Correlation of unit recordings with regional cell counts in epileptogenic human temporal lobe

Exploring human epileptic activity at the single-neuron level

Today, localization of the seizure focus heavily relies on EEG monitoring (scalp or intracranial). However, current technology enables much finer resolutions. The activity of hundreds of single neurons in the human brain can now be simultaneously explored before, during, and after a seizure or in association with an interictal discharge. This technology opens up new horizons to understanding epilepsy at a completely new level. This review therefore begins with a brief description of the basis of the technology, the microelectrodes, and the setup for their implantation in patients with epilepsy. Using these electrodes, recent studies provide novel insights into both the time domain and firing patterns of epileptic activity of single neurons. In the time domain, seizure-related activity may occur even minutes before seizure onset (in its current, EEG-based definition). Seizure-related neuronal interactions exhibit complex heterogeneous dynamics. In the seizure-onset zone, changes in firing patterns correlate with cell loss; in the penumbra, neurons maintain their spike stereotypy during a seizure. Hence, investigation of the extracellular electrical activity is expected to provide a better understanding of the mechanisms underlying the disease; it may, in the future, serve for a more accurate localization of the seizure focus; and it may also be employed to predict the occurrence of seizures prior to their behavioral manifestation in order to administer automatic therapeutic interventions.

Decreased neuronal burst discharge near site of seizure onset in epileptic human temporal lobes

We examined auto-correlation and interval distribution characteristics of neuronal discharge from patients with complex partial seizures. The objective was to compare the interictal firing patterns of neurons in mesial temporal structures ipsilateral to the site of seizure onset with firing patterns of neurons in homologous contralateral structures. Spontaneous interictal recordings of 258 single neurons were acquired from 23 patients. A "burst area" measure was derived from the neuronal auto-correlation to assess the likelihood of grouped action potential discharge (burst discharge). Large burst area measures indicate a tendency for single neuronal burst discharge, but do not disclose information about interspike intervals within bursts. Although several measures based on single neuronal interspike interval distributions showed no overall difference between hemispheres, burst area was significantly reduced in mesial temporal structures ipsilateral to the site of seizure onset. Possible mechanisms of decreased burst discharge in epileptogenic regions include selective loss of burst-discharging neurons and increased recurrent inhibition.