Binocular advantages in reading revisited: attenuating effects of individual horizontal heterophoria

Reading with two eyes necessitates efficient processes of binocular vision, which provide a single percept of the text. These processes come with a binocular advantage: binocular reading shows shorter average fixation durations and sentence reading times when compared to monocular reading. A couple of years ago, we showed for a small sample (N=13) that binocular advantages critically relate to the individual heterophoria (the resting state of vergence). In the present, large-scale replication we collected binocular eye movements (Eyelink II) for 94 participants who read 20 sentences monocularly and 20 sentences binocularly. Further, individual heterophorias were determined using three different optometric standards: objective eye tracking (EyeLink II at 60 cm), Maddox wing test (at 30 cm) and measures following the “Guidelines for the application of the Measuring and Correcting Methodology after H.-J. Haase” (MCH; at 6 m). Binocular eye movements showed typical pattern and we replicated (1) binocular advantages of about 25 ms for average fixation durations and (2) a reduction in binocular advantages when heterophoria increased – but only when heterophoria was identified by EyeLink II or Maddox wing measures; MCH measures of heterophoria did not affect binocular advantages in reading. For large heterophorias binocular reading even turned into a disadvantage. Implications for effect estimations and optometric testing will be discussed

“Trait” and “state” aspects of fixation disparity during reading

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

Comparison of shutter glasses and mirror stereoscope for measuring dynamic and static vergence

Vergence eye movement recordings in response to disparity step stimuli require to present different stimuli to the two eyes. The traditional method is a mirror stereoscope. Shutter glasses are more convenient, but have disadvantages as limited repetition rate, residual cross task, and reduced luminance. Therefore, we compared both techniques measuring (1) dynamic disparity step responses for stimuli of 1 and 3 deg and (2) fixation disparity, the static vergence error. Shutter glasses and mirror stereoscope gave very similar dynamic responses with correlations of about 0.95 for the objectively measured vergence velocity and for the response amplitude reached 400 ms after the step stimulus (measured objectively with eye movement recordings and subjectively with dichoptic nonius lines). Both techniques also provided similar amounts of fixation disparity, tested with dichoptic nonius lines

The effect of calibration errors on the accuracy of the eye movement recordings

For calibrating eye movement recordings, a regression between spatially defined calibration points and corresponding measured raw data is performed. Based on this regression, a confidence interval (CI) of the actually measured eye position can be calculated in order to quantify the measurement error introduced by inaccurate calibration coefficients. For calculating this CI, a standard deviation (SD) - depending on the calibration quality and the design of the calibration procedure - is needed. Examples of binocular recordings with separate monocular calibrations illustrate that the SD is almost independent of the number and spatial separation between the calibration points – even though the later was expected from theoretical simulation. Our simulations and recordings demonstrate that the SD depends critically on residuals at certain calibration points, thus robust regressions are suggested

Microsaccades under monocular viewing conditions

Among the eye movements during fixation, the function of small saccades occuring quite commonly at fixation is still unclear. It has been reported that a substantial number of these microsaccades seem to occur in only one of the eyes. The aim of the present study is to investigate microsaccades in monocular stimulation conditions. Although this is an artificial test condition which does not occur in natural vision, this monocular presentation paradigm allows for a critical test of a presumptive monocular mechanism of saccade generation. Results in these conditions can be compared with the normal binocular stimulation mode. We checked the statistical properties of microsaccades under monocular stimulation conditions and found no indication for specific interactions for monocularly detected small saccades, which might be present if they were based on a monocular physiological activation mechanism

Akkomodation und Kognition:eine Frage der vegetativen Balance?

