Increased Intrinsic Activity of Medial-Temporal Lobe Subregions is Associated with Decreased Cortical Thickness of Medial-Parietal Areas in Patients with Alzheimer’s Disease Dementia

Abstract

In Alzheimer’s disease (AD), disrupted connectivity between medial-parietal cortices and medial-temporal lobes (MTL) is linked with increased MTL local functional connectivity, and parietal atrophy is associated with increased MTL memory activation. We hypothesized that intrinsic activity in MTL subregions is increased and associated with medial-parietal degeneration and impaired memory in AD. To test this hypothesis, resting-state-functional and structural-MRI was assessed in 22 healthy controls, 22 mild cognitive impairment patients, and 21 AD-dementia patients. Intrinsic activity was measured by power-spectrum density of blood-oxygenation-level-dependent signal, medial-parietal degeneration by cortical thinning. In AD-dementia patients, intrinsic activity was increased for several right MTL subregions. Raised intrinsic activity in dentate gyrus and cornu ammonis 1 was associated with cortical thinning in posterior cingulate cortices, and at-trend with impaired delayed recall. Critically, increased intrinsic activity in the right entorhinal cortex was associated with ipsilateral posterior cingulate degeneration. Our results provide evidence that in AD, intrinsic activity in MTL subregions is increased and associated with medial-parietal atrophy. Results fit a model in which medial-parietal degeneration contributes to MTL dysconnectivity from medial-parietal cortices, potentially underpinning disinhibition-like changes in MTL activity.

Keywords

Alzheimer’s disease atrophy functional magnetic resonance imaging hippocampus medial-parietal cortex medial-temporal lobe neurodegeneration neuroimaging physiopathology resting-state activity

INTRODUCTION

Medial-temporal lobe (MTL) pathophysiology plays a central role in Alzheimer’s disease (AD) [1 –4]. Based on the high degree of ‘normal’ MTL-parietal cortex interactions via prominent structural and functional connectivity, MTL pathophysiology has been suggested to interact with parietal cortex pathophysiology in AD [5 –8]. In particular, the current study suggests that intrinsic activity in MTL subregions is increased in AD, with increases being associated with degeneration in medial-parietal cortices.

The MTLs are characterized by the hippocampal-entorhinal system, which subserves navigation and memory functions via highly coordinated oscillatory dynamics among subregions [9]. The hippocampal-entorhinal system includes a main excitatory feedforward loop of subregions, starting from lower layers of entorhinal cortex (EC) to dentate gyrus (DG), cornu ammonis (CA) 3, CA1, and finally to upper layers of EC [10]. This loop interacts extensively with two cortical systems, whereas one of these two systems is mainly constituted by posterior cingulate cortex and precuneus i.e., medial-parietal cortices [11].

In AD, MTL pathophysiology is characterized by tau-based pathological changes, structural degradation and substantial cell loss; amyloid-β-based pathology is less prominent— at least in early stages of the disease [1, 4]. Tau-based pathological changes and substantial cell loss start typically in the transition zone between EC and adjacent temporal cortices, then spreading to limbic (including MTLs) and, later, to cortical regions [1]. Start of MTL changes is commonly accompanied by the start of first clinical symptoms such as impaired memory [1]. During memory processes, MTL activity particularly in the hippocampus, is paradoxically increased in disease stages of mild cognitive impairment (MCI) [12]. Such activity increase seems to be dysfunctional since both hyperactivity and memory performance normalize after anti-epileptic medication [13]. Beyond memory related activity, intrinsic activity within the MTL (i.e., local intrinsic functional connectivity), is progressively increased in MCI and dementia stages of AD, with coherence increases being linked with impaired memory function [6 , 14–16].

