Role of cortico-cortical connections in mediating cerebral cortex plasticity: visual cortex model

在哺乳动物中，视网膜的损伤不仅会影响眼睛内的神经元，还会引起大脑其他部分的变化，特别是视觉皮质。研究发现，视网膜损伤后，视皮层细胞通常接受来自视网膜受损部分的输入，现在则接受来自邻近正常视网膜(异位感受野)的输入。所有视力发育良好的哺乳动物都有大量独立的视觉皮质区域(灵长类动物中有30多个)。这些区域被安排在层次中，其中被认为随着视觉刺激的特征变得更复杂，它们在层次中更高的区域中被区分。这些高阶区域也投射回低阶区域。这种来自较高区域的反馈活动可以通过将给定区域冷却到约10摄氏度，然后将其重新升温到其正常温度来可逆地取消。我们将试图确定在猫(视力发育良好的动物)视网膜小区域受损后，来自较高视觉皮质区域的反馈活动是否会影响较低视觉皮质区域的异位感受野。另一种可能性是，视网膜损伤后出现的异位感受野可能依赖于特定皮质区域内的水平连接，该连接从正常皮质延伸到受损伤影响的皮质区域。我们建议通过可逆地(用化学试剂)阻断这些水平纤维的传递并确定受病变影响区域的神经元的特征来测试这一想法。了解反馈和水平皮质-皮质连接在视网膜损伤后建立新的异位感受野中的作用，将有助于我们理解视网膜损伤或某些年龄相关性视网膜变性后发生知觉扭曲和视觉幻觉的机制。

In mammals injury to the retina not only affects the neurones within the eye but also induces changes in the other parts of the brain, particularly in the visual cortex. It has been found that after retinal injury cells in the visual cortex, normally receiving an input from the injured part of the retina, now receive an input from adjacent normal retina ( ectopic receptive field ). All mammals with well developed vision have a large number of separate visual cortical areas (more than 30 in primates). These areas are arranged in a hierarchy in which it is thought that as features of the visual stimuli become more complex they are discriminated in the areas higher in the hierarchy. These higher-order areas also project back to lower-order areas. This feedback activity from the higher areas can be reversibly abolished by cooling a given area to about 10oC and then rewarming it back to its normal temperature. We will try to determine if in cats (animals with well developed vision) following damage to a small region of the retina the feedback activity from higher visual cortical areas affects the ectopic receptive fields in the lower visual cortical areas. Another possibility is that the ectopic receptive field apparent following retinal damage might depend on horizontal connections within the particular cortical area, running from normal cortex to the area of cortex affected by the lesion. We propose to test this idea by blocking reversibly (with chemical agents) transmission of these horizontal fibres and determining the characteristics of neurones in the area affected by the lesion. Understanding the role of feedback and horizontal cortico-cortical connections in establishing new ectopic receptive fields following spatially delineated damage to the retina will help us to understand the mechanisms underlying perceptual distortions and visual hallucinations which occur following retinal traumas or some age-related retinal degenerations.