Wang Chao, Rajagovindan Rajasimhan, Han Sahng-Min, Ding Mingzhou
J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida Gainesville, FL, USA.
Front Hum Neurosci. 2016 Jan 20;10:15. doi: 10.3389/fnhum.2016.00015. eCollection 2016.
Alpha oscillations (8-12 Hz) are thought to inversely correlate with cortical excitability. Goal-oriented modulation of alpha has been studied extensively. In visual spatial attention, alpha over the region of visual cortex corresponding to the attended location decreases, signifying increased excitability to facilitate the processing of impending stimuli. In contrast, in retention of verbal working memory, alpha over visual cortex increases, signifying decreased excitability to gate out stimulus input to protect the information held online from sensory interference. According to the prevailing model, this goal-oriented biasing of sensory cortex is effected by top-down control signals from frontal and parietal cortices. The present study tests and substantiates this hypothesis by (a) identifying the signals that mediate the top-down biasing influence, (b) examining whether the cortical areas issuing these signals are task-specific or task-independent, and (c) establishing the possible mechanism of the biasing action. High-density human EEG data were recorded in two experimental paradigms: a trial-by-trial cued visual spatial attention task and a modified Sternberg working memory task. Applying Granger causality to both sensor-level and source-level data we report the following findings. In covert visual spatial attention, the regions exerting top-down control over visual activity are lateralized to the right hemisphere, with the dipoles located at the right frontal eye field (FEF) and the right inferior frontal gyrus (IFG) being the main sources of top-down influences. During retention of verbal working memory, the regions exerting top-down control over visual activity are lateralized to the left hemisphere, with the dipoles located at the left middle frontal gyrus (MFG) being the main source of top-down influences. In both experiments, top-down influences are mediated by alpha oscillations, and the biasing effect is likely achieved via an inhibition-disinhibition mechanism.
阿尔法振荡（8 - 12赫兹）被认为与皮层兴奋性呈负相关。针对目标的阿尔法调制已得到广泛研究。在视觉空间注意中，与被注意位置相对应的视觉皮层区域的阿尔法降低，这意味着兴奋性增加，以促进对即将到来的刺激的处理。相反，在言语工作记忆的保持过程中，视觉皮层上的阿尔法增加，这意味着兴奋性降低，以阻止刺激输入，从而保护在线保存的信息免受感觉干扰。根据流行的模型，感觉皮层的这种针对目标的偏向是由来自额叶和顶叶皮层的自上而下的控制信号实现的。本研究通过以下方式检验并证实了这一假设：（a）识别介导自上而下偏向影响的信号；（b）检查发出这些信号的皮层区域是任务特定的还是任务独立的；（c）确定偏向作用的可能机制。在两个实验范式中记录了高密度的人类脑电图数据：一个逐次试验的提示视觉空间注意任务和一个修改后的斯特恩伯格工作记忆任务。将格兰杰因果关系应用于传感器层面和源层面的数据，我们报告了以下发现。在隐蔽视觉空间注意中，对视觉活动施加自上而下控制的区域偏向右侧半球，位于右额叶眼区（FEF）和右下额叶回（IFG）的偶极是自上而下影响的主要来源。在言语工作记忆的保持过程中，对视觉活动施加自上而下控制的区域偏向左侧半球，位于左额中回（MFG）的偶极是自上而下影响的主要来源。在这两个实验中，自上而下的影响均由阿尔法振荡介导，并且偏向效应可能是通过抑制 - 去抑制机制实现的。
