Distinct timing mechanisms are implicated in distinct circle drawing tasks.

That individual timing variability is correlated across some tasks but not others has instigated the notion of distinct timing processes, referred to as 'event' timing and 'emergent' timing for tasks with and without salient events, respectively. The delineation of the event-emergent framework owes much to the circle drawing task as it can be performed with or without such events, all other factors being equal. We investigated continuous and intermittent circle drawing from a principled perspective allowing for the classification of timing mechanisms based on mathematical theorems. We propose a one-dimensional dynamical model to portray the essential dynamics of circle drawing that exhibits either fixed point or oscillator dynamics, each associated with distinct timing mechanisms. Eight participants drew circles under three different instruction conditions (no specific instructions, to draw as a fast or as smoothly as possible) at seven frequencies ranging from 0.5Hz to 3.5Hz. We computed the angle between the circle drawing's horizontal and vertical component and reconstructed the corresponding vector fields prescribing the temporal evolution. Fixed point dynamics were present only in the 'fast' condition at low movement frequencies (i.e., in intermittent circle drawing representative of event timing). All other conditions were realized via oscillator dynamics representative of emergent timing. The transition between both dynamical mechanisms involved a saddle-node on invariant circle bifurcation, which was accompanied by increased trajectory variability, which is a key signature of phase transitions. These findings demonstrate the involvement of distinct timing mechanisms in different circle drawing tasks. Copyright 2010 Elsevier Ireland Ltd. All rights reserved.