Reference frames in saccadic targeting

We attempt to determine the egocentric reference frame used in directing saccades to remembered targets when landmark-based (exocentric) cues are not available. Specifically, we tested whether memory-guided saccades rely on a retina-centered frame, which must account for eye movements that intervene during the memory period (thereby accumulating error) or on a head-centered representation that requires knowledge of the position of the eyes in the head. We also examined the role of an exocentric reference frame in saccadic targeting since it would not need to account for intervening movements. We measured the precision of eye movements made by human observers to target locations held in memory for a few seconds. A variable number of saccades intervened between the visual presentation of a target and a later eye movement to its remembered location. A visual landmark that allowed for exocentric encoding of the memory target appeared in half the trials. Variable error increased slightly with a greater number of intervening saccades. The landmark aided targeting precision, but did not eliminate the increase in variable error with additional intervening saccades. We interpret these results as evidence for a representation that relies on knowledge of eye position with respect to the head and not one that relies solely 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 available to the saccadic system during short memory periods at 1.4 degrees for saccades of about 10 degrees.