Iemi Luca, Chaumon Maximilien, Crouzet Sébastien M, Busch Niko A
Berlin School of Mind and Brain, Humboldt-Universität zu Berlin, 10099 Berlin, Germany,
Institute of Medical Psychology, Charité-Universitätsmedizin, 10117 Berlin, Germany.
J Neurosci. 2017 Jan 25;37(4):807-819. doi: 10.1523/JNEUROSCI.1432-16.2016.
The brain exhibits organized fluctuations of neural activity, even in the absence of tasks or sensory input. A prominent type of such spontaneous activity is the alpha rhythm, which influences perception and interacts with other ongoing neural activity. It is currently hypothesized that states of decreased prestimulus α oscillations indicate enhanced neural excitability, resulting in improved perceptual acuity. Nevertheless, it remains debated how changes in excitability manifest at the behavioral level in perceptual tasks. We addressed this issue by comparing two alternative models describing the effect of spontaneous α power on signal detection. The first model assumes that decreased α power increases baseline excitability, amplifying the response to both signal and noise, predicting a liberal detection criterion with no effect on sensitivity. The second model predicts that decreased α power increases the trial-by-trial precision of the sensory response, resulting in improved sensitivity. We tested these models in two EEG experiments in humans where we analyzed the effects of prestimulus α power on visual detection and discrimination using a signal detection framework. Both experiments provide strong evidence that decreased α power reflects a more liberal detection criterion, rather than improved sensitivity, consistent with the baseline model. In other words, when the task requires detecting stimulus presence versus absence, reduced α oscillations make observers more likely to report the stimulus regardless of actual stimulus presence. Contrary to previous interpretations, these results suggest that states of decreased α oscillations increase the global baseline excitability of sensory systems without affecting perceptual acuity.
Spontaneous fluctuations of brain activity explain why a faint sensory stimulus is sometimes perceived and sometimes not. The prevailing view is that heightened neural excitability, indexed by decreased α oscillations, promotes better perceptual performance. Here, we provide evidence that heightened neural excitability instead reflects a state of biased perception, during which a person is more likely to see a stimulus, whether or not it is actually present. Therefore, we propose that changes in neural excitability leave the precision of sensory processing unaffected. These results establish the link between spontaneous brain activity and the variability in human perception.
即使在没有任务或感觉输入的情况下，大脑也会表现出有组织的神经活动波动。这种自发活动的一种突出类型是阿尔法节律，它会影响感知并与其他正在进行的神经活动相互作用。目前有假设认为，刺激前α振荡减少的状态表明神经兴奋性增强，从而导致感知敏锐度提高。然而，关于兴奋性变化在感知任务的行为层面如何表现仍存在争议。我们通过比较描述自发α功率对信号检测影响的两种替代模型来解决这个问题。第一个模型假设α功率降低会增加基线兴奋性，放大对信号和噪声的反应，预测检测标准放宽且对灵敏度无影响。第二个模型预测α功率降低会增加感觉反应的逐次试验精度，从而提高灵敏度。我们在两项针对人类的脑电图实验中测试了这些模型，在实验中我们使用信号检测框架分析了刺激前α功率对视觉检测和辨别力的影响。两项实验都提供了强有力的证据，表明α功率降低反映了更宽松的检测标准，而不是提高了灵敏度，这与基线模型一致。换句话说，当任务要求检测刺激是否存在时，α振荡减少会使观察者更有可能报告刺激，而不管实际刺激是否存在。与之前的解释相反，这些结果表明α振荡减少的状态会增加感觉系统的整体基线兴奋性，而不会影响感知敏锐度。
大脑活动的自发波动解释了为什么微弱的感觉刺激有时能被感知到，有时却不能。普遍的观点是，以α振荡减少为指标的神经兴奋性增强会促进更好的感知表现。在这里，我们提供证据表明，神经兴奋性增强反而反映了一种有偏差的感知状态，在此期间，一个人更有可能看到刺激，无论它实际上是否存在。因此，我们提出神经兴奋性的变化不会影响感觉处理的精度。这些结果建立了自发脑活动与人类感知变异性之间的联系。
