The brain timewise: how timing shapes and supports brain function

Abstract

We discuss the importance of timing in brain function: how temporal dynamics of the world has left its traces in the brain during evolution and how we can monitor the dynamics of the human brain with non-invasive measurements. Accurate timing is important for the interplay of neurons, neuronal circuitries, brain areas and human individuals. In the human brain, multiple temporal integration windows are hierarchically organized, with temporal scales ranging from microseconds to tens and hundreds of milliseconds for perceptual, motor and cognitive functions, and up to minutes, hours and even months for hormonal and mood changes. Accurate timing is impaired in several brain diseases. From the current repertoire of non-invasive brain imaging methods, only magnetoencephalography (MEG) and scalp electroencephalography (EEG) provide millisecond time-resolution; our focus in this paper is on MEG. Since the introduction of high-density whole-scalp MEG/EEG coverage in the 1990s, the instrumentation has not changed drastically; yet, novel data analyses are advancing the field rapidly by shifting the focus from the mere pinpointing of activity hotspots to seeking stimulus- or task-specific information and to characterizing functional networks. During the next decades, we can expect increased spatial resolution and accuracy of the time-resolved brain imaging and better understanding of brain function, especially its temporal constraints, with the development of novel instrumentation and finer-grained, physiologically inspired generative models of local and network activity. Merging both spatial and temporal information with increasing accuracy and carrying out recordings in naturalistic conditions, including social interaction, will bring much new information about human brain function.

Keywords: brain imaging; hierarchical order; magnetoencephalography; networks; prediction; time.

Figure 1.

Genesis of EEG and MEG signals. (a) Electric currents (red arrow) in active neurons drive volume currents (yellow lines) within the head, which gives rise to a potential distribution (V) on the scalp. The currents also generate a magnetic field (green lines; B) outside of the head; here the direction of the magnetic field follows (according to the right-hand rule) the direction of the net intracellular currents. (b) The main contribution to EEG and MEG signals comes from post-synaptic currents (red arrows) in the apical dendrites of pyramidal neurons. (c) A highly schematic illustration of electrophysiological (MEG/EEG) and haemodynamic (fMRI) response time courses to stimuli of three different durations. Evoked responses are phase-locked to the stimuli while induced responses reflect amplitude changes in the non-phase-locked oscillatory brain activity.