Neural Substrates of Stimulus Recognition and Association Memory

我们一生的学习能力是由大脑的结构变化所调节的。然而，这种变化在学习过程中的时空动态尚不清楚。在这里，我们在两个任务中依次训练了10只幼稚的成年动物。首先，这些动物被训练在一地任务中达到标准（90%），这是一项视觉运动任务，要求动物到达并触摸电脑屏幕上的物体。接下来，这些动物被训练去触摸放置在由多个几何元素组成的人工视觉场景中的目标前景物体。动物们同时学习了几种独特的场景；目标物体的身份和位置在不同的场景中不同，但在场景中是固定的。我们使用Bruker 4.7T MRI系统获得了动物在两个时间点（训练前和在一个或两个任务中达到标准后）的多层高级弥散MRI （dMRI）图像。从行为上看，这些动物在学习不同任务的能力上表现出了很大的个体差异。全脑白质分析显示，在时间点之间，水扩散率（径向（RD），轴向（AD），实质（PD））的测量有显著变化，但各向异性（分数，线性，平面）的测量没有显著变化。有趣的是，PD（一种没有自由水污染的组织密度测量方法）的下降与视觉场景任务的更快学习有关。一项比较掌握场景任务的动物和那些没有掌握场景任务的动物的分析表明，沿穹窿脚的RD发生了焦点变化。进一步的研究表明，小腿部位扩散系数的降低（可能是由于受限制/阻碍的水扩散的增加）往往与更快的场景学习有关。总的来说，dMRI测量的变化模式表明，长时间学习复杂的视觉场景会引起白质微观结构的整体和局部变化。

The ability to learn throughout our lifetime is known to be mediated by structural changes in the brain. However, the spatiotemporal dynamics of such changes during learning are unclear. Here, we trained 10 nave, adult animals in two tasks sequentially. The animals were first trained to criterion (90%) in the one-place task, a visuomotor task which required the animal to reach and touch an object on a computer screen. Next, the animals were trained to touch a target foreground object placed in an artificial visual scene composed of multiple geometric elements. The animals learned several unique scenes concurrently; the identity and location of the target object differed across scenes but was fixed within scenes. We acquired multishell advanced Diffusion MRI (dMRI) images from the animals across two timepoints (before training and after reaching criterion in one or both tasks) using a Bruker 4.7T MRI system. Behaviorally, the animals showed wide individual variability in their ability to learn the different tasks. Whole brain analyses of white matter revealed significant changes in measures of water diffusivity (Radial (RD), Axial (AD), Parenchymal (PD)) but not measures of anisotropy (Fractional, Linear, Planar), between the timepoints. Interestingly, the decrease in PD, a measure of tissue density without freewater contamination, was correlated with faster learning of the visual scenes task. An analysis comparing animals that mastered the scenes task and those that did not indicated a focal change in RD along the crus of the fornix. Further examination revealed that a decrease in diffusivity measures in the crus, possibly due to an increase in restricted/hindered water diffusion, tends to be correlated with faster scene learning. Overall, the pattern of changes in the dMRI measures suggest that prolonged learning of complex visual scenes evokes global as well as local changes in white matter microstructure.