Deficits in egocentric-updating and spatial context memory in a case of developmental amnesia

Abstract

Patients with developmental amnesia usually suffer from both episodic and spatial memory deficits. DM, a developmental amnesic, was impaired in her ability to process self-motion (i.e., idiothetic) information while her ability to process external stable landmarks (i.e., allothetic) was preserved when no self-motion processing was required. On a naturalistic and incidental episodic task, DM was severely and predictably impaired on both free and cued recall tasks. Interestingly, when cued, she was more impaired at recalling spatial context than factual or temporal information. Theoretical implications of that co-occurrence of deficits and those dissociations are discussed and testable cerebral hypothesis are proposed.

Keywords:

• hippocampus
• parietal
• spatial memory
• allocentric
• episodic memory

Acknowledgements

We thank all participants, in particular Ms. DM, for their time and effort. The authors would also like to thank Elise Antoine, Mélanie Cerles, and Sophie Lemmonier for their precious experimental contribution.

Notes

^1. Surprisingly, this deficit spares the very recent period (last few months). On inquiry DM also appeared perfectly oriented in time and space. Both preservations appear to be at odds with what is generally observed in acquired amnesia following hippocampal lesions and were probably underpinned by compensatory strategies: in fact, helped by her entourage and relatives, DM uses rehearsal strategies for significant events of her recent life which might explain her performance. This reliance on compensatory strategies could have been favored by the developmental nature of her amnesia.

^2. To control for the encoding-test delay: (a) a Rotation trial was always tested before a no-rotation trial, (b) during each rotation trial, the experimenter recorded the encoding-test delay, (c) this delay was then replicated in the following no-rotation trial.

^3. Data analysis: For each condition or subtest, modified t-tests (Crawford, Howell, & Garthwaite, 1998) were carried out to compare DM’s performance with that of the CG.When a classical functional dissociation was assessed between conditions, a modified Z-score was calculated following Crawford, Garthwaite, and Gray’s recommendations (Crawford, Garthwaite, & Howell, 2009). This score respect the three following criterion for two tasks X and Y:

• Criterion 1 – Patient’s score on task X is significantly lower than controls using Crawford et al. (1998) modified t-test method.

• Criterion 2 – Patient’s score on task Y is not significantly lower than in the control group (i.e., score fails to meet threshold for a deficit and is therefore considered to be within normal limits).

• Criterion 3 – Patient’s score on task X is significantly lower than patient’s score on task Y using Crawford et al. (1998) test.

^4. DM only reported two very unspecific spatial elements of the event (“one of the rooms was slightly bigger”, “I was standing”). Such elements were never included for control participants who always provided clear spatial elements such as: “the room was not larger than 15 m²”, “there was a window on the opposite side of the door”, “there was a bookshelf on the left”, “a chair on the right”… Excluding the two very unspecific spatial elements from the analysis does not qualitatively modify the results (T = −2.17, p = 0.0581).