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Hippocampal spectral degradation

Citation

Zhou, Yuchen et al. (2021), Hippocampal spectral degradation, Dryad, Dataset, https://doi.org/10.5061/dryad.vmcvdncs6

Abstract

The hippocampal local field potential (LFP) exhibits a strong correlation with behavior. During rest, the theta rhythm is not prominent, but during active behavior, there are strong rhythms in the theta, theta harmonics, and gamma ranges. With increasing running velocity, theta, theta harmonics and gamma increase in power and in cross-frequency coupling, suggesting that neural entrainment is a direct consequence of the total excitatory input. While it is common to study the parametric range between the LFP and its complementing power spectra between deep rest and epochs of high running velocity, it is also possible to explore how the spectra degrades as the energy is completely quenched from the system. Specifically, it is unknown whether the 1/f slope is preserved as synaptic activity becomes diminished, as low frequencies are generated by large pools of neurons while higher frequencies comprise the activity of more local neuronal populations. To test this hypothesis, we examined rat LFPs recorded from the hippocampus and entorhinal cortex during barbiturate overdose euthanasia. Within the hippocampus, the initial stage entailed a quasi-stationary LFP state with a power-law feature in the power spectral density. In the second stage, there was a successive erosion of power from high- to low-frequencies in the second stage that continued until the only dominant remaining power was less than 20 Hz. This stage was followed by a rapid collapse of power spectrum towards the absolute electrothermal noise background. As the collapse of activity occurred later in hippocampus compared with medial entorhinal cortex, it suggests that the ability of a neural network to maintain the 1/f slope with decreasing energy is a function of general connectivity. Broadly, these data support the energy cascade theory where there is a cascade of energy from large cortical populations into smaller loops, such as those that supports the higher frequency gamma rhythm. As energy is pulled from the system, neural entrainment at gamma frequency (and higher) decline first. The larger loops, comprising a larger population, are fault-tolerant to a point capable of maintaining their activity before a final collapse.

Methods

After completing all other behavioral experiments, animals were recorded in the usual resting container to establish a baseline for the LFP data and then received a lethal dose of SomnaSol (390 mg/ml pentobarbital sodium, 50 mg/ml phenytoin sodium; Henry Schein; Melville, NY) injected intraperitoneally. LFP recording continued throughout the injection and for 10 to 15 minutes after the animal no longer exhibited a nociceptive withdrawal reflex. The LFP data were analyzed in MATLAB (The MathWorks, Natick, MA) using custom-written code . Raw LFP records sampled at 24 kHz (Tucker-Davis system) were low-pass filtered down to 2 kHz. 

Usage Notes

Data can be directly loaded into MATLAB (The MathWorks, Natick, MA). The number of the rat indicated in the name of dataset. When loaded in to MATLAB, each "eeg" structure has four fields:

eeg.data is the a matrix of LFP recordings (in the unit of mV). The rows correspond to channels and columns correspond to time steps.

eeg.ts is a vector which indicates the time steps (in the unit of second, starting from 0). The length of eeg.ts is equal to the number of columns in eeg.data)

eeg.fs is a scalar which indicates the sampling rate (in the unit of Hz).

eeg.t_inj is a scalar which indicates the time of SomnaSol administration (in the unit of second).

For rat 730 and rat 782, channel 1 to channel 32 are hippocampal channels while channel 33 to channel 64 are entorhinal cortex channels.

For other rats, all channels are in hippocampus.

Funding

Evelyn F. McKnight Brain Research Foundation

National Institutes of Health

National Institute on Aging, Award: AG055544

National Institute of Mental Health, Award: MH109548