Trial-to-Trial Variability in Evoked Neural Responses Exhibits a Very Low Frequency Temporal Signature

In functional magnetic resonance (fMRI) studies, the blood oxygen level dependent (BOLD) signal displays intrinsic spontaneous and task-independent very low frequency (VLF) oscillations (< 0.1 Hz). Most prominent during rest, when they persist into task sessions they can predict trial-to-trial variability in both evoked behavior and brain responses by providing a baseline onto which deterministic responses elicited by the task are superimposed. Moreover, evidence in the literature tentatively suggests that this VLF activity may not be present in the data as distinct, independent source(s) per se, but rather as a mechanism that modulates and perhaps even governs underlying brain processes. Here, we use electrophysiology to investigate the intertrial variability observed in magnetoencephalographic (MEG) event-related field (ERF) components, and to examine whether this variability exhibits a VLF time signature in order to indirectly infer information about the underlying slow waves. The focus is on the visual component, the M100, understood to be regulated by attention. We also explored whether individual differences in the M100 VLF pattern varies as a function of attention deficit/hyperactivity disorder (ADHD) by comparing 11 cases against 11 controls. The M100 component was extracted from the data using a recently introduced blind-source separation technique – space-time independent component analysis (ST-ICA) – which allowed trial-by-trial analysis to be performed on the M100 for proper assessment of VLF modulation. Our results demonstrate, for the first time, the ability of this signal-processing method to isolate relevant components from multidimensional, noisy, ERF data recorded from a highly dense 148-channel MEG system. The intertrial variability in the amplitude and latency of the M100 responses exhibits a slow wave pattern (< 0.1 Hz). However, there was no evidence that the degree of VLF modulation was different in ADHD participants. The role of this VLF activity in brain function is discussed.