Introduction

The function of neural systems, from the feeding and avoidance behaviors of the simplest multicellular organisms to the highest cognitive functions of the human brain, depends on dynamic spatial and temporal patterns of activation within linked networks of excitable cells. In the human brain, most purposeful function is mediated by correlated activity in substantial populations of neurons. The physical and physiological consequences of this activity can be detected with noninvasive measurement techniques, including electroencephalography (EEG) and magne-toencephalography (MEG). These techniques measure the integrated activity of thousands to hundreds of thousands of neurons. Many cells operating in synchrony are required to generate fields that can be detected centimeters away from the source. Fortunately, neural activation typically involves large clusters of neurons with similar response properties.

The information processing activities of individual neurons depend on a chain of biophysical processes: the integration of synaptic input (both excitatory and inhibitory) throughout the dendritic tree; electrical excitation mediated by the biophysical properties of ionic channel proteins in the cell membrane; transmission mediated by active and passive physical processes in the neuronal axon; and chemical or electrical relay of activation at the synapses on target cells. By linking together collections of excitable cells, networks can achieve complex behaviors beyond the capacity of individual cells.

MEG and EEG are not the only methods that can be employed with event-related and evoked response techniques. Essentially any technique that records a consistent transient response to neural activation can be used. Optical imaging methods employing sensitive video cameras have been used to record event-related responses from exposed brain tissue in experimental animals and in humans undergoing neurosurgery. With small animals such as rats or mice, it is possible to acquire images of reasonable quality though the skull. Most studies to date have exploited changes in blood flow and oxygenation associated with neural activation—the same processes that serve as the basis of fMRI. Studies have shown that it is possible to image fast intrinsic responses of neural tissue with highperformance video technology in vivo. Fast optical responses tightly coupled with the electrophysiological processes of neural activation were described at least 30 years ago, and the roots of such work go back much farther.

Other investigators have recorded optical evoked responses from human subjects using noninvasive methods, i.e., by injecting and recording light at the head surface to detect changes in the optical properties of tissue buried deep beneath the skull. Impedance tomography techniques have been used in a similar way to detect transient changes in tissue conductivity associated with neural activity. Even sensitive temperature measurement techniques, based on thermal emission of infrared photons, or, more recently, MRI-

based methods have been used to record event-related responses associated with neural activation.

Event-related experimental paradigms have been demonstrated with functional MRI (fMRI), in spite of the facts that the earliest detectable hemodynamic responses require several hundred milliseconds and that the response peaks several seconds after neural electrophysiological activation. It is possible to use sophisticated selective averaging or correlation techniques to identify fMRI evoked responses that are highly overlapping due to rapid stimulation rates.

MRI has other important and unique roles in functional neuroimaging. MRI is used to identify and visualize the anatomical substrate of functional activation through coregistration with other imaging modalities. MRI can be used to define the computational geometries required to model the physics of techniques such as MEG and EEG, facilitating three-dimensional (3D) source localization on the basis of data that is surface-based and, thus, topographic. By defining the geometry of cortex, MRI provides useful constraints on ill-posed source localization procedures. Functional MRI provides an alternative measure of neural activation that can increase confidence in the results of comparative or integrated analysis based on multiple imaging modalities. Event-related fMRI techniques allow the same experimental paradigms to be used for NEM and fMRI experiments, a useful feature for most purposes and a prerequisite for simultaneous measurements, for example, combining EEG and fMRI.

We will consider a range of methods within this article, but the major focus will be on neural electromagnetic measurement (NEM) techniques (MEG and EEG) and noninvasive measurements of neural population responses. These methods have challenges and limitations for localizing the source of neural responses, but the excellent temporal resolution of these responses can be exploited by clever experimental paradigms to probe the dynamic interactions between multiple cortical regions. These interactions serve as the basis of information processing and control by the human brain.

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