Dynamic Functional Brain Connectivity for Face Perception

Abstract

Face perception is mediated by a distributed brain network comprised of the core system at occipito-temporal areas and the extended system at other relevant brain areas involving bilateral hemispheres. In this study we explored how the brain connectivity changes over the time for face-sensitive processing. We investigated the dynamic functional connectivity in face perception by analyzing time-dependent EEG phase synchronization in four different frequency bands: theta (4-7 Hz), alpha (8-14 Hz), beta (15-24 Hz), and gamma (25-45 Hz) bands in the early stages of face processing from 30 to 300 ms. High-density EEG were recorded from subjects who were passively viewing faces, buildings, and chairs. The dynamic connectivity within the core system and between the extended system were investigated. Significant differences between faces and non-faces mainly appear in theta band connectivity: (1) at the time segment of 90-120 ms between parietal area and occipito-temporal area in the right hemisphere, and (2) at the time segment of 150-180 ms between bilateral occipito-temporal areas. These results indicate (1) the importance of theta-band connectivity in the face-sensitive processing, and (2) that different parts of network are involved for the initial stage of face categorization and the stage of face structural encoding.

Keywords: ERP; dynamic functional connectivity; face perception; high-density EEG; phase lag index.

FIGURE 1

Examples of the visual stimuli. Three categories of photographs (224 pixels × 189 pixels) including 10 human faces (5 women), 10 buildings, 10 chairs were used for visual perception. The face photographs were taken from “the database of faces” from AT&T Laboratories Cambridge (Samaria and Harter, 1994) and the facial hair was removed. Additionally, three photographs of butterflies were introduced as target stimuli to help subjects concentrate on the experiment and to eliminate the effect of selective attention on the stimuli. The background of all stimuli was tuned to an 8-bit uniform gray level to uniform the luminance.

FIGURE 2

Event-related potentials (ERPs) at the P7/8 following the presentation of face and non-face stimuli. The solid lines are the grand averages of 10 subjects. The dashed lines indicate the standard error of the mean. The ERP components P1 and N170 are larger in response to faces (red lines) than non-faces (buildings: blue lines, chairs: green lines) in both hemispheres, with larger amplitudes in the right hemisphere (P8).

FIGURE 3

Dynamic functional connectivity matrices in face perception and the difference between time segments for one representative subject. The electrodes in the matrices are listed in the ascending order of electrode numbers from top to bottom, and from left to right. For clarity, non-significant phase lag index (PLI) values are set to zero in the maps.

FIGURE 4

Grand average of topography of dynamical functional connectivity between the right occipito-temporal electrode P8 (marked out by arrow) and other electrodes over subjects. Non-significant PLI values are set to zero in the topographies.

FIGURE 5

Grand average of topography of dynamical functional connectivity between the left occipito-temporal electrode PO7 (marked out by arrow) and other electrodes over subjects. Non-significant PLI values are set to zero in the topographies.

FIGURE 6

Grand average of functional connectivity between electrode pairs. PO7-P8 (between electrodes in bilateral occipito-temporal areas), CP4-P8 (between electrodes in right parietal and occipito-temporal areas), CP4-P7 (between electrodes in right parietal and occipito-temporal areas), and Fz-Oz (between electrodes in prefrontal and occipital areas). The time segment with significant differences between face and non-face stimuli in PLI value are indicated by the vertical lines and stars, where a ‘^∗’ indicates 0.01 ≤P < 0.05, ‘^∗∗’ 0.001 ≤P < 0.01, and ‘^∗∗∗’P < 0.001.

FIGURE 7

Grand average of topography of dynamical functional connectivity between the occipito-temporal electrode Oz (marked out by arrow) and other electrodes over subjects. Non-significant PLI values are set to zero in the topographies.

FIGURE 8

The dynamic functional network in theta band for different stages of face processing. PF, prefrontal area; RP, right parietal area; ROT, right occipito-temporal area; LOT, left occipito-temporal area; O, occipital area. Red lines indicate significant stronger connectivity for faces than non-faces, while green lines represent comparable or weaker connectivity for faces.