Buia Calin, Tiesinga Paul
Computational Neurophysics Laboratory, Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill, North Carolina 27599, USA.
J Comput Neurosci. 2006 Jun;20(3):247-64. doi: 10.1007/s10827-006-6358-0. Epub 2006 Apr 22.
The response of a neuron in the visual cortex to stimuli of different contrast placed in its receptive field is commonly characterized using the contrast response curve. When attention is directed into the receptive field of a V4 neuron, its contrast response curve is shifted to lower contrast values (Reynolds et al., 2000). The neuron will thus be able to respond to weaker stimuli than it responded to without attention. Attention also increases the coherence between neurons responding to the same stimulus (Fries et al., 2001). We studied how the firing rate and synchrony of a densely interconnected cortical network varied with contrast and how they were modulated by attention. The changes in contrast and attention were modeled as changes in driving current to the network neurons. We found that an increased driving current to the excitatory neurons increased the overall firing rate of the network, whereas variation of the driving current to inhibitory neurons modulated the synchrony of the network. We explain the synchrony modulation in terms of a locking phenomenon during which the ratio of excitatory to inhibitory firing rates is approximately constant for a range of driving current values. We explored the hypothesis that contrast is represented primarily as a drive to the excitatory neurons, whereas attention corresponds to a reduction in driving current to the inhibitory neurons. Using this hypothesis, the model reproduces the following experimental observations: (1) the firing rate of the excitatory neurons increases with contrast; (2) for high contrast stimuli, the firing rate saturates and the network synchronizes; (3) attention shifts the contrast response curve to lower contrast values; (4) attention leads to stronger synchronization that starts at a lower value of the contrast compared with the attend-away condition. In addition, it predicts that attention increases the delay between the inhibitory and excitatory synchronous volleys produced by the network, allowing the stimulus to recruit more downstream neurons.
视觉皮层中神经元对其感受野内不同对比度刺激的反应通常用对比度响应曲线来表征。当注意力指向V4神经元的感受野时，其对比度响应曲线会向较低对比度值偏移（雷诺兹等人，2000年）。因此，与无注意力时相比，该神经元能够对更弱的刺激做出反应。注意力还会增强对同一刺激做出反应的神经元之间的同步性（弗里斯等人，2001年）。我们研究了紧密互联的皮层网络的放电率和同步性如何随对比度变化，以及它们如何受到注意力的调制。对比度和注意力的变化被建模为网络神经元驱动电流的变化。我们发现，增加对兴奋性神经元的驱动电流会提高网络的整体放电率，而对抑制性神经元驱动电流的变化则会调制网络的同步性。我们用一种锁定现象来解释同步性调制，即在一定范围的驱动电流值内，兴奋性与抑制性放电率的比值大致恒定。我们探讨了这样一种假设，即对比度主要表现为对兴奋性神经元的驱动，而注意力则对应于对抑制性神经元驱动电流的降低。利用这一假设，该模型再现了以下实验观察结果：（1）兴奋性神经元的放电率随对比度增加；（2）对于高对比度刺激，放电率达到饱和且网络同步；（3）注意力将对比度响应曲线向较低对比度值偏移；（4）与注意力分散状态相比，注意力导致更强的同步性，且这种同步性在较低对比度值时就开始出现。此外，它还预测注意力会增加网络产生的抑制性和兴奋性同步脉冲之间的延迟，使刺激能够招募更多下游神经元。