MTL pathophysiology of AD seems to be linked with both cortical pathology and MTL dysconnectivity from the cortex. In patients with MCI, increased hippocampus activation during memory processing is associated with cortical thinning in parietal lobes [17], and in cognitive normal older adults, the amount of cortical amyloid-β pathology measured by in vivo PET correlates with decreased functional connectivity of the perirhinal cortex within the MTL [18]. The MTLs are part of the so-called default mode network, which is characterized by functional connectivity of intrinsic activity in MTLs, medial-prefrontal, medial-parietal and lateral-parietal cortices [19]. In patients with AD-dementia, intrinsic functional connectivity between hippocampus and medial-parietal parts of the default mode network is reduced, and such decrease is associated with both hippocampus increased local intrinsic functional connectivity and glucose metabolism [6, 7]. Furthermore, in a longitudinal study in patients with early AD, baseline hippocampus atrophy was associated with decreasing cingulum bundle integrity (the cingulum bundle is a white matter tract connecting the MTLs with medial-parietal parts of the default mode network), and cingulum bundle integrity itself was linked to decreasing medial-parietal glucose metabolism along disease progression, supporting a consistent interaction between MTL and medial-parietal pathophysiology in AD [5].

Based on these studies, we suggest increased MTL intrinsic activity in early AD. Such activity increases might be of disinhibition-like nature due to both local and remote factors. Candidates for local factors are pathological alterations like tau- or amyloid-β pathology, whereas both tau and amyloid-β have been demonstrated –at least in animal models of AD –to induce epileptiform discharges at local network level [20 –22]. Candidates for remote factors are pathological changes in medial-parietal cortical regions and related MTL dysconnectivity, which may include dysfunctional signals (e.g., discharges or signal loss) [23] that in turn contribute to misbalanced excitation and inhibition of MTL’s main excitatory loop from EC to hippocampus and back. The current study tested several aspects of this model. First, we hypothesized that in early AD, MTL subregions, which build up the main excitatory loop, have increased intrinsic activity. Second, we expected intrinsic activity increases to be associated with cortical degeneration in medial-parietal regions of the default mode network. Finally, we suggested that MTL intrinsic activity increases are linked with patients’ memoryimpairment.

MATERIALS AND METHODS

Overview

We used structural MRI and resting-state-fMRI with both whole-brain and partial-brain field-of-view in healthy controls (HC) and patients with MCI andAD-dementia. Partial-brain fMRI data were of increased spatial resolution (in comparison to whole-brain fMRI) with a limited field-of-view focused on the MTL and medial-parietal cortices. Imaging data have been analyzed previously but with a focus on local and global intrinsic functional connectivity within the hippocampus and larger default mode network [6]. In contrast to the previous analysis, the current study focuses on intrinsic activity within MTL subregions (i.e., subregions constituting the hippocampus but also adjacent regions such as entorhinal cortex) and its relation to medial-parietal atrophy of the default mode network. By the use of automated brain parcellation of each subject’s structural MRI data, regions-of-interest were defined in MTLs and medial-parietal cortices of the default mode network, whereas the default mode network was defined by the analysis of whole brain resting-state-fMRI data. Outcome measures were (1) the power spectrum density of low frequency fluctuations (PSD of LFF) in the blood-oxygenation-level-dependent (BOLD) signal of partial-brain fMRI data [24] for intrinsic activity in MTL subregions and (2) cortical thickness for structural degeneration of medial-parietal cortices. ANOVA was used for group comparisons in both PSD of LFF and cortical thickness, and Spearman correlation coefficients to analyze the link between intrinsic activity and both cortical thinning and memory performance (i.e., delayed recall scores). Across-subject analyses were controlled for MTL subregions’ atrophy to ensure the functional nature of potential PSD changes and for multiple comparisons to account for errors in inferences due to simultaneous multiple testing.

Participants

22 HC (16 females, age 56–85), 22 MCI patients (11 females, ages 48–80) and 21 AD-dementia patients (13 females, ages 57–83) participated in this study (Table 1). The study was approved and registered by the medical ethical board of Technische Universität München (TUM) in line with Human Research Committee guidelines of TUM. All participants provided informed consent in accordance with the standard protocol approvals. Patients were recruited from the Memory Clinic of the Department of Psychiatry, controls by word-of-mouth advertising. Examination of every participant included medical history, neurological examination, informant interview (Clinical Dementia Rating, CDR) [25], neuropsychological assessment (Consortium to establish a registry for Alzheimer’s disease, CERAD battery) [26], structural MRI and blood tests (for patients only). MCI patients (CDR-global = 0.5) met criteria for MCI including reported and neuropsychologically assessed cognitive impairments, largely intact activities of daily living, and excluded dementia [27]. AD patients fulfilled criteria for mild dementia (CDR-global = 1) and the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer’s Disease and Related Disorders Association (NINCIDS-ADRDA) criteria for AD [28]. Exclusion criteria for entry into the study were other neurological, psychiatric, or systemic diseases (e.g., stroke, depression, alcoholism) or clinically remarkable structural MRI alterations (e.g., stroke lesions) potentially related to cognitive impairment. 9 AD patients/8 MCI patients/6 healthy controls were treated for hypertension (Beta-blockers, ACE-inhibitors, and Calcium channel blockers), 4/5/3 for hypercholesterolemia (statins), 2/2/0 had diabetes mellitus, 6/3/0 received antidepressant medication (Mirtazapine, Citalopram), and 21/0/0 received cholinesterase inhibitors.

