Neural Substrates of Stimulus Recognition and Association Memory

众所周知，在我们的一生中学习的能力是由大脑的结构变化调节的。然而，学习过程中这种变化的时空动力学尚不清楚。在这里，我们按照两个任务顺序训练了10只幼稚的成年动物。这些动物首先接受了一处任务的标准(90%)，这是一种视觉运动任务，要求动物接触并触摸电脑屏幕上的一个物体。接下来，训练这些动物触摸放置在由多个几何元素组成的人造视觉场景中的目标前景对象。这些动物同时学习了几个独特的场景；目标物体的身份和位置在不同的场景中有所不同，但在场景中是固定的。我们使用Bruker 4.7T磁共振系统采集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)的多壳高级扩散磁共振成像(DMRI)图像。在行为上，这些动物在学习不同任务的能力上表现出很大的个体差异。对白质的全脑分析显示，水的扩散率(径向(RD)、轴向(AD)、实质(PD))的测量在时间点之间发生了显著变化，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的减少与视觉场景任务的更快学习相关。PD是一种衡量组织密度的指标，而不受自由水污染。一项分析比较了掌握场景任务的动物和那些没有显示穹隆小腿RD焦点变化的动物。进一步的检查表明，小腿的扩散率指标的降低，可能是由于限制/阻碍水扩散的增加，往往与更快的场景学习相关。总体而言，dMRI测量的变化模式表明，对复杂视觉场景的长期学习会引起脑白质微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.