Sharika K M, Ramakrishnan Arjun, Murthy Aditya
National Brain Research Centre, Nainwal More, Manesar 122051, Haryana, India, and.
Centre for Neuroscience, Indian Institute of Science, Bengaluru 560012, Karnataka, India
J Neurosci. 2014 Nov 26;34(48):16009-21. doi: 10.1523/JNEUROSCI.0328-14.2014.
The concurrent planning of sequential saccades offers a simple model to study the nature of visuomotor transformations since the second saccade vector needs to be remapped to foveate the second target following the first saccade. Remapping is thought to occur through egocentric mechanisms involving an efference copy of the first saccade that is available around the time of its onset. In contrast, an exocentric representation of the second target relative to the first target, if available, can be used to directly code the second saccade vector. While human volunteers performed a modified double-step task, we examined the role of exocentric encoding in concurrent saccade planning by shifting the first target location well before the efference copy could be used by the oculomotor system. The impact of the first target shift on concurrent processing was tested by examining the end-points of second saccades following a shift of the second target during the first saccade. The frequency of second saccades to the old versus new location of the second target, as well as the propagation of first saccade localization errors, both indices of concurrent processing, were found to be significantly reduced in trials with the first target shift compared to those without it. A similar decrease in concurrent processing was obtained when we shifted the first target but kept constant the second saccade vector. Overall, these results suggest that the brain can use relatively stable visual landmarks, independent of efference copy-based egocentric mechanisms, for concurrent planning of sequential saccades.
连续扫视的并发规划提供了一个简单模型，用于研究视觉运动转换的本质，因为在第一次扫视之后，第二次扫视向量需要重新映射，以便使中央凹对准第二个目标。重新映射被认为是通过自我中心机制发生的，该机制涉及在第一次扫视开始时可用的第一次扫视的传出副本。相比之下，如果存在相对于第一个目标的第二个目标的外中心表示，则可用于直接编码第二个扫视向量。当人类志愿者执行修改后的双步任务时，我们通过在动眼系统能够使用传出副本之前很久就移动第一个目标位置，来研究外中心编码在并发扫视规划中的作用。通过检查在第一次扫视期间第二个目标移动之后第二个扫视的终点，测试了第一个目标移动对并发处理的影响。与没有第一个目标移动的试验相比，在有第一个目标移动的试验中，第二个扫视到第二个目标的旧位置与新位置的频率以及第一次扫视定位误差的传播（这两个并发处理指标）都显著降低。当我们移动第一个目标但保持第二个扫视向量不变时，也获得了类似的并发处理减少。总体而言，这些结果表明，大脑可以使用相对稳定的视觉地标，独立于基于传出副本的自我中心机制，来进行连续扫视的并发规划。
