Imaging of Neuromodulation and Surgical Interventions for Epilepsy.

About one-third of epilepsy cases are refractory to medical therapy. During the past decades, the availability of surgical epilepsy interventions has substantially increased as therapeutic options for this group of patients. A wide range of surgical interventions and electrophysiologic neuromodulation techniques are available, including lesional resection, lobar resection, thermoablation, disconnection, multiple subpial transections, vagus nerve stimulation, responsive neurostimulation, and deep brain stimulation. The indications and imaging features of potential complications of the newer surgical interventions may not be widely appreciated, particularly if practitioners are not associated with comprehensive epilepsy centers. In this article, we review a wide range of invasive epilepsy treatment modalities with a particular focus on their postoperative imaging findings and complications. A state-of-the-art treatment algorithm provides context for imaging findings by helping the reader understand how a particular invasive treatment decision is made.

Independence of firing correlates of anatomically proximate hippocampal pyramidal cells.

In neocortex, neighboring neurons frequently exhibit correlated encoding properties. There is conflicting evidence whether a similar phenomenon occurs in hippocampus. To assess this quantitatively, a comparison was made of the spatial and temporal firing correlations within and between local groups of hippocampal cells, spaced 350-1400 microm apart. No evidence of clustering was found in a sample of >3000 neurons. Moreover, cells active in two environments were uniformly interspersed at a scale of <100 microm, as assessed by the activity-induced gene Arc. Independence of encoding characteristics implies uncorrelated inputs, which could enhance the capacity of the hippocampus to store arbitrary associations.

Reactivation of hippocampal ensemble activity patterns in the aging rat.

In young rats, the pattern of neuronal ensemble activity correlations expressed among hippocampal pyramidal cells during behavior persists during subsequent quiet wakefulness and slow-wave sleep, a process that may facilitate the consolidation of episodic memories. The present study explored the hypothesis that age-related changes in this process might contribute to memory impairments observed during normal aging. Neuronal activity was recorded from CA1 pyramidal cells, and in both young and old rats, there was a strong similarity between the resting epoch activity patterns and those from the preceding behavior epoch. This similarity was strongest during sharp-wave events. There were no detectable differences in the reactivation process or the decay rate between the young and old age groups. Thus, age differences in spatial memory do not appear to be explainable by differences in the spontaneous reactivation of familiar patterns within the hippocampus during the immediate postbehavior period.

Effect of age on burst firing characteristics of rat hippocampal pyramidal cells.

During behavior, hippocampal pyramidal cells emit high frequency bursts, modulated by the animal's location and the 7 Hz theta rhythm. During rest, CA1 EEG exhibits large irregular activity (LIA), containing sharp-wave/ripple complexes, during which pyramidal cells exhibit burst discharge. Aging results in altered intracellular calcium homeostasis, increased electrical coupling and reduced cholinergic modulation within CA1, all of which might affect burst discharge characteristics. During LIA, old rats exhibited more short (3-7 ms) inter-spike intervals, with no change in mean firing rate. During behavior induced theta rhythm, however, interval distributions were not affected by age. Thus, different mechanisms must underlie burst discharge in theta and LIA states. Moreover, age related changes in the cholinergic system appear not to play a major role in shaping the temporal discharge characteristics of CA1 pyramidal cells. The mechanism and significance of the higher frequency bursting in old rats during LIA remains to be determined.

Deciphering the hippocampal polyglot: the hippocampus as a path integration system.

Hippocampal 'place' cells and the head-direction cells of the dorsal presubiculum and related neocortical and thalamic areas appear to be part of a preconfigured network that generates an abstract internal representation of two-dimensional space whose metric is self-motion. It appears that viewpoint-specific visual information (e.g. landmarks) becomes secondarily bound to this structure by associative learning. These associations between landmarks and the preconfigured path integrator serve to set the origin for path integration and to correct for cumulative error. In the absence of familiar landmarks, or in darkness without a prior spatial reference, the system appears to adopt an initial reference for path integration independently of external cues. A hypothesis of how the path integration system may operate at the neuronal level is proposed.