Hippocampal spectral degradation

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.