DOI: 10.5061/dryad.vmcvdncs6
title: Hippocampal spectral degradation

description: 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.
description: 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. 
description: 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 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.