Abstract
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.
Highlights
The origin of binocular coordination of saccades and the role of learning remain controversial (Hering vs Helmholtz)
One can consider the case of horizontal rightward saccades: because of the asymmetry of the oculomotor plant, a central saccade command would result in a saccade that was larger in the abducting than the adducting eye (Fig. 1A)
This study was the first to quantify the quality of binocular coordination during and after saccades in dyslexic participants, showing that the coordination is poor in both tasks whether it be reading single words or making saccades to LED targets
Summary
The origin of binocular coordination of saccades (central, peripheral) and the role of learning remain controversial (Hering vs Helmholtz). On the other hand, Zhou and King (1998) reported monocular coding of saccades in the saccade generator located at the paramedian reticular formation Based on such electrophysiological observations, King and Zhou (2002) proposed a model of control in line with Helmholtz’ thinking: the site of learning of binocular coordination would be downstream at the level of oculomotor nuclei rather than at the central saccade command level. Our driving hypothesis is that the CNS uses natural existing mechanisms of saccade-vergence interaction to render saccades equal for the two eyes To illustrate this hypothesis, one can consider the case of horizontal rightward saccades: because of the asymmetry of the oculomotor plant (difference in muscles and/or in relays of innervation), a central saccade command would result in a saccade that was larger in the abducting than the adducting eye (Fig. 1A). We will review studies of healthy subjects and patients, supporting the above mentioned conceptual model, that is, supporting the contention that binocular coordination depends on vergence
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