Response-time dynamics: Evidence for linear and low-dimensional nonlinear structure in human choice sequences

Response time (RT) is a commonly used measure of cognitive performance, whi ch is usually characterized as stochastic. However, useful information may be hidden in the apparently random fluctuations of RT. Dynamical systems an alysis techniques allow an exploration of the alternative hypothesis that R T fluctuations are deterministic, albeit in a complex manner. We applied ca reful task construction and noise-reduction and surrogate series tests to s how that RT series from a forced-pace serial response-time task have low-di mensional chaotic characteristics. In Experiment 1, 80% of subjects' filter ed RT series had low dimensionality, sensitive dependence on initial condit ions, spectra close to I If, and stable attractor geometry across sessions. In Experiment 2, we showed that the size of the inter-stimulus interval (I SI) determined the number of subjects with low-dimensional chaotic series. A small ISI caused 100% of subjects to respond in the chaotic regime, where as only 25% had a low-dimensional chaotic RT component when the ISI was lar ge. We argue that demanding task requirements cause a reduction in the dime nsionality of the dynamics, producing RT fluctuations that may reflect a re sponse strategy for controlling RT.