A model of hippocampal spiking responses to items during learning of a context-dependent task.

Single unit recordings in the rat hippocampus have demonstrated shifts in the specificity of spiking activity during learning of a contextual item-reward association task. In this task, rats received reward for responding to different items dependent upon the context an item appeared in, but not dependent upon the location an item appears at. Initially, neurons in the rat hippocampus primarily show firing based on place, but as the rat learns the task this firing became more selective for items. We simulated this effect using a simple circuit model with discrete inputs driving spiking activity representing place and item followed sequentially by a discrete representation of the motor actions involving a response to an item (digging for food) or the movement to a different item (movement to a different pot for food). We implemented spiking replay in the network representing neural activity observed during sharp-wave ripple events, and modified synaptic connections based on a simple representation of spike-timing dependent synaptic plasticity. This simple network was able to consistently learn the context-dependent responses, and transitioned from dominant coding of place to a gradual increase in specificity to items consistent with analysis of the experimental data. In addition, the model showed an increase in specificity toward context. The increase of selectivity in the model is accompanied by an increase in binariness of the synaptic weights for cells that are part of the functional network.

在大鼠海马体的单细胞记录表明，在学习情境 - 物品 - 奖励关联任务期间，神经元放电活动的特异性发生了变化。在该任务中，大鼠根据物品出现的情境对不同物品做出反应会得到奖励，而不是根据物品出现的位置。最初，大鼠海马体中的神经元主要根据位置放电，但随着大鼠学习任务，这种放电对物品变得更具选择性。我们使用一个简单的电路模型模拟了这种效应，该模型具有离散输入，驱动代表位置和物品的放电活动，随后依次是对涉及对物品做出反应（挖掘食物）或移动到不同物品（移动到不同的食盆）的运动行为的离散表征。我们在网络中实现了放电重放，代表在尖波涟漪事件期间观察到的神经活动，并基于对尖峰时间依赖的突触可塑性的简单表征修改突触连接。这个简单的网络能够持续学习情境依赖的反应，并从对位置的主导编码转变为对物品特异性的逐渐增加，这与实验数据分析结果一致。此外，该模型对情境的特异性也有所增加。模型中选择性的增加伴随着作为功能网络一部分的细胞的突触权重二值性的增加。