You or me? Disentangling perspectival, perceptual, and integrative mechanisms in heterotopagnosia.

Heterotopagnosia-without-Autotopagnosia (HwA) is characterized by the incapacity to point to body parts on others, but not on one's own body. This has been classically interpreted as related to a self-other distinction, with impaired visual representations of other bodies seen in third person perspective (3PP), besides spared own body somatosensory representations in 1PP. However, HwA could be impacted by a deficit in the integration of visual and somatosensory information in space, that are spatially congruent in the case of one's own body, but not for others' body. Here, we test this hypothesis in a rare neurological patient with HwA, H+, as well as in a control patient with a comparable neuropsychological profile, but without HwA, and in age-matched healthy controls, in two experiments. First, we assessed body part recognition in a new task where somatosensory information from the participant's body and visual information from the target body shown in virtual reality was never aligned in space. Results show that, differently from the flawless performance in controls, H+ committed errors for not only the body of others in 3PP, but for all conditions where the information related to the real and the target body was not spatially congruent. Then, we tested whether the integration between these multisensory bodily cues in space, as during visuo-tactile stimulation in the full-body illusion, improves the patient's performance. Data show that after the stimulation prompting visuo-tactile integration, but not in control conditions, the patient's abilities to process body parts improved up to normal level, thus confirming and extending the first findings. Altogether, these results support a new interpretation of HwA as linked to the matching between somatosensory inputs from one's body and visual information from a body seen at a distance, and encourage the application of multisensory stimulation and virtual reality for the treatment of body-related disorders.

An electrophysiological dissociation between orbitofrontal reality filtering and context source monitoring.

Memory influences behavior in multiple ways. One important aspect is to remember in what precise context in the past a piece of information was acquired (context source monitoring). Another important aspect is to sense whether an upcoming thought, composed of fragments of memories, refers to present reality and can be acted upon (orbitofrontal reality filtering). Whether these memory control processes share common underlying mechanisms is unknown. Failures of both have been held accountable for false memories, including confabulation. Electrophysiological and imaging studies suggest a dissociation but used very different paradigms. In this study, we juxtaposed the requirements of context source monitoring and reality filtering within a unique continuous recognition task, which healthy participants performed while high-resolution evoked potentials were recorded. The mechanisms dissociated both behaviorally and electrophysiologically: Reality filtering induced a frontal positivity, absence of a specific electrocortical configuration, and posterior medial orbitofrontal activity at 200-300 msec. Context source monitoring had no electrophysiological expression in this early period. It was slower and less accurate than reality filtering and induced a prolonged positive potential over frontal leads starting at 400 msec. The study demonstrates a hitherto unrecognized separation between orbitofrontal reality filtering and source monitoring. Whereas deficient orbitofrontal reality filtering is associated with reality confusion in thinking, the behavioral correlates of deficient source monitoring should be verified with controlled experimental exploration.

Neural Correlate of Anterograde Amnesia in Wernicke-Korsakoff Syndrome.

The neural correlate of anterograde amnesia in Wernicke-Korsakoff syndrome (WKS) is still debated. While the capacity to learn new information has been associated with integrity of the medial temporal lobe (MTL), previous studies indicated that the WKS is associated with diencephalic lesions, mainly in the mammillary bodies and anterior or dorsomedial thalamic nuclei. The present study tested the hypothesis that amnesia in WKS is associated with a disrupted neural circuit between diencephalic and hippocampal structures. High-density evoked potentials were recorded in four severely amnesic patients with chronic WKS, in five patients with chronic alcoholism without WKS, and in ten age matched controls. Participants performed a continuous recognition task of pictures previously shown to induce a left medial temporal lobe dependent positive potential between 250 and 350 ms. In addition, the integrity of the fornix was assessed using diffusion tensor imaging (DTI). WKS, but not alcoholic patients without WKS, showed absence of the early, left MTL dependent positive potential following immediate picture repetitions. DTI indicated disruption of the fornix, which connects diencephalic and hippocampal structures. The findings support an interpretation of anterograde amnesia in WKS as a consequence of a disconnection between diencephalic and MTL structures with deficient contribution of the MTL to rapid consolidation.

Mechanism of disorientation: reality filtering versus content monitoring.

Disorientation is frequent after brain damage. It is a constituent component of post-traumatic amnesia and was part of the original definition of the Korsakoff syndrome, together with amnesia and confabulations. Orbitofrontal reality filtering is a pre-conscious memory control process that has been held accountable for disorientation and a specific type of confabulations that patients act upon. A recent study questioned the specificity of this process and suggested that confabulating patients who failed in orbitofrontal reality filtering similarly failed to monitor the precise content of memories, a critical step within the strategic retrieval account, which describes a series of processes leading up to the recollection of memories. In the present study we combined the proposed experimental requirements of both processes in a single continuous recognition task and tested a group of 21 patients with a matched deficit of delayed free recall. We found that only deficient reality filtering, but not content monitoring, significantly correlated with disorientation and distinguished between confabulators and non-confabulators. Thus, reality confusion, as evident in disorientation and behaviourally spontaneous confabulation, primarily reflects an inability to monitor memories' relation with ongoing reality rather than to monitor their precise content.

Adaptive reorganization of cortical networks in Alzheimer's disease.

OBJECTIVE: The impact of neuronal cell loss associated with Alzheimer's disease (AD) on the network organization of the brain is poorly understood. Here we investigated whether modifications in resting-state functional connectivity (FC) are associated with cognitive function of AD patients. METHODS: High-density electroencephalograms (EEGs) were obtained from patients with early stages of AD and elderly healthy controls. Cortical oscillations were reconstructed with an adaptive spatial filter. Maps of imaginary coherence (IC) between brain areas were compared between groups and correlated with cognitive performance. RESULTS: Parietal and medial temporal lobes of AD patients showed a disruption of alpha band FC to the rest of the brain. However, an adaptive extension of the language network to the right hemisphere could be observed in AD patients and was correlated with better verbal fluency. A shift of FC from alpha frequencies to theta frequencies could be observed in a memory network and was associated with better verbal memory performance. CONCLUSIONS: Not only dysfunctional but also adaptive network reorganization occurs in early AD. SIGNIFICANCE: The network mechanisms for preserved cognitive functioning may inform novel treatment strategies for AD in the future.

Forms of confabulation: dissociations and associations.

Confabulation denotes the emergence of memories of experiences and events which never took place. Whether there are distinct forms with distinct mechanisms is still debated. In this study, we explored 4 forms of confabulation and their mechanisms in 29 amnesic patients. Patients performed tests of explicit memory, executive functions, and two test of orbitofrontal reality filtering (memory selection and extinction capacity in a reversal learning task) previously shown to be strongly associated with confabulations that patients act upon and disorientation. Results indicated the following associations: (1) Intrusions in a verbal memory test (simple provoked confabulations) dissociated from all other forms of confabulation and were not associated with any specific cognitive measure. (2) Momentary confabulations, defined as confabulatory responses to questions and measured with a confabulation questionnaire, were associated with impaired mental flexibility, a tendency to fill gaps in memory, and with one measure of reality filtering. Momentary confabulations, therefore, may emanate from diverse causes. (3) Behaviourally spontaneous confabulation, characterized by confabulations that the patients act upon and disorientation, was strongly associated with failure in the two reality filtering tasks. Behaviourally spontaneous confabulation may be seen as a specific instance of momentary confabulations with a distinct mechanism. (4) A patient producing fantastic confabulations with nonsensical, illogical content had wide-spread cognitive dysfunction and failed in the reality filtering tasks. The results support the presence of truly or partially dissociable types of confabulation with different mechanisms.