Der Ziliarmuskel und das Herz erfüllen unterschiedliche Aufgaben - werden aber beide über das vegetative Nervensystem gesteuert. Eine Steigerung der Herzrate bei einer leichten Leseaufgabe geht mit einer Verlagerung der Akkommodationsruhelage in die Nähe einher und die Akkommodation verlagert sich mit Zunahme der kognitiven Anforderung in die Ferne, während die Herzperiode abnimmt. Ursache ist eine Verschiebung in der vegetativen Balance. Ziel war es, die Akkommodationsveränderung dem Sympathikus bzw. Parasympathikus zu zuordnen. Es konnte allerdings keine signifikante Änderung der Akkommodation erneut gezeigt werden. Weder das "Lesen/Addieren" einer Zahlenmatrix noch ein "Lesen", "Addieren" und "Multiplizieren" mehrstelliger Zahlen führte zu einer Akkommodationsänderung. (Die Verhaltensdaten, Pupillengröße, Herzperiode sowie die Pulswellenlaufzeit zeigten eine Reaktion.) Geringe Akkommodationsänderungen gingen auf eine Blickrichtungsänderung und die Richtigkeit der Aufgabenbearbeitung zurück

Does text contrast mediate binocular advantages in reading?

Humans typically make use of both of their eyes in reading and efficient processes of binocular vision provide a stable, single percept of the text. Binocular reading also comes with an advantage: reading speed is high and word frequency effects (i.e., faster lexical processing of words that are more often encountered in a language) emerge during fixations, which is not the case for monocular reading (Jainta, Blythe, & Liversedge, 2014). A potential contributor to this benefit is the reduced contrast in monocular reading: reduced text contrasts in binocular reading are known to slow down reading and word identification (Reingold & Rayner, 2006). To investigate whether contrast reduction mediates the binocular advantage, we first replicated increased reading time and nullified frequency effects for monocular reading (Experiment 1). Next, we reduced the contrast for binocular but whole sentences to 70% (Weber-contrast); this reading condition resembled monocular reading, but found no effect on reading speed and word identification (Experiment 2). A reasonable conclusion, therefore, was that a reduction in contrast is not the (primary) factor that mediates less efficient lexical processing under monocular reading. In a third experiment (Experiment 3) we reduced the sentence contrast to 40% and the pattern of results showed that, globally, reading was slowed down but clear word frequency effects were present in the data. Thus, word identification processes during reading (i.e., the word frequency effect) were qualitatively different in monocular reading compared with effects observed when text was read with substantially reduced contrast

The Pupil Reflects Motor Preparation for Saccades – Even before the Eye Starts to Move

The eye produces saccadic eye movements whose reaction times are perhaps the shortest in humans. Saccade latencies reflect ongoing cortical processing and, generally, shorter latencies are supposed to reflect advanced motor preparation. The dilation of the eye’s pupil is reported to reflect cortical processing as well. Eight participants made saccades in a gap and overlap paradigm (in pure and mixed blocks), which we used in order to produce a variety of different saccade latencies. Saccades and pupil size were measured with the EyeLink II. The pattern in pupil dilation resembled that of a gap effect: for gap blocks, pupil dilations were larger compared to overlap blocks; mixing gap and overlap trials reduced the pupil dilation for gap trials thereby inducing a switching cost. Furthermore, saccade latencies across all tasks predicted the magnitude of pupil dilations post hoc: the longer the saccade latency the smaller the pupil dilation before the eye actually began to move. In accordance with observations for manual responses, we conclude that pupil dilations prior to saccade execution reflect advanced motor preparations and therefore provide valid indicator qualities for ongoing cortical processes

Using a dichoptic moving window presentation technique to investigate binocular advantages during reading

Reading comes with a clear binocular advantage, expressed in shorter fixation times and fewer regressions in binocular relative to monocular visual presentations. Little is known, however, about whether the cost associated with monocular viewing derives primarily from the encoding of foveal information or in obtaining a preview benefit from upcoming parafoveal text. In the present sentence reading eye tracking experiment, the authors used a novel dichoptic binocular gaze-contingent moving window technique to selectively manipulate the amount of text made available to the reader both binocularly and monocularly in the fovea and parafovea on a fixation-by-fixation basis. This technique allowed the authors to quantify disruption to reading caused by prevention of binocular fusion during direct fixation of words and parafoveal preprocessing of upcoming text. Sentences were presented (a) binocularly; (b) monocularly; (c) with monocular text to the left of fixation; (d) with monocular text to the right of fixation; or (e) with all words other than the fixated word presented binocularly. A robust binocular advantage occurred for average fixation duration and regressions. Also, while there was a limited cost associated with monocular foveal processing, the restriction of parafoveal processing to monocular information was particularly disruptive. The findings demonstrate the critical importance of a unified binocular input for the efficient preprocessing text to the right of fixation