Electrophysiological imaging of brain activity and connectivity-challenges and opportunities

Abstract

Unlocking the dynamic inner workings of the brain continues to remain a grand challenge of the 21st century. To this end, functional neuroimaging modalities represent an outstanding approach to better understand the mechanisms of both normal and abnormal brain functions. The ability to image brain function with ever increasing spatial and temporal resolution has made a significant leap over the past several decades. Further delineation of functional networks could lead to improved understanding of brain function in both normal and diseased states. This paper reviews recent advancements and current challenges in dynamic functional neuroimaging techniques, including electrophysiological source imaging, multimodal neuroimaging integrating fMRI with EEG/MEG, and functional connectivity imaging.

Fig. 1

Schematic illustration of the ranges of spatial and temporal resolution of various noninvasive (in blue) imaging techniques and invasive (in red) experimental techniques (adapted from [5], with permission).

Fig 2

When cortical pyramidal neurons (A) are excited, the synaptic currents flowing across the cell membranes induces local excitatory postsynaptic potentials as well as magnetic fluxes. The synchronized activity of cortical neurons in a columnar vicinity can be approximated by a current dipole located on cortical surface and oriented perpendicular to the local cortical surface (B). Through the forward modeling of brain source and head volume conductor (C, e.g. Finite element model and Boundary element model), the relationship between the EEG/MEG (D) measurements and underlying dipolar sources can be approximated through a linear system.

Fig. 3

Illustration of multimodal imaging approaches based on the spatial and temporal integrations. Waveforms of a typical EEG event-related potential and a block-designed BOLD change are shown. Notice the disparate temporal scales of the responses in the EEG and BOLD signals. Also, responses of both modalities are widely distrusted in the brain.

Fig. 4

In an effort to image continuous rhythmic activity, a recent study developed an EEG-fMRI integrated imaging methodologies (A). The technique was used to reconstruct spatial distribution and temporal modulation of alpha-band oscillation in an eyes-open-eyes-closed task. (From [79] with permission)

Fig. 5

Model depicting functional (A) and effective (B) connectivity metrics. As can be observed from the figure, functional connectivity denotes regions which are linked, but does not specify the direction of this coupling, a property which is provided by effective connectivity methods. A type of effective connectivity measure, the DTF, is shown in (C) during a motor task. From the directionality information provided by the DTF, the degree to which each cortical region acts as either a source or sink of cortical activity during the task can be calculated