科学家近日揭示了由伽利略发现的视觉错觉的神经机制。该发现提高了我们对大脑识别物体工作机制的认识。该研究由纽约大学Jens Kremkow博士领导的研究团队完成，相关报道发表在近期的PNAS杂志上。
伽利略在观测行星的时候发现了一个奇怪的现象，他发现裸眼和望远镜观察行星的形态是不一样的。直接观察行星，行星看起来会变大，并具有发光光环。这就使得金星看起来比木星大八至十倍，而事实上木星比金星大四倍。伽利略意识到该物体大小的错觉是由于观察者眼睛造成的，但是他并不知道其中的原因。
伽利略猜测该现象的原因可能是由于光线在角膜处发生折射，或者是光线在眼皮边缘发生反射到角膜，或者其他原因。一代又一代的科学家研究着伽利略发现的现象，认为该错觉是由于物体模糊或其他类似光学效果产生的。但是尽管物体模糊能够干扰我们对物体大小的感知，但是还不能解释裸眼情况下为什么金星看起来比木星大。赫尔曼·冯·亥姆霍兹是19世纪德国著名的物理学家和生理学家，他首次意识到该现象可能并不是单纯的物理现象。
直到今天，Kremkow博士领导的科学团队才真正的解决了该问题。科学家分别检测了视觉相关脑区神经元对光亮和黑暗的反应，发现黑暗会产生与物体大小相同的，真实的神经元反应；而光亮刺激会使得神经元产生非线性放大作用，导致刺激比实物本身看起来更大。这也就是为什么黑色背景上的白点比白色背景上相同大小的黑点看起来要大的原因。所以伽利略看到的金星大小并不是其实际的大小。
该机制是我们识别背景和前景的普遍机制。这也解释了一个众所周知但无法解释的现象，我们看白色纸张上的黑字比黑色纸张上的白字更容易。通过比较视网膜和视觉中枢的反应，科学家发现该视觉错觉源自于视觉最初级，即光感受器部分。
doi:10.1073/pnas.1310442111
Neuronal nonlinearity explains greater visual spatial resolution for darks than lights
Jens Kremkow,Jianzhong Jin,Stanley J. Komban,Yushi Wang,Reza Lashgari,Xiaobing Li,Michael Jansen,Qasim Zaidi,and Jose-Manuel Alonso
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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