Stimuli and experimental protocol.

<p>A. Stimuli consisted of four disconnected bars arranged in a square shape whose contrast was modulated along the bars. When in motion these stimuli can be perceived as incoherent (unbound) pairs of bars oscillating along cardinal axes at different frequencies (2.3 and 3 Hz) or as a single (bound) square shape translating along a Lissajou's trajectory (inset). B. Time course of a trial: a trial started with the presentation of a fixation point for 1500 ms, followed by the presentation of the static bars for a variable duration (450–550 ms) after which the oscillatory motion began and lasted for 1200 ms. At the end of the motion stimulation, a screen appeared with response color codes scrambled on each trial, which implied remapping the motor response on each trial thus avoiding motor preparation and anticipation that could contaminate the data. C. Individual behavioral responses (dotted lines) and average responses (black line) for the 4 stimulus conditions. Bars with low contrast line-ends (conditions 1 and 2) are mostly seen as a bound shape, bars with high contrast line-ends (conditions 3 and 4) are mostly seen as unbound segments (Filled circles: percentage of the trials seen as bound; filled squares percentage of the trials seen as unbound; filled triangles: percentage of unclassified trials). Errors bars represent ±1 standard error.