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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. [ABSTRACT FROM AUTHOR]

Humans have two, frontally placed eyes and during reading oculomotor and sensory processes are needed to combine the two inputs into a unified percept of the text. Generally, slight vergence errors, i.e., fixation disparities, occur but do not cause double vision since disparate retinal inputs fall into Panum’s fusional area, that is, a range of disparity wherein sensory fusion of the two retinal images is achieved. In this study, we report benchmark data with respect to the mean magnitude and range of vertical compared to horizontal fixation disparities for natural reading. Our data clearly fit to an elliptical pattern of Panum’s fusional area that corresponds with theoretical estimates. Furthermore, when we examined disparity-driven vergence adjustments during fixations by comparing monocular with binocular reading conditions, we found that only horizontal fixation disparities increased significantly under conditions of monocular stimulation. Also, no significant vertical fine-tuning (vergence adjustment) was observed for vergence eye movements during reading fixations. Thus, horizontal and vertical fixation disparities and vergence adjustments during reading showed quite different characteristics, and this dissociation is directly related to the functional role of vergence adjustments: vertical fusion – and vertical vergence – subserve the maintenance of a single percept and stereopsis by keeping the eyes in register and allowing for horizontal fusional processes to successfully operate over a vertically aligned input. A reliable and stable vertical alignment is, thus, a pre-requisite over which horizontal fusional responses (and depth perception) can work most efficiently – even in a task like reading. [ABSTRACT FROM AUTHOR]

Studies dealing with developmental aspects of binocular eye movement behavior during reading are scarce. In this study we have explored binocular strategies during reading and visual search tasks in a large population of dyslexic and typical readers. Binocular eye movements were recorded using a video-oculography system in 43 dyslexic children (aged 8-13) and in a group of 42 age-matched typical readers. The main findings are: (i) ocular motor characteristics of dyslexic children are impaired in comparison to those reported in typical children in reading task; (ii) a developmental effect exists in reading in control children, in dyslexic children the effect of development was observed only on fixation durations; and (iii) ocular motor behavior in the visual search tasks is similar for dyslexic children and for typical readers, except for the disconjugacy during and after the saccade: dyslexic children are impaired in comparison to typical children. Data reported here confirms and expands previous studies on children's reading. Both reading skills and binocular saccades coordination improve with age in typical readers. The atypical eye movement's patterns observed in dyslexic children suggest a deficiency in the visual attentional processing as well as an impairment of the ocular motor saccade and vergence systems interaction. [ABSTRACT FROM AUTHOR]

Previous studies examining binocular coordination during reading have reported conflicting results in terms of the nature of disparity (e.g. Kliegl, Nuthmann, & Engbert (Journal of Experimental Psychology General 135:12-35, ); Liversedge, White, Findlay, & Rayner (Vision Research 46:2363-2374, ). One potential cause of this inconsistency is differences in acquisition devices and associated analysis technologies. We tested this by directly comparing binocular eye movement recordings made using SR Research EyeLink 1000 and the Fourward Technologies Inc. DPI binocular eye-tracking systems. Participants read sentences or scanned horizontal rows of dot strings; for each participant, half the data were recorded with the EyeLink, and the other half with the DPIs. The viewing conditions in both testing laboratories were set to be very similar. Monocular calibrations were used. The majority of fixations recorded using either system were aligned, although data from the EyeLink system showed greater disparity magnitudes. Critically, for unaligned fixations, the data from both systems showed a majority of uncrossed fixations. These results suggest that variability in previous reports of binocular fixation alignment is attributable to the specific viewing conditions associated with a particular experiment (variables such as luminance and viewing distance), rather than acquisition and analysis software and hardware. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the effective fusional range for written stimuli in children and adults in a natural viewing situation. We recorded binocular eye movements in children and adults during processing of stereoscopically presented words in a lexical decision task. The effect of disparity magnitude on ease of fusion caused decreased response accuracy, increasing numbers of fixations and increased trial viewing times when retinal disparity exceeded one character space. The data suggest that retinal inputs of a word that are disparate by up to one character (0.37°) fall within the effective fusional range such that lexical decisions are not impaired. [Copyright &y& Elsevier]

Abstract: We examined the influence of a variety of visual factors on binocular coordination during saccadic orienting. Some experimental conditions placed similar demands on the oculomotor system as those that occur during reading, but in the absence of linguistic processing. We examined whether saccade target extent, preceding saccade magnitude, preceding saccade direction, and parafoveal availability of saccade target influenced fixation disparity. Disparities similar in magnitude and frequency to those obtained in previous binocular reading experiments occurred. Saccade magnitude had a robust influence upon fixation disparities. The results are very similar to those obtained in investigations of binocular coordination during reading, and indicate that similar patterns occur during reading-like eye scanning behaviour, in the absence of linguistic processing. [Copyright &y& Elsevier]

