A Nonlinear Model of the Behavior of Simple Cells in Visual Cortex

Despite their structured receptive fields (RFs) and the strong linear components in their responses, most simple cells in mammalian visual cortex exhibit nonlinear behaviors. Besides the contrast-response function, nonlinearities are evident in various types of failure at superposition tasks, in the disagreement between direction indices computed from drifting and counterphase flickering gratings, in various forms of response suppression (including end- and side-stopping, spatial-frequency-specific inhibition and cross-orientation inhibition), in the advance of phase with increasing contrast, and in phase-insensitive and frequency-doubled responses to counterphase flickering gratings. These behaviors suggest that nonlinearities are involved in the operation of simple cells, but current models fail to explain them. A quantitative model is presented here that purports to describe basic and common principles of operation for all visual cortical cells. Simple cells are described as receiving afferents from multiple subunits that differ in their individual RFs and temporal impulse responses (TIRs). Subunits are independent and perform a spatial integration across their RFs followed by halfwave rectification and temporal convolution with their TIRs. This parallel operation yields a set of temporal functions representing each subunit's contribution to the membrane potential of the host cell, whose final form is given by the weighted sum of all subunits' contributions. By varying the number of subunits and their particular characteristics, different instances of the model are obtained each of which displays a different set of behaviors. Extensive simulation results are presented that illustrate how all of the reported nonlinear behaviors of simple cells arise from these multi-subunit organizations.     

Spatiotemporal organization of simple-cell receptive fields in area 18 of cat''s cortex

Spatiotemporal structures of receptive-fields (RF) have been studied for simple cells in area 18 of eat by measuring the temporal transfer function (TTF) over different locations (subregions) within the RF. The temporal characteristics of different subregions differed from each other in the absolute phase shift (APS) to visual stimuli. Two types of relationships can be seen: (i)The APS varied continuously from one subregion to the next: (ii) A 180°-phase jump was seen as the stimulus position changed somewhere within the receptive field. Spatiotemporal receptive field profiles have been determined by applying reverse Fourier analysis to responses in the frequency domain. For the continuous type, spatial and temporal characteristics cannot be dissociated (space time inseparable) and the spatiotemporal structure is oriented. On the contrary, the spatial and temporal characteristics for the jumping type can be dissociated (space-time separable) and the structure is not oriented in the space-time plane. Based on the APSs measured at different subregions, the optimal direction of motion and optimal spatial frequency of neurons can be predicted. Project supported by the National Natural Science Foundation of China (Grant Nos. 39570206, 39330110) and the Laboratory of Visual Information Processing, Chinese Academy of Sciences.     

Spatiotemporal organization of simple-cell receptive fields in area 18 of cat''''s cortex

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猫V1区不同外周整合细胞的经典感受野属性研究

以家猫为动物模型,采用细胞外记录的方法,测试了82个初级视皮层细胞的方位和方向调谐以及感受野大小.基于细胞的面积整合特性,区分出52个外周抑制型细胞和30个外周无抑制型细胞.所有被测细胞均存在强的方位选择性,而外周无抑制型细胞比抑制型细胞有更强的方位选择性.两类细胞的方向选择性没有显著性差异.外周抑制型细胞比外周无抑制型细胞有着更大的动作电位发放率.采用两种不同方法测量两类细胞的感受野范围,却产生了不同的结果:用最小反应区测量发现抑制型细胞的经典感受野更大,而用面积整合曲线测量时外周无抑制型细胞的感受野更大.     

Retino-Cortical Mapping Ratio Predicts Columnar and Salt-and-Pepper Organization in Mammalian Visual Cortex

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