Abstract
It had been proposed that spatial landmarks are encoded either categorically, such that the relative positions of objects are defined in prepositional terms (e.g., to the left/right, in front/behind); or in terms of visual coordinates, such that the precise distances between objects are represented. In humans, it has been assumed that categorical representations are subserved by a left hemisphere neural network, whereas coordinate representations are largely right-lateralized. However, evidence in support of this distinction has been garnered almost exclusively from tasks that involved static, two-dimensional arrays (e.g., objects arranged on a table-top), rather than in the context of active navigation. In the present study, we used functional magnetic resonance imaging (fMRI) to identify neural circuits underlying categorical and coordinate representations during active spatial navigation of a virtual environment. In the encoding phase of each trial, participants navigated to the location of a target object and encoded either its spatial quadrant relative to a reference landmark (Categorical condition) or its distance from a reference landmark (Coordinate condition). After a short delay, participants re-entered the arena and were required either to navigate back to the appropriate quadrant of the arena, disregarding their distance from the landmark, or to navigate to an appropriate distance from the landmark ignoring the quadrant. We examined fMRI activity patterns during the encoding period, in which visual exposure to the virtual environment was matched across conditions. Activity in the categorical versus coordinate condition was significantly greater in the posterior and medial parietal cortex bilaterally, and in the left middle temporal gyrus. The complementary contrast (coordinate minus categorical) revealed greater activity in the right hippocampus, parahippocampus and dorsal striatum. These findings are broadly consistent with analogous studies in rodents, and support the suggestion of distinct neural circuits underlying categorical and coordinate representations of object location during active spatial navigation.
This research was supported by funds from the Australian Research Council (ARC) Thinking Systems grant.