Timing of Allocentric and Egocentric Spatial Processing in Human Intracranial EEG

Spatial reference frames (RFs) play a key role in spatial cognition, especially in perception, spatial memory, and navigation. There are two main types of RFs: egocentric (self-centered) and allocentric (object-centered). Although many fMRI studies examined the neural correlates of egocentric and allocentric RFs, they could not sample the fast temporal dynamics of the underlying cognitive processes. Therefore, the interaction and timing between these two RFs remain unclear. Taking advantage of the high temporal resolution of intracranial EEG (iEEG), we aimed to determine the timing of egocentric and allocentric information processing and describe the brain areas involved. We recorded iEEG and analyzed broad gamma activity (50-150 Hz) in 37 epilepsy patients performing a spatial judgment task in a three-dimensional circular virtual arena. We found overlapping activation for egocentric and allocentric RFs in many brain regions, with several additional egocentric- and allocentric-selective areas. In contrast to the egocentric responses, the allocentric responses peaked later than the control ones in frontal regions with overlapping selectivity. Also, across several egocentric or allocentric selective areas, the egocentric selectivity appeared earlier than the allocentric one. We identified the maximum number of egocentric-selective channels in the medial occipito-temporal region and allocentric-selective channels around the intraparietal sulcus in the parietal cortex. Our findings favor the hypothesis that egocentric spatial coding is a more primary process, and allocentric representations may be derived from egocentric ones. They also broaden the dominant view of the dorsal and ventral streams supporting egocentric and allocentric space coding, respectively.

Tackling the challenges of group network inference from intracranial EEG data

INTRODUCTION

Intracranial EEG (iEEG) data is a powerful way to map brain function, characterized by high temporal and spatial resolution, allowing the study of interactions among neuronal populations that orchestrate cognitive processing. However, the statistical inference and analysis of brain networks using iEEG data faces many challenges related to its sparse brain coverage, and its inhomogeneity across patients.

METHODS

We review these challenges and develop a methodological pipeline for estimation of network structure not obtainable from any single patient, illustrated on the inference of the interaction among visual streams using a dataset of 27 human iEEG recordings from a visual experiment employing visual scene stimuli. 100 ms sliding window and multiple band-pass filtered signals are used to provide temporal and spectral resolution. For the connectivity analysis we showcase two connectivity measures reflecting different types of interaction between regions of interest (ROI): Phase Locking Value as a symmetric measure of synchrony, and Directed Transfer Function-asymmetric measure describing causal interaction. For each two channels, initial uncorrected significance testing at p < 0.05 for every time-frequency point is carried out by comparison of the data-derived connectivity to a baseline surrogate-based null distribution, providing a binary time-frequency connectivity map. For each ROI pair, a connectivity density map is obtained by averaging across all pairs of channels spanning them, effectively agglomerating data across relevant channels and subjects. Finally, the difference of the mean map value after and before the stimulation is compared to the same statistic in surrogate data to assess link significance.

RESULTS

The analysis confirmed the function of the parieto-medial temporal pathway, mediating visuospatial information between dorsal and ventral visual streams during visual scene analysis. Moreover, we observed the anterior hippocampal connectivity with more posterior areas in the medial temporal lobe, and found the reciprocal information flow between early processing areas and medial place area.

DISCUSSION

To summarize, we developed an approach for estimating network connectivity, dealing with the challenge of sparse individual coverage of intracranial EEG electrodes. Its application provided new insights into the interaction between the dorsal and ventral visual streams, one of the iconic dualities in human cognition.

