Neurobiology Select

Ongoing efforts to link patterns of neural activity to cognition and behavior entail the use of an impressive range of techniques, from single-cell recording to functional magnetic resonance imaging. Recent discoveries in systems neuroscience, highlighted in this issue's Neurobiology Select, include evidence that humans use grid cells for spatial navigation and a report linking small variations in membrane potential, known as spikelets, to spatial representations in-the hippocampus of the rat. Other advances implicate theta wave synchronization between distant brain regions in anxiety behavior and suggest that the degree of correlated firing in cortical microcircuits may be much lower than previously thought. In the rat and mouse entorhinal cortex, the firing of particular neurons known as grid cells shows a distinctive pattern of activity as an animal freely explores its environment. When the occurrence of firing of a grid cell is placed on a map of the animal's location, what emerges is a triangular grid, with peak activities at the vertices. By cleverly taking advantage of the unique collective properties of grid cells, Doeller et al. (2010) now provide evidence that humans also have and use grid cells during navigation tasks. The authors imaged neural activity using functional magnetic resonance imaging (fMRI) of participants as they explored a virtual landscape. Because of the six-fold symmetry of the activity map of the population of grid cells, the modulation of firing of some grid cells by running direction, and the fact that grid cell activity is more pronounced at faster running speeds than at slow speeds, the authors predicted how neuronal activity should vary as a function of the speed and direction of a partici-pant's virtual exploration. The authors observed these predicted activation patterns with the most pronounced signature of grid cell activity found in the human entorhinal cortex. In a larger context, these efforts demonstrate the potential of fMRI to infer the fine-scale properties of neural networks in humans by building on animal models where, unlike in humans, it is possible to measure the activity of individual neurons. Spikelets are variations in the membrane potential that are much smaller than the action potentials that mediate neurotransmission. Long appreciated , but little understood, work by Epsztein et al. (2010) now suggests that spikelets do not occur randomly but appear to contribute in a systematic fashion to neural representations of space and have a role in driving full-blown action potentials. The authors make intracellular …