MRI data acquisition

All participants underwent structural MRI, 10 min of whole-brain resting-state-fMRI and 6 min of partial-brain resting-state-fMRI. For resting-state-fMRI, subjects were instructed to keep their eyes closed and not to fall asleep. We verified that subjects stayed awake by interrogating via intercom immediately after each scan. MRI was performed on a 3T MRI scanner using 8-channel phased-array head coil (Achieva, Philips, Netherland). Whole-brain resting-state-fMRIused a gradient echo EPI sequence (TE = 35 ms; TR = 2000 ms; flip angle = 82°; FoV = 220×220 mm²; matrix = 80×80; 32 slices; slice thickness = 4 mm; 0 mm interslice gap). Partial-brain resting-state-fMRI used a gradient echo EPI sequence (TE = 35 ms; TR =2000 ms; flip angle = 82°; FoV = 220×196.4 mm²; matrix = 112×96; 26 slices; slice thickness = 2 mm; 0.2 mm interslice gap) and we used a limited field-of-view centered on MTL to increase spatial resolution of the data (see reference [6] for more details). To optimally cover MTL, the field-of-view orientation of resting-state-fMRI scans was systematically adapted for each individual. T1-weighted anatomical data were obtained by using a magnetization-prepared rapid acquisition gradient echo sequence (TE = 4 ms, TR =9 ms, TI = 100 ms, flip angle = 5°, FoV = 240×240 mm², matrix = 240×240, 170 slices, voxel size =1.0×1.0×1.0 mm³).

Parcellation and volumetry of MTL and parietal subregions

For each participant, structural MRI data were parcellated in an automated mode as implemented by FreeSurfer software (http://surfer.nmr.mgh.harvard.edu) (see Supplementary Material for more details). FreeSurfer provides a highly automated structural MRI image-processing pipeline resulting in regional parcellation of subcortical and cortical regions including MTL (i.e., entorhinal cortex and hippocampus subregions), and isocortex [29]. FreeSurfer provides volumetric and thickness estimates for parcellated regions [30]. Manually based segmentation and volumetry of MTL subregions in patients with MCI and AD are comparable with FreeSurfer-based procedures [31]. FreeSurfer segmentation was thoroughly inspected for each subject. Quality checks included extensive visual inspection of pial and white matter boundaries along the cortical mantle as well as location and extend of segmented medial-parietal and MTL regions-of-interest for each subject. Furthermore, we compared both volumetry of MTL subregions and cortical thickness of medial-parietal regions-of-interest, with estimates derived from existing studies involving both automatic and manual segmentation procedures in healthy subjects and patients (see also Supplementary Tables 1 and 2). None of the subjects failed for these criteria and was excluded from the analysis.

Figure 1A and B show the current study’s regions-of-interest based on FreeSurfer analysis. MTL regions-of-interest included bilateral parahippocampal cortices (PHC), EC, DG-CA4, CA2-CA3, CA1, subiculum, and presubiculum [32]. In the following, we abbreviate DG-CA4 and CA2-CA3 with DG and CA3, respectively. Medial-parietal regions-of-interest included bilateral dorsal posterior cingulate cortex (PCC), ventral PCC, and precuneus. These areas are part of the medial posterior default mode network, which is primarily affected by earliest changes in AD such as amyloid plaque deposition, hypometabolism, and atrophy [33, 34]. For each subject, overlap with the default mode network was verified by comparing individual segmented medial-parietal regions with individual default mode networks. More specifically, we used subjects’ previously defined individual posterior default mode network maps (see reference [6]) and compared them with individually segmented ventral and dorsal PCC and precuneus (example in Fig. 1B). Individual default mode maps were derived from group-independent component analysis of whole-brain resting-state-fMRI data and back reconstructed from the group map into individual subject space. Overlap was counted as relevant when the network covered more than 75% of segmented areas, with none of the subjects failing this criterion.

