 There has also been much recent work applying the perspective of dynamical systems to cognition and action, drawing explicitly on such properties as attractors, bifurcations, SDIC and other denizens of the nonlinear zoo (e.g., van Gelder 1995; Kelso 1995; Port and van Gelder 1995; Juarrero 1999; Tsuda 2001). The basic idea is to deploy the framework of nonlinear dynamics for interpreting neural and cognitive activity as well as complex behavior. It is then hoped that patterns of neural, cognitive and human activity can be explained as the results of nonlinear dynamical processes involving causal interactions and constraints at multiple levels (e.g., neurons, brains, bodies, physical environments). Such approaches are highly suggestive, but also face challenges. For instance, as mentioned in the previous section, the nature of neural and cognitive dynamics is still much disputed. Ultimately, it is an empirical matter whether these dynamics are largely nonlinear or not. Moreover, the explanatory power of dynamical systems approaches relative to rival computational approaches has been challenged (e.g., Clark 1998). Again, questions about realism and explanation in chaotic dynamics  (§5)  are relevant here as well as the faithful model assumption. 