During exploration, the activity of place 1 and grid cells 2 represent self-location. Together these cells have been hypothesized to support spatial memory 1,3 and navigation 3,4 . Hippocampal replay 5,6 -the reactivation of place cell sequences during immobility and sleep -has been proposed as a mechanism for consolidation 5 and route planning 7 , yet the involvement of grid cells remains unknown. Potentially, spatially coherent place and grid cell activity may emerge during replay as the hippocampus broadcasts memory traces to the cortex 8 . To study the involvement of grid cells in replay we recorded concurrently from rodent medial entorhinal cortex layers 5/6 (MECV&VI) and hippocampus (CA1) during trackrunning and subsequent rest. We report robust coherence between place and grid cell spatial representations during hippocampal replay.A total of 43 grid cells were recorded across 11 sessions from 6 rats. Concurrently, 34-72 place cells were recorded in each session, 592 in total (Fig. 1a-d, Supplementary Fig. 1,2, Supplementary Table 1). During subsequent rest, we identified putative replay events based on place cell activity (Figure 1e, see Online Methods). A Bayesian decoding algorithm 9 and a trajectory-fitting procedure was used to reconstruct position and score the replay 9 ( Supplementary Fig. 3). Robust replay events exhibiting clear, straight trajectories (each p<0.2 vs own shuffle) were used for further analyses. On average, during replay events, grid cell activity was higher than during non-replay periods 10 (2.19Hz (SD =1.74) vs 1.33Hz (SD=1.61), t(43)=3.24, p=0.00023), with peak grid cell activity lagging that of place cells by 10ms (Fig. 1f, Supplementary Fig. 4). To investigate grid-place cell spatial coherence, we assessed the similarity of the grid and place cell representations during replay events in which both were active. 2Specifically, we superimposed the trajectory derived from a hippocampal replay event onto the decoded representation from concurrently recorded grid cell spikes ( Supplementary Fig. 5). The position represented by grid cells during replay events was similar to that represented by the place cell trajectories (Fig. 2a, Supplementary Fig. 6), exceeding the coherence obtained by pairing a grid cell event with a random place cell event from the same session (p<0.0001 area under the curve (AUC) test, see Online Methods, Fig. 2b, Supplementary Fig. 5); comparisons against shuffled distributions generated by permuting grid cell ratemaps (p<0.0001 AUC) and spike times (p<0.001 AUC) corroborated this finding ( Supplementary Fig. 7,8). Importantly, grid-place coherence also exceeded chance levels when analyses was limited to just the strongest (p<0.025 vs own shuffle) place cell replay events (p<0.0001 AUC, Supplementary Fig. 9) and did not exceed chance levels for the least robust (p>0.5) place cell events (p=0.17 AUC, Supplementary Fig. 10).To confirm these results, we constructed an 'event-ratemap' for each grid cell using the spikes emitted during replay events and the posit...

SummaryThe firing patterns of grid cells in medial entorhinal cortex (mEC) and associated brain areas form triangular arrays that tessellate the environment [1, 2] and maintain constant spatial offsets to each other between environments [3, 4]. These cells are thought to provide an efficient metric for navigation in large-scale space [5–8]. However, an accurate and universal metric requires grid cell firing patterns to uniformly cover the space to be navigated, in contrast to recent demonstrations that environmental features such as boundaries can distort [9–11] and fragment [12] grid patterns. To establish whether grid firing is determined by local environmental cues, or provides a coherent global representation, we recorded mEC grid cells in rats foraging in an environment containing two perceptually identical compartments connected via a corridor. During initial exposures to the multicompartment environment, grid firing patterns were dominated by local environmental cues, replicating between the two compartments. However, with prolonged experience, grid cell firing patterns formed a single, continuous representation that spanned both compartments. Thus, we provide the first evidence that in a complex environment, grid cell firing can form the coherent global pattern necessary for them to act as a metric capable of supporting large-scale spatial navigation.

