| Objective:Working memory(WM) is necessary for higher cognitive functions.It is one of the hot topics for the investigation of working memory mechanism in the field of cognitive neuroscience. The aim of this study is to investigate the coordination mechanism between the two modes of neural signals, local field potentials(LFPs) and spikes in rat prefrontal cortex during a Y-maze working memory task via coherence and mutual information. It is expected to provide new insights for the investigation the neural coding mechanism in working memory.Methods:1. Acquisition of LFPs and spikes in rat PFC during a Y-maze working memory task The experimental data were multi-channel LFPs and spikes obtained from SD rat prefrontal cortex(PFC) during the working memory task in Y-maze. Rats were trained in a Y-maze working memory task until the rats can reach 85% correct rate on two consecutive days. Then multi-channel neural signals were obtained from rat PFC through the implanted microelectrode array during the task(4 rats 40 correct trials). Neural signals were 0.3-500 Hz filtered to obtain the LFPs. Neural signals were 500-7500 Hz filtered and pre-set voltage threshold was used for spike detection. The spikes and LFPs from one channel were joined in a pair. The channels for each rat could be kept same in the 10 correct trials. The number of effective channels was 14 for all rat. LFPs and spikes recorded at resting state were further analyzed to compare with the results in working memory.2. Time-frequency analysis of LFPs for working memory related frequency bands Recorded LFPs were filtered by a 50 Hz notching filter and baseline drifts were removed.The short-time Fourier transform(STFT) was applied to analyze the power changes of working memory related frequency bands(θ-band: 4-12 Hz, γ-band: 30-60 Hz) in the LFPs. Then the frequency components(θ-band and γ-band) were obtained via band-pass filtering.3. Construction of a continuous time series from a spike trains A continuous time series was constructed via instantaneous firing rate.4. LFPs- spikes coherence during working memory Coherence analysis was applied to calculate the spatial-temporal distribution of LFPs(θ-band and γ-band)- spikes coherence during the working memory task(a 500-ms sliding window with a 125-ms step). Coherence analysis was further applied to calculate the spatial-temporal distribution of LFPs-spikes coherence at resting state. The LFPs-spikes coherence in working memory and resting state was compared. The coherence values in 40 working memory trials and 40 resting trials from 4 rats were normalized. The corresponding maximum was 1 and the minimum was 0. Based on the spatial-temporal distribution characteristics of coherence, to investigate the LFPs-spikes coordination mechanism in working memory.5. LFPs-spikes mutual information during working memory The spatial-temporal distribution of LFPs(θ-band and γ-band)- spikes mutual information during the working memory task was calculated(a 500-ms sliding window with a 125-ms step). The spatial-temporal distribution of LFPs-spikes mutual information at resting state was also calculated. The LFPs-spikes mutual information in working memory and resting state was further compared. The mutual information values in 40 working memory trials and 40 resting trials from 4 rats were normalized. The corresponding maximum was 1 and the minimum was 0. Based on the spatial-temporal distribution characteristics of mutual information, to investigate the LFPs-spikes coordination mechanism in working memory.6. Statistical analysis T-test was used to compare the LFPs-spikes coherence and mutual information values in working memory and resting state.Results: 1. Behavior results Rats were trained on a Y-maze working memory task, until the rats could reach the criterion. We recorded 51 trials(45 correct trials and 6 incorrect trials from 4 rats) while they performed the working memory task, and the correct rate was 88.2%. In total, we described 40 correct trials(10 correct trials from each rats) in the present thesis. 2. Power changes of LFPs during the working memory task The power in the θ-band(4-12Hz) and γ-band(30-60Hz) of LFPs were high during working memory. The θ and γ power peaked before the rats arrived at the reference point. The dominant frequencies in the LFPs were the θ-band and γ-band in working memory. 3. LFPs- spikes coherence during working memory(1) LFPs θ-band-spikes coherence during working memory In the working memory task(10 correct trials from each rats), the LFPs θ band-spikes coherence values in working memory state are 0.79043±0.0158, 0.6701 ± 0.0163, 0.6825 ± 0.0104, 0.6013 ± 0.1361; The values in resting state are 0.3347 ± 0.0130, 0.3475 ± 0.0168, 0.4402 ± 0.0543,0.3224 ± 0.0528. The LFPs θ band-spikes coherence value in working memory state was significantly higher than that in resting state(t test,P<0.05). The LFPs θ-band-spikes coherence value peaked before the reference point(0.9715 ± 0.2496 s).(2) LFPs γ-band-spikes coherence during working memory In the working memory task(10 correct trials from each rats), the LFPs γ-band-spikes coherence values in working memory state are 0.8003 ± 0.0185, 0.7902 ± 0.0254, 0.8125 ± 0.0141, 0.7884 ± 0.1221; The values in resting state are 0.4557 ± 0.0120, 0.4754 ± 0.0093, 0.4301 ± 0.0413, 0.4311 ± 0.0322. The LFPs γ-band-spikes coherence value in working memory state was significantly higher than in resting state(t test,P<0.05). The LFPs γ-band-spikes coherence value peaked before the reference point(0.7715 ± 0.1393 s).4. LFPs-spikes mutual information during working memory(1) LFPs θ-band-spikes Mutual Information coding during In the working memory task(10 correct trials from each rats), the LFPs θ-band-spikes mutual information values in working memory state are 0.6812±0.0302,0.6739±0.0527,0.6449±0.0432,0.6103±0.0500. The values in resting state are 0.3341±0.0116,0.3257±0.0214,0.3152±0.0523,0.3824±0.0435; The LFPs θ-band-spikes mutual information value in working memory state was significantly higher than in resting state(t test,P<0.05). The LFPs θ-band-spikes coherence value peaked before the reference point(0.6818±0.1435s).(2) LFPs γ-band-spikes Mutual Information coding during In the working memory task(10 correct trials from each rats), the LFPs γ-band-spikes mutual information values in working memory state are 0.7912±0.0464,0.6725±0.0390, 0.7592±0.0504, 0.6947±0.0421.The value in resting state are 0.4021±0.0146,0.3602±0158,0.3923±0.0415,0.3802±0.0325. The LFPs θ-band-spikes mutual information value in working memory state was significantly higher than in resting state(t test,P<0.05). The LFPs γ-band-spikes coherence value peaked before the reference point(0.7055±0.2274s).Conclusions: This study investigated the coordination mechanism between the LFPs and spikes in rat prefrontal cortex during a Y-maze working memory task via coherence and mutual information.The results show as follows:1. The power in the θ-band(4-12Hz) and γ-band(30-60Hz) of LFPs increased to peaks before the reference point during working memory, indicating that the θ-band and γ-band are the characteristic frequency bands. 2. LFPs and spikes cooperatively encoded the working memory, from the view of coherence and mutual information(1) The LFPs θ-band-spikes coherence encoded the working memory, as did the LFPs γ-band-spikes coherence.(2) The mutual information of LFPs θ-band-spikes encoded the working memory, as did the LFPs γ band-spikes mutual information. In particular, when the neural signals showed stronger non-linear features, the mutual information could encode the working memory information more effectively. |
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