Tolhurst David J, Smyth Darragh, Thompson Ian D
Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG, United Kingdom.
J Neurosci. 2009 Feb 25;29(8):2355-70. doi: 10.1523/JNEUROSCI.3869-08.2009.
Various arguments suggest that neuronal coding of natural sensory stimuli should be sparse (i.e., individual neurons should respond rarely but should respond reliably). We examined sparseness of visual cortical neurons in anesthetized ferret to flashed natural scenes. Response behavior differed widely between neurons. The median firing rate of 4.1 impulses per second was slightly higher than predicted from consideration of metabolic load. Thirteen percent of neurons (12 of 89) responded to <5% of the images, but one-half responded to >25% of images. Multivariate analysis of the range of sparseness values showed that 67% of the variance was accounted for by differing response patterns to moving gratings. Repeat presentation of images showed that response variance for natural images exaggerated sparseness measures; variance was scaled with mean response, but with a lower Fano factor than for the responses to moving gratings. This response variability and the "soft" sparse responses (Rehn and Sommer, 2007) raise the question of what constitutes a reliable neuronal response and imply parallel signaling by multiple neurons. We investigated whether the temporal structure of responses might be reliable enough to give additional information about natural scenes. Poststimulus time histogram shape was similar for "strong" and "weak" stimuli, with no systematic change in first-spike latency with stimulus strength. The variance of first-spike latency for repeat presentations of the same image was greater than the latency variance between images. In general, responses to flashed natural scenes do not seem compatible with a sparse encoding in which neurons fire rarely but reliably.
各种观点表明，对自然感觉刺激的神经元编码应该是稀疏的（即单个神经元应该很少响应，但应该可靠地响应）。我们在麻醉的雪貂中检查了视觉皮层神经元对闪现自然场景的稀疏性。神经元之间的反应行为差异很大。每秒4.1次冲动的中值放电率略高于从代谢负荷考虑所预测的值。13%的神经元（89个中的12个）对<5%的图像有反应，但一半的神经元对>25%的图像有反应。对稀疏值范围的多变量分析表明，67%的方差是由对移动光栅的不同反应模式造成的。图像的重复呈现表明，自然图像的反应方差夸大了稀疏性测量；方差与平均反应成比例，但Fano因子低于对移动光栅的反应。这种反应变异性和“软”稀疏反应（Rehn和Sommer，2007）提出了一个问题，即什么构成了可靠的神经元反应，并暗示了多个神经元的并行信号传递。我们研究了反应的时间结构是否可能足够可靠，以提供有关自然场景的额外信息。“强”刺激和“弱”刺激的刺激后时间直方图形状相似，首次放电潜伏期没有随刺激强度的系统变化。同一图像重复呈现时首次放电潜伏期的方差大于不同图像之间的潜伏期方差。一般来说，对闪现自然场景的反应似乎与神经元很少但可靠地放电的稀疏编码不相符。
