Oommen Brian S, Stahl John S
Department of Neurology, Louis Stokes Cleveland Department of Veterans Affairs Medical Center and Case Western Reserve University, 11100 Euclid Avenue, Cleveland, OH 44106, USA.
Exp Brain Res. 2005 Nov;167(2):276-86. doi: 10.1007/s00221-005-0036-8. Epub 2005 Jul 21.
The ability to dissociate eye movements from head movements is essential to animals with foveas and fovea-like retinal specializations, as these species shift the eyes constantly, and moving the head with each gaze shift would be impractical and energetically wasteful. The processes by which the dissociation is effected remain unclear. We hypothesized that the dissociation is accomplished by means of a neural gate, which prevents a common gaze-shift command from reaching the neck circuitry when eye-only saccades are desired. We further hypothesized that such a gate would require a finite period to reset following opening to allow a combined eye-head saccade, and thus the probability of generating a head movement during a saccade would be augmented when a new visual target (the 'test' target) appeared during, or soon after, a combined eye-head saccade made to an earlier, 'conditioning' target. We tested human subjects using three different combinations of targets-a horizontal conditioning target followed by a horizontal test target (H/H condition), horizontal conditioning followed by vertical test (H/V), and vertical conditioning followed by horizontal test (V/H). We varied the delay between the onset of the conditioning head movement and the presentation of the test target, and determined the probability of generating a head movement to the test target as a function of target delay. As predicted, head movement probability was elevated significantly at the shortest target delays and declined thereafter. The half-life of the increase in probability averaged 740, 490, and 320 ms for the H/H, H/V, and V/H conditions, respectively. For the H/H condition, the augmentation appeared to outlast the duration of the conditioning head movement. Because the augmentation could outlast the conditioning head movement and did not depend on the head movements to the conditioning and test targets lying in the same directions, we could largely exclude the possibility that the augmentation arises from mechanical effects. These results support the existence of the hypothetical eye-head gate, and suggest ways that its constituent neurons might be identified using neurophysiological methods.
对于具有中央凹和类似中央凹视网膜特化的动物而言，将眼球运动与头部运动分离的能力至关重要，因为这些物种会不断转动眼睛，而每次注视转移时都移动头部既不切实际又会造成能量浪费。实现这种分离的过程仍不清楚。我们推测这种分离是通过一个神经门来完成的，当只需要眼球扫视时，该神经门会阻止共同的注视转移指令到达颈部神经回路。我们进一步推测，这样一个神经门在打开后需要一段有限的时间来重置，以便进行联合眼球-头部扫视，因此当在对先前的“条件”目标进行联合眼球-头部扫视期间或之后不久出现新的视觉目标（“测试”目标）时，在扫视期间产生头部运动的概率会增加。我们使用三种不同的目标组合对人类受试者进行了测试——水平条件目标后接水平测试目标（H/H条件）、水平条件后接垂直测试（H/V）以及垂直条件后接水平测试（V/H）。我们改变了条件性头部运动开始与测试目标呈现之间的延迟，并确定了作为目标延迟函数的向测试目标产生头部运动的概率。正如预测的那样，在最短的目标延迟时，头部运动概率显著升高，此后下降。对于H/H、H/V和V/H条件，概率增加的半衰期分别平均为740、490和320毫秒。对于H/H条件，这种增加似乎持续到条件性头部运动的持续时间之后。由于这种增加可以持续到条件性头部运动之后，并且不依赖于向条件和测试目标的头部运动方向相同，我们可以在很大程度上排除这种增加是由机械效应引起的可能性。这些结果支持了假设的眼球-头部神经门的存在，并提出了使用神经生理学方法识别其组成神经元的方法。
