DOI: 10.7280/d14x30
title: Hippocampal ensembles represent sequential relationships among an extended sequence of nonspatial events

description: The hippocampus is critical to the temporal organization of our
 experiences. Although this fundamental capacity is conserved across
 modalities and species, its underlying neuronal mechanisms remain unclear.
 Here we recorded hippocampal activity as rats remembered an extended
 sequence of nonspatial events and, using novel statistical methods,
 uncovered new forms of sequential organization in ensemble activity
 important for order memory judgments. Specifically, we discovered that
 hippocampal ensembles provide significant temporal coding throughout
 nonspatial event sequences, differentiate distinct types of task-critical
 information sequentially within events, exhibit theta-associated replay of
 the sequential relationships among events, and can represent past, present
 and future events within individual theta cycles. These findings suggest a
 fundamental function of the hippocampal network is to encode, preserve,
 and predict the sequential order of experiences.
description: <strong id="docs-internal-guid-e648bdc1-7fff-579b-72f9-c6691764aa86">Recording device and surgery</strong> Each chronically implanted custom microdrive contained 20 independently drivable tetrodes. Each tetrode consisted of four twisted nichrome wire (13 µm in diameter; California Fine Wire) and gold-plated to achieve final tip impedance of ~250 kΩ (measured at 1 kHz). Voltage signals recorded from the tetrode tips were referenced to a ground screw positioned over the cerebellum, and filtered for single-unit activity (154 Hz to 8.8 kHz). The neural signals were then amplified (10,000–32,000X), digitized (40 kHz) and recorded to disk with the data acquisition system (MAP, Plexon). The microdrive was implanted over the left hippocampus, centered on coordinates: -4.0 mm AP, 3.5 mm ML.  <strong id="docs-internal-guid-e648bdc1-7fff-579b-72f9-c6691764aa86">Unit preprocessing and spike sorting</strong> Action potentials from individual neurons were manually isolated offline using a combination of standard waveform features across the four channels of each tetrode (Offline Sorter, Plexon). Proper isolation was verified using interspike interval distributions for each isolated unit (assuming a minimum refractory period of 1 ms) and cross-correlograms for each pair of simultaneously recorded units on the same tetrode. <strong id="docs-internal-guid-e648bdc1-7fff-579b-72f9-c6691764aa86">Odor sequence task</strong> Five naïve rats were initially trained to nosepoke and reliably hold their nose for 1.2 s in the odor port for a water reward. Odor sequences of increasing length were then introduced in successive stages (A, AB, ABC, ABCD, and ABCDE) upon reaching behavioral criterion of 80% correct over three sessions per training stage. In each stage, rats were trained to correctly identify each presented item as either InSeq (by holding their nosepoke response for at least 1.2 s to receive a water reward) or OutSeq (by withdrawing their nose before 1.2 s to receive a reward). Note that OutSeq items could be presented in any sequence position except the first (i.e., sequences always began with odor A, though odor A could also be presented later in the sequence as an OutSeq item). All procedures were conducted in accordance with the Institutional Animal Care and Use Committee (Boston University and University of California, Irvine). <strong id="docs-internal-guid-e648bdc1-7fff-579b-72f9-c6691764aa86">For more information on data collection and procedures, please see the included references or feel free to contact us! </strong>
description: Please see the included README file.
description: # Hippocampal ensembles represent sequential relationships among an
 extended sequence of nonspatial events
 Shahbaba et al., 2021 Data
 repository Readme drafted on October XX, 2021 ## Contents Project summary
 Contact information Dataset Citation References using this data Data
 collection methods and behavior Description of files Associated code
 Directory Manifest ## 1. Project summary The hippocampus is critical to
 the temporal organization of our experiences. Although this fundamental
 capacity is conserved across modalities and species, its underlying
 neuronal mechanisms remain unclear. Our work and others suggest a
 fundamental function of the hippocampal network is to encode, preserve,
 and predict the sequential order of experiences. Here, this dataset
 contains tetrode extracellular recordings from hippocampus (local field
 potentials and sorted unit spikes) from five male Long–Evans rats. These
 recordings were performed using a custom electrode microdrive array,
 positioned along the dorsal axis of the hippocampus, while the rats
 performed a complex odor sequence memory task. ## 2. Contact information
 Norbert Fortin (Principal investigator) 
 Keiland Cooper (For questions regarding the dataset or documentation)
 ## 3. Dataset Citation TBD ## 4. References using
 this data Allen, T. A., Salz, D. M., McKenzie, S. & Fortin, N. J.
 Nonspatial sequence coding in CA1 neurons. J. Neurosci. 36, 1547–1563
 (2016). Shahbaba, B., Li, L., Agostinelli, F., Saraf, M., Cooper, K. W.,
 Haghverdian, D., Elias, G. A., Baldi, P., & Fortin, N. J. (2021).
 Hippocampal ensembles represent sequential relationships among an extended
 sequence of nonspatial events. Nature Communications. (In press) ## 5.
 Data collection methods and behavior For more information on data
 collection and procedures, please see the included references. Recording
 device and surgery Each chronically implanted custom microdrive contained
 20 independently drivable tetrodes. Each tetrode consisted of four twisted
 nichrome wire (13 µm in diameter; California Fine Wire) and gold-plated to
 achieve final tip impedance of ~250 kΩ (measured at 1 kHz). Voltage
 signals recorded from the tetrode tips were referenced to a ground screw
 positioned over the cerebellum, and filtered for single-unit activity (154
 Hz to 8.8 kHz). The neural signals were then amplified (10,000–32,000X),
 digitized (40 kHz) and recorded to disk with the data acquisition system
 (MAP, Plexon). The microdrive was implanted over the left hippocampus,
 centered on coordinates: -4.0 mm AP, 3.5 mm ML. Unit preprocessing and
 spike sorting Action potentials from individual neurons were manually
 isolated offline using a combination of standard waveform features across
 the four channels of each tetrode (Offline Sorter, Plexon). Proper
 isolation was verified using interspike interval distributions for each
 isolated unit (assuming a minimum refractory period of 1 ms) and
 cross-correlograms for each pair of simultaneously recorded units on the
 same tetrode. Odor sequence task Naïve rats were initially trained to
 nosepoke and reliably hold their nose for 1.2 s in the odor port for a
 water reward. Odor sequences of increasing length were then introduced in
 successive stages (A, AB, ABC, ABCD, and ABCDE) upon reaching behavioral
 criterion of 80% correct over three sessions per training stage. In each
 stage, rats were trained to correctly identify each presented item as
 either InSeq (by holding their nosepoke response for at least 1.2 s to
 receive a water reward) or OutSeq (by withdrawing their nose before 1.2 s
 to receive a reward). Note that OutSeq items could be presented in any
 sequence position except the first (i.e., sequences always began with odor
 A, though odor A could also be presented later in the sequence as an
 OutSeq item). All procedures were conducted in accordance with the
 Institutional Animal Care and Use Committee (Boston University and
 University of California, Irvine). ## 6. Description of files Please see
 the included README file (within Shahbaba_2021_dataset folder).