SummaryReactivation of hippocampal place cell sequences during behavioral immobility and rest has been linked with both memory consolidation and navigational planning. Yet it remains to be investigated whether these functions are temporally segregated, occurring during different behavioral states. During a self-paced spatial task, awake hippocampal replay occurring either immediately before movement toward a reward location or just after arrival at a reward location preferentially involved cells consistent with the current trajectory. In contrast, during periods of extended immobility, no such biases were evident. Notably, the occurrence of task-focused reactivations predicted the accuracy of subsequent spatial decisions. Additionally, during immobility, but not periods preceding or succeeding movement, grid cells in deep layers of the entorhinal cortex replayed coherently with the hippocampus. Thus, hippocampal reactivations dynamically and abruptly switch between operational modes in response to task demands, plausibly moving from a state favoring navigational planning to one geared toward memory consolidation.

Medial septal inputs to the hippocampal system are crucial for aspects of temporal and spatial processing, such as theta oscillations and grid cell firing. However, the precise contributions of the medial septum’s cholinergic neurones to these functions remain unknown. Here, we recorded neuronal firing and local field potentials from the medial entorhinal cortex of freely foraging mice, while modulating the excitability of medial septal cholinergic neurones. Alteration of cholinergic activity produced a reduction in the frequency of theta oscillations, without affecting the slope of the non-linear theta frequency vs running speed relationship observed. Modifying septal cholinergic tone in this way also led mice to exhibit behaviours associated with novelty or anxiety. However, grid cell firing patterns were unaffected, concordant with an absence of change in the slopes of the theta frequency and firing rate speed signals thought to be used by grid cells.

SummaryReactivation of hippocampal place cell sequences during behavioural immobility and rest has been linked with both memory consolidation and navigational planning. Yet it remains to be investigated whether these functions are temporally segregated; occurring during different behavioural states.During a self-paced spatial task, awake hippocampal replay occurring immediately before movement towards a reward location, or just after arrival at a reward location, preferentially involved cells consistent with the current trajectory. In contrast, during periods of extended immobility, no such biases were evident. Notably, the occurrence of task-focused reactivations predicted the accuracy of subsequent spatial decisions. Additionally, during immobility but not periods preceding or succeeding movement, grid cells in deep layers of entorhinal cortex replayed coherently with the hippocampus.Thus, hippocampal reactivations dynamically and abruptly switch operational mode in response to task demands -plausibly moving from a state favouring navigational planning to one geared towards memory consolidation.

Grid cells have been proposed to encode both the self-location of an animal and the relative position of locations within an environment. We reassess the validity of these roles in light of recent evidence demonstrating grid patterns to be less temporally and spatially stable than previously thought.

Observational trials at the UC Kearney Research and Extension Center indicate that new southern highbush blueberry cultivars, which require fewer "chill hours" to produce fruit, are well adapted to the San Joaquin Valley climate. In a replicated cultivar evaluation, we quantified yields and identified several productive and flavorful varieties. These initial trials and ongoing studies on irrigation, plant spacing, mulches and pruning will improve the likelihood of establishing this promising new crop in the semiarid valley. Because blueberries are acid-loving, the soil must be extensively treated before planting, at considerable expense. Growers considering planting or expanding blueberry acreage should develop sound business plans, accounting for lower future prices and improved growing, harvesting and packing efficiencies. * Productivity represents third-year harvest: low < 5 lb., moderate 5-10 lb., high > 10 lb. † Harvest period indicates initiation of harvest: early = initial harvest; early/midseason = 7 days later; midseason = 14 days later; late = 21 days later. ‡ Fruit size represents the number of berries per 6 oz. cup. Although fruit size is genetically predetermined, pruning and other cultural practices can affect fruit size significantly. Field trials showed that Southern highbush blueberry cultivars are well adapted to the San Joaquin Valley. These new cultivars, such as 'Reveille', above, begin flowering in late winter and open a harvest window from early May through the Fourth of July.

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