FMRI data analysis: Preprocessing and PSD of LFF

Preprocessing. After discarding the first three volumes of each person’s partial-brain fMRI scans, data were motion corrected, spatially smoothed (3×3×3 mm³ Gaussian kernel), and co-registered with anatomical data. To control for motion-induced artifacts, point-to-point head motion was estimated for each participant [35]. Excessive head motion (cumulative translation or rotation >3 mm or 3° and mean point-to-point translation or rotation >0.15 mm or 0.1°) was applied as initial exclusion criterion. None of the participants had to be excluded. ANOVA and related post-hoc t-test yielded no significant differences between groups regarding translational and rotational head movements of any direction. In the same way, root mean square of the translational ad rotational head movement parameters was not different across groups. Furthermore, we replicated our analysis of PSD of LFF on “scrubbed” fMRI data, which were censored for movement-induced artifacts via exclusion of volumes potentially contaminated by movement [35]. Censoring was based on the root mean square of the translational and rotational head movement parameters reflecting volume-to-volume head displacement. We identified volumes were the root mean square of translational and rotational head movement parameters exceeded a specific threshold (0.25 mm + 2 standard deviations of all subjects) and removed them from further analyses [36] (see Supplementary Material for more details and results, Supplementary Figure 1).

Outcome measure PSD of LFF. For each subject, movement parameters in six directions (linear and rotational), the global grey matter, white matter, and cerebrospinal fluid signal were extracted from partial-brain resting-state-fMRI data by linear regression. As the global grey matter signal is thought to reflect a combination of physiological processes (including cardiac and respiratory fluctuations) and scanner drift, it was extracted as a nuisance signal [37]. To get time series for grey matter, white matter, and cerebrospinal fluid, each individual’s segmented T1-weighted image was segmented to define corresponding tissues. Time courses of each voxel within each region-of-interest were extracted and bandpass-filtered (0.009 to 0.09 Hz). To derive a representative time course of the region-of-interest, the BOLD signal of all voxels of the region-of-interest at a single time point was multiplied with the probability of corresponding voxels to belong to the specific region-of-interest (based on the probabilistic maps provided by FreeSurfer). Weighted time courses were averaged across voxels in the concerned region-of-interest. The power spectrum density was subsequently calculated for each time course using the Welch’s method (TR: 0.5 Hz; windows: 8; overlap: 50% ) [24]. To control for atrophy, we regressed out individual volumes of MTL subregions from the corresponding spectra for each subject via regression analysis. For quantification, the spectra were divided into three frequency bands (0.00–0.03; 0.03–0.06 and 0.06–0.09 Hz) and the integral calculated following the approach of Malinen et al. [24]. A frequency band specific approach was chosen since a recent study indicated selective changes across different frequency bands of intrinsic activity in early AD [38].

Cortical thickness asymmetry index

To foreshadow results, we found mainly right-sided MTL subregions of increased intrinsic activity in dementia patients. We suggested that such asymmetric activity increase in MTL might be associated with asymmetric medial-parietal cortex atrophy. To evaluate the specificity of the relationship between increased intrinsic MTL activity and medial-parietal cortex degeneration in AD, we investigated whether asymmetric inter-hemispheric cortical thinning in medial-parietal regions could account for lateralized increased PSD of LFF in right MTL subregions. Therefore, we defined an Asymmetry Index (AsI) for thickness of medial-parietal region-of-interests: $AsI = (RCTr - RCTl) / (RCTr + RCTl)$ where RCTr is the relative cortical thickness of the right region-of-interest and RCTl is the relative cortical thickness of the homologous left region-of-interest. Relative cortical thickness measures where derived by dividing the cortical thickness of a region-of-interest with its volume. AsI ranges between 1 and –1, with positive values indicating a right >left asymmetry and negative values indicating a right <left asymmetry.

Statistical analysis

Comparison between patients and control regarding PSD of LFF in MTL subregions were performed in a one-way ANOVA model with subsequent post-hoc t-tests (p < 0.05, FWE corrected for multiple comparisons). Spearman correlation (p < 0.05) was used to relate PSD of LFF in MTL subregions to both cortical thickness of medial-parietal regions-of-interest and delayed recall scores in patients with AD-dementia only. Correlation analysis was restricted to MTL subregions with increased PSD in patients.

