Ringach Dario L, Malone Brian J
Department of Psychology and Neurobiology, Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90095-1563, USA.
J Neurosci. 2007 Jul 18;27(29):7673-83. doi: 10.1523/JNEUROSCI.1048-07.2007.
Spiking neurons translate analog intracellular variables into a sequence of action potentials. A simplified model of this transformation is one in which an underlying "generator potential," representing a measure of overall neuronal drive, is passed through a static nonlinearity to produce an instantaneous firing rate. An important question is how adaptive mechanisms adjust the mean and SD of the generator potential to define an "operating point" that controls spike generation. In early sensory pathways adaptation has been shown to rescale the generator potential to maximize the amount of transmitted information. In contrast, we demonstrate that the operating point in the cortex is tuned so that cells respond only when the generator potential executes a large excursion above its mean value. The distance from the mean of the generator potential to spike threshold is, on average, 1 SD of the ongoing activity. Signals above threshold are amplified linearly and do not reach saturation. The operating point is adjusted dynamically so that it remains relatively invariant despite changes in stimulus contrast. We conclude that the operating regimen of the cortex is suitable for the detection of signals in background noise and for enhancing the selectivity of spike responses relative to those of the generator potential (the so-called "iceberg effect"), but not to maximize the transmission of total information.
发放脉冲的神经元将模拟的细胞内变量转化为一系列动作电位。这种转化的一个简化模型是，一个潜在的“发生器电位”（代表整体神经元驱动的一种度量）通过一个静态非线性函数，以产生瞬时发放率。一个重要的问题是，适应性机制如何调整发生器电位的均值和标准差，以定义一个控制脉冲产生的“工作点”。在早期感觉通路中，适应性已被证明可重新调整发生器电位，以最大化传输的信息量。相比之下，我们证明，皮层中的工作点是经过调整的，以便细胞仅在发生器电位执行高于其均值的大幅偏移时才做出反应。从发生器电位均值到脉冲阈值的距离平均为正在进行的活动的1个标准差。高于阈值的信号被线性放大且不会达到饱和。工作点会动态调整，以便尽管刺激对比度发生变化，它仍保持相对不变。我们得出结论，皮层的工作机制适用于在背景噪声中检测信号，以及相对于发生器电位增强脉冲反应的选择性（所谓的“冰山效应”），但并非为了最大化总信息的传输。
