【作者】 叶翔；

【导师】 周逸峰；

【作者基本信息】 中国科学技术大学 ， 生物物理学， 2009， 博士

【摘要】 视觉系统对于连续的刺激会产生一种自我校准的机制,这种自我校准机制会使得视觉系统对输入信号中占优势的统计量产生适应。适应是视觉系统的一个普遍现象。自从适应现象被发现以来,人们就从心理物理学和生理学等方面对其进行了广泛的研究。适应之所以这样令人感兴趣主要有两个原因:一是由于适应产生的速度足够快,这样它就贡献于实时感觉信息处理;二是由于适应是研究正常神经组织可塑性的一种有用的工具。因此,适应也被称为心理物理学家的电极或探针。在生理学上,人们发现适应现象存在于视觉通路的各个阶段。最初,对适应的研究主要集中在初级视皮层区域。随着研究的深入,人们逐渐在视网膜、外侧膝状体(外膝体,lateral geniculate nucleus, LGN)和高级视皮层发现适应现象。在这些后来发现的适应现象中,研究的最为详尽的是视网膜上的适应。与此同时,人们在猴和猫上发现了一些知觉后效的神经基础:时间频率、空间频率、对比度、方位、方向和速度适应都逐步的被发现。在知觉上,适应能够形成各种各样的后效应。在生理学领域,人们研究适应的一个任务就是要找出这些后效应的生理学基础及其发生的位点。现在已了解方位适应是著名的倾斜后效应的生理学基础。人们在猫的皮层中已经观察到在方位适应后会排斥性的改变细胞的最优方位,从而得出倾斜效应起源于初级视皮层的观点。LGN通常被认为是视觉信息到皮层的一个被动的中继站。然而,近来的研究表明LGN可能扮演重要的作用:作为早期视觉通路上的“守门人”控制注意反应的增益和视知觉。这表明LGN在视觉通路上可能不仅仅是被动的传递视觉信息。因此,在这里我们研究了适应对猫LGN细胞反应特性的影响,这其中包括方位适应对高方位选择性的外膝体细胞反应特性的影响以及对比度适应对外膝体X和Y细胞反应影响的差异。1.方位适应对高方位选择性的外膝体细胞反应特性的影响对刺激方位适应通常被认为是皮层细胞的特性,很少有研究表明方位适应是否影响皮层下神经元的活动。先前的研究表明,LGN的一些神经元具有方位选择性,同时外膝体神经元也具有适应现象。这提示我们在具有高方位选择性的LGN细胞中可能发生方位适应并对外膝体细胞反应产生影响。同时,在中央视觉系统,每一阶段的神经元反应特性都受到更高阶段的反馈调节。对于猫和灵长类,视网膜输入仅占LGN中继细胞接收输入的10%左右,然而皮层反馈占据了接收输入的30%左右。先前的研究表明皮层反馈影响LGN神经元感受野的空间结构和中央周边交互作用,控制LGN神经元发放的时间同步性,提高视觉信息的传递。这提示了我们:皮层–外膝体反馈可能对LGN神经元方位适应有贡献。我们用单细胞记录技术和皮层冷冻使皮层失活的方法,在有、无皮层反馈的条件下研究了在麻醉、麻痹猫中方位适应对外膝体(LGN)高方位选择性神经元反应的影响。我们发现在用某一方位的刺激进行适应后,适应方位的反应下降,神经元的最优方位相对于适应方位发生排斥性改变。这个效应和在猫的初级视皮层观察到的方位适应效应相似,并且当适应方位处于相对于适应前最优方位的合适的位置时最显著。此外,当初级视皮层失活后,方位适应仍然能导致反应的下降,但是最优方位没有表现出系统的改变。这个结果表明皮层反馈对高方位选择性LGN细胞的方位适应效应有贡献。我们的数据提供了皮层丘脑循环如何调节视觉信息处理的一个例子,并提示LGN不仅仅是视觉通路上一个简单被动的中继站,它也参与了对视觉信息的调控。2.对比度适应对外膝体X和Y细胞不同的效应对比度适应是众所周知的心理物理现象。盯着高对比度刺激一段时间,会导致我们的对比敏感度就降低。对比度适应提供了两个潜在的好处:它能保护细胞不至于被高对比度刺激饱和;而且可以确保一个细胞的操作范围仅分布于特定图像中的对比度范围内。在电生理领域中,对比度适应首先在猫和灵长类的初级视皮层被发现,而早期的生理学研究在猫和猴的LGN很少发现对比度适应。这些结果把对比度适应严格限制在了初级皮层。然而,此后慢对比度适应在蝾螈、兔子和猴的视网膜上被发现。此外,人们重新评估皮层神经元的适应认为对比度适应差不多只发生在单眼视觉通路。这些发现提示对比度适应可能也发生在LGN水平。Solomon发现在猴的LGN中对比度适应主要影响M细胞而对P细胞影响很少。Duong的结果也从一个侧面提示了对比度适应现象可能在猫的LGN中也存在。然而,很少有实验证据表明在猫的外膝体中对比度适应是否对X和Y细胞有同样的效应。因此,我们用单细胞记录方法,在麻醉麻痹猫上研究了上述问题。我们发现对比度适应主要改变LGN细胞的对比度增益而很少影响细胞最大反应,同时对比度适应提高了LGN细胞的对比度检测阈值。Y细胞比X细胞表现出更大的对比度增益的改变从而表现出更强的对比度适应。而在对比度检测阈值提高方面,X和Y细胞没有显著的差异。我们的结果表明对比度适应在LGN细胞中是分离处理的,从而提供了视觉信息分离处理的例子。更多还原

