High levels of GABA (gamma-aminobutyric acid, the brain's primary inhibitory neurotransmitter) are associated with enhanced cognitive and perceptual performance. It has been proposed that these effects result from GABA reducing neural noise or variability, but the precise mechanisms remain unknown. We have measured how individual differences in GABA concentration in the visual cortex are related to performance on a visual contrast discrimination task. Our results reveal that the facilitatory strength of the typical “dipper” function elicited by this task is strongly correlated with GABA concentration. A simple, biologically plausible, network model comprising excitatory and suppressive neural populations accounts for the data well and indicates that the strength of suppression increases as GABA concentration increases. Inter-individual variations in GABA were correlated both with the inhibition strength of the model (mimicking the effect of GABA) and, inversely, with the magnitude of the response criterion. This enhanced suppression has the dual effect of suppressing noise and reducing the gain of the neural response. Our findings thus suggest that the changes in performance conferred by high GABA concentration are mediated by both a reduction of noise and, paradoxically, a reduction in neural, but not perceptual, sensitivity.

高水平的GABA（γ-氨基丁酸，大脑的主要抑制性神经递质）与增强的认知和知觉性能有关。已经提出这些效应是由GABA降低神经噪声或变异性引起的，但是确切的机制仍然未知。我们已经测量了视觉皮层中GABA浓度的个体差异如何与视觉对比识别任务的表现相关。我们的结果表明，此任务引起的典型“浸染”功能的促进强度与GABA浓度密切相关。由兴奋性和抑制性神经群体组成的简单的，生物学上可行的网络模型很好地说明了数据，并表明抑制强度随GABA浓度的增加而增加。GABA的个体间差异既与模型的抑制强度（模拟GABA的作用）相关，又与响应标准的大小相关。这种增强的抑制具有抑制噪声和减少神经反应增益的双重作用。因此，我们的发现表明，高GABA浓度所带来的性能变化是由噪声的降低和神经（但不是感知）敏感性的降低介导的。