Vergence responses to vertical binocular disparity during lexical identification

Large-scale cortico-cerebellar computations for horizontal and vertical vergence in humans

Horizontal and vertical vergence eye movements play a central role in binocular coordination. Neurophysiological studies suggest that cortical and subcortical regions in animals and humans are involved in horizontal vergence. However, little is known about the extent to which the neural mechanism underlying vertical vergence overlaps with that of horizontal vergence. In this study, to explore neural computation for horizontal and vertical vergence, we simultaneously recorded electrooculography (EOG) and whole-head magnetoencephalography (MEG) while presenting large-field stereograms for 29 healthy human adults. The stereograms were designed to produce vergence responses by manipulating horizontal and vertical binocular disparities. A model-based approach was used to assess neural sensitivity to horizontal and vertical disparities via MEG source estimation and the theta-band (4 Hz) coherence between brain activity and EOG vergence velocity. We found similar time-locked neural responses to horizontal and vertical disparity in cortical and cerebellar areas at around 100–250 ms after stimulus onset. In contrast, the low-frequency oscillatory neural activity associated with the execution of vertical vergence differed from that of horizontal vergence. These findings indicate that horizontal and vertical vergence involve partially shared but distinct computations in large-scale cortico-cerebellar networks.

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.

Vertical disconjugacy during reading in dyslexic and non-dyslexic children

PURPOSE

The objective of this study was to explore vertical binocular coordination in dyslexic and non-dyslexic children during saccades and post-saccadic fixation period while reading a text.

METHODS

Binocular eye movements were recorded by an infrared system (Mobile T2^®, SuriCog) in thirty-six dyslexic children from 7.3 to 13.6 years of age (mean age: 10.4 ± 0.3 years) who were asked to silently read a four-line text during binocular viewing. Data were compared to those of thirty-six age-matched non-dyslexic children.

RESULTS

Vertical disconjugacy during post-saccadic fixation was higher in dyslexic children with respect to non-dyslexic children group. Vertical disconjugacy was not age-dependent either for dyslexic children or for non-dyslexic children.

CONCLUSIONS

The poor binocular vertical coordination observed in dyslexic children while reading could suggest a deficiency in the cerebellum and/or extra-ocular muscles involved in vertical eye alignment. Moreover, the fact that this vertical binocular coordination was not age-dependent could be due to an abnormal eye position and/or to a dysfunction of midbrain structures involved in vertical vergences.

The word frequency effect during sentence reading: A linear or nonlinear effect of log frequency?

The effect of word frequency on eye movement behaviour during reading has been reported in many experimental studies. However, the vast majority of these studies compared only two levels of word frequency (high and low). Here we assess whether the effect of log word frequency on eye movement measures is linear, in an experiment in which a critical target word in each sentence was at one of three approximately equally spaced log frequency levels. Separate analyses treated log frequency as a categorical or a continuous predictor. Both analyses showed only a linear effect of log frequency on the likelihood of skipping a word, and on first fixation duration. Ex-Gaussian analyses of first fixation duration showed similar effects on distributional parameters in comparing high- and medium-frequency words, and medium- and low-frequency words. Analyses of gaze duration and the probability of a refixation suggested a nonlinear pattern, with a larger effect at the lower end of the log frequency scale. However, the nonlinear effects were small, and Bayes Factor analyses favoured the simpler linear models for all measures. The possible roles of lexical and post-lexical factors in producing nonlinear effects of log word frequency during sentence reading are discussed.