Mapping the Scene and Object Processing Networks by Intracranial EEG

Human perception and cognition are based predominantly on visual information processing. Much of the information regarding neuronal correlates of visual processing has been derived from functional imaging studies, which have identified a variety of brain areas contributing to visual analysis, recognition, and processing of objects and scenes. However, only two of these areas, namely the parahippocampal place area (PPA) and the lateral occipital complex (LOC), were verified and further characterized by intracranial electroencephalogram (iEEG). iEEG is a unique measurement technique that samples a local neuronal population with high temporal and anatomical resolution. In the present study, we aimed to expand on previous reports and examine brain activity for selectivity of scenes and objects in the broadband high-gamma frequency range (50–150 Hz). We collected iEEG data from 27 epileptic patients while they watched a series of images, containing objects and scenes, and we identified 375 bipolar channels responding to at least one of these two categories. Using K-means clustering, we delineated their brain localization. In addition to the two areas described previously, we detected significant responses in two other scene-selective areas, not yet reported by any electrophysiological studies; namely the occipital place area (OPA) and the retrosplenial complex. Moreover, using iEEG we revealed a much broader network underlying visual processing than that described to date, using specialized functional imaging experimental designs. Here, we report the selective brain areas for scene processing include the posterior collateral sulcus and the anterior temporal region, which were already shown to be related to scene novelty and landmark naming. The object-selective responses appeared in the parietal, frontal, and temporal regions connected with tool use and object recognition. The temporal analyses specified the time course of the category selectivity through the dorsal and ventral visual streams. The receiver operating characteristic analyses identified the PPA and the fusiform portion of the LOC as being the most selective for scenes and objects, respectively. Our findings represent a valuable overview of visual processing selectivity for scenes and objects based on iEEG analyses and thus, contribute to a better understanding of visual processing in the human brain.

The use of egocentric and allocentric reference frames in static and dynamic conditions in humans

The dissociation between egocentric and allocentric reference frames is well established. Spatial coding relative to oneself has been associated with a brain network distinct from spatial coding using a cognitive map independently of the actual position. These differences were, however, revealed by a variety of tasks from both static conditions, using a series of images, and dynamic conditions, using movements through space. We aimed to clarify how these paradigms correspond to each other concerning the neural correlates of the use of egocentric and allocentric reference frames. We review here studies of allocentric and egocentric judgments used in static two- and three-dimensional tasks and compare their results with the findings from spatial navigation studies. We argue that neural correlates of allocentric coding in static conditions but using complex three-dimensional scenes and involving spatial memory of participants resemble those in spatial navigation studies, while allocentric representations in two-dimensional tasks are connected with other perceptual and attentional processes. In contrast, the brain networks associated with the egocentric reference frame in static two-dimensional and three-dimensional tasks and spatial navigation tasks are, with some limitations, more similar. Our review demonstrates the heterogeneity of experimental designs focused on spatial reference frames. At the same time, it indicates similarities in brain activation during reference frame use despite this heterogeneity.

Scopolamine disrupts place navigation in rats and humans: a translational validation of the Hidden Goal Task in the Morris water maze and a real maze for humans

RATIONALE

Development of new drugs for treatment of Alzheimer's disease (AD) requires valid paradigms for testing their efficacy and sensitive tests validated in translational research.

OBJECTIVES

We present validation of a place-navigation task, a Hidden Goal Task (HGT) based on the Morris water maze (MWM), in comparable animal and human protocols.

METHODS

We used scopolamine to model cognitive dysfunction similar to that seen in AD and donepezil, a symptomatic medication for AD, to assess its potential reversible effect on this scopolamine-induced cognitive dysfunction. We tested the effects of scopolamine and the combination of scopolamine and donepezil on place navigation and compared their effects in human and rat versions of the HGT. Place navigation testing consisted of 4 sessions of HGT performed at baseline, 2, 4, and 8 h after dosing in humans or 1, 2.5, and 5 h in rats.

RESULTS

Scopolamine worsened performance in both animals and humans. In the animal experiment, co-administration of donepezil alleviated the negative effect of scopolamine. In the human experiment, subjects co-administered with scopolamine and donepezil performed similarly to subjects on placebo and scopolamine, indicating a partial ameliorative effect of donepezil.

CONCLUSIONS

In the task based on the MWM, scopolamine impaired place navigation, while co-administration of donepezil alleviated this effect in comparable animal and human protocols. Using scopolamine and donepezil to challenge place navigation testing can be studied concurrently in animals and humans and may be a valid and reliable model for translational research, as well as for preclinical and clinical phases of drug trials.

Real-space path integration is impaired in Alzheimer's disease and mild cognitive impairment

INTRODUCTION

Path integration (PI) is an important component of spatial navigation that integrates self-motion cues to allow the subject to return to a starting point. PI depends on the structures affected early in the course of Alzheimer's disease (AD) such as the medial temporal lobe and the parietal cortex.

