EEG Sharp Waves and Sparse Ensemble Unit Activity in the Macaque Hippocampus

Abstract

Neural unit activity and EEGs were recorded from inferior temporal regions of three rhesus macaques chronically implanted with "hyperdrives" holding 12 individually movable tetrodes. Recordings were made from each monkey over a period of approximately 3 mo, while the electrodes were moved by small increments through the hippocampus and neighboring structures. After recording, the monkeys were necropsied, and the brains were sectioned and Nissl-stained, permitting identification of individual electrode tracks. The results establish that hippocampal pyramidal cells are "complex spike cells," firing at overall average rates of approximately 0.3 Hz, with spike trains consisting of long periods of silence interspersed with bursts of activity. The results also establish that the monkey hippocampal EEG shows "sharp wave" events consisting of a high-frequency "ripple" oscillation ( approximately 110 Hz) together with a large slow-wave EEG deflection lasting several hundred milliseconds. The evidence suggests that monkey sharp waves are probably generated mainly in the CA1 region and that sharp waves are associated with an inactive/drowsy-or-sleeping behavioral state, which is also associated with increased hippocampal pyramidal cell activity and increased hippocampal EEG amplitude. The results of this initial study of ensembles of primate hippocampal neurons are consistent with previous studies in rodents and consistent with the hypothesis that theories and models of hippocampal memory function developed on the basis of rat data may be applicable to a wide range of mammalian species.

Odor-induced modulation of electroencephalogram waves in healthy controls

“…Various researchers have found significant sharp waves in monkeys and humans. [15,16] It seems that appearance of sharp wave during odor administration, in the temporal region perhaps indicates their association with memory as shown in previous research. [17] The EEG abnormalities (slowing and sharp waves) were mainly observed in right temporal region and occasionally in frontotemporal, centro-parieto-temporal, and centro-parietal regions.…”

Grid cells in mice

“…Such oscillations have been reported not only in rats (Vanderwolf, 1969) but also in a wide range of other mammals including mice (Buzsaki et al, 2003), bats (Ulanovsky and Moss, 2007), monkeys (Skaggs et al, 2007), and humans (Kahana et al, 1999). Sharp-wave related ripple activity was also first reported in the rat (Buzsaki, 1986) but has since been observed in cats (Grenier et al, 2003), monkeys (Skaggs et al, 2007), and humans (Bragin et al, 1999). Altogether, the many similarities in connectivity and network activity suggest that hippocampal areas perform similar computational functions across a wide range of mammalian species.…”

Mechanisms of physiological and epileptic HFO generation

“…Experiments on cats and monkeys have extended detailed investigation of HFOs to more highly developed mammals (Grenier et al, 2001;Skaggs et al, 2007). Recordings from cats demonstrated that HFOs with ripple frequency occur during active network states and when the membrane potential of cortical neurons is depolarized.…”

“…However, the level of the membrane potential is not the sole factor; in normal behavioural conditions the maximal power of cortical ripples occurs during active "UP" phases of slow-wave sleep, smaller during passive wakefulness and REM sleep and lowest (virtually absent) during silent "DOWN" phases of slow wave sleep (Grenier et al, 2001). In hippocampus of both rats and monkeys, ripples are typically recorded during inactive, drowsy or sleeping behavioral states, all of which are associated with increased hippocampal pyramidal cell activity (Skaggs et al, 2007;Ylinen et al, 1995).…”