Time course of egocentric decay, allocentric decay, and allocentric-to-egocentric conversion in memory-guided reach

Allocentric cues can be used to encode target locations in visuospatial memory, but it is not known how these representations are converted into egocentric commands for behaviour. We tested the influence of different memory intervals on reach performance toward targets defined in either egocentric or allocentric coordinates, and then compared this to performance in a task where subjects were implicitly free to choose when to convert from allocentric to egocentric representations (the allo-to-ego conversion condition). Nine subjects reached to place their right index finger at a remembered target location in complete darkness after a variable memory delay (2.5sec, 5.5sec, or 8.5sec in total). Our results confirm that egocentric representations degrade over a delay of several seconds, wheras allocentric representations remained relatively stable over the same time scale. Despite a cost in precision, when subjects were free to choose, they converted allocentric into egocentric representations as soon as possible. This suggests that the brain prefers to use egocentric coordinates for visuomotor transformations. Furthermore, this early conversion appears to provide a benefit in reaction time that is perhaps more important in real-world circumstances.