REFERENCE FRAMES IN SACCADIC TARGETING

We attempt to determine the egocentric reference frame used in directi ng saccades to remembered targets when landmark-based (exocentric) cue s are not available. Specifically, we tested whether memory-guided sac cades rely on a retina-centered frame, which must account for eye move ments that intervene during the memory period (thereby accumulating er ror) or on a head-centered representation that requires knowledge of t he position of the eyes in the head. We also examined the role of an e xocentric reference frame in saccadic targeting since it would not nee d to account for intervening movements. We measured the precision of e ye movements made by human observers to target locations held in memor y for a few seconds. A variable number of saccades intervened between the visual presentation of a target and a later eye movement to its re membered location. A visual landmark that allowed for exocentric encod ing of the memory target appeared in half the trials. Variable error i ncreased slightly with a greater number of intervening saccades. The l andmark aided targeting precision, but did not eliminate the increase in variable error with additional intervening saccades. We interpret t hese results as evidence for a representation that relies on knowledge of eye position with respect to the head and not one that relies sole ly on updating in a retina-centered frame. Our results allow us to set an upper bound on the standard deviation of an eye position signal av ailable to the saccadic system during short memory periods at 1.4 degr ees for saccades of about 10 degrees.