Figure 3.

Sample amplitude-shortening session: FTD paradigm. (A) Before adaptation, saccades to a single target are accurate and have a small scatter (95% confidence ellipses are shown, together with actual saccade end points). (B) After adaptation, the amplitude of saccades to target B are considerably reduced (by 28% in this case); saccades aimed at A and D are only mildly affected. Thin ellipses indicate pre-adaptation end points, whereas thick ellipses indicate post-adaptation end points. Arrows indicate mean shift. (C) Before adaptation, the end points of the first saccade in the FP–A–B sequence are accurate; end points of the second saccade are considerably more scattered (note that the horizontal scatter due to the target jitter has already been compensated for). (D) After adaptation, we observed a large change in the end points of the second saccade, but not the first saccade, in the FP–A–B sequence. (E) Based on the pre-adaptation end points of the second saccades, the motor and goal updating hypotheses make clear predictions about the post-adaptation end points. The motor updating hypothesis (blue ellipse and arrow) predicts that they will shift like the end points of single saccades aimed at B, whereas the goal updating hypothesis (red ellipse and arrow) essentially predicts that they will mimic the changes of single saccades aimed at D. (F) In this session, the post-adaptation second saccade end points (black dots) obviously match the motor prediction (blue ellipse) much better than the goal prediction (red ellipse). We tested this statistically by projecting these points onto the axis that connects the centers of the two predictions. We normalize this axis to compute an index (GMUI) such that a value of 0.0 (1.0) corresponds to the center of the goal (motor) prediction. A distribution mean significantly smaller (larger) than 0.5 supports the goal (motor) hypothesis. In this case, the mean is 1.06. Data from subject S1.