Abstract
In our study, 14 subjects read 60 sentences from the Potsdam Sentence Corpus twice (viewing distance: 60 cm), while eye movements were measured with the EyeLink II. We analyzed fixation disparities for complete sentence replications (N=388). After subtracting the average fixation disparity of each sentence from each observation (which gave the “state” fixation disparity), 99% of all remaining fixation disparities were aligned, i.e. smaller than one character width (20 min arc) – depending mostly on incoming saccade amplitude and fixation position. Additionally, we measured the heterophoria for each subject during calibration and found a qualitative relationship between average, individual measures of fixation disparity (“trait” fixation disparity) and heterophoria, after dividing the sample in 3 groups of esophore, exophore and orthophore subjects. We showed that the magnitude of “trait” fixation disparity was biased by the direction of heterophoria: the more eso the heterophoria, the more eso the average sentence fixation disparity. In sum, despite a large “trait” fixation disparity (in the range of -6.6 to +33.6 min arc), “state” fixation disparities within a sentence were on average -0.9 (± 8.7) min arc and, thus, as precise as needed, i.e. within the expected extent of Panum’s area.
Highlights
Eye movement research in reading has traditionally been associated with the investigation of visual processing and language comprehension (see, for example: (Kliegl, Nuthmann & Engbert, 2006, Liversedge, White, Findlay & Rayner, 2006b, Rayner, 1998))
We read with both eyes, and besides saccadic eye movements our eyes perform vergence eye movements
Regarding the vergence movements during the saccade and during the postsaccadic drift, we found the same pattern, that were described in previous research (Hendriks, 1996, Liversedge et al, 2006b, Nuthmann & Kliegl, 2009, Vernet & Kapoula, 2009)
Summary
Eye movement research in reading has traditionally been associated with the investigation of visual processing and language comprehension (see, for example: (Kliegl, Nuthmann & Engbert, 2006, Liversedge, White, Findlay & Rayner, 2006b, Rayner, 1998)). In (theoretically) optimal binocular vision, the principal visual directions of both eyes intersect at the fixation point; slight deviations - fixation disparities (FD) or vergence errors - from this optimal state typically amount to a few minutes of arc and are smaller than the Panum’s area (i.e. the range of disparity where sensory fusion of the two retinal images is performed), not leading to double vision. These fixation disparities are called exo or eso when the visual axes of the eyes converge slightly behind or in front of the fixation point, respectively
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