Figure 1.

The visuospatial rhythm task. (A) In the 2-choice task a visual stimulus (colored disk) alternates left and right of a central fixation point. After three visible intervals (presentation phase) the disk disappears and subjects must internally track its location as a function of elapsed time (continuation phase). The fixation point can disappear at the midpoint of any pseudo-randomly selected continuation interval (Go-time), instructing the subjects to indicate the estimated disk location (left or right). (B) In the 6-choice task the disk moves sequentially to six marked locations along a circular path. Taking into account the direction of rotation, this version of the task allows estimating whether the subjects' responses are ahead of or behind the correct target location. (C) In the continuous version the disk moves smoothly along a gray path at a velocity that matches the position of the disk on the 6-choice task. Angle units were transformed to time units for the analyses described in the text.