OBJECTIVES

To assess whether PI is impaired in patients with mild AD and amnestic mild cognitive impairment (aMCI) and to investigate the role of the hippocampus, entorhinal and inferior parietal cortex in this association.

METHODS

27 patients with aMCI, 14 with mild AD and 18 controls completed eight trials of Arena Path Integration Task. The task required subjects with a mask covering their eyes to follow an enclosed triangle pathway through two previously seen places: start-place1-place2-start. Brains were scanned at 1.5T MRI and respective volumes and thicknesses were derived using FreeSurfer algorithm.

RESULTS

Controlling for age, education, gender and Mini-Mental State Examination score the aMCI and AD subjects were impaired in PI accuracy on the pathway endpoint (p=0.042 and p=0.013) compared to controls. Hippocampal volume and thickness of entorhinal and parietal cortices explained separately 36-45% of the differences in PI accuracy between controls and aMCI and 28-31% of the differences between controls and AD subjects.

CONCLUSIONS

PI is affected in aMCI and AD, possibly as a function of neurodegeneration in the medial temporal lobe structures and the parietal cortex. PI assessment (as a part of spatial navigation testing) may be useful for identification of patients with incipient AD.

Comparison of Visuospatial and Verbal Abilities in First Psychotic Episode of Schizophrenia Spectrum Disorder: Impact on Global Functioning and Quality of Life

Objectives: Deficit in visuospatial functions can influence both simple and complex daily life activities. Despite the fact that visuospatial deficit was reported in schizophrenia, research on visuospatial functions as an independent entity is limited. Our study aims to elucidate the impact of visuospatial deficit in comparison with verbal deficit on global functioning and quality of life in the first psychotic episode of schizophrenia spectrum disorder (FES). The significance of clinical symptoms and antipsychotic medication was also studied.

Methods: Thirty-six FES patients and a matched group of healthy controls (HC group) were assessed with a neuropsychological battery focused on visuospatial (VIS) and verbal (VERB) functions. Using multiple regression analysis, we evaluated the cumulative effect of VERB and VIS functions, psychiatric symptoms (PANSS) and antipsychotic medication on global functioning (GAF) and quality of life (WHOQOL-BREF) in the FES group.

Results: The FES group demonstrated significant impairment both in VIS and VERB cognitive abilities compared to the HC group. Antipsychotic medication did not significantly affect either VIS or VERB functioning. PANSS was not related to cognitive functioning, apart from the Trail Making Test B. In the FES group, the GAF score was significantly affected by the severity of positive symptoms and VERB functioning, explaining together 60% of GAF variability. The severity of negative and positive symptoms affected only the Physical health domain of WHOQOL-BREF. The degree of VERB deficit was associated with both Physical and Psychological health. Although we did not find any relation between VIS functioning, GAF, and WHOQOL-BREF, a paradoxical finding emerged in the Environment quality domain, where a worse quality of the environment was associated with better VIS functioning.

Conclusions: Our results suggest that the deficit in VIS functions is an integral part of cognitive deficit in schizophrenia spectrum disorders, rather than a side effect of symptomatology or antipsychotic medication. Moreover, VERB functioning was a better predictor of GAF and WHOQOL-BREF than VIS functioning. Given the findings of negative or missing effect of VIS deficit on WHOQOL-BREF and GAF, the accuracy of these measures in evaluating the impact of global cognitive deficit on everyday life in schizophrenia could be questioned.

Famous landmark identification in amnestic mild cognitive impairment and Alzheimer's disease

BACKGROUND

Identification of famous landmarks (FLI), famous faces (FFI) and recognition of facial emotions (FER) is affected early in the course of Alzheimer's disease (AD). FFI, FER and FLI may represent domain specific tasks relying on activation of distinct regions of the medial temporal lobe, which are affected successively during the course of AD. However, the data on FFI and FER in MCI are controversial and FLI domain remains almost unexplored.

OBJECTIVES

To determine whether and how are these three specific domains impaired in head to head comparison of patients with amnestic MCI (aMCI) single domain (SD-aMCI) and multiple domain (MD-aMCI). We propose that FLI might be most reliable in differentiating SD-aMCI, which is considered to be an earlier stage of AD pathology spread out, from the controls.

