Assuming that scenes would be visually scanned by chunking information, we partitioned fixation sequences of web page viewers into chunks using isolate gaze point(s) as the delimiter. Fixations were coded in terms of the segments in a mesh imposed on the screen. The identified chunks were mostly short, consisting of one or two fixations. These were analyzed with respect to the within- and between-chunk distances in the overall records and the patterns (i.e., subsequences) frequently shared among the records. Although the two types of distances were both dominated by zero- and one-block shifts, the primacy of the modal shifts was less prominent between chunks than within them. The lower primacy was compensated by the longer shifts. The patterns frequently extracted at three threshold levels were mostly simple, consisting of one or two chunks. The patterns revealed interesting properties as to segment differentiation and the directionality of the attentional shifts. 1. Introduction Eyes seldom stay completely still. They continually move even when one tries to fixate one’s gaze on an object because of the tremors, drifts, and microsaccades that occur on a small scale [1]. Hence, researchers need to infer a fixation from consecutive gaze points clustered in space [2]. We may regard such a cluster of gaze points as a perceptual chunk, a familiar term in psychology after Miller [3] in referring to a practically meaningful unit of information processing. During fixation, people closely scan a limited part of the scene they are interested in. They then quickly move their eyes to the next fixation area by saccade, which momentarily disrupts vision. However, it normally goes unnoticed thanks to our vision system that produces continuous transsaccades perception [4–6]. It means that successive fixations constitute a higher order chunking over and above the primary chunking of gaze points. Put metaphorically, the relationship is analogous to the relationship. For the sake of brevity, a chunk of fixations will be referred to as a chunk. In viewing natural scenes or displays, a chunk continues to grow until interrupted by one or more isolate gaze points resulting from drifting attention or by accident. These do not participate in any fixation. Whatever causes the interruption, we believe that such isolate points serve as chunk delimiters, like the pauses in speech. As a pause can be either short or long, interruptions by isolate points can vary in length. Figure 1 illustrates two levels of chunking: (a) chunking of gaze points into fixations and (b) chunking of
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