Binocular yoking of saccades is essential for single vision of words during reading. This study examines the quality of binocular coordination in individuals with dyslexia, independent of the process of reading. Fifteen dyslexia children (11.2±1.4 years) and 15 non-dyslexia individuals (8 children, aged 11.1±1.3 years, and 7 adults, 24±3 years) were studied. Eye movements were recorded in two conditions. In the control condition, participants made saccades to a single target where the saccade direction and magnitude were controlled. In the experimental condition saccades were allowed to move freely while viewing paintings. The results indicated that, compared with the non-dyslexia group, the dyslexia group showed a larger saccade amplitude difference between the two eyes, as well as a larger conjugate post-saccadic drift, during painting exploration than that for saccades to a single target. While both groups showed a larger disconjugate post-saccadic drift during painting exploration relative to the control condition, this showed a negative correlation with saccade disconjugacy (i.e. disconjugate drift reduced the disparity) only for the non-dyslexia group. These results indicate that individuals with dyslexia have problems of binocular coordination, both during the saccade and fixations, which are independent of the reading process. It is suggested that this reflects an immaturity of the normal oculomotor learning mechanisms. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Disconjugate oculomotor adaptation is driven by the need to maintain binocular vision. Since binocular vision in Duane Retraction Syndrome (DRS) patients is normal in half of their horizontal field of gaze (i.e., sound-side of gaze), we wondered whether oculomotor adaptive capabilities are efficient despite such a severe impairment of eye motility towards the other half of the horizontal field of gaze (i.e., affected-side gaze). We compared properties of horizontal saccades of patients with congenital unilateral Duane Retraction Syndrome type I in binocular viewing and monocular viewing conditions by simultaneously recording both eyes with the search coil technique. Our results show a mismatch between the pulse and the step signal of the innervation for saccades. When tested in the affected eye viewing condition (sound eye covered), the eyes showed not only similarly-directed increases of the saccadic gain (pulse signal) in the two eyes but also disjunctive post-saccadic drifts (step signal). This behavior suggests that visuomotor errors presented only to the affected eye were transferred to the sound eye, producing conjugate changes of the saccadic command. The post-saccadic command remained unchanged, however, and controlled the final position of each eye separately. This suggests that monocular adaptation is possible only for the step of innervation (i.e., controlling the final eye position) but not for the pulse of innervation (i.e., controlling the saccadic gain), even though the peculiarity of unilateral DRS type I offers a clear advantage for separate pathways of control for the two eyes. [Copyright &y& Elsevier]

Abstract: Binocular coordination of the eyes during reading was examined. Fixation disparity greater than one character occurred on 47% of fixations, with the disparity being predominantly uncrossed (39%), though a small proportion of fixations were crossed. The average magnitude of disparity, measured at the end of fixation, was 1.1 characters for all fixations. For the 47% of non-aligned fixations the average magnitude of disparity was 1.9 characters. Vergence movements that reduced fixation disparity occurred during fixations, and their magnitude was positively correlated with fixation duration. Finally, eye dominance did not modulate fixation disparity magnitude or the proportion of disparate fixations. [Copyright &y& Elsevier]

Stabilization of images on the fovea during either fore/aft translation of a subject or fore/aft movement of a visual target in front of a stationary observer imposes complex geometrical requirements that depend upon the eccentricity of the object of interest with respect to the eyes. Each eye needs to be rotated independently with varying proportions of conjugate (version) and disconjugate (vergence) eye movements to maintain fixation of the target. Here, we describe binocular coordination in the early response to translational movements of normal subjects along their naso-occipital axis. We recorded the responses evoked by small (about 4 cm), abrupt (about 0.7 g), fore/aft translations in four normal subjects while they viewed a near target. In the forward and backward starting positions the target was 15 or 10.5 cm away, respectively. Each subject was tested with the target centered between the eyes, aligned on the right eye, and placed to the right of the right eye by ∼3 cm. The three conditions differed only in the lateral eccentricity of the target, yet the geometrical requirements for image stabilization are very different: pure vergence, one eye still, or mostly version. We found that the eye-movement responses closely matched what was needed for visual stabilization of the target, though responses to stimuli calling for divergence were less accurate than those for convergence. The latency of these responses ranged from 40 to 65 ms and achieved about 80% of the ideal response by 90 to 100 ms after the onset of the stimulus. Next, we asked whether these eye movements were generated by the vestibular system or by high-level strategies for image stabilization, such as pursuit. Thus, in a second set of experiments we used the mean profile of fore\aft body motion computed for each subject to drive a small visual target across the same distances and in the same eccentricities used during body translations. We found that visually driven responses had longer latencies (by at least 80 ms, ranging from 144 to 155 ms) and slower dynamics (with significantly lower peak eye velocities), highlighting the different subsystems producing the two types of responses. Saccades were also an important component of the response to both visual and vestibular stimuli, less frequent during the centered- target configuration and more frequent during viewing of eccentric targets. Visual stimuli evoked saccadic corrections more often and at shorter latencies than did vestibular stimuli. Both smooth and saccadic eye movements were appropriately disconjugate and their pattern depended on whether the eyes were converging or diverging. [ABSTRACT FROM AUTHOR]

We examined three-dimensional binocular positions in the alert and sleepy monkeys. In contrast to the tightly yoked eye movements observed in alertness, the eyes were usually converged, vertically misaligned and had a much larger torsional variability during light sleep. While in alertness eye position vectors were confined to fronto-parallel planes, the corresponding planes were rotated temporally (e.g. leftward for the left eye) in light sleep. There was no correlation between temporal rotation of the eye position planes and horizontal vergence. All these observations can be explained by randomly innervated extraocular muscles that are rotating the two eyes about anatomically determined axes. [Copyright &y& Elsevier]