Visual memory system and mechanisms of associative learning : Interaction between the temporal neocortex and the hippocampus

神经心理学理论认为，内侧颞叶和新皮质之间的相互作用在形成对事实和事件的长期记忆中起着关键作用，这一点经常通过对人类一系列配对的单词或图形的学习来检验。本研究采用视觉配对联想任务的动物模型，通过单单位记录的方法，对猴子记忆形成的神经机制进行了研究。首先，我验证了一种假说，即视觉物体的长期表征是通过在前颞下皮质(IT)的神经网络中学习而获得的。我确定了两种机制；一种是调优，另一种是关联。在执行视觉配对联想任务的猴子的IT皮层中，我发现一组神经元对这两个配对联想都表现出选择性反应(配对编码神经元)。这为单个IT神经元通过联想LEAR…获得刺激选择性提供了强有力的证据更多的宁静。然后，我通过询问反复接触一张图片是否会产生选择性地对特定形式做出反应的IT神经元来测试调谐机制。在几何相似的模式中比较单个神经元的活动，其中只有一个在单位记录(学习模式)之前反复暴露给猴子。我发现IT神经元对习得的模式反应最好。然而，对学习模式的反应是最优的，只是局部的，而且在单个神经元的整个形式空间中可能有几个局部极大值。一个成对编码的神经元在形式空间中至少有两个独立的局部最大值，对应于每个成对的伙伴。第二，我测试了从内侧颞叶到IT皮质的反向信号的作用。将鹅膏油酸单侧注射到内嗅皮层和周边皮质，向同侧的IT皮质提供大量的后向投射。在损伤后，猴子可以学习一组新的配对伙伴。我发现(I)尽管有病变，神经元对视觉刺激的选择性反应，这表明调节机制的保存，以及(Ii)配对的伙伴未能在与病变测试的细胞中引起相关反应。我的结论是，边缘病变扰乱了IT神经元在配对的伙伴之间的联系代码，而不会损害对每个刺激的视觉反应。这些结果为灵长类长期记忆系统分解为不同的神经机制提供了具体证据。较少

Neuropsychological theories proposed a critical role of the interaction between the medial temporal lobe and neocortex in the formation of long-term memory for facts and events, which has often been tested by learning of a series of paired words or figures in humans. In this research project, I have been identifying neural mechanisms underlying the memory formation by single-unit recording with an animal model of visual pair-association task in monkeys.First, I tested a hypothesis that long-term representations of visual objects are acquired through learning in the neural network of the anterior inferotemporal (IT) cortex. I identified two mechanisms ; one is tuning and the other is association. In the IT cortex of monkeys performing the visual pair-association task, I found a group of neurons that manifested selective responses to both of the paired associates (pair-coding neuron). It provides strong evidence that single IT neurons acquire stimulus-selectivity through associative lear … More ning. Then I tested tuning mechanism by asking whether repeated exposures to a picture produce IT neurons that selectively respond to that particular form. Single-neuron activity was compared among geometrically similar patterns, only one of which had been exposed repeatedly to the monkey before the unit recording (learned pattern). I found that IT neurons responded best to the learned pattern. The reponse to a learned pattern is optimal, however, only locally, and there are presumably several local maxima in the whole form space of a single neuron. A pair-coding neuron would have at least two separate local maxima, correponding to each of the paired associates, in the form space.Second, I tested the role of the backward signal from medial temporal lobe to IT cortex. Ibotenic acid was injected unilaterally into the entorhinal and perirhinal cortex which provide massive backward projections ipsilaterally to IT cortex. After the lesion, the monkeys could learn a new set of paired associates. I found that (i) in spite of the lesion, the neurons responded selectively to the visual stimuli, which suggests preservation of the tuning mechanism, and (ii) the paired associates failed to elicit correlated responses in the cells tested with the lesion. I conclude that the limbic lesion disrupted the associative code of the IT neurons between the paired associates, without impairing the visual response to each stimulus. These results give concrete evidence for decomposition of the primate long-term memory system into different neural mechanisms. Less