RESULTS

Progressively reduced volumes in MTL subregions and cortical thinning in medial-parietal areas in patients

Cortical thickness of medial-parietal regions-of-interest and volumes of MTL subregions of healthy controls were comparable with thickness and volume estimates from previous studies using FreeSurfer and comparable to volumes from manual segmentations of the EC and PHC (Supplementary Tables 1 and 2) [32 , 40]. Both progressive volume loss of MTL subregions and cortical thinning of medial-parietal areas were present in MCI and AD (Supplementary Tables 1 and 2) and in line with previous studies [41 –43].

Increased PSD of LFF in MTL subregions in patients with AD

For each subject, PSD of LFF was calculated for 14 MTL regions-of-interest by the use of extracted time series of BOLD signal with power spectra being divided into three frequency bands (0.00–0.03; 0.03–0.06 and 0.06–0.09 Hz) and adjusted for corresponding atrophy levels. Comparison between patients and controls were performed by a one-way ANOVA (p < 0.05 family wise error corrected for multiple comparisons; Fig. 2). We found increased PSD of LFF in AD patients compared to healthy controls and MCI patients in the left CA1 and right DG, CA3, CA1, and EC (Fig. 2 A1, B1–B5). For right CA1 and EC all frequency bands were concerned; for right CA3 the two highest frequency bands were concerned; for right DG and left CA1 the two lowest frequency bands were concerned. We did not find any significant changes of PSD in MCI patients. One should note that performing all these group comparisons but without controlling for subregions volume yields consistently stronger group differences for MTL subregions PSD, indicating relevant effects of subregional volume loss on subregional activity increases. For example, we found an increased number of MTL subregions with significant group differences in PSD (right DG 0.06–0.09 Hz AD versus HC; right CA3 0.06–0.09 Hz AD versus MCI) as indicated by Supplementary Figure 2. Furthermore, in right parahippocampal cortex or left DG there was no PSD difference across groups, indicating that observed increase of PSD is specific for mentioned MTL subregions and not a general phenomenon in AD.

To control for potential movement-induced effects, we implemented a second analysis with alternative movement control based on “censoring” procedures (see Supplementary Material). Results of control analyses are consistent with the current findings, demonstrating robustness against movement effects (Supplementary Figure 1).

Increased PSD of LFF in MTL subregions correlates with thinning of the medial-parietal cortex

To investigate the link between MTL increased intrinsic activity and medial-parietal degeneration, we correlated PSD of LFF and cortical thickness of medial-parietal regions, restricted to MTL subregions of significantly increased PSD in AD (Fig. 3A; Spearman correlation, p < 0.05). In AD, increased PSD of LFF of right CA1 and DG correlated with cortical thinning of right and left ventral PCC, respectively, but the relationship was not significant after multiple-comparison correction controlling for family wise error.

To test for more specific links between cortical degeneration and increased MTL intrinsic activity, we investigated whether lateralized increases in intrinsic MTL activity correspond with asymmetrically pronounced medial-parietal degeneration. First, we found positive Asymmetry Index (AsI) for the ventral PCC, indicating stronger right-sided degeneration in the vPCC (AsI [SD] = 0.09 [0.14]; p < 0.05 FWE corrected), while dorsal PCC showed negative AsI, indicating stronger left sided degeneration (AsI [SD] = –0.05 [0.06]; p < 0.05 FWE corrected). Second, we found that increased PSD of LFF in the right EC correlated negatively with cortical thickness AsI of the ventral PCC (Fig. 3B, Spearman correlation, p < 0.05 uncorrected). This result suggests that the stronger degeneration of right vPCC, the higher increased intrinsic activity of right EC. Further medial-parietal regions with significant negative AsI (i.e., dorsal PCC) did not correlate with raised intrinsic MTL activity.

