Functional brain activity and connectivity have been studied by calculating intersubject and seed-based correlations of hemodynamic data acquired with functional magnetic resonance imaging (fMRI). To inspect temporal dynamics, these correlation measures have been calculated over sliding time windows with necessary restrictions on the length of the temporal window that compromises the temporal resolution. Here, we show that it is possible to increase temporal resolution by using instantaneous phase synchronization (PS) as a measure of dynamic (time-varying) functional connectivity. We applied PS on an fMRI dataset obtained while 12 healthy volunteers watched a feature film. Narrow frequency band (0.04-0.07 Hz) was used in the PS analysis to avoid artifactual results. We defined three metrics for computing time-varying functional connectivity and time-varying intersubject reliability based on estimation of instantaneous PS across the subjects: (1) seed-based PS, (2) intersubject PS, and (3) intersubject seed-based PS. Our findings show that these PS-based metrics yield results consistent with both seed-based correlation and intersubject correlation methods when inspected over the whole time series, but provide an important advantage of maximal single-TR temporal resolution. These metrics can be applied both in studies with complex naturalistic stimuli (e.g., watching a movie or listening to music in the MRI scanner) and more controlled (e.g., event-related or blocked design) paradigms. A MATLAB toolbox FUNPSY (http://becs .aalto.fi/bml/software.html) is openly available for using these metrics in fMRI data analysis.

Event related potentials (ERPs) were recorded from 11 subjects watching photographs of angry and happy faces with different gaze directions. ERPs to right averted gaze differed from those to straight and left averted gaze at 85 and 460 ms whereas ERPs to happy and angry expressions differed at 115, 330 and 380 ms. We suggest that short-latency effects, maximal over occipital cortex, reflect the involvement of visual cortex in the early analysis of socially-relevant stimuli. Interaction of gaze and expressions was reflected in ERPs at 270 - 450 ms. We conclude that gaze and emotional expressions are analyzed in parallel at the early stages of visual processing. The interaction of these two processing streams starts no earlier than at 270 ms.

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