von der Malsburg and Vasishth Version 2

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A New Method for Analyzing Eye Movements in Reading that Is Sensitive to Fixation Patterns and Fixation Durations

Titus von der Malsburg and Shravan Vasishth

In the investigation of reading, word-based measures such as gaze duration and regression probability, are commonly used for analyzing empirical data. However, these measures are not always adequate when the effect of an experimental manipulation is expected to appear in patterns of fixations rather than in local measures. One example is the study of reanalysis strategies in temporarily ambiguous sentences. We address this shortcoming by proposing alternative methods for analyzing eye movements that are based on the notion of similarity of scanpaths. For this purpose we introduce a measure that quantifies the similarity of two scanpaths with respect to their fixation patterns and their temporal dynamics.

Our metric is a member of the class of edit-distances of which the Levenshtein metric is an instance (Levenshtein 1968). Edit-distances have already been used for analysing eye tracking data in a variety of studies outside reading research, for example in experiments studying scene perception or the usability of graphical user interfaces [citation]. While edit-distances capture the topological similarities of scanpaths, they operate on a representation of scanpaths that neglects two crucial properties: The first is fixation duration, which is one of the most important and best-studied sources of evidence in online reading research. The second is the spatial distance of regions of interest. The observation of parafoveal preview effects, however, suggest that the spatial distance does play an important role and should be accounted for [cite something that demonstrates preview effects].

We addressed these deficits in standard edit-distance measures by replacing the cost function of the Levenshtein metric with a function that compares fixations with respect to their duration and distance. The way we incorporate distance is motivated by findings about acuity in human visual perception (Rovamo, Virsu, Näsänen, 1978). The resulting measure operates on a parsimonious representation of eye movement data and can efficiently be computed (O(m*n), where n, m are the numbers of fixations of the two compared scanpaths).

Possible approaches to analysing data based on this measure are: 1.) Cluster analysis of sets of scanpaths in order to identify reading strategies, 2.) analysis of variance of the pairwise similarities in sets of scanpaths for identifying different populations, 3.) application of string interpolation methods and optimization techniques for computing prototypical scanpaths for sets of scanpaths.

Areas of investigation where these methods might also be useful comprise the evaluation of models of oculo-motor control, classification of reading disorders, analysis of cognitive strategies in experiments using the visual world paradigm.

As opposed to approaches using word-based measures, the proposed similarity-based methods enable the analysis of the full available data and do not necessitate averaging of variables thereby preserving possibly crucial information. The main benefit of our similarity measure is that it is sensitive to fixation patterns and the temporal dynamics in a scanpaths at the same time.