Increased MTL intrinsic activity and reduced delayed recall in AD

We additionally investigated the relationship between memory impairments and PSD of LFF in patients with AD. Delayed recall scores from the CERAD battery were significantly lower in patients with AD-dementia (Table 1) and were used as a memory measure. Correlation analyses were restricted to MTL subregions with increased PSD of LFF and to AD-dementia patients. We found negative correlations between delayed recall and PSD of LFF of several MTL subregions with significant effects (p < 0.05) for the left CA1 (frequency bands 0.00–0.03 Hz and 0.03–0.06 Hz) and at-trend results (p < 0.10) for right DG (frequency bands 0.00–0.03 Hz and 0.03–0.06 Hz), CA3 (frequency band 0.03–0.06 Hz) and CA1 (frequency band 0.06–0.09 Hz) (Table 2). Relationship between left CA1 and delayed recall was not significant after multiple-comparison correction controlling for family wise error.

DISCUSSION

Power spectrum density of low frequency fluctuations in the BOLD signal of the left CA1 and right DG, CA3, CA1, and EC was increased in patients with AD-dementia. For right DG and CA1, increased intrinsic activity was associated with reduced cortical thickness in the ventral PCC and, at-trend, with impaired delayed recall. Particularly, increased intrinsic activity in the right EC was associated with ipsilateral PCC degeneration, suggesting a specific link between dysfunctional intrinsic MTL activity and medial-parietal degeneration. Results provide first evidence for both increased intrinsic activity in MTL subregions and its link with degeneration in medial-parietal cortices in AD. Data suggest that medial-parietal degeneration may contribute to disinhibition-like changes in MTL activity, possibly via parietal-MTL dysconnectivity.

Intrinsic activity is increased in MTL subregions

PSD of LFF was increased in the left and right CA1, right CA3, DG, and EC of patients with AD compared to MCI and healthy controls (Fig. 2).Findings were controlled for multiple testing, regional atrophy, and head movement, indicating robustness against these effects (Supplementary Figure 1). Particularly, adjustment for atrophy indicates the functional nature of observed PSD increases. Interestingly, PSD increases were more prominent in the right hemisphere (Fig. 2B1–5), concerning all subregions of MTL’s main excitatory loop from EC to DG, CA3, and CA1 back to EC [10]. This lateralized but total effect suggests that increase of MTL subregions intrinsic activity may be an all-or-nothing effect concerning all parts of excitatory MTL loop. In line with this finding, we found in the same patients, increased local functional connectivity of intrinsic hippocampus activity within MTL [6 , 15] and previous studies reported increased local functional connectivity among MTL subregions in patients with MCI and AD [6 , 14–16], which may indicate a ‘coherent’ increase of activity across MTL subregions.

While in right CA1 and EC, PSD increases in AD patients’ intrinsic activity were found for all frequency bands (0.00–0.03; 0.03–0.06 and 0.06–0.09 Hz), in right CA3 the two highest and in right DG and left CA1 the two lowest frequency bands were affected (Fig. 2). This result suggests specific effects on frequency bands of intrinsic MTL subregions activity in AD. Currently, though several studies report frequency-band specific effects on regional and network-specific intrinsic activity for several brain disorders such as Parkinson’s disease or chronic pain [24, 44], consensus about the function of different frequency bands in intrinsic BOLD fluctuations is missing. A recent study in early AD, combined functional connectivity of intrinsic activity at rest and effective connectivity of attention-related activity during flankers task, and found frequency-band specific impairments in patients for the relation between lower frequent functional connectivity at rest (e.g., 0.001–0.03 Hz) and effective bottom-up connectivity to prefrontal areas during attention [38]. Given this background, our result gives reason to suggest that in AD, frequency-band specific increase in MTL subregions intrinsic activity might be relevant for distinct processes in task-related activity such as memory. Future studies are necessary, which combine resting-state and memory-related fMRI in order to investigate such relationship.

We did not find increased PSD of LFF in MTL subregions of patients with MCI, albeit previous studies have reported changes in both task-related activity and coherence of intrinsic activity within the MTL of patients with MCI [6 , 17]. The lack of such finding might be due to several factors. First, in the same MCI patients, we found increased local hippocampus functional connectivity but significant less than in AD-dementia patients [6 , 15]. This result might indicate that while coherence of intrinsic activity is already increased in MCI, intrinsic activity of single subregions is still at normal levels. This finding suggests that coherence of intrinsic activity in the whole MTL subregions is more sensitive to MTL impairments than the power of intrinsic activity in single subregions. Second, we might fail to detect changes in MTL intrinsic activity in MCI due to potential heterogeneity of our MCI sample. Since MCI patients’ characterization was based on clinical-neuropsychological testing without the use of additional biomarkers such as positron emission tomography or liquor diagnostics, our sample could include other neurodegenerative diseases than AD, lowering power of our approach in MCI patients.

