Objective: Deficits in egocentric (subject-to-object) and allocentric (object-to-object) spatial
representations, with a mainly allocentric impairment, characterize the first stages of the Alzheimer's
disease (AD).
Methods:
To identify early cognitive signs of AD conversion, some studies focused on amnestic-Mild
Cognitive Impairment (aMCI) by reporting alterations in both reference frames, especially the allocentric
ones. However, spatial environments in which we move need the cooperation of both reference
frames. Such cooperating processes imply that we constantly switch from allocentric to egocentric
frames and vice versa. This raises the question of whether alterations of switching abilities might also
characterize an early cognitive marker of AD, potentially suitable to detect the conversion from aMCI to
dementia. Here, we compared AD and aMCI patients with Normal Controls (NC) on the Ego-Allo-
Switching spatial memory task. The task assessed the capacity to use switching (Ego-Allo, Allo-Ego)
and non-switching (Ego-Ego, Allo-Allo) verbal judgments about relative distances between memorized
stimuli.
Results:
The novel finding of this study is the neat impairment shown by aMCI and AD in switching
from allocentric to egocentric reference frames. Interestingly, in aMCI when the first reference frame
was egocentric, the allocentric deficit appeared attenuated.
Conclusion:
This led us to conclude that allocentric deficits are not always clinically detectable in aMCI
since the impairments could be masked when the first reference frame was body-centred. Alongside, AD
and aMCI also revealed allocentric deficits in the non-switching condition. These findings suggest that
switching alterations would emerge from impairments in hippocampal and posteromedial areas and from
concurrent dysregulations in the locus coeruleus-noradrenaline system or pre-frontal cortex.
Link of the Hipparcos Stellar Reference Frame to the Distant Quasars
