Diversity in periodic pattern of firing in human hippocampal neurons

Firing periodicity was examined in human hippocampal neurons using autocorrelation analysis. Extracellular single-unit activities were recorded from the anterior hippocampus through fine platinum microelectrodes, and the typical firing pattern in an entire recording period was reconstructed statistically in autocorrelograms (average number of firings analyzed: 5639.0 +/- 968.1 SE, range: 1158 to 31,203; number of single-unit trains was 57). Three types of periodic firing were identified as highly consistent. The first pattern consisted of a random recurrence of high-frequency action potentials (100 to 300 Hz) and was observed as an intermittent burst. In this burst, the first 10 to 30 ms after the onset of the burst was the patterned firing of several action potentials, suggesting that the generation of this stereotyped portion of the burst is primarily due to intrinsic membrane characteristics. The second pattern was the continuous rhythmical firing with a lower frequency ranging from 1 to 30 Hz. The third pattern was a clustered rhythmical firing in which a series of short rhythmical firings recurred with regular intervals; the frequency of short rhythmical firing varied from 6.7 to 17 Hz between neurons, and the interval of the regular recurrence of these rhythmical firings ranged from 0.5 to 10 s among neurons. These firing periodicities not only cover a cellular rhythm in the theta frequency reported in the lower mammalian hippocampus but also appear to be more diverse than those previously reported for hippocampal neurons in the animal literature.