Hallum Luke E, Movshon J Anthony
Center for Neural Science, New York University, New York, NY 10003, United States.
Center for Neural Science, New York University, New York, NY 10003, United States.
Vision Res. 2014 Nov;104:24-35. doi: 10.1016/j.visres.2014.10.004. Epub 2014 Oct 23.
Single neurons in areas V1 and V2 of macaque visual cortex respond selectively to luminance-modulated stimuli. These responses are often influenced by context, for example when stimuli extend outside the classical receptive field (CRF). These contextual phenomena, observed in many sensory areas, reflect a fundamental cortical computation and may inform perception by signaling second-order visual features which are defined by spatial relationships of contrast, orientation and spatial frequency. In the anesthetized, paralyzed macaque, we measured single-unit responses to a drifting preferred sinusoidal grating; low spatial frequency sinusoidal contrast modulations were applied to the grating, creating contrast-modulated, second-order forms. Most neurons responded selectively to the orientation of the contrast modulation of the preferred grating and were therefore second-order orientation-selective. Second-order selectivity was created by the asymmetric spatial organization of the excitatory CRF and suppressive extraclassical surround. We modeled these receptive field subregions using spatial Gaussians, sensitive to the modulation of contrast (not luminance) of the preferred carrier grating, that summed linearly and were capable of recovering asymmetrical receptive field organizations. Our modeling suggests that second-order selectivity arises both from elongated excitatory CRFs, asymmetrically organized extraclassical surround suppression, or both. We validated the model by successfully testing its predictions against conventional surround suppression measurements and spike-triggered analysis of second-order form responses. Psychophysical adaptation measurements on human observers revealed a pattern of second-order form selectivity consistent with neural response patterns. We therefore propose that cortical cells in primates do double duty, providing signals about both first- and second-order forms.
猕猴视觉皮层V1和V2区域的单个神经元对亮度调制刺激有选择性反应。这些反应通常受上下文影响，例如当刺激延伸到经典感受野（CRF）之外时。在许多感觉区域观察到的这些上下文现象反映了一种基本的皮层计算，并且可能通过发出由对比度、方向和空间频率的空间关系定义的二阶视觉特征信号来为感知提供信息。在麻醉、瘫痪的猕猴中，我们测量了对漂移的首选正弦光栅的单单元反应；对光栅施加低空间频率正弦对比度调制，产生对比度调制的二阶形式。大多数神经元对首选光栅对比度调制的方向有选择性反应，因此是二阶方向选择性的。二阶选择性是由兴奋性CRF和抑制性非经典周边的不对称空间组织产生的。我们使用空间高斯函数对这些感受野子区域进行建模，该函数对首选载波光栅的对比度（而非亮度）调制敏感，这些高斯函数线性相加，能够恢复不对称的感受野组织。我们的建模表明，二阶选择性既来自拉长的兴奋性CRF，也来自不对称组织的非经典周边抑制，或者两者兼而有之。我们通过成功地将其预测与传统的周边抑制测量和二阶形式反应的峰触发分析进行对比测试，验证了该模型。对人类观察者的心理物理学适应测量揭示了与神经反应模式一致的二阶形式选择性模式。因此，我们提出灵长类动物的皮层细胞具有双重功能，既提供关于一阶形式的信号，也提供关于二阶形式信号。
