Abstracts

Rationale: Ripples (100-200 Hz) are oscillations of local field potentials that occur in normal CA1. In epileptic rats, they are normal or pathological, and there is currently no way to distinguish between the two. We previously described asymmetric ripples in normal and epileptic rats, which were defined morphologically, as an oscillation of at least three cycles with an upslope in each cycle that is no more than half the downslope. In normal rats, 10% of ripples were asymmetric, but in epileptic rats only 2% of ripples were asymmetric. We previously described that asymmetric ripples were significantly more likely to be present in normal rats. The aim of this study was to explore whether asymmetric ripples have a distinct spatial distribution.Methods: Tungsten microelectrodes were implanted in bilateral piriform and entorhinal cortex, anterior and posterior hippocampus of 3 normal adult male Sprague-Dawley rats and moved into position after a 2-3 day recovery period. Recordings were made at 10 kHz at 16 bit precision. 50 ripples were serially examined in each rat, and asymmetric were distinguished from symmetric ripples. Spatial distributions of asymmetric and symmetric ripples were determined and then compared using Cramer-von Mises statistics. Cramer-von Mises statistics were used, because this is the way to compare two data sets, and determine if they are from the same distribution, without making assumptions on the shape of either the data or the underlying distribution. Results: Although symmetric ripples were present in all four areas examined, asymmetric ripples was found in anterior hippocampus, posterior hippocampus, and piriform cortex, but none were found in the entorhinal cortex. The spatial distributions of symmetric and asymmetric ripples were significantly different. Conclusions: While asymmetric ripples may reflect an asymmetric relationship between the electrode and the source of generation of the signal, asymmetric ripples appear to have a different spatial distribution than symmetric ripples, and occur less frequently in epileptic rats. This suggests they may be a feature that can be used to distinguish normal from pathological ripples. These results also suggest that different types of ripples may have different functions in normal cortex, since asymmetric ripples have a unique spatial localization and morphology. The reason for the lack of asymmetric ripples in the entorhinal cortex should be examined in the future. The major weakness of the study is the small numbers of included ripples. We would like to thank Dr. Anatol Bragin for supplying the data for this study.