A fundamental question in neuroscience concerns the relation between the spiking of individual neurons and the aggregate electrical activity of neuronal ensembles as seen in local field potentials (LFPs). Because LFPs reflect both spiking activity and subthreshold events, this question is not simply one of data aggregation. Recording from 20 neurosurgical patients, we directly examined the relation between LFPs and neuronal spiking. Examining 2030 neurons in widespread brain regions, we found that firing rates were positively correlated with broadband (2–150 Hz) shifts in the LFP power spectrum. In contrast, narrowband oscillations correlated both positively and negatively with firing rates at different recording sites. Broadband power shifts were a more reliable predictor of neuronal spiking than narrowband power shifts. These findings suggest that broadband LFP power provides valuable information concerning neuronal activity beyond that contained in narrowband oscillations.

Grid cell activity in the rodent and non-human primate entorhinal cortex is thought to provide spatial location information to the hippocampus for navigation and spatial processing. Here, Jacobs et al. examined single neuron spiking activities from human subjects performing a virtual spatial navigation task and show the presence of grid-like firing activity. Grid cells in the entorhinal cortex appear to represent spatial location via a triangular coordinate system. Such cells, which have been identified in rats, bats and monkeys, are believed to support a wide range of spatial behaviors. Recording neuronal activity from neurosurgical patients performing a virtual-navigation task, we identified cells exhibiting grid-like spiking patterns in the human brain, suggesting that humans and simpler animals rely on homologous spatial-coding schemes.

A growing body of animal research suggests that neurons represent information not only in terms of their firing rates but also by varying the timing of spikes relative to neuronal oscillations. Although researchers have argued that this temporal coding is critical in human memory and perception, no supporting data from humans have been reported. This study provides the first analysis of the temporal relationship between brain oscillations and single-neuron activity in humans. Recording from 1924 neurons, we find that neuronal activity in various brain regions increases at specific phases of brain oscillations. Neurons in widespread brain regions were phase locked to oscillations in the theta- (4–8 Hz) and gamma- (30– 90 Hz) frequency bands. In hippocampus, phase locking was prevalent in the delta- (1–4 Hz) and gamma-frequency bands. Individual neurons were phase locked to various phases of theta and delta oscillations, but they only were active at the trough of gamma oscillations. These findings provide support for the temporal-coding hypothesis in humans. Specifically, they indicate that theta and delta oscillations facilitate phase coding and that gamma oscillations help to decode combinations of simultaneously active neurons.

Remembrance of Places Past: The hippocampus has two major roles in cognition. Place-responsive neurons form a context-sensitive cognitive map, firing more strongly when an animal traverses specific regions of its environment. Both humans and animals thus need the hippocampus to learn their way around novel environments. Similarly, the hippocampus is critical for our ability to remember a specific event in space and time. It has thus been suggested that the spatial and memory functions of the hippocampus reflect a common architecture. Recording from neurosurgical patients playing a virtual reality memory game, Miller et al. (p. 1111 ) found that the recall of events was indeed associated with reinstatement of the place-firing of neurons activated as the subjects navigated through the environment.