Motion Adaptation and the Velocity Coding of Natural Scenes
Motion Adaptation and the Velocity Coding of Natural Scenes
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DOI：
10.1016/j.cub.2010.03.072
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发表时间：
2010-06-08
期刊：
影响因子：
9.2
通讯作者：
O'Carroll, David C.
中科院分区：
文献类型：
--
作者：
Barnett, Paul D.;Nordstroem, Karin;O'Carroll, David C.
由于自然场景的对比度和空间结构差异很大，因此估计自然环境中的相对速度是具有挑战性的。被广泛接受的基于相关性的基本运动检测器(EMDS)模型对对比度和空间结构敏感，因此会产生速度的模糊估计[1]。尽管直接从这样的EMD阵列[3，4]接收输入，但在飞蝇的第三个光叶中识别的神经元可以可靠地编码自然图像的速度，在很大程度上与对比度无关[2]。这种对比度不变性表明，额外的神经过程在图像运动的稳健编码中起着重要作用[2，5，6]。然而，目前还不清楚哪些神经过程对对比度不变性有贡献。通过对苍蝇小叶水平系统神经元的记录，我们展示了两种活动依赖的适应机制，它们对不同对比度的图像起到了近乎理想的归一化作用，否则会产生高度可变的反应幅度。对最初是弱神经驱动的图像的反应会在数百毫秒内增强。对最初是强大神经驱动因素的图像的反应在较长的时间尺度上会减少。这些自适应机制似乎与更高阶的自然图像统计相匹配，使神经元对图像速度的准确编码与基于相关的运动检测器固有的模糊性相一致。
Estimating relative velocity in the natural environment is challenging because natural scenes vary greatly in contrast and spatial structure. Widely accepted correlation-based models for elementary motion detectors (EMDs) are sensitive to contrast and spatial structure and consequently generate ambiguous estimates of velocity [1]. Identified neurons in the third optic lobe of the hoverfly can reliably encode the velocity of natural images largely independent of contrast [2], despite receiving inputs directly from arrays of such EMDs [3, 4]. This contrast invariance suggests an important role for additional neural processes in robust encoding of image motion [2, 5, 6]. However, it remains unclear which neural processes are contributing to contrast invariance. By recording from horizontal system neurons in the hoverfly lobula, we show two activity-dependent adaptation mechanisms acting as near-ideal normalizers for images of different contrasts that would otherwise produce highly variable response magnitudes. Responses to images that are initially weak neural drivers are boosted over several hundred milliseconds. Responses to images that are initially strong neural drivers are reduced over longer time scales. These adaptation mechanisms appear to be matched to higher-order natural image statistics reconciling the neurons' accurate encoding of image velocity with the inherent ambiguity of correlation-based motion detectors.