Investigating spatial reference systems through distortions in visual memory

Memory representations of spatial information require the choice of one or more reference systems to specify spatial relations. In two experiments we investigated the role of different reference systems for the encoding of sp atial information in human memory. In Experiment 1, participants had to rep roduce the location of a previously seen dot in relation to two landmarks o n a computer screen. The placement of the two landmarks was varied so that they were horizontally or vertically aligned in half of the trials, and dia gonally aligned in the other half of the trials. Reproductions showed a sim ilar pattern of distortions for all four different orientations of the land marks, indicating the use of the landmarks as an allocentric reference syst em. In Experiment 2, the influence of this allocentric reference system for very brief retention intervals (100 and 400 ms) was demonstrated in a visu al discrimination task, extending previous work. The results suggest that l andmark-based spatial reference systems are functional within 100 ms of sti mulus presentation for most of the observers. Allocentric reference sytems therefore are an essential part even of early mental representations of spa tial information.