Increased PSD of several MTL subregions negatively correlated with delayed recall in AD patients. A significant effect was found for the left CA1 of AD patients, and a trend for such correlation was found for right DG, CA3, and CA1 (Table 2). Correlations did not survive correction for multiple comparisons, increasing the risk for false positive findings. Though being of weak significance, this result contains a reference that increased intrinsic activity in patients’ MTL subregions might be relevant for impaired memory performance. Interestingly, in a previous study with the same patients we found an analogous link between memory impairment and hippocampus raised local functional connectivity within the MTL [6]. This finding suggests that both intrinsic functional connectivity within the whole MTL and intrinsic activity of single MTL subregions, albeit reflecting different processes, are functionally meaningful measures of MTL activity at rest with both being related to memoryperformance.

In summary, we conclude that MTL subregions intrinsic activity is increased in AD; furthermore, data suggest that subregions of the main excitatory loop of the MTL might be affected as a whole and that their increased activity might contribute to impaired delayed recall in patients.

Link between increased intrinsic activity of MTL subregions and cortical thinning of medial-parietal cortices

We found that increased PSD of LFF of both the right DG and CA1 was negatively correlated with cortical thickness in ventral PCC (Fig. 3A). This finding fits previous results which link MTL activity/local functional connectivity increases and parietaldegradation/impaired parietal-hippocampal connectivity [6 , 17]. For example, in the same patients we found a link between raised local and eased global hippocampal functional connectivity in AD and MCI [6, 14]. In a sample of MCI patients whose cognitive impairment is comparable with that of the current AD sample, hippocampus hyperactivity during memory performance was associated with cortical thinning in parietal areas, in line with our results [17]. Similarly, a recent study demonstrated that in patients with mild AD, hippocampus glucose metabolism is higher when reductions in functional connectivity between hippocampus and medial posterior cortex are stronger [7].

Increased PSD of MTL subregions was largely lateralized to the right side (Fig. 2B1–5). We found further that such lateralized PSD increases link with asymmetric medial-parietal cortical degeneration (Fig. 3B). In more detail, significant asymmetry in cortical thickness was present in the ventral PCC, with relative higher cortical thickness in the right hemisphere compared to the left in patients. This result is in line with previous findings of right weighted thickness asymmetry in the PCC in healthy younger subjects and is even more pronounced in older adults [45 –47]. Critically, in AD patients, we found that the smaller thickness asymmetry in the PCC, the higher right EC intrinsic activity (Fig. 3B). This result indicates that relative stronger degenerative processes in the right PCC compared to the left PCC are associated with increased right EC activity. Having in mind that PCC and EC [48] are highly connected, this finding suggests a specific link between right hemispheric medial-parietal degeneration and increased right EC intrinsic activity.

Models of MTL pathophysiology in AD

When considering the bigger picture, our findings can be placed within a model, which we like to call MTL dysconnectivity hypothesis (precursors of this idea can be found in [2 , 49]). This hypothesis states that in AD, due to foregoing medial-parietal pathophysiology, disrupted medial-parietal input contributes to disinhibition-like changes in the EC-hippocampus circuit with detrimental consequences for MTL physiology and function. In this model, cortical pathology is driven by amyloid-β pathology, which starts years or decades before substantial MTL changes [1 , 50] and which affects preferentially the default mode network [51, 52]. MTL pathophysiology is driven by tau-pathology [4, 53], which is assumed to be facilitated by remote medial-parietal dysconnectivity.Dysconnectivity, in turn, consists of missing ordischarging signaling [23], and may contribute to misbalanced excitation/inhibition relationship within main MTL circuits with resulting disinhibition-like changes in MTL subregions. Our data can be interpreted in light of such a model, as we report increased intrinsic activity in MTL subregions, which affects all main regions of the right excitatory MTL circuit and which is associated with medial-parietal degeneration.

