Investigating the Limits of Allocentric Memory

There are at least two distinct ways in which the brain encodes spatial information. In egocentric representations locations are encoded relative to the observer, whereas in allocentric representations locations are encoded relative to the environment. Both inform spatial memory, but their contributions to behaviour are not fully understood. Each system has specific advantages and disadvantages for different tasks, and these strengths and weaknesses relate to fundamental characteristics of the underlying representation.
This thesis uses a novel method (developed in Chapter 2), combining approaches from spatial memory research and psychophysics to measure spatial precision in change detection tasks where the observer’s viewpoint changes between presentation and testing (viewpoint-independent memory, which relies more on allocentric representation).
Chapter 3 uses these methods to investigate the effect of parametric changes in viewpoint on spatial change detection thresholds. A monotonic but non-linear effect of viewpoint on precision was found, consistent with a preregistered model that shows how the precision of spatial memory changes lawfully as a function of viewpoint shift. The model separately quantifies viewpoint-dependent and -independent parameters reflecting the way ego- and allocentric representations combine to determine performance.
Chapter 4 builds on these results to investigate spatial memory precision with regards to changes in the scale of the stimulus and environment. In both viewpoint-dependent and -independent memory, precision is found to scale with the extent of the stimulus in the observer’s field of view rather than its absolute dimensions. This finding suggests that egocentric encoding plays a part in limiting the precision of viewpoint-independent memory.
Chapter 5 investigates the limits of capacity in viewpoint-dependent and -independent memory with two distinct tasks. Here, working memory-like capacity limits determine how many items can be retained in both viewpoint-dependent and -independent spatial memory with some indications that these limits are distinct, perhaps due to additional task-specific demands.
Overall, these studies highlight the way that ego- and allocentric systems interact to determine the limits of spatial memory.