Abstracts

Rationale: We study, in vitro, the electrophysiology of human brain tissue from temporal neocortex removed at operation for temporal lobe epilepsy. To date, we have studied 19 patients, mainly with epilepsy emanating from the mesial temporal structures. The neocortex that we study is not directly within the most epileptogenic zone, suggesting physiological normalcy of the tissue with respect to cellular electrophysiology and neural network activity. The goal was to produce, in vitro, similar physiological rhythms that have been described in vivo in humans and in animals. Coherence between supra- and infra-granular layers was also investigated, reflecting feed-forward and feed-back integration. Methods: Temporal neocortical slices (500 μm) were made from a section of the resected middle temporal gyrus, and placed into a sucrose-based holding solution at 4oC with a glutamate receptor blocker (kynurenic acid). The slices were placed into artificial cerebrospinal fluid (ACSF) at 35oC. Extracellular field, whole cell intracellular, and cell attached recordings were performed in chambers with ACSF flow rates of 12-15 cc/min at 35oC. Results: Normal physiological rhythms were observed with extracellular recordings as follows. In ACSF alone, only delta rhythms were noted. With the addition of a low concentration of kainate (50 nM), and carbachol (50uM) to mimic endogenous activation, a number of physiological oscillations were observed. Delta rhythms were observed with higher amplitude in the deep layers, whereas the higher frequency rhythms were predominant in superficial layers. Broadband and narrowband (rhythmic) activity was observed well up into the high-gamma frequency range. Theta and beta oscillations were also observed. The rhythmic activity was correlated to the IPSCs recorded intracellularly in layer II/III pyramidal cells. "Nesting" of gamma oscillations within theta oscillations was observed within the supragranular layers. Entrainment between deep and superficial layers was found almost exclusively in the beta frequency band (15 -25 Hz) as measured by wavelet phase coherence. Such entrainment had non-zero phase offset and was significant with an alpha-value of 0.05 using surrogate methods. Phase lag shifted directionality between measurements, possibly indicating bidirectional information transfer. Conclusions: Physiological rhythms are demonstrated in a novel in vitro model of human in vivo oscillations. In particular, high gamma rhythms, theta-gamma modulation and beta-phase coherence have been observed and quantified.