Department of Neurology, University of Ioannina, Ioannina, Greece.
Akutnahe Rehabilitation, Kantonsspital Baden, Baden, Switzerland.
J Neurophysiol. 2019 Nov 1;122(5):1928-1936. doi: 10.1152/jn.00343.2019. Epub 2019 Sep 4.
The functional significance of vestibular information for the generation of gaze shifts is controversial and less well established than the vestibular contribution to gaze stability. In this study, we asked seven bilaterally avestibular patients to execute voluntary, whole body pivot turns to visual targets up to 180° while standing. In these conditions, not only are the demands imposed on gaze transfer mechanisms more challenging, but also neck proprioceptive input represents an inadequate source of head-in-space motion information. Patients' body segment was slower and jerky. In the absence of visual feedback, gaze advanced in small steps, closely resembling normal multiple-step gaze-shift patterns, but as a consequence of the slow head motion, target acquisition was delayed. In ~25% of trials, however, patients moved faster but the velocity of prematurely emerging slow-phase compensatory eye movements remained lower than head-in-space velocity due to vestibuloocular failure. During these trials, therefore, gaze advanced toward the target without interruption but, again, taking longer than when normal controls use single-step gaze transfers. That is, even when patients attempted faster gaze shifts, exposing themselves to gaze instability, they acquired distant targets significantly later than controls. Thus, while patients are upright, loss of vestibular information disrupts not only gaze stability but also gaze transfers. The slow and ataxic head and trunk movements introduce significant foveation delays. These deficits explain patients' symptoms during upright activities and show, for the first time, the clinical significance of losing the so-called "anticompensatory" (gaze shifting) function of the vestibuloocular reflex. Previous studies in sitting avestibular patients concluded that gaze transfers are not substantially compromised. Still, clinicians know that patients are impeded (e.g., looking side to side before crossing a road). We show that during large gaze transfers while standing, vestibularly derived head velocity signals are critical for the mechanisms governing reorientation to distant targets and multisegmental coordination. Our findings go beyond the traditional role of the vestibular system in gaze stability, extending it to gaze transfers, as well.
前庭信息对于眼动扫视的产生具有何种功能意义仍存在争议，且不如前庭对眼动稳定的贡献那样得到广泛认可。在本研究中，我们要求七名双侧前庭功能丧失的患者在站立时执行向视觉目标的全身 180°旋转。在这些条件下，不仅对眼球传递机制的要求更具挑战性，而且颈部本体感觉输入代表了头在空间运动信息的不足来源。患者的身体运动较慢且不流畅。在没有视觉反馈的情况下，眼动以小步进行，非常类似于正常的多步扫视模式，但由于头部运动缓慢，目标获取会延迟。然而，在大约 25%的试验中，患者移动得更快，但由于前庭眼反射失败，过早出现的慢相补偿性眼球运动的速度仍低于头在空间中的速度。因此，在这些试验中，眼球朝着目标前进而没有中断，但由于头在空间中的运动缓慢，眼球运动的速度较低，眼球的扫视需要更长的时间。也就是说，即使患者试图更快地进行眼动扫视，从而暴露于眼动不稳定中，他们获取远距离目标的时间也明显长于正常对照组使用单步眼动转移时。因此，即使在患者直立时，失去前庭信息不仅会破坏眼动稳定性，还会破坏眼动转移。缓慢且共济失调的头部和躯干运动会导致注视点明显延迟。这些缺陷解释了患者在直立活动期间的症状，并首次表明失去前庭眼反射的所谓“反补偿”（眼动扫视）功能的临床意义。先前在坐姿前庭功能丧失患者中的研究得出结论，眼动转移并未受到严重影响。尽管如此，临床医生知道患者会受到阻碍（例如，在过马路前先向侧面看）。我们表明，在站立时进行大的眼动转移时，由前庭衍生的头部速度信号对于控制向远距离目标的重新定向和多节段协调的机制至关重要。我们的发现超越了前庭系统在眼动稳定中的传统作用，将其扩展到眼动转移中。
