Detecting Functional Connectivity During Audiovisual Integration with MEG: A Comparison of Connectivity Metrics

Abstract

In typical magnetoencephalography and/or electroencephalography functional connectivity analysis, researchers select one of several methods that measure a relationship between regions to determine connectivity, such as coherence, power correlations, and others. However, it is largely unknown if some are more suited than others for various types of investigations. In this study, the authors investigate seven connectivity metrics to evaluate which, if any, are sensitive to audiovisual integration by contrasting connectivity when tracking an audiovisual object versus connectivity when tracking a visual object uncorrelated with the auditory stimulus. The authors are able to assess the metrics' performances at detecting audiovisual integration by investigating connectivity between auditory and visual areas. Critically, the authors perform their investigation on a whole-cortex all-to-all mapping, avoiding confounds introduced in seed selection. The authors find that amplitude-based connectivity measures in the beta band detect strong connections between visual and auditory areas during audiovisual integration, specifically between V4/V5 and auditory cortices in the right hemisphere. Conversely, phase-based connectivity measures in the beta band as well as phase and power measures in alpha, gamma, and theta do not show connectivity between audiovisual areas. The authors postulate that while beta power correlations detect audiovisual integration in the current experimental context, it may not always be the best measure to detect connectivity. Instead, it is likely that the brain utilizes a variety of mechanisms in neuronal communication that may produce differential types of temporal relationships.

Keywords: MEG; all-to-all; audiovisual integration; beta; coherence; functional connectivity; multimodal; oscillation.

FIG. 1.

Hummingballs task design. Eye tracker data and a visually inspected algorithm determine when a subject locates the correct target ball. Subjects are instructed to find the ball as quickly as possible and are given feedback on every trial detailing how long it took them to locate the correct ball. Only periods of time after the correct ball is located and the subject is tracking it are utilized in this report.

FIG. 2.

Beta band sound tracking minus color tracking R of Stockwell power mean frequency (RSP-PF) results; visualization of all connections q<0.1. Connections in red indicate active (sound tracking) is significantly larger than control (color tracking), and connections in blue indicate control is significantly larger than active.

FIG. 3.

Beta band sound tracking minus color tracking RSP-PF results; region of interest (ROI) breakdown of the all-to-all connections in Figure 2. Thickness of the lines indicates the amount of significant connections between ROIs, shown in the legend at upper left. The color of the lines (legend upper right) indicates the average strength of all significant connections between the ROIs. Within ROI connectivity is shown by a percentage directly to the right of the ROI, colored red for active>control and blue for control>active. If any connections between two ROIs include significant positive and negative values, the line is colored black. If any significant within ROI connectivity has both positive and negative connections, the percentage to the right of the ROI is also colored black.

FIG. 4.

Beta band sound tracking minus color tracking results for the six other tested metrics aside from RSP-PF. Metrics are grouped via power (A–C) and phase (D–F) accordingly. Refer to Figure 3 for node and legend labeling.

FIG. 5.

Gamma (A, B), alpha (C, D), and theta (E, F) sound tracking minus color tracking for the RSP-PF (A, C, E) and coherence (B, D, F) metrics. Refer to Figure 3 for node and legend labeling.

FIG. 6.

Power differences between sound and vision tracking. Significant q<0.05 differences were seen in the beta and alpha bands. No significant differences were seen in the theta and gamma bands, trends p<0.05 are presented. Blue indicates power was greater in the vision condition, while red signifies higher power in the sound tracking condition. Crosshairs in all Figures are centered on right auditory cortex (−41, 25, 8).