Tamás Laszlo T, Weber Konrad P, Bockisch Christopher J, Straumann Dominik, Lasker David M, Büki Béla, Tarnutzer Alexander A
1Department of Otolaryngology, Petz Aladár County Hospital, Györ, Hungary; 2Department of Neurology, University Hospital Zurich, University of Zurich, Zurich, Switzerland; 3Department of Ophthalmology, University Hospital Zurich, University of Zurich, Zurich, Switzerland; 4Department of Otorhinolaryngology, University Hospital Zurich, University of Zurich, Zurich, Switzerland; 5Department of Otolaryngology-Head and Neck Surgery, Johns Hopkins University School of Medicine, Baltimore, MD, USA; and 6Department of Otolaryngology, Karl Landsteiner University Hospital Krems, Krems, Austria.
Ear Hear. 2017 May/Jun;38(3):e193-e199. doi: 10.1097/AUD.0000000000000398.
During head rotations, neuronal firing rates increase in ipsilateral and decrease in contralateral vestibular afferents. At low accelerations, this "push-pull mechanism" is linear. At high accelerations, however, the change of firing rates is nonlinear in that the ipsilateral increase of firing rate is larger than the contralateral decrease. This mechanism of stronger ipsilateral excitation than contralateral inhibition during high-acceleration head rotation, known as Ewald's second law, is implemented within the nonlinear pathways. The authors asked whether caloric stimulation could provide an acceleration signal high enough to influence the contribution of the nonlinear pathway to the rotational vestibulo-ocular reflex gain (rVOR gain) during head impulses.
Caloric warm (44°C) and cold (24, 27, and 30°C) water irrigations of the left ear were performed in 7 healthy human subjects with the lateral semicircular canals oriented approximately earth-vertical (head inclined 30° from supine) and earth-horizontal (head inclined 30° from upright).
With the lateral semicircular canal oriented earth-vertical, the strongest cold caloric stimulus (24°C) significantly decreased the rVOR gain during ipsilateral head impulses, while all other irrigations, irrespective of head position, had no significant effect on rVOR gains during head impulses to either side.
Strong caloric irrigation, which can only be achieved with cold water, reduces the rVOR gain during ipsilateral head impulses and thus demonstrates Ewald's second law in healthy subjects. This unilateral gain reduction suggests that cold-water caloric irritation shifts the set point of the nonlinear relation between head acceleration and the vestibular firing rate toward a less acceleration-sensitive zone.
在头部旋转过程中，同侧前庭传入神经的神经元放电频率增加，而对侧前庭传入神经的放电频率降低。在低加速度时，这种“推挽机制”是线性的。然而，在高加速度时，放电频率的变化是非线性的，即同侧放电频率的增加大于对侧放电频率的降低。这种在高加速度头部旋转过程中同侧兴奋强于对侧抑制的机制，即埃瓦尔德第二定律，是在非线性通路中实现的。作者们探讨了冷热刺激是否能提供足够高的加速度信号，以影响头部脉冲期间非线性通路对旋转性前庭眼反射增益（rVOR增益）的贡献。
对7名健康人类受试者的左耳进行冷热（44°C）和冷（24、27和30°C）水灌注，此时外侧半规管大致呈地球垂直方向（头部从仰卧位倾斜30°）和地球水平方向（头部从直立位倾斜30°）。
当外侧半规管呈地球垂直方向时，最强的冷刺激（24°C）显著降低了同侧头部脉冲期间的rVOR增益，而所有其他灌注，无论头部位置如何，对向两侧的头部脉冲期间的rVOR增益均无显著影响。
只有冷水才能实现的强冷热灌注，会降低同侧头部脉冲期间的rVOR增益，从而在健康受试者中证明了埃瓦尔德第二定律。这种单侧增益降低表明，冷水冷热刺激将头部加速度与前庭放电频率之间非线性关系的设定点向加速度敏感性较低的区域移动。
