Murthy A, Humphrey A L, Saul A B, Feidler J C
Department of Neurobiology, University of Pittsburgh School of Medicine, PA 15261, USA.
Vis Neurosci. 1998 Mar-Apr;15(2):239-56. doi: 10.1017/s0952523898152045.
Previous studies of cat visual cortex have shown that the spatiotemporal (S-T) structure of simple cell receptive fields correlates with direction selectivity. However, great heterogeneity exists in the relationship and this has implications for models. Here we report a laminar basis for some of the heterogeneity. S-T structure and direction selectivity were measured in 101 cells using stationary counterphasing and drifting gratings, respectively. Two procedures were used to assess S-T structure and its relation to direction selectivity. In the first, the S-T orientations of receptive fields were quantified by fitting response temporal phase versus stimulus spatial phase data. In the second procedure, conventional linear predictions of direction selectivity were computed from the amplitudes and phases of responses to stationary gratings. Extracellular recording locations were reconstructed histologically. Among direction-selective cells, S-T orientation was greatest in layer 4B and it correlated well (r = 0.76) with direction selectivity. In layer 6, S-T orientation was uniformly low, overlapping little with layer 4B, and it was not correlated with directional tuning. Layer 4A was intermediate in S-T orientation and its relation (r = 0.46) to direction selectivity. The same laminar patterns were observed using conventional linear predictions. The patterns do not reflect laminar differences in direction selectivity since the layers were equivalent in directional tuning. We also evaluated a model of linear spatiotemporal summation followed by a static nonlinear amplification (exponent model) to account for direction selectivity. The values of the exponents were estimated from differences between linearly predicted and actual amplitude modulations to counterphasing gratings. Comparing these exponents with another exponent--that required to obtain perfect matches between linearly predicted and measured directional tuning--indicates that an exponent model largely accounts for direction selectivity in most cells in layer 4, particularly layer 4B, but not in layer 6. Dynamic nonlinearities seem essential for cells in layer 6. We suggest that these laminar differences may partly reflect the differential involvement of geniculocortical and intracortical mechanisms.
以往对猫视觉皮层的研究表明，简单细胞感受野的时空（S-T）结构与方向选择性相关。然而，这种关系存在很大的异质性，这对模型有一定影响。在此，我们报告了部分异质性的层状基础。分别使用静止反相光栅和漂移光栅对101个细胞的S-T结构和方向选择性进行了测量。采用了两种方法来评估S-T结构及其与方向选择性的关系。第一种方法是通过拟合响应时间相位与刺激空间相位数据来量化感受野的S-T方向。在第二种方法中，根据对静止光栅响应的幅度和相位计算方向选择性的传统线性预测值。通过组织学方法重建细胞外记录位置。在方向选择性细胞中，S-T方向在4B层最大，且与方向选择性密切相关（r = 0.76）。在6层，S-T方向普遍较低，与4B层几乎没有重叠，且与方向调谐无关。4A层的S-T方向及其与方向选择性的关系处于中间水平（r = 0.46）。使用传统线性预测也观察到了相同的层状模式。这些模式并不反映方向选择性的层间差异，因为各层在方向调谐方面是等效的。我们还评估了一个先进行线性时空总和然后进行静态非线性放大的模型（指数模型）来解释方向选择性。指数值是根据线性预测和反相光栅实际幅度调制之间的差异估计得出的。将这些指数与另一个指数（即获得线性预测和测量的方向调谐之间完美匹配所需的指数）进行比较表明，指数模型在很大程度上解释了4层大多数细胞（特别是4B层）的方向选择性，但不能解释6层细胞的方向选择性。动态非线性似乎对6层细胞至关重要。我们认为这些层间差异可能部分反映了丘脑皮质和皮质内机制的不同参与程度。