PATIENTS AND METHODS

A total of 114 patients, 13 with single domain (SD-aMCI) and 30 with multiple domains (MD-aMCI), 29 with mild AD and 42 controls underwent standard neurological and neuropsychological evaluations as well as tests of FLI, FER and FFI.

RESULTS

Compared to the control group, AD subjects performed worse on FFI (p = 0.020), FER (p<0.001) and FLI (p<0.001), MD-aMCI group had significantly worse scores only on FLI (p = 0.002) and approached statistical significance on FER (0.053). SD-aMCI group performed significantly worse only on FLI (p = 0.028) compared to controls.

CONCLUSIONS

Patients with SD-aMCI had an isolated impairment restricted to FLI, while patients with MD-aMCI showed impairment in FLI as well as in FER. Patients with mild dementia due to AD have more extensive impairment of higher visual perception. The results suggest that FLI testing may contribute to identification of patients at risk of AD. We hypothesize that clinical examination of all three domains might reflect the spread of the disease from transentorhinal cortex, over amygdala to fusiform gyrus.

Risk factors for spatial memory impairment in patients with temporal lobe epilepsy

At present, the risk factors for world-centered (allocentric) navigation impairment in patients with temporal lobe epilepsy (TLE) are not known. There is some evidence on the importance of the right hippocampus but other clinical features have not been investigated yet. In this study, we used an experimental human equivalent to the Morris water maze to examine spatial navigation performance in patients with drug-refractory unilateral TLE. We included 47 left-hemisphere speech dominant patients (25 right sided; 22 left sided). The aim of our study was to identify clinical and demographic characteristics of TLE patients who performed poorly in allocentric spatial memory tests. Our results demonstrate that poor spatial navigation is significantly associated with younger age at epilepsy onset, longer disease duration, and lower intelligence level. Allocentric navigation in TLE patients was impaired irrespective of epilepsy lateralization. Good and poor navigators did not differ in their age, gender, or preoperative/postoperative status. This study provides evidence on risk factors that increase the likelihood of allocentric navigation impairment in TLE patients. The results indicate that not only temporal lobe dysfunction itself but also low general cognitive abilities may contribute to the navigation impairment.

Spatial navigation-a unique window into physiological and pathological aging

Spatial navigation is a skill of determining and maintaining a trajectory from one place to another. Mild progressive decline of spatial navigation develops gradually during the course of physiological ageing. Nevertheless, severe spatial navigation deficit can be the first sign of incipient Alzheimer's disease (AD), occurring in the stage of mild cognitive impairment (MCI), preceding the development of a full blown dementia. Patients with amnestic MCI, especially those with the hippocampal type of amnestic syndrome, are at very high risk of AD. These patients present with the same pattern of spatial navigation impairment as do the patients with mild AD. Spatial navigation testing of elderly as well as computer tests developed for routine clinical use thus represents a possibility for further investigation of this cognitive domain, but most of all, an opportunity for making early diagnosis of AD.

From Morris Water Maze to computer tests in the prediction of Alzheimer's disease

BACKGROUND

Spatial navigation performance in the Hidden Goal Task (HGT), a real-space human analogue of the Morris Water Maze, can identify amnestic mild cognitive impairment (aMCI) patients with memory impairment of the hippocampal type, a known indicator of incipient Alzheimer's disease (AD).

OBJECTIVE

Contrast results from computer versus real-space versions of the HGT.

METHODS

A total of 42 aMCI patients were clinically and neuropsychologically classified into: (1) memory impairment of the hippocampal type--the hippocampal aMCI (HaMCI; n = 10) and (2) isolated retrieval impairment--the nonhippocampal aMCI (NHaMCI; n = 32). Results were compared to the control (n = 28) and AD (n = 21) groups.

RESULTS

The HaMCI group, although similar to the NHaMCI group with respect to overall cognitive impairment, performed poorer on the computer version of the HGT and yielded parallel results to the real-space version. The two versions were strongly correlated.

