Accuracies of saccades to moving targets during pursuit initiation and maintenance.

The overall goals of the studies presented here were to compare (1) the accuracies of saccades to moving targets with either a novel or a known target motion, and (2) the relationships between the measures of target motion and saccadic amplitude during pursuit initiation and maintenance. Since resampling of position error just prior to saccade initiation can confound the interpretation of results, the target ramp was masked during the planning and execution of the saccade. The results suggest that saccades to moving targets were significantly more accurate if the target motion was known from the early part of the trial (e.g., during pursuit maintenance) than in the case of novel target motion (e.g., during pursuit initiation); both these types of saccades were more accuate than those when target motion information was not available. Using target velocity in space as a rough estimate of the magnitude of the extra-retinal signal during pursuit maintenance, the saccadic amplitude was significantly associated with the extra-retinal target motion information after accounting for the position error. In most subjects, this association was stronger than the one between retinal slip velocity and saccadic amplitude during pursuit initiation. The results were similar even when the smooth eye motion prior to the saccade was controlled. These results suggest that different sources of target motion information (retinal image velocity vs internal representation of previous target motion in space) are used in planning saccades during different stages of pursuit. The association between retinal slip velocity and saccadic amplitude is weak during initiation, thus explaining poor saccadic accuracy during this stage of pursuit.