Kayser A, Priebe N J, Miller K D
Department of Physiology, University of California, San Francisco, California 94143-0444, USA.
J Neurophysiol. 2001 May;85(5):2130-49. doi: 10.1152/jn.2001.85.5.2130.
We study a recently proposed "correlation-based," push-pull model of the circuitry of layer 4 of cat visual cortex. This model was previously shown to explain the contrast-invariance of cortical orientation tuning. Here we show that it can simultaneously account for several contrast-dependent (c-d) "nonlinearities" in cortical responses. These include an advance with increasing contrast in the temporal phase of response to a sinusoidally modulated stimulus; a change in shape of the temporal frequency tuning curve, so that higher temporal frequencies may give little or no response at low contrast but reasonable responses at high contrast; and contrast saturation that occurs at lower contrasts in cortex than in the lateral geniculate nucleus (LGN). In the context of the model circuit, these properties arise from a mixture of nonlinear cellular and synaptic mechanisms: short-term synaptic depression, spike-rate adaptation, contrast-induced changes in cellular conductance, and the nonzero spike threshold. The former three mechanisms are sufficient to explain the experimentally observed increase in c-d phase advance in cortex relative to LGN. The c-d changes in temporal frequency tuning arise as a threshold effect: voltage modulations in response to higher-frequency inputs are only slightly above threshold at lower contrast, but become robustly suprathreshold at higher contrast. The other three nonlinear mechanisms also play a crucial role in this result, allowing contrast dependence of temporal frequency tuning to coexist with contrast-invariance of orientation tuning. Contrast saturation, and the observation that responses to stimuli of increasing temporal frequency saturate at increasingly high contrasts, can be induced both by the model's push-pull inhibition and by synaptic depression. Previous proposals explained these nonlinear response properties by assuming contrast-invariant orientation tuning as a starting point, and adding normalization by shunting inhibition derived equally from cells of all preferred orientations. The present proposal simultaneously explains both contrast-invariant orientation tuning and these contrast-dependent nonlinearities and requires only processing that is local in orientation, in agreement with intracellular measurements.
我们研究了最近提出的猫视觉皮层第4层电路的“基于相关性”的推拉模型。该模型先前已被证明可以解释皮层方向调谐的对比度不变性。在这里，我们表明它可以同时解释皮层反应中几种依赖于对比度（c-d）的“非线性”。这些包括对正弦调制刺激的反应时间相位随对比度增加而提前；时间频率调谐曲线形状的变化，使得较高的时间频率在低对比度下可能几乎没有反应或没有反应，但在高对比度下有合理的反应；以及在皮层中比在外侧膝状体（LGN）中更低的对比度下出现的对比度饱和。在模型电路的背景下，这些特性源于非线性细胞和突触机制的混合：短期突触抑制、放电率适应、对比度引起的细胞电导变化以及非零的放电阈值。前三种机制足以解释实验观察到的皮层相对于LGN中c-d相位提前的增加。时间频率调谐中的c-d变化是作为一种阈值效应出现的：对高频输入的电压调制在低对比度下仅略高于阈值，但在高对比度下变得稳健地高于阈值。其他三种非线性机制在这一结果中也起着关键作用，使得时间频率调谐的对比度依赖性与方向调谐的对比度不变性共存。对比度饱和以及对时间频率增加的刺激的反应在越来越高的对比度下饱和的观察结果，既可以由模型的推拉抑制引起，也可以由突触抑制引起。先前的提议通过假设对比度不变的方向调谐作为起点，并通过同样来自所有偏好方向的细胞的分流抑制进行归一化来解释这些非线性反应特性。本提议同时解释了对比度不变的方向调谐和这些依赖于对比度的非线性，并且只需要在方向上局部的处理，这与细胞内测量结果一致。