CONCLUSIONS

Both versions of the HGT can reliably identify aMCI with pronounced memory impairment of the hippocampal type. The computer version of the HGT may be a useful, relatively inexpensive screening tool for early detection of individuals at a high risk of AD.

Spatial navigation testing discriminates two types of amnestic mild cognitive impairment

The hippocampus is essential for consolidation of declarative information and spatial navigation. Alzheimer's disease (AD) diagnosis tends to be preceded by a long prodromal period and mild cognitive impairment (MCI). Our goal was to test whether amnestic MCI comprises two different subgroups, with hippocampal and non-hippocampal memory impairment, that vary with respect to spatial navigation ability. A total of 52 patients were classified into two subgroups: non-amnestic MCI (naMCI) (n=10) and amnestic MCI (aMCI) (n=42). The aMCI subgroup was further stratified into memory impairment of hippocampal type-hippocampal aMCI (HaMCI) (n=10) (potential preclinical AD) and isolated retrieval impairment-non-hippocampal (NHaMCI) (n=32). Results were compared to control (n=28) and AD (n=21) groups. We used the Hidden Goal Task, a human analogue of the Morris Water Maze, to examine spatial navigation either dependent (egocentric) or independent of individual's position (allocentric). Overall, the HaMCI group performed poorer on spatial navigation than the NHaMCI group, especially in the latter trials when the HaMCI group exhibited limited capacity to learn and the NHaMCI group exhibited a learning effect. Finally, the HaMCI group performed almost identically as the AD group. Spatial navigation deficit is particularly pronounced in individuals with hippocampus-related memory impairment and may signal preclinical AD.

Subtype-dependence of N-methyl-d-aspartate receptor modulation by pregnenolone sulfate

N-methyl-D-aspartate receptors play a critical role in synaptogenesis, synaptic plasticity, and excitotoxicity. They are heteromeric complexes of NR1 combined with NR2A-D and/or NR3A-B subunits. The subunit composition determines the biophysical and pharmacological properties of the N-methyl-D-aspartate receptor channel complex. In this study, we report that responses mediated by recombinant rat N-methyl-D-aspartate receptors expressed in human embryonic kidney HEK293 cells are differentially affected by naturally occurring neurosteroid pregnenolone sulfate. We show that responses induced by 1mM glutamate in NR1-1a/NR2A and NR1-1a/NR2B receptors are potentiated five- to eight-fold more by pregnenolone sulfate than responses of NR1-1a/NR2C and NR1-1a/NR2D receptors with no differences in the concentration of pregnenolone sulfate that produced 50% potentiation. In addition to potentiation, pregnenolone sulfate also has an inhibitory effect at recombinant N-methyl-D-aspartate receptors, with values of the concentration of pregnenolone sulfate that produces 50% inhibition of NR1/NR2D=NR1/NR2C<NR1/NR2B<NR1/NR2A. In addition, we show that the structure of the extracellular loop between the third and fourth transmembrane domains of the NR2 subunit is critical for both the potentiating and inhibitory effects of pregnenolone sulfate. The modulatory effects of pregnenolone sulfate are consistent with a model in which this neurosteroid acts at two distinct binding sites on the N-methyl-D-aspartate receptor. These data provide insight into the mechanisms by which pregnenolone sulfate and related sulfated neurosteroids modulate activity of N-methyl-D-aspartate receptor channels.

Molecular mechanism of pregnenolone sulfate action at NR1/NR2B receptors

NMDA receptors are highly expressed in the CNS and are involved in excitatory synaptic transmission and synaptic plasticity as well as excitotoxicity. They have several binding sites for allosteric modulators, including neurosteroids, endogenous compounds synthesized by the nervous tissue and expected to act locally. Whole-cell patch-clamp recording from human embryonic kidney 293 cells expressing NR1-1a/NR2B receptors revealed that neurosteroid pregnenolone sulfate (PS) (300 microm), when applied to resting NMDA receptors, potentiates the amplitude of subsequent responses to 1 mm glutamate fivefold and slows their deactivation twofold. The same concentration of PS, when applied during NMDA receptor activation by 1 mm glutamate, has only a small effect. The association and dissociation rate constants of PS binding and unbinding from resting NMDA receptors are estimated to be 3.3 +/- 2.0 mm(-1)sec(-1) and 0.12 +/- 0.02 sec(-1), respectively, corresponding to an apparent affinity K(d) of 37 microm. The results of experiments indicate that the molecular mechanism of PS potentiation of NMDA receptor responses is attributable to an increase in the peak channel open probability (P(o)). Responses to glutamate recorded in the continuous presence of PS exhibit marked time-dependent decline. Our results indicate that the decline is induced by a change of the NMDA receptor affinity for PS after receptor activation. These results suggest that the PS is a modulator of NMDA receptor P(o), the effectiveness of which is lowered by glutamate binding. This modulation may have important consequences for the neuronal excitability.

