Leopold David A, Logothetis Nikos K
Max Planck Institut für biologische Kybernetik, Tübingen, Germany.
Rev Neurosci. 2003;14(1-2):195-205. doi: 10.1515/revneuro.2003.14.1-2.195.
Spontaneous activity among visually responsive neurons is often considered to consist of random neural events, or perhaps to reflect an irrelevant by-product of brain homeostasis. However, recent studies have emphasized that such ongoing activity is strongly synchronized over large cortical distances, and can have a marked impact on the responsiveness of neurons to visual stimuli, suggesting that such activity may indeed be highly relevant to the brain's interpretation of its sensory input. In the current study, we examined the spatiotemporal nature of local field potential (LFP) fluctuations in the visual cortex of two macaque monkeys that were awake, but in a state of relaxation with minimal visual stimulation. Using an array of 16 electrodes spaced by several millimeters, we simultaneously monitored the LFP at many sites over a large region of the visual cortex. In agreement with the literature, we found that the coherence in the raw LFP signal fell off quickly with both frequency and distance. However, when we examined slower fluctuations in the LFP power, we found that power signals, including those derived from the high y-range frequencies, had high coherence that fell off only very slowly with cortical distance. Finally, we performed an additional experiment, with several electrodes placed on either side of a sulcus, to demonstrate that the decline in local field synchrony with cortical distance was so reliable that the interruption in the cortical sheet corresponding to the opening of the sulcus could be easily identified by monitoring just a few minutes of spontaneous LFP activity. These experiments reveal that a significant portion of spontaneous LFP fluctuations in the visual cortex is contributed by global mechanisms, imposing synchrony that is, first and foremost, a function of cortical separation between any two points.
视觉反应神经元的自发活动通常被认为是由随机神经事件组成，或者可能反映了大脑内稳态的一种无关副产品。然而，最近的研究强调，这种持续的活动在大的皮质距离上强烈同步，并且对神经元对视觉刺激的反应性有显著影响，这表明这种活动可能确实与大脑对其感觉输入的解释高度相关。在当前的研究中，我们研究了两只清醒但处于放松状态且视觉刺激最小的猕猴视觉皮层中局部场电位（LFP）波动的时空特性。使用一组间隔几毫米的16个电极，我们同时在视觉皮层的一个大区域的多个部位监测LFP。与文献一致，我们发现原始LFP信号中的相干性随着频率和距离迅速下降。然而，当我们研究LFP功率中较慢的波动时，我们发现功率信号，包括那些来自高γ频段频率的信号，具有高相干性，并且仅随着皮质距离非常缓慢地下降。最后，我们进行了一项额外的实验，在一条脑沟的两侧放置了几个电极，以证明局部场同步性随皮质距离的下降是如此可靠，以至于通过监测仅仅几分钟的自发LFP活动，就可以很容易地识别出与脑沟开口相对应的皮质层中断。这些实验表明，视觉皮层中自发LFP波动的很大一部分是由全局机制引起的，这种机制施加的同步性首先是任意两点之间皮质距离的函数。