However, our findings could also be explained by alternative mechanisms driving pathological increases of intrinsic activity in MTL subregions. First, local versus remote factors: One should note that beside MTL dysconnectivity, local MTL pathology might also contribute to increased intrinsic activity. For instance, recent studies in rodents demonstrated amyloid-β-plaque-induced hyperactivity within the hippocampus [20 , 54]. Tau-pathology affects the hippocampus in very early AD and is associated with local network hyper-excitability in mice corresponding with seizure frequency [22]. Our data support such suggested influence of local pathology on intrinsic MTL activity, since we found that strength of group differences in MTL subregions intrinsic activity between controls and AD patients was reduced when accounting for subregions volume as co-variate-of-no-interest (Supplementary Figure 2). This group difference reduction indicates that subregions volume loss in patients (i.e., local substance loss due to assumed tau-associated pathology) is associated with subregions intrinsic activity. These data suggest that local and remote factors are relevant for increased intrinsic activity in the MTL circuit. Second, interaction between MTL and parietal pathophysiology: Based on our data (i.e., cross-sectional approach with analysis of correlation between MTL activity increase and parietal atrophy), we cannot exclude that MTL changes precedes medial-parietal degradation (see for example [5]) or that MTL pathology occurs independently from medial-parietal degeneration [4, 52]. For example, Villain and colleagues showed that baseline MTL atrophy is associated with the rate of cingulum degradation linking MTL and medial-parietal cortices, suggesting MTL pathological changes as cause for remote degeneration (‘classical hippocampus disconnection hypothesis’). Future longitudinal studies are necessary to carefully reveal the direction of effects between MTL and medial-parietal pathophysiology. In particular, future studies should relate characteristic measures for medial-parietal pathology as amyloid load with more specific measures for MTL dysconnectivity based e.g. on diffusion tensor imaging or on MTL intrinsic functional connectivity to medial-parietal brain areas.

In summary, our findings can be related to three models of MTL pathophysiology in AD-MTL dysconnectivity, local factor model, and hippocampus disconnection-which are probably not exclusive but rather complementary mechanisms involved in the degenerative process. Longitudinal multimodal studies, which integrate local and remote factors of MTL pathophysiology, are necessary to complete our picture about MTL changes in AD.

Limitations

Our study has several limitations. First, albeit strongly hypothesis driven, our approach is of cross-sectional nature, so no causation can be inferred from our findings regarding degenerative processes in AD. Second, MCI is an etiologically heterogeneous syndrome, it is possible that some of the current MCI patients did not suffer from AD. Although AD is the most frequent cause underlying MCI [55], results of patients with MCI should be interpreted carefully. Third, patients with AD-dementia were treated with cholinesterase inhibitors, which may influence BOLD activity and consequently correspondent differences in intrinsic activity [56]. Even if our results are consistent with both previous findings in dementia [6 , 17], they have to be interpreted carefully. Forth, the use of FreeSurfer-based anatomical parcellation is an objective method to identify subregions of interest particularly in the MTL. However, FreeSurfer-based MTL segmentation has not yet been validated in aging and neurodegenerative populations and manual segmentation is still considered as the golden standard for MTL subregional segmentation and volumetry [57, 58]. Fifth, our approach suffers from very general problems when comparing BOLD signals across groups of controls and patients, since observed group differences might stem from non-neuronal sources such as altered neurovascular coupling or movement artifacts [36, 59]. While differences in neurovascular coupling across groups are hard to control for, we accounted extensively for potential movement-induced artifacts. However, novel approaches enabling the computation of PSD of LFF on unequally sampled time points are needed when frame censoring is used to control for head movement artifacts.

CONCLUSION

In AD patients, intrinsic activity of MTL subregions is increased which is associated with medial-parietal cortex thinning. Medial-parietal degradation might contribute to disinhibition-like changes in the MTL.

Footnotes

ACKNOWLEDGMENTS

This work was supported by the German Federal Ministry of Education and Research (BMBF 01EV0710 to A.M.W., 01ER0803 to C.S.), the Alzheimer Forschungs-Initiative (AFI 08860 to V.R., 12819 to C.S.), the Wellcome Trust (to N.E.M.), and the Kommission für Klinische Forschung of the Klinikum rechts der Isar der Technischen Universität München (KKF 8765162 to C.S). We are grateful to the participants of the study and the staff of the Departments of Psychiatry and Neuroradiology for their help in recruitment and data collection.

Authors’ disclosures available online ().

Appendix

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