[Severe lead intoxication after ingestion of lead shots]

The authors describe severe lead intoxication in a male patient who swallowed about 20 lead shots by accident. It caused an acute lead intoxication with highest blood lead reaching about 2.4 fold value of biological exposure limit for blood lead concentration for occupational exposure (0.97 mg/l), coproporphyrines in urine reaching 30 fold increase of biological exposure limit (1000 nmol/mmol creatinine), and 5-aminolevulic acid about 2.7 fold increase of biological limit (35.0 micromol/mmol creatinine). After first dose of chelating antidote (calcium disodium edetate, EDTA) the patient excreted 9.0 mg of lead in urine during 24 hours. Clinical symptoms and results of examinations led to suspicion of gastroduodenal ulcer at first. Diagnosis was defined after detailed examination and completing of the patient's history. Typical symptoms of intoxication developed--normocytic normochromic anemia and saturnine colics. Elimination of shots from digestive tract and treatment with 8 doses of antidote led to crucial change and improvement in the course of one month. The article should serve as an instruction to early recognition of lead intoxication.

Allothetic orientation and sequential ordering of places is impaired in early stages of Alzheimer's disease: corresponding results in real space tests and computer tests

Spatial disorientation and learning problems belong to the integral symptoms of Alzheimer's disease (AD). A circular arena for human subjects (2.9 m diameter, 3 m high) was equipped with a computerized tracking system, similar to that used in animals. We studied navigation in 11 subjects diagnosed with early stages of Alzheimer's disease (AD), 27 subjects with subjective problems with memory or concentration, and 10 controls. The task was to locate one or several unmarked goals using the arena geometry, starting position and/or cues on the arena wall. Navigation in a real version and a computer map view version of the tests yielded similar results. The AD group was severely impaired relative to controls in navigation to one hidden goal in eight rotated positions. The impairment was largest when only the cues on the wall could be used for orientation. Also, the AD group recalled worse than controls the order of six sequentially presented locations, though they recalled similarly to controls the positions of the locations. The group with subjective problems was not impaired in any of the tests. Our results document the spatial navigation and non-verbal episodic memory impairment in the AD. Similar results in real and map view computer tests support the use of computer tests in diagnosis of cognitive disturbances.

Intracellular spermine decreases open probability of N-methyl-D-aspartate receptor channels

Spermine and related polyamines have been shown to be endogenous regulators of several ion channel types including ionotropic glutamate receptors. The effect of spermine on N-methyl-d-aspartate (NMDA) receptors in cultured rat hippocampal neurons was studied using single-channel and whole-cell patch clamp recordings. Intracellular spermine resulted in the dose-dependent inhibition of NMDA-induced responses. Spermine reversibly inhibited the single NMDA receptor channel activity in inside-out patches suggesting a membrane-delimited mechanism of action. Open probability of NMDA receptor channels was decreased in a dose-dependent manner. Mechanism of spermine-induced inhibition of NMDA receptor was different from that of intracellular Ca(2+)-induced NMDA receptor inactivation. Both pharmacological studies and single channel analysis indicate that in contrast to the effect of extracellular spermine the intracellular spermine effect is not dependent on the NMDA receptor subunit composition. We propose that intracellular spermine has a direct inhibitory effect on NMDA receptors that is different from calcium-induced NMDA receptor inactivation and spermine-induced voltage-dependent inhibition of AMPA/kainate receptors. Spermine-induced tonic change in the open probability of NMDA receptor channels may play a role in mechanisms underlying short-term changes in the synaptic efficacy.