One might be tempted to ask whether dynamic stimuli are any different from static stimuli. After all, if they are not, then why should one go through all of the extra effort of using dynamic stimuli? At the extreme, one might go so far as to suggest that dynamic sequences are nothing more than a collection of static snapshots. Indeed, the application of FACS to video sequences explicitly requires this assumption, as it has no method of encoding spatiotemporal information. There is, however, a lot of evidence demonstrating that dynamic stimuli are fundamentally different from static stimuli in general (e.g., Gibson,
1979) as well as for facial expressions in specific. Possibly the first evidence that there is some temporal information specific to facial expressions comes from Bassili's (
1978,
1979) seminal work using point-light displays. Subsequent research has clearly shown that dynamic information can improve the recognition of facial expressions even in more natural displays (Ambadar, Schooler, & Cohn,
2005; Constantini, Pianesi, & Prete,
2005; Cunningham & Wallraven,
2008; Ehrlich, Schiano, & Sheridan,
2000; Harwood, Hall, & Shinkfield,
1999; Katsyri, Klucharev, Frydrych, & Sams,
2003; Wallraven, Breidt, Cunningham, & Bülthoff,
2007; Wehrle, Kaiser, Schmidt, & Scherer,
2000; Weyers, Mühlberger, Hefele, & Pauli,
2006). A recent series of perceptual experiments shows that not only are dynamic expressions recognized more easily than static expressions, but also that no explanation based solely on static information can explain the difference (Cunningham & Wallraven,
2008). That is, there is some characteristic information for specific expressions that is
only available over time. Finally, there is anatomical work suggesting that static and dynamic facial expressions are processed differently in the human brain, possibly involving completely different brain structures (Adolphs,
2002; Humphreys, Donnelly, & Riddoch,
1993; LaBar, Crupain, Voyvodic, & McCarthy,
2003, Schwaninger et al,
2006). Regardless of whether dynamic expressions are different from static expressions or not, it remains true that dynamic expressions represent the normal case in everyday life and thus are the focus of the present work.