Enter the ratrix

Abstract

Understanding the computations the brain performs to guide behavior largely relies on electrical recordings of action potentials generated by neurons in awake, behaving animals. Despite their widespread usage, the high density of neurons in cortex poses a significant limitation on the number of neighboring cells whose signals can be resolved even with state-of-the-art electrode-based methods. Thus current methods can only go so far in understanding neuronal processing. Optical methods to image the activity of groups of neurons promise to ameliorate many of the problems faced by electrical recordings, but typically these methods are restricted in their range of application. Traditional systems are too bulky to mount on freely moving animals and have had only qualified success in monitoring action potentials with sufficient temporal precision. In this issue of PNAS, Sawinski et al. (1) successfully use two-photon microscopy to image cortical neurons in freely moving rats and resolve the Ca2+ transients that accompany action potentials in populations of up to tens of neurons (Fig. 1). The current technology represents a critical step forward in the use of optical tools to assess the spiking activity of cells in the brain—at least those in the upper layers of cortex. The success of Sawinski et al. (1) appears to remove a significant barrier to the more widespread use of imaging to study the neuronal mechanisms of behavior.