Maurer C, Mergner T, Becker W, Jürgens R
Department of Neurology, Neurozentrum, University of Freiburg, Germany.
Exp Brain Res. 1998 Oct;122(3):260-74. doi: 10.1007/s002210050514.
Eye-head coordination during saccadic gaze shifts normally relies on vestibular information. A vestibulo-saccadic reflex (VSR) is thought to reduce the eye-in-head saccade to account for current head movement, and the vestibulo-ocular reflex (VOR) stabilizes postsaccadic gaze while the head movement is still going on. Acute bilateral loss of vestibular function is known to cause overshoot of gaze saccades and postsaccadic instability. We asked how patients suffering from chronic vestibular loss adapt to this situation. Eye and head movements were recorded from six patients and six normal control subjects. Subjects tracked a random sequence of horizontal target steps, with their heads (1) fixed in primary position, (2) free to move, or (3) preadjusted to different head-to-target offsets (to provoke head movements of different amplitudes). Patients made later and smaller head movements than normals and accepted correspondingly larger eye eccentricities. Targeting accuracy, in terms of the mean of the signed gaze error, was better in patients than in normals. However, unlike in normals, the errors of patients exhibited a large scatter and included many overshoots. These overshoots cannot be attributed to the loss of VSR because they also occurred when the head was not moving and were diminished when large head movements were provoked. Patients' postsaccadic stability was, on average, almost as good as that of normals, but the individual responses again showed a large scatter. Also, there were many cases of inappropriate postsaccadic slow eye movements, e.g., in the absence of concurrent head movements, and correction saccades, e.g., although gaze was already on target. Performance in patients was affected only marginally when large head movements were provoked. Except for the larger lag of the head upon the eye, the temporal coupling of eye and head movements in patients was similar to that in normals. Our findings show that patients with chronic vestibular loss regain the ability to make functionally appropriate gaze saccades. We assume, in line with previous work, three main compensatory mechanisms: a head movement efference copy, an active cervico-ocular reflex (COR), and a preprogrammed backsliding of the eyes. However, the large trial-to-trial variability of targeting accuracy and postsaccadic stability indicates that the saccadic gaze system of patients does not regain the high precision that is observed in normals and which appears to require a vestibular head-in-space signal. Moreover, this variability also permeates their gaze performance in the absence of head movements.
扫视性眼动过程中的眼头协调通常依赖于前庭信息。前庭-扫视反射（VSR）被认为可减少眼在头内的扫视幅度，以适应当前的头部运动，而前庭-眼反射（VOR）则在头部运动仍在进行时稳定扫视后的注视。已知急性双侧前庭功能丧失会导致扫视性眼动的过冲和扫视后不稳定。我们研究了患有慢性前庭功能丧失的患者如何适应这种情况。记录了6名患者和6名正常对照受试者的眼动和头部运动。受试者跟踪一系列随机的水平目标步移，头部状态分别为：（1）固定在初始位置；（2）可自由移动；（3）预先调整到不同的头-目标偏移量（以引发不同幅度的头部运动）。患者的头部运动比正常人更迟且幅度更小，相应地接受了更大的眼偏心率。就有符号注视误差的平均值而言，患者的目标定位准确性优于正常人。然而，与正常人不同的是，患者的误差表现出较大的离散度，且包括许多过冲情况。这些过冲不能归因于VSR的丧失，因为它们在头部不运动时也会出现，而在引发大幅度头部运动时会减小。患者扫视后的稳定性平均而言与正常人几乎一样好，但个体反应再次表现出较大的离散度。此外，还存在许多不适当的扫视后缓慢眼动情况，例如在没有同时发生的头部运动时，以及校正性扫视情况，例如尽管注视已经在目标上。当引发大幅度头部运动时，患者的表现仅受到轻微影响。除了头部相对于眼睛的延迟更大外，患者眼动和头部运动的时间耦合与正常人相似。我们的研究结果表明，患有慢性前庭功能丧失的患者重新获得了做出功能上适当的扫视性眼动的能力。我们与之前的研究一致，假定有三种主要的补偿机制：头部运动传出副本、主动颈-眼反射（COR）和眼睛的预编程回退。然而，目标定位准确性和扫视后稳定性在不同试验之间的巨大变异性表明，患者的扫视性注视系统没有恢复到正常人所观察到的高精度，而这种高精度似乎需要前庭头部在空间中的信号。此外，这种变异性也渗透到了他们在没有头部运动时的注视表现中。
