Abstract
Spontaneous eye movements of zebrafish larvae in the dark consist of centrifugal saccades that move the eyes from a central to an eccentric position and postsaccadic centripetal drifts. In a previous study, we showed that the fitted single-exponential time constants of the postsaccadic drifts are longer in the temporal-to-nasal (T->N) direction than in the nasal-to-temporal (N->T) direction. In the present study, we further report that saccadic peak velocities are higher and saccadic amplitudes are larger in the N->T direction than in the T->N direction. We investigated the underlying mechanism of this ocular disconjugacy in the dark with a top-down approach. A mathematic ocular motor model, including an eye plant, a set of burst neurons and a velocity-to-position neural integrator (VPNI), was built to simulate the typical larval eye movements in the dark. The modeling parameters, such as VPNI time constants, neural impulse signals generated by the burst neurons and time constants of the eye plant, were iteratively adjusted to fit the average saccadic eye movement. These simulations suggest that four pools of burst neurons and four pools of VPNIs are needed to explain the disconjugate eye movements in our results. A premotor mechanism controls the synchronous timing of binocular saccades, but the pools of burst and integrator neurons in zebrafish larvae seem to be different (and maybe separate) for both eyes and horizontal directions, which leads to the observed ocular disconjugacies during saccades and postsaccadic drifts in the dark.
Highlights
Many lateral-eyed afoveate animals, such as rabbit (Collewijn, 1969; Baarsma and Collewijn, 1974), rat (Hess et al, 1985; van Alphen et al, 2010), goldfish (Easter, 1971; Beck et al, 2004), and zebrafish (Beck et al, 2004; Huang and Neuhauss, 2008), display yoked eye movements: the two eyes move in the same direction and the timings of binocular saccadesAbbreviations: velocity-to-position neural integrator (VPNI), Velocity-to-position neural integrator; N->T, Nasal-to-temporal; T->N, Temporal-to nasal.Disconjugacy in Zebrafish Larvae are synchronous
It has been shown that, in the dark, 5 days post fertilization zebrafish larvae display spontaneous eye movements consisting of centrifugal saccades and subsequent postsaccadic centripetal drifts
Since the medium used to restrain body movements could affect the eye movement recording, our results can be compared to those by Beck et al (2004), who restricted only the body movement with agarose. In this measuring method with a higher recording frame rate (60 Hz), the slopes of maximum velocity vs. amplitude in zebrafish larvae were 12–13 (Beck et al, 2004, Figure 5B), which are similar to values in our study
Summary
Many lateral-eyed afoveate animals, such as rabbit (Collewijn, 1969; Baarsma and Collewijn, 1974), rat (Hess et al, 1985; van Alphen et al, 2010), goldfish (Easter, 1971; Beck et al, 2004), and zebrafish (Beck et al, 2004; Huang and Neuhauss, 2008), display yoked eye movements: the two eyes move in the same direction and the timings of binocular saccadesDisconjugacy in Zebrafish Larvae are synchronous. It has been shown that, in the dark, 5 days post fertilization (dpf) zebrafish larvae display spontaneous eye movements consisting of centrifugal saccades and subsequent postsaccadic centripetal drifts (see typical eye traces in Figure 1 of Chen et al, 2014). We re-analyzed the data from our previous study (Chen et al, 2014) by calculating saccadic peak velocities and saccadic amplitudes After confirming both the disconjugate saccadic and postsaccadic eye movements in our results, we raised the following question: what is the underlying mechanism responsible for the yoked but disconjugate eye movements of zebrafish larvae in the dark?
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