Corneil Brian D, Elsley James K
Department of Physiology and Pharmacology and Psychology, University of Western Ontario, London, Canada.
J Neurophysiol. 2005 Jul;94(1):883-95. doi: 10.1152/jn.01171.2004. Epub 2005 Feb 23.
The countermanding task requires subjects to cancel a planned movement on appearance of a stop signal, providing insights into response generation and suppression. Here, we studied human eye-head gaze shifts in a countermanding task with targets located beyond the horizontal oculomotor range. Consistent with head-restrained saccadic countermanding studies, the proportion of gaze shifts on stop trials increased the longer the stop signal was delayed after target presentation, and gaze shift stop-signal reaction times (SSRTs: a derived statistic measuring how long it takes to cancel a movement) averaged approximately 120 ms across seven subjects. We also observed a marked proportion of trials (13% of all stop trials) during which gaze remained stable but the head moved toward the target. Such head movements were more common at intermediate stop signal delays. We never observed the converse sequence wherein gaze moved while the head remained stable. SSRTs for head movements averaged approximately 190 ms or approximately 70-75 ms longer than gaze SSRTs. Although our findings are inconsistent with a single race to threshold as proposed for controlling saccadic eye movements, movement parameters on stop trials attested to interactions consistent with a race model architecture. To explain our data, we tested two extensions to the saccadic race model. The first assumed that gaze shifts and head movements are controlled by parallel but independent races. The second model assumed that gaze shifts and head movements are controlled by a single race, preceded by terminal ballistic intervals not under inhibitory control, and that the head-movement branch is activated at a lower threshold. Although simulations of both models produced acceptable fits to the empirical data, we favor the second alternative as it is more parsimonious with recent findings in the oculomotor system. Using the second model, estimates for gaze and head ballistic intervals were approximately 25 and 90 ms, respectively, consistent with the known physiology of the final motor paths. Further, the threshold of the head movement branch was estimated to be 85% of that required to activate gaze shifts. From these results, we conclude that a commitment to a head movement is made in advance of gaze shifts and that the comparative SSRT differences result primarily from biomechanical differences inherent to eye and head motion.
反指令任务要求受试者在停止信号出现时取消计划中的动作，这有助于深入了解反应的产生和抑制。在此，我们研究了在反指令任务中，目标位于水平眼动范围之外时的人眼-头注视转移。与头部固定的扫视反指令研究一致，在停止试验中，停止信号在目标呈现后延迟的时间越长，注视转移的比例就越高，并且在七名受试者中，注视转移停止信号反应时间（SSRTs：一种衡量取消动作所需时间的衍生统计量）平均约为120毫秒。我们还观察到在相当比例的试验中（占所有停止试验的13%），注视保持稳定，但头部朝着目标移动。这种头部运动在中等停止信号延迟时更为常见。我们从未观察到相反的情况，即注视移动而头部保持稳定。头部运动的SSRTs平均约为190毫秒，比注视SSRTs长约70 - 75毫秒。尽管我们的发现与控制扫视眼动所提出的单一阈值竞争模型不一致，但停止试验中的运动参数证明了与竞争模型架构一致的相互作用。为了解释我们的数据，我们测试了扫视竞争模型的两个扩展。第一个假设注视转移和头部运动由并行但独立的竞争控制。第二个模型假设注视转移和头部运动由单一竞争控制，在不受抑制控制的终端弹道间隔之前，并且头部运动分支在较低阈值下被激活。尽管两个模型的模拟都能较好地拟合实证数据，但我们更倾向于第二种选择，因为它与眼动系统的最新发现更简洁。使用第二个模型，注视和头部弹道间隔的估计值分别约为25毫秒和90毫秒，这与最终运动路径的已知生理学一致。此外，估计头部运动分支的阈值是激活注视转移所需阈值的85%。从这些结果中，我们得出结论，在注视转移之前就已经做出了进行头部运动的承诺，并且比较SSRT差异主要源于眼动和头部运动固有的生物力学差异。
