Fig 1.

Panel A: Experimental paradigm. The trial starts when the monkey fixes the gaze to the central target (FT). A brief pre-stimulus delay follows (PSD). The reference bar is presented for 500 ms with one of three possible orientations (Reference). During the subsequent delay the subject has to maintain fixation (delay). The test bar is shown tilted to the left or to the right in relation to the reference bar. The amount of orientation of the test bar relative to the reference (TRO) manipulates the difficulty of the trial. When the test bar disappears the subject decides whether the test was tilted right or left in relation to the reference bar by making a saccade towards the right or left choice targets respectively. For more details see [4]. Panel B: Average probability to select the “left” option for the two monkeys as a function of the TRO. To each value of TRO a different color is assigned: Colors from dark blue (red) to light blue (yellow) correspond to left (right) responses with increasing difficulty, i.e., relative orientation from 4° (−4°) to 1° (−1°). Panel C: Average Movement time (MT) for the two monkeys as a function of the TRO separated for correct (black) and error (gray) trials.