The hippocampus frequently replays memories of past experiences during sharp-wave ripple (SWR) events. These events can represent spatial trajectories extending from the animal’s current location to ...distant locations, suggesting a role in the evaluation of upcoming choices. While SWRs have been linked to learning and memory, the specific role of awake replay remains unclear. Here we show that there is greater coordinated neural activity during SWRs preceding correct, as compared to incorrect, trials in a spatial alternation task. As a result, the proportion of cell pairs coactive during SWRs was predictive of subsequent correct or incorrect responses on a trial-by-trial basis. This effect was seen specifically during early learning, when the hippocampus is essential for task performance. SWR activity preceding correct trials represented multiple trajectories that included both correct and incorrect options. These results suggest that reactivation during awake SWRs contributes to the evaluation of possible choices during memory-guided decision making.
► Greater SWR reactivation of past experience precedes correct decisions ► Increased reactivation is specifically seen during learning ► Possible future paths are reactivated most strongly ► Hippocampal reactivation is related to memory-guided decision making
How does the brain use past experience to make decisions? Singer et al. report that, during learning, stronger hippocampal replay predicts subsequent correct decisions. Their findings provide a link between replay and an internal exploration of possible options during decision making.
The replay of previously stored memories during hippocampal sharp wave ripples (SWRs) is thought to support both memory retrieval and consolidation in distributed hippocampal-neocortical circuits. ...Replay events consist of precisely timed sequences of spikes from CA3 and CA1 neurons that are coordinated both within and across hemispheres. The mechanism of this coordination is not understood. Here, we show that during SWRs in both awake and quiescent states there are transient increases in slow gamma (20-50 Hz) power and synchrony across dorsal CA3 and CA1 networks of both hemispheres. These gamma oscillations entrain CA3 and CA1 spiking. Moreover, during awake SWRs, higher levels of slow gamma synchrony are predictive of higher quality replay of past experiences. Our results indicate that CA3-CA1 gamma synchronization is a central component of awake memory replay and suggest that transient gamma synchronization serves as a clocking mechanism to enable coordinated memory reactivation across the hippocampal network.
► Gamma oscillations transiently increase during hippocampal sharp wave ripples ► Gamma is synchronous across CA3 and CA1 of both hemispheres ► CA3 and CA1 neurons are entrained by gamma during sharp wave ripples ► Gamma synchrony is predictive of high quality memory replay
Spatially distributed hippocampal neurons replay stored memories, a process thought to support memory consolidation and retrieval. Carr et al. show that synchronized gamma oscillations bind distributed cell assemblies together and coordinate memory replay across the hippocampal circuit in both hemispheres.
Hippocampal replay is thought to be essential for the consolidation of event memories in hippocampal-neocortical networks. Replay is present during both sleep and waking behavior, but although sleep ...replay involves the reactivation of stored representations in the absence of specific sensory inputs, awake replay is thought to depend on sensory input from the current environment. Here, we show that stored representations are reactivated during both waking and sleep replay. We found frequent awake replay of sequences of rat hippocampal place cells from a previous experience. This spatially remote replay was as common as local replay of the current environment and was more robust when the rat had recently been in motion than during extended periods of quiescence. Our results indicate that the hippocampus consistently replays past experiences during brief pauses in waking behavior, suggesting a role for waking replay in memory consolidation and retrieval.
Trends for the atmospheric deposition of sulphur (S) and inorganic nitrogen (inorg-N) to forests and changes in the forest soil water chemistry in Sweden have been assessed since 1985, with special ...focus on the last 25 years, based on measurements within the Swedish Throughfall Monitoring Network (SWETHRO). The reductions in the deposition of S and inorg-N in the southern part of Sweden corresponded relatively well with the pollutant emission reductions for S and inorg-N from both EU27 + UK and Sweden during 1996/97–2021/22. For northern Sweden the deposition of S and inorg-N decreased to a lesser extent than both European and Swedish emissions. The bulk deposition of NO3-N has decreased more than the deposition of NH4-N over the last 25-year period, which is consistent with the much larger emission reductions for NOx compared to NH3 from EU27 + UK and Sweden. The S concentrations in the soil water, at 50 cm below soil surface, have decreased during the last 25 years, however somewhat less than the S deposition. At sites with low ANC and pH in the beginning of the period, the increase in ANC was generally greater and the increase in pH was smaller, but at sites with high pH and ANC above zero, the increase in pH was dominant, in line with the nonlinear relationship between pH and ANC in the soil water. The incidence of elevated concentrations of NO3-N in the soil water was highest in southwest Sweden, ranging between 4 and 19 % of all measuring occasions since 1985/86. The reduced deposition of N over the 35-year period was not reflected in the incidence of elevated concentrations of NO3-N in the soil water over time.
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•Deposition of S and N in south Sweden has decreased in line with emission reductions.•Bulk deposition of NO3-N decreased more than NH4-N in south Sweden.•S concentrations in soil water have decreased substantially, but less than S deposition.•Soil water pH and ANC indicate slow recovery from acidification.•A change in incidence of elevated NO3-N in soil water could not be demonstrated.
