentence. Beyond these uncontroversial facts, however, much still needs to be learned about perceptual and attentional processes and properties of words that guide the eyes through a sentence. Starr and Rayner (2001, p. 156) highlighted the following three issues as particularly controversial: "(1) the extent to which eye-movement behavior is affected by low-level oculomotor factors versus higher-level cognitive processes; (2) how much information is extracted from the right of fixations; and (3) whether readers process information from more than one word at a time." Distributed Processing in Fixation Durations 4In this article, we report new empirical results relating to each of these issues. We also present a data-analytic framework within which these issues can be addressed simultaneously and propose a set of theoretical principles which account for a complex set of experimental observations....

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Even during visual fixation of a stationary target, our eyes perform rather erratic miniature movements, which represent a random walk. These ''fixational'' eye movements counteract perceptual fading, a consequence of fast adaptation of the retinal receptor systems to constant input. The most important contribution to fixational eye movements is produced by microsaccades; however, a specific function of microsaccades only recently has been found. Here we show that the occurrence of microsaccades is correlated with low retinal image slip Ϸ200 ms before microsaccade onset. This result suggests that microsaccades are triggered dynamically, in contrast to the current view that microsaccades are randomly distributed in time characterized by their rate-of-occurrence of 1 to 2 per second. As a result of the dynamic triggering mechanism, individual microsaccade rate can be predicted by the fractal dimension of trajectories. Finally, we propose a minimal computational model for the dynamic triggering of microsaccades.random walks ͉ visual fixation ͉ eye movements ͉ saccade detection

The understanding of the control of eye movements has greatly benefited from the analysis of mathematical models. Currently most comprehensive models include sequential shifts of visual attention. Here we propose an alternative model of eye movement control, which includes three new principles: spatially distributed lexical processing, a separation of saccade timing from saccade target selection, and autonomous (random) generation of saccades with foveal inhibition. These three features provide a common control mechanism for fixations, refixations, and regressions. Consequently, the model is called SWIFT (Saccade-generation with inhibition by foveal targets). Results from numerical simulations are in good agreement with effects of word frequency on single-fixation, first-fixation, and gaze durations as well as fixation and word skipping probabilities in first-pass analysis. The model inherently produces complex eye movement patterns including refixations and regressions due to its underlying dynamical principles.

Eye-movement control during scene viewing can be represented as a series of individual decisions about where and when to move the eyes. While substantial behavioral and computational research has been devoted to investigating the placement of fixations in scenes, relatively little is known about the mechanisms that control fixation durations. Here we propose a computational model (CRISP) that accounts for saccade timing and programming and thus for variations in fixation durations in scene viewing.

Microsaccades are one component of the small eye movements that constitute fixation. Their implementation in the oculomotor system is unknown. To better understand the physiological and mechanistic processes underlying microsaccade generation, we studied microsaccadic inhibition, a transient drop of microsaccade rate, in response to irrelevant visual and auditory stimuli. Quantitative descriptions of the time course and strength of inhibition revealed a strong dependence of microsaccadic inhibition on stimulus characteristics. In Experiment 1, microsaccadic inhibition occurred sooner after auditory than after visual stimuli and after luminance-contrast than after color-contrast visual stimuli. Moreover, microsaccade amplitude strongly decreased during microsaccadic inhibition. In Experiment 2, the latency of microsaccadic inhibition increased with decreasing luminance contrast. We develop a conceptual model of microsaccade generation in which microsaccades result from fixation-related activity in a motor map coding for both fixation and saccades. In this map, fixation is represented at the central site. Saccades are generated by activity in the periphery, their amplitude increasing with eccentricity. The activity at the central, fixation-related site of the map predicts the rate of microsaccades as well as their amplitude and direction distributions. This model represents a framework for understanding the dynamics of microsaccade behavior in a broad range of tasks.