PNAS2014-02-24 5:20 PM

神经元非线性解释了其相较于光亮而对黑暗具有更大视觉空间分辨率  Neuronal nonlinearity explains greater visual spatial resolution for darks than lights 

论文摘要 
百年前天文学家和物理学家就发现视觉空间分辨率对黑暗的刺激比光亮更高,但我们对于这种感知不对称性的神经机制尚不清楚。在此我们表明不对称性是由神经元在早期视觉通路中的非线性作用引起的。我们表示独立于背景亮度,由黑暗驱动的神经元(神经关闭)的反应分辨率会随亮度递减大致遵循线性提升。然而,光亮所驱动的神经元(神经开启)的反应分辨率随着光亮的小幅增加很快达到饱和,且需要明亮背景才能接近之前神经关闭状态下的的线性度。我们表明这种线性差异后果使得感知区域面积在丘脑神经开启状态下比关闭状态下更大,在低空间频率中,皮层神经受黑暗刺激驱动比光亮刺激更强烈。这种(神经)开启 / 关闭不线性对称可以在猫,猴和人的视觉皮层以及在猫视觉丘脑中得到证实。此外,我们表明在猫视觉丘脑中神经非线性表现在(神经)开启中央神经元的(神经)开启感受野中心以及环绕神经(关闭)神经元的(神经)开启感受野, 这表明了感光体层面的起点。这些结果表明在(神经)开启和关闭模式之间视觉处理的根本区别,并揭示了低空间频率中(神经元)关闭相较于(神经元)开启模式的竞争优势,当婴儿眼内视网膜图像因光学系统发育不成熟而表现模糊时,这一发现可能在皮质发育过程中显得非常重要。

Abstract 
Astronomers and physicists noticed centuries ago that visual spatial resolution is higher for dark than light stimuli, but the neuronal mechanisms for this perceptual asymmetry remain unknown. Here we demonstrate that the asymmetry is caused by a neuronal nonlinearity in the early visual pathway. We show that neurons driven by darks (OFF neurons) increase their responses roughly linearly with luminance decrements, independent of the background luminance. However, neurons driven by lights (ON neurons) saturate their responses with small increases in luminance and need bright backgrounds to approach the linearity of OFF neurons. We show that, as a consequence of this difference in linearity, receptive fields are larger in ON than OFF thalamic neurons, and cortical neurons are more strongly driven by darks than lights at low spatial frequencies. This ON/OFF asymmetry in linearity could be demonstrated in the visual cortex of cats, monkeys, and humans and in the cat visual thalamus. Furthermore, in the cat visual thalamus, we show that the neuronal nonlinearity is present at the ON receptive field center of ON-center neurons and ON receptive field surround of OFF-center neurons, suggesting an origin at the level of the photoreceptor. These results demonstrate a fundamental difference in visual processing between ON and OFF channels and reveal a competitive advantage for OFF neurons over ON neurons at low spatial frequencies, which could be important during cortical development when retinal images are blurred by immature optics in infant eyes. 

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