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In reading, binocular eye movements are required for optimal visual processing and thus, in case of asthenopia or reading problems, standard orthoptic and optometric routines check individual binocular vision by a variety of tests. The present study therefore examines the predictive value of such standard measures of heterophoria, accommodative and vergence facility, AC/A-ratio, NPC and symptoms for binocular coordination parameters during reading. Binocular eye movements were recorded (EyeLink II) for 65 volunteers during a typical reading task and linear regression analyses related all parameters of binocular coordination to all above-mentioned optometric measures: while saccade disconjugacy was weakly predicted by vergence facility (15% explained variance), vergence facility, AC/A and symptoms scores predicted vergence drift (31%). Heterophoria, vergence facility and NPC explained 31% of fixation disparity and first fixation duration showed minor relations to symptoms (18%). In sum, we found only weak to moderate relationships, with expected, selective associations: dynamic parameter related to optometric tests addressing vergence dynamics, whereas the static parameter (fixation disparity) related mainly to heterophoria. Most surprisingly, symptoms were only loosely related to vergence drift and fixation duration, reflecting associations to a dynamic aspect of binocular eye movements in reading and potentially non-specific, overall but slight reading deficiency. Thus, the efficiency of optometric tests to predict binocular coordination during reading was low – questioning a simple, straightforward extrapolation of such test results to an overlearned, complex task.

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.

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 to 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 ; (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.

We hypothesize that the high quality of binocular coordination of saccades in reading is progressively learned during childhood, and this oculomotor learning is based on a synergy between saccades and vergence. In present work deficits in the binocular control of saccades in six dyslexic children (mean age was 11±2.48 years) are studied for two tasks (text reading and Xs-C scanning), and at two viewing distances (40 cm and 100 cm). Fixation durations resulting are longer in Xs-C scanning task than in text reading task. We postulate that while reading motor preparation processes are executed with less demand for attentional resources. Importantly all physiological parameters of the saccades were the same for the two conditions and in either distance. Namely disconjugacy of saccades and disconjugate post-saccadic drifts were high but similar for the two conditions. Time analysis applied on saccade amplitude disconjugacy, on disconjugate post-saccadic drift and on fixation duration showed no significant effect of repetition or time. we believe that the binocular coordination deficits in dyslexic children reflect some type of microdyspraxia due to reduced oculomotor learning, perhaps related to inefficiency of the magnocellular visual system and the cerebellar functions.

Reading is an activity based on complex sequences of binocular saccades and fixations. During saccades, the eyes do not move together perfectly: saccades could end with a misalignment, compromising fused vision. During fixations, small disconjugate drift can partly reduce this misalignment. We hypothesized that maintaining eye alignment during reading involves active monitoring from posterior parietal cortex (PPC); this goes against traditional views considering only downstream binocular control.Nine young adults read a text; transcranial magnetic stimulation (TMS) was applied over the PPC every 5±0.2 sec. Eye movements were recorded binocularly with Eyelink II. Stimulation had three major effects: (1) disturbance of eye alignment during fixation; (2) increase of saccade disconjugacy leading to eye misalignment; (3) absence of eye alignment reduction during fixation drift. The effects depend on the side; the right PPC was more involved in maintaining alignment over the motor sequence.Thus, the PPC is actively involved in the control of binocular eye alignment during reading, allowing clear vision. Cortical activation during reading is related to linguistic processes and motor control per se. The study might be of interest for the understanding of deficits of binocular coordination, encountered in several populations, e.g. in children with dyslexia.

Smooth pursuit (SP) was explored in dyslexics and non-dyslexics. Dyslexic children show similar gain of SP, and number and amplitude of catch-up saccades (CUS) as non-dyslexic children. The quality of binocular coordination is good for both groups; the only significant exception is for pursuit to the right for both smooth phase and CUS; dyslexics show higher disconjugacy. Decrement of binocular control during rightward pursuit only could reflect immaturity of oculomotor learning mechanisms needed to optimize binocular coordination for all directions. Yet, these observations need to be confirmed in a larger population including older children and compared with other populations, e.g. with right-to-left 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.

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.

In this paper I present a brief review of some recent studies my colleagues and I have carried out to investigate binocular coordination during reading. These studies demonstrate that the eyes are often not perfectly aligned during reading, with fixation disparities of approximately one character on average. Both crossed and uncrossed disparities are common and vergence movements during fixations serve to reduce, but not eliminate disparity. Fixation disparity results in different retinal inputs from each eye, yet a single non diplopic visual representation of the text is perceived when we read. A further experiment, with dichoptically presented target words in normally presented sentence frames, showed that a mechanism of fusion rather than suppression operates at an early stage during visual processing. Saccade metrics appear to be computed according to a unified visual representation based on input from both eyes. based on input from both eyes.

The origin of binocular coordination of saccades (central, peripheral) and the role of learning remain controversial (Hering vs Helmholtz). We will present evidence for learning: in young children (5 years) horizontal saccades are poorly yoked, coordination improves slowly with age particularly at near viewing distances. In dyslexic teenagers coordination of horizontal saccades is poor relative to non-dyslexics, suggesting slower learning. On the other hand, in healthy elderly participants (73 years) coordination of vertical saccades is intact, an example of a non ageing sub-system. To assess further central mechanisms, we applied TMS over the posterior parietal cortex of healthy adults, 100 ms after target onset. TMS impaired coordination particularly for rightward and downward saccades. Thus binocular coordination of saccades relies partially on cerebral function.experimental stages.