【Abstract】 Visual system will produce a self-calibration mechanism in a continuous work, and such a self-calibration mechanism will allow the visual system adapt to the dominated statistics component of the signal input. Adaptation is a common phenomenon in visual system. Since the adaptation phenomenon has been found, it has been studied comprehensively in psychophysics and physiology. Adaptation is of interesting for two reasons: one is that adaptation occurs rapidly enough to contribute to real time sensory information processing; another is that adaptation is a useful tool for studying the plasticity of normal issues. Therefore,adaptation is known as the electrode or probe of psychophysicist.In physiology, the adaptation phenomenon has been found at all levels of visual system. At first, adaptation studying focuses on the primary visual cortex. With the study going on, the adaptation phenomenon has gradually been found in retina, LGN and the advanced visual cortex.In perception, adaptation could bring about various after-effects. For physiology, one task of study is to find the physiological basis of these after-effects and the site of its occurring. Orientation adaptation is thought to be the physiological substrate of the well-known tilt after-effect (TAE). In primary cortex of cat, the observation of the repulsive shift of orientation preference after orientation adaptation leads to the view that the TAE originates from the V1.The LGN is often thought of as a passive relay of visual information to the cortex. However, recent studies have demonstrated that the LGN could play an important role as an early“gatekeeper”for controlling the gain of attentional responses and visual awareness. This suggested that LGN is not only a passive relay of visual information transmitting. Therefore, we studied the effects of adaptation on the responses properties of LGN cells, which include the effects of orientation adaptation on the responses properties of LGN cells with high orientation bias in cats and contrast adaptation has different effects on X and Y cells of the lateral geniculate nucleus.1. The effects of orientation adaptation on the responses properties of LGN cells with high orientation bias in catsAdaptation to stimulus orientation is assumed to have a cortical basis, but few studies have addressed whether it affects the activity of subcortical neurons. Previous studies demonstrated that some LGN neurons exhibited an orientation bias and also showed adaptation phenomena. In the light of these studies, it seems that orientation adaptation could have specific effects on the activities of LGN neurons with high orientation bias.In the central visual system, the properties of neurons at each stage are modulated by feedback from higher structures. For cats and primates, retinal afferents comprise 10% of the inputs to LGN relay cells, whereas corticofugal feedback comprises 30% of their inputs. Previous studies have shown that cortical feedback influences the spatial structure and centre-surround interaction of LGN receptive fields, controls temporal synchronization of LGN spiking activity. Thus, it is possible that corticogeniculate feedback contributes to the orientation adaptation of LGN neurons.Using single-unit recording and inactivating cortex by freezing, we studied the effects of orientation adaptation on the responses of lateral geniculate nucleus (LGN) neurons with high orientation bias in anesthetized and paralyzed cats under with feedback or without feedback condition. Following adaptation to one stimulus orientation, the response at the adapting orientation was decreased and the preferred orientation was shifted away from the adapting orientation. This phenomenon was similar to the effects observed for orientation adaptation in the primary visual cortex, and was obvious when the adapting orientation was at an appropriate location relative to the original preferred orientation. Moreover, when the primary visual cortex was inactivated, the response at the adapting orientation was also decreased but the preferred orientation did not show a systematic shift after orientation adaptation in LGN. This result indicates that cortical feedback contributes to the effect of orientation adaptation on LGN neurons, which have a high orientation bias. These data provide an example of how the corticothalamic loop modulates the processing of visual information, and suggest that the LGN is not only a simply passive relay but also a modulator of visual information.2. Contrast adaptation has different effects on X and Y cells of the lateral geniculate nucleusContrast adaptation is well known psychophysical phenomenon. When we stare at a high contrast stimulus for a time, our subsequent contrast sensitivity will decrease. Contrast adaptation provides two potential benefits: it may protect a cell from saturation by high contrast stimuli and it may ensure that the operating range of a cell spans only the range of contrasts contained in a particular image.In electrophysiology, Contrast adaptation first has been found in primary cortex of cat and primate, and early physiological studies has found little contrast adaptation in the LGN of cat or monkey. These results would place contrast adaptation strictly into early cortex. However, a slow contrast adaptation has been found in the retina of salamander, rabbit, and macaque, and furthermore a reevaluation of adaptation in cortical neurons has concluded that contrast adaptation occurs almost exclusively in that part of the visual pathway which is monocular. These finding suggest contrast adaptation could occur at the level of LGN.Solomon et al have found contrast adaptation affects most on the M cells and little on the P cells in the LGN of macaque. Recently Duong’s results have also confirmed contrast adaptation phenomenon exists in LGN of cat from the other side. However, whether contrast adaptation has same effects on X and Y cells of lateral geniculate nucleus (LGN) of cat are not clear documented. Using single-unit recording, we here studied this issue in anesthetized and paralyzed cats. We found that contrast adaptation mostly shifted contrast gain of LGN cells, caused maximum response to change little, and increased the contrast detection threshold of LGN cells. Y cells express stronger contrast adaptation than X cells for contrast gain change. However, there was no significant difference between X and Y cells for the increase of contrast detection threshold. These results indicate contrast adaptation is separated to process by LGN cells and further provide an example of separate processing of visual information.更多还原