|Human brain dynamics accompanying use of egocentric and allocentric refrence frames during spatial orienting|
(Ying Wu) What differences in brain dynamics did you discover between individuals using ego- or allocentric reference frames in determining their homing direction after a tunnel navigation task?
(Klaus Gramann) During navigation the differences between subjects using an egocentric reference frame, so called Turners, and subjects using an allocentric reference frame, so called Nonturners, are not so much 'qualitative' differences in the number and location of brain areas underlying navigation. That is, both strategy groups show activation in a wide spread network including occipital, parietal, motor/premotor and frontal areas during the task. This simply reflect the fact that both reference frames are active in all subjects during spatial tasks but that only one is dominantly used for a specific task. The parallel computation and use of distinct reference frames is necessary for solving spatial task in our natural environment and more and more studies reveal individual variability in using one or the other reference frame dependent on the task. However, the preference to use an egocentric or an allocentric reference frame during navigation in the tunnel task is accompanied by significant differences in spectral modulation in a wide frequency range and confined to a subset of brain regions, revealing a network of areas associated with the dominant use of an allocentric frame of reference. The differences can be found most prominently in the alpha frequency band demonstrating increased alpha desynchronization during task relevant periods within occipital MT+, inferior parietal, and retrosplenial cortex. Since the differences are already observable during the navigation process is is plausible to assume that the differences in adjusting the homing vector after the passage are due to the reference frame used during navigation.
Most differences that you report between Turners and Nonturners occur within the alpha frequency band. Were you surprised that differences within theta bandwidths weren't observed as well? What do you make of this outcome?
It is correct that Nonturners show a prominent alpha power decrease compared to baseline activation (watching only straight tunnels) that was more pronounced for this strategy group as compared to Turners. Further, we observed strong theta modulation during distinct stages of the task in both strategy groups which did not differ dependent on the reference frame used. One might expect that the use of an allocentric reference frame would lead to prominent theta activity within the medial temporal cortex, i.e. In hippocampal cortex and adjacent structures. However, studies describing this kind of theta activity (usually associated with the computation of the cognitive map) i) did not dissociate between different navigation strategies, ii) used material that included prominent landmarks that could be stored in a cognitive map, iii) used different methods to record brain dynamics (i.e., ECOG, MEG). Even if the use of an allcoentric reference frame was accompanied by increased theta activity within hippocampus in the tunnel task we do not know whether this activity can be measured with EEG since the anatomical structure of the hippocampus might make it impossible to measure field potentials with scalp electrodes. The fact that alpha modulations were the most prominent indicator for the use of distinct reference frames points to an underestimation of the functional role of alpha activity in spatial tasks.
What can your findings tell us about spatial navigation in the real world?
The use of distinct reference frames is a common phenomenon that can be observed in everyday spatial cognition. Imagine you try to find the train station in an unknown city using a map. Some people will rotate the map to align it with their actual physical heading. Other people will mentally rotate themselves to align their heading with the north/south orientation of the map instead. That is not to say that both groups are unable to use the other strategy as well and in some cases will do so. However, there seems to be a proclivity to use one or the other strategy. Furthermore, dependent on the information given people will switch to a strategy that might be best to solve the task. Individual proclivities in interaction with the existant information to solve a spatial task will determine what strategy is used. Interestingly, the inability to use an alternative strategy/reference frame is associated with decreased grey matter in the brain regions supporting this alternative strategy (see VD Bohbot, J Lerch, B Thorndycraft, G Iaria, AP, et al. Journal of Neuroscience, 2007). The results of the study can tell us which frame of reference is preferentially used and whether subjects are able to use alternative reference frames for solving the task (see Gramann et al, 2005). This might lead to new diagnostics and training methods for subject with deficits in spatial orienting.