We used an ambiguous stimulus that consisted of 450 yellow square
dots moving coherently on a black background with central white fixation
cross and framed by a white square (Fig. 1). Due to the structurefrom-
motion phenomenon this stimulus is perceived as a sphere rotating
in depth around a vertical axis (diameter 4.1° of visual angle, rotation
speed 1/6 revolutions/s). The rotation direction of the sphere is ambiguous
so that during prolonged viewing perception keeps fluctuating every
few seconds between the percept of a sphere rotating towards the left
and a sphere rotating towards the right. To familiarize participants
with the two possible percepts evoked by the sphere, the experiment
started with an initial training session during which the sphere was continuously
presented for four minutes in which participants' perception
fluctuated between the two percepts.