The effects of landmarks on the performance of delayed and real-time pointing movements.

Converging lines of evidence suggest that the presence of non-target landmarks affects the performance of delayed target-directed movements (e.g., Diedrichsen et al. 2004; Sheth and Shimojo 2004). In the present experiment, we examined the effects of non-target landmarks on the accuracy and precision of delayed and immediate target-directed pointing movements. In our experiment, the landmarks were present just prior to and during the presentation of the target; they were never present during the execution of the movement. Absolute errors were significantly reduced when the landmarks were available during target presentation for both delayed and immediate action conditions. In contrast, the presence of landmarks improved the precision of delayed but not immediate movements (as indexed by the variable error). The locus of this "landmark benefit" appears to be in the encoding of target position since landmarks were never available after target offset. We suggest that, when available, information provided by landmarks is used to improve the accuracy of the estimation of target location. Since the targets were presented for only 100 ms, it is apparent that the spatial information available from landmarks can be quite rapidly used to estimate target position. Further, with respect to the precision of movements, we suggest that the presence of landmarks serves to improve the stability of the estimation of target position. This particular reliance on landmark information becomes more critical as the movement is delayed.