Abstract
It has been suggested that both egocentric and allocentric cues can be used for memory-guided movements, and that allocentric memory dominates during longer memory intervals (Obhi & Goodale, 2005; Hay & Redon, 2006). In the present study we examined 1) at what point in the reach plan allocentric representations are converted to egocentric representations and 2) the rates of decay of egocentric and allocentric memory. Nine subjects reached for a remembered target in complete darkness after a variable memory delay (2.5s, 5.5s, or 8.5 seconds in total). In the Ego Task the target was presented alone in the periphery on a CRT screen. In the Allo Task the target was presented along with four nearby blue disks (visual landmarks). After the variable delay, the landmarks reappeared at a shifted location, and subjects were instructed to reach to the target relative to the landmarks. In the Allo-Ego Conversion Task the shifted landmarks re-appeared twice: once before the variable delay and once immediately after (just before the reach cue). We analyzed the variance of reaching errors and reaction time (RT) for each memory delay in the three tasks. In the Ego Task, variance increased significantly in medium and long delays compared to the short delay; RT was longer in the short delay than medium and long delays, and the latter was significant. In the Allo Task there was no significant difference in variance and RT across the delays. In the Allo-Ego Conversion Task, there were significant increase in variance and decrease in RT for the medium and long delays compared to the short delay, which was similar to Ego Task. These results confirm that egocentric memory for reaching degrades more rapidly than allocentric memory, but despite this, in our Allo-Ego Task subjects preferred to convert allocentric into egocentric representations at the first possible opportunity.
Canada Research Chairs Program.