Abstract
Allocentric cues can be used to encode target location in visuo-spatial memory (Chen et al., 2011; Obhi & Goodale, 2005), and the allocentric representation is converted into egocentric representation at the first possible opportunity for reach (Chen et al., 2011). However, neural substrates for allocentric-to-egocentric conversion have not been explored yet. Here we used fMRI to investigate brain areas involved in allocentric-to-egocentric conversion for memory-guided reach. Ten participants reached towards a remembered target location represented in allocentric frames of reference. Participants fixated a central point while a target was presented along with an allocentric cue for 2s. The concurrent presentation of target and cue was followed by a delay period (6s), after which an auditory instruction ("Same cue" or "Different cue") instructed participants that the allocentric cue would re-appear at the same location, allowing for a conversion of target location from allocentric to egocentric, or at a different location, requiring participants to wait for the re-appearance of the cue before they knew the target location to reach. A second delay period (10s) followed the auditory instruction. Next, the allocentric cue re-appeared for 2s and was followed by the go-signal to reach towards the remembered target location relative to the location of the re-displayed allocentric cue. We hypothesized that brain areas involved in allocentric-to-egocentric conversion would show higher activation when the allocentric representation of target location could be turned into an egocentric representation. This would be revealed by higher activation in the "Same cue" as compared to the "Different cue" condition during the second delay period. This pattern was revealed in bilateral dorsal precuneus, left angular gyrus and bilateral inferior frontal gyrus. Our results suggest that posterior parietal cortex and frontal areas play a critical role in converting allocentric representation of target location into egocentric representation for reach planning.
Meeting abstract presented at VSS 2014