Recall of sequences based on the position of the first cue stimulus in rats

A new paradigm for investigating temporal order memory shows higher order associations are present in recent but not in remote retrieval

Memory of a sequence of distinct events requires encoding the temporal order as well as the intervals that separates these events. In this study, using order-place association task where the animal learns to associate the location of the food pellet to the order of entry into the event arena, we probe the nature of temporal order memory in mice. In our task, individual trials become distinct events, as the animal is trained to form a unique association between entry order and a correct location. The inter-trial intervals (> 30 min) are chosen deliberately to minimize the inputs from working memory. We develop this paradigm initially using four order-place associates and later extend it to five paired associates. Our results show that animals not only acquire these explicit (entry order to place) associations but also higher order associations that can only be inferred implicitly (temporal relation between the events) from the temporal order of these events. As an indicator of such higher order learning during the probe trial, the mice exhibit predominantly prospective errors that decline proportionally with temporal distance. On the other hand, prior to acquiring the sequence, the retrospective errors are dominant. In addition, we also tested the nature of such acquisitions when temporal order CS is presented along with flavored pellet as a compound stimulus comprising of order and flavor both simultaneously being paired with location. Results from these experiments indicate that the animal learns both order-place and flavor-place associations. Comparing with pure order-place training, we find that the additional flavor stimulus in a compound training paradigm did not interfere with the ability of the animals to acquire the order-place associations. When tested remotely, pure order-place associations could be retrieved only after a reminder training. Further higher order associations representing the temporal relationship between the events is markedly absent in the remote time.

Effects of uninterrupted sinusoidal LF-EMF stimulation on LTP induced by different combinations of TBS/HFS at the Schaffer collateral-CA1 of synapses

Long-term potentiation (LTP) is an important aspect of synaptic plasticity and is one of the main mechanisms involved in memory. Low-frequency electromagnetic fields (LF-EMFs) such as transcranial magnetic stimulation are emerging neuromodulation tools for the regulation of LTP. However, whether LF-EMFs have different effects on different types of LTP has not yet been verified. Herein, we studied the regulatory effects of 15 Hz/2 mT sinusoidal magnetic field as pre-magnetic stimulation on several types of LTP, which were induced by theta-burst(TBS) or high-frequency stimulation (HFS) or some combination of them, and applied N-methyl-D-aspartate receptor(NMDAR) antagonists to observe the relationship between the regulation of LTP by LF-EMFs and NMDAR in the Schaffer collateral pathway of rat brain slices in vitro. The results presented in this paper are the performance of TBS and HFS was not exactly the same and the recovery speed of TBS-LTP was faster than HFS-LTP after receiving the regulation of LF-EMFs; moreover, the LTP level was affected by the order of combination and the effect of pre-magnetic stimulation could maintain the entire process of the combined induction experiment, while NMDAR antagonists could not completely offset the influence of LF-EMFs. The memory patterns are diverse, and this study has shown LF-EMFs can regulate LTP such as TBS-LTP and HFS-LTP and can continuously affect multiple LTP induction processes. However, different memory processes may have different performance in the face of LF-EMFs regulation. In terms of the mechanism of LF-EMFs-induced LTP regulation, NMDARs may be involved in the process of LF-EMF regulation of LTP, but are not the only factor.

Hippocampal-prefrontal theta phase synchrony in planning of multi-step actions based on memory retrieval

Planning of multi-step actions based on the retrieval of acquired information is essential for efficient foraging. The hippocampus (HPC) and prefrontal cortex (PFC) may play critical roles in this process. However, in rodents, many studies investigating such roles utilized T-maze tasks that only require one-step actions (i.e., selection of one of two alternatives), in which memory retrieval and selection of an action based on the retrieval cannot be clearly differentiated. In monkeys, PFC has been suggested to be involved in planning of multi-step actions; however, the synchrony between HPC and PFC has not been evaluated. To address the combined role of the regions in planning of multi-step actions, we introduced a task in rats that required three successive nose-poke responses to three sequentially illuminated nose-poke holes. During the task, local field potentials (LFP) and spikes from hippocampal CA1 and medial PFC (mPFC) were simultaneously recorded. The position of the first hole indicated whether the following two holes would be presented in a predictable sequence or not. During the first nose-poke period, phase synchrony of LFPs in the theta range (4-10 Hz) between the regions was not different between predictable and unpredictable trials. However, only in trials of predictable sequences, the magnitude of theta phase synchrony during the first nose-poke period was negatively correlated with latency of the two-step ahead nose-poke response. Our findings point to the HPC-mPFC theta phase synchrony as a key mechanism underlying planning of multi-step actions based on memory retrieval rather than the retrieval itself.

A Comparative Study of the Impact of Theta-Burst and High-Frequency Stimulation on Memory Performance

The transformation of the information stored in the working memory into the system of long-term memory depends on the physiological mechanism, long-term potential (LTP). In a large number of experimental studies, theta-burst stimulation (TBS) and high-frequency stimulation (HFS) are LTP induction protocols. However, they have not been adapted to the model related to memory. In this paper, the improved Camperi–Wang (C–W) model with Ca²⁺ subsystem-induced bi-stability was adopted, and TBS and HFS were simulated to act as the initial stimuli of this working memory model. Evaluating the influence of stimuli properties (cycle, amplitude, duty ration) on memory mechanism of the model, it is found that both TBS and HFS can be adopted to activate working memory model and produce long-term memory. Moreover, the different impacts of two types of stimuli on the formation of long-term memory were analyzed as well. Thus, the importance of this study lies firstly in describing the link and interaction between working memory and long-term memory from the quantitative view, which provides a theoretical basis for the study of neural dynamics mechanism of long-term memory formation in the future.