The brain is a massive neuronal network, organized into anatomically distributed sub-circuits, with functionally relevant activity occurring at timescales ranging from milliseconds to years. Current ...methods to monitor neural activity, however, lack the necessary conjunction of anatomical spatial coverage, temporal resolution, and long-term stability to measure this distributed activity. Here we introduce a large-scale, multi-site, extracellular recording platform that integrates polymer electrodes with a modular stacking headstage design supporting up to 1,024 recording channels in freely behaving rats. This system can support months-long recordings from hundreds of well-isolated units across multiple brain regions. Moreover, these recordings are stable enough to track large numbers of single units for over a week. This platform enables large-scale electrophysiological interrogation of the fast dynamics and long-timescale evolution of anatomically distributed circuits, and thereby provides a new tool for understanding brain activity.
•Modular polymer electrode-based system capable of recording up to 1,024 channels•Recording from 375 single units across multiple regions in freely behaving rats•Single-unit recording longevity for 160 or more days post-implantation•System capable of tracking populations of single units continuously for over a week
Chung et al. present a large-scale, multi-site, polymer electrode-based recording platform. The modular system is capable of data collection from up to 1,024 channels in freely behaving rats, enabling long-term studies of distributed networks.
Cognitive faculties such as imagination, planning, and decision-making entail the ability to represent hypothetical experience. Crucially, animal behavior in natural settings implies that the brain ...can represent hypothetical future experience not only quickly but also constantly over time, as external events continually unfold. To determine how this is possible, we recorded neural activity in the hippocampus of rats navigating a maze with multiple spatial paths. We found neural activity encoding two possible future scenarios (two upcoming maze paths) in constant alternation at 8 Hz: one scenario per ∼125-ms cycle. Further, we found that the underlying dynamics of cycling (both inter- and intra-cycle dynamics) generalized across qualitatively different representational correlates (location and direction). Notably, cycling occurred across moving behaviors, including during running. These findings identify a general dynamic process capable of quickly and continually representing hypothetical experience, including that of multiple possible futures.
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•Firing across hippocampal neurons can regularly “take turns” (cycle) every ∼125 ms•Cycle firing is seen at single-cell, cell-pair, and population levels•Cycle firing encodes hypothetical experience, including multiple possible futures•Cycle coding generalizes across representational correlates, implying common process
Imagination, planning, and decision-making require the ability to generate representations of hypothetical experience. Kay et. al. find that neurons in the rat hippocampus can represent alternative hypothetical scenarios both regularly and quickly (every ∼125 ms). Further, the underlying activity has a temporal structure that is equivalent across different representational correlates (location and direction), implying a common process.
The yearly, total (dry+wet) deposition of inorganic nitrogen (inorg-N) to Norway spruce forests was estimated with a full spatial coverage over Sweden for a twenty-year period, 2001–2020, based on ...combined measurements with Teflon string samplers, throughfall deposition and bulk deposition to the open field. The results were based on a novel method to apply estimates of the dry deposition based on measurements at a limited number of sites, to a larger number of sites with only bulk deposition measurements, in turn based on the existence of a strong geographical gradient in the dry deposition of inorg-N from southwest to northeast Sweden. The method should be applicable for other geographical regions where gaseous NH3, NO2 and HNO3 are not main drivers of N dry deposition and where geographical gradients in dry deposition could be defined. It was shown that Norway spruce forests in south Sweden receive more N from deposition than has been previously estimated, based on modelling. Clear time trends were demonstrated for decreased deposition of inorg-N to Norway spruce forests in all parts of Sweden. The decreases were somewhat larger than what could be expected from the decrease in the reported emissions of inorg-N from Europe. The results emphasize that estimates of the total deposition are necessary in order to map levels and follow the development of N deposition in forests.
Total deposition of inorganic nitrogen to Norway spruce forests in Sweden, 2001 and 2020. Display omitted
•The dry+wet deposition of inorganic nitrogen to Norway spruce forests was estimated.•Combined measurements with Teflon string samplers, throughfall and bulk deposition•Full spatial coverage over Sweden for a twenty-year period, 2001–2020•Depositions were higher than has been previously estimated, based on modelling.•Clear time trends were demonstrated for decreased deposition in all parts of Sweden.
How does an animal know where it is when it stops moving? Hippocampal place cells fire at discrete locations as subjects traverse space, thereby providing an explicit neural code for current location ...during locomotion. In contrast, during awake immobility, the hippocampus is thought to be dominated by neural firing representing past and possible future experience. The question of whether and how the hippocampus constructs a representation of current location in the absence of locomotion has been unresolved. Here we report that a distinct population of hippocampal neurons, located in the CA2 subregion, signals current location during immobility, and does so in association with a previously unidentified hippocampus-wide network pattern. In addition, signalling of location persists into brief periods of desynchronization prevalent in slow-wave sleep. The hippocampus thus generates a distinct representation of current location during immobility, pointing to mnemonic processing specific to experience occurring in the absence of locomotion.
Hippocampal information processing is often described as two-state, with a place cell state during movement and a reactivation state during stillness. Relatively little is known about how the network ...transitions between these different patterns of activity during exploration. Here we show that hippocampal network changes quickly and continuously as animals explore and become familiar with initially novel places. We measured the relationship between moment-by-moment changes in behavior and information flow through hippocampal output area CA1 in rats. We examined local field potential (LFP) patterns, evoked potentials and ensemble spiking and found evidence suggestive of a smooth transition from strong CA3 drive of CA1 activity at low speeds to entorhinal cortical drive of CA1 activity at higher speeds. These changes occurred with changes in behavior on a timescale of less than a second, suggesting a continuous modulation of information processing in the hippocampal circuit as a function of behavioral state.
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