The daily proportion of light and dark hours (photoperiod) changes annually and plays an important role in the synchronization of seasonal biological phenomena, such as reproduction, hibernation, and migration. In mammals, the first step of photoperiod transduction occurs in the suprachiasmatic nuclei (SCN), the circadian pacemaker that also coordinates 24-h activity rhythms. Thus, in parallel with its role in annual synchronization, photoperiod variation acutely shapes day/night activity patterns, which vary throughout the year. Systematic studies of this behavioral modulation help understand the mechanisms behind its transduction at the SCN level. To explain how entrainment mechanisms could account for daily activity patterns under different photoperiods, Colin Pittendrigh and Serge Daan proposed a conceptual model in which the pacemaker would be composed of 2 coupled, evening (E) and morning (M), oscillators. Although the E-M model has existed for more than 40 years now, its physiological bases are still not fully resolved, and it has not been tested quantitatively under different photoperiods. To better explore the implications of the E-M model, we performed computer simulations of 2 coupled limit-cycle oscillators. Four model configurations were exposed to systematic variation of skeleton photoperiods, and the resulting daily activity patterns were assessed. The criterion for evaluating different model configurations was the successful reproduction of 2 key behavioral phenomena observed experimentally: activity psi-jumps and photoperiod-induced changes in activity phase duration. We compared configurations with either separate light inputs to E and M or the same light inputs to both oscillators. The former replicated experimental results closely, indicating that the configuration with separate E and M light inputs is the mechanism that best reproduces the effects of different skeleton photoperiods on day/night activity patterns. We hope this model can contribute to the search for E and M and their light input organization in the SCN.

中文翻译：

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不同骨架光周期下双昼夜节律振荡器模型的定量研究。

每天的明暗时间比例（光周期）每年都在变化，并且在季节性生物现象（例如繁殖，冬眠和迁徙）的同步中起着重要作用。在哺乳动物中，光周期转导的第一步发生在视交叉上核（SCN）中，这是昼夜节律起搏器，也可以协调24小时活动节律。因此，与其在年度同步中的作用并行，光周期变化会急剧影响白天/夜晚的活动模式，而这种活动模式全年都会变化。对这种行为调节的系统研究有助于在SCN级别了解其转导的机制。为了解释夹带机制如何解释不同光周期下的日常活动模式，Colin Pittendrigh和Serge Daan提出了一种概念模型，其中的起搏器将由两个耦合的傍晚（E）和早晨（M）振荡器组成。尽管EM模型已经存在了40多年，但其生理基础仍未完全解决，并且尚未在不同的光周期下进行定量测试。为了更好地探索EM模型的含义，我们对2个耦合的极限周期振荡器进行了计算机仿真。四种模型配置暴露于骨骼光周期的系统变化，并评估了由此产生的日常活动模式。评估不同模型配置的标准是成功再现实验观察到的两个关键行为现象：活动psi跳跃和活动期持续时间中光周期引起的变化。我们比较了配置和E和M的单独光输入或两个振荡器的相同光输入。前者紧密复制了实验结果，表明具有单独的E和M光输入的配置是最能重现不同骨架光周期对昼夜活动模式影响的机制。我们希望该模型可以有助于在SCN中搜索E和M及其光输入组织。