Figure 8.

Choices are independent of small drifts in eye position during motion presentation. (A) Horizontal and vertical eye position from a representative trial from one monkey (Monkey H, SS = 0) plotted in space (upper panel) and over time (relative to motion stimulus onset; lower panel). We often observed small drifts in eye position during presentation of the motion stimulus (green symbols in the upper panel and green shaded epoch in the lower position-vs.-time panel). (B) Eye speed versus time relative to motion stimulus onset, projected onto the motion stimulus direction and averaged over all trial for each SS. On average, the eyes remain stationary for approximately 75 ms after motion onset (longer, ~125 ms, in human observers) before drifting slowly until onset of the saccade indicating the subject’s choice. Drift speed decreased systematically as motion SS was reduced. (C) Magnitude of the drift in the direction of the stimulus motion. (D) Proportion of correct trials as a function of SS, based on the drift vector direction. Proportion correct for both marmosets and humans decreased as SS was reduced. To aid comparison, dashed lines show proportion correct for each subject based on their choices (reproduced from Fig. 5B). (E) Density plots of saccade error versus drift error to assess the extent to which systematic drift in eye position could account for the subject’s subsequent choice. For both marmosets and humans, errors in the subject’s choices were independent of errors in drift direction. Same conventions as in Figs 5–7.