Phenotypic Variability in Autosomal Dominant Familial Alzheimer Disease due to the S170F Mutation of Presenilin-1

BACKGROUND

In rare cases, patients with Alzheimer disease (AD) present at an early age and with a family history suggestive of an autosomal dominant mode of inheritance. Mutations of the presenilin-1 (PSEN1) gene are the most common causes of dementia in these patients. Early-onset and particularly familial AD patients frequently present with variable non-amnestic cognitive symptoms such as visual, language or behavioural changes as well as non-cognitive, e.g. motor, symptoms.

OBJECTIVE

To investigate the phenotypic variability in carriers of the PSEN1 S170F mutation.

METHODS

We report a family with 4 patients carrying the S170F mutation of whom 2 underwent detailed clinical examinations. We discuss our current findings in the context of previously reported S170F cases.

RESULTS

The clinical phenotype was consistent regarding initial memory impairment and early onset in the late twenties found in all S170F patients. There were frequent non-amnestic cognitive changes and, at early stages of the disease, indications of a more pronounced disturbance of visuospatial abilities as compared to face and object recognition. Non-cognitive symptoms most often included myoclonus and cerebellar ataxia. A review of the available case reports indicates some phenotypic variability associated with the S170F mutation including different constellations of symptoms such as parkinsonism and delusions.

CONCLUSION

The variable clinical findings associated with the S170F mutation highlight the relevance of atypical phenotypes in the context of research and under a clinical perspective. CSF sampling and detection of Aβ species may be essential to indicate AD pathology in unclear cases presenting with cognitive and motor symptoms at a younger age.

The 170ms Response to Faces as Measured by MEG (M170) Is Consistently Altered in Congenital Prosopagnosia

Modularity of face processing is still a controversial issue. Congenital prosopagnosia (cPA), a selective and lifelong impairment in familiar face recognition without evidence of an acquired cerebral lesion, offers a unique opportunity to support this fundamental hypothesis. However, in spite of the pronounced behavioural impairment, identification of a functionally relevant neural alteration in congenital prosopagnosia by electrophysiogical methods has not been achieved so far. Here we show that persons with congenital prosopagnosia can be distinguished as a group from unimpaired persons using magnetoencephalography. Early face-selective MEG-responses in the range of 140 to 200ms (the M170) showed prolonged latency and decreased amplitude whereas responses to another category (houses) were indistinguishable between subjects with congenital prosopagnosia and unimpaired controls. Latency and amplitude of face-selective EEG responses (the N170) which were simultaneously recorded were statistically indistinguishable between subjects with cPA and healthy controls which resolves heterogeneous and partly conflicting results from existing studies. The complementary analysis of categorical differences (evoked activity to faces minus evoked activity to houses) revealed that the early part of the 170ms response to faces is altered in subjects with cPA. This finding can be adequately explained in a common framework of holistic and part-based face processing. Whereas a significant brain-behaviour correlation of face recognition performance and the size of the M170 amplitude is found in controls a corresponding correlation is not seen in subjects with cPA. This indicates functional relevance of the alteration found for the 170ms response to faces in cPA and pinpoints the impairment of face processing to early perceptual stages.

Dysfunctional action control as a specific feature of Parkinson's disease

Parkinson's disease (PD) is characterised by motor deficits as well as cognitive alterations, particularly concerning frontal lobe control. Here, we were interested in whether executive function is abnormal already early in PD, as well as whether this dysfunction worsens as a part of the dementia in PD. The following groups engaged in tasks addressing action control: PD patients with mild and advanced motor symptoms (aPD) without dementia, PD patients with dementia (PDD), patients with Alzheimer's disease (AD) and healthy subjects (CON). Subjects either had to perform or inhibit button presses upon go and no-go cues, respectively. These cues were preceded by pre-cues, either randomly instructive of right or left hand preparation (switch condition), or repetitively instructive for one side only (non-switch condition). PDD and aPD omitted more go responses than CON. Furthermore, PDD disproportionally committed failures upon no-go cues compared to CON. In the non-switch condition, PDD performed worse than AD, whose deficits increased to the level of PDD in the switch condition. Over all PD patients, task performance correlated with disease severity. Under the switch condition, task performance was low in both PDD and AD. In the non-switch condition, this also held true for advanced PD patients (with and without dementia), but not for AD. Thus, the deficits evident in PDD appear to develop from imbalanced inhibitory-to-excitatory action control generally inherent to PD. These results specify the concept of dysexecution in PD and differentiate the cognitive profile of PDD from that of AD patients.

Faces as objects of non-expertise: processing of thatcherised faces in congenital prosopagnosia

Congenital prosopagnosia (cPA) is a severe disorder in recognising familiar faces, a human characteristic that is presumably innate, without any macro-spatial brain anomalies. Following the idea that cPA is based on deficits of configural face processing, we used a speeded grotesqueness decision task with thatcherised faces, since the Thatcher illusion can serve as a test of configural disruption (Lewis and Johnston, 1997 Perception 26 225-227). The time needed to report the grotesqueness of a face in relation to orientation showed dissociate patterns between a group of fourteen people with cPA and a group of matched controls: whereas the RTs of controls followed a strong sigmoid function depending on rotation from the upright orientation, the RTs of people with cPA showed a much weaker sigmoid trend approaching a linear function. The latter result is interpreted as a diagnostic sign of impaired configural processing, being the primary cause of the absence of 'face expertise' in prosopagnosia.

MEG/EEG sources of the 170-ms response to faces are co-localized in the fusiform gyrus

The 170-ms electrophysiological processing stage (N170 in EEG, M170 in MEG) is considered an important computational step in face processing. Hence its neuronal sources have been modelled in several studies. The current study aimed to specify the relation of the dipolar sources underlying N170 and M170. Whole head EEG and MEG were measured simultaneously during the presentation of unfamiliar faces. An Independent Component Analysis (ICA) was applied to the data prior to localization. N170 and M170 were then modelled with a pair of dipoles in a four-shell ellipse (EEG)/homogeneous sphere (MEG) arranged symmetrically across midline. The dipole locations were projected onto the individual structural MR brain images. Dipoles were localized in fusiform gyri in ten out of eleven individuals for EEG and in seven out of eleven for MEG. N170 and M170 were co-localized in the fusiform gyrus in six individuals. The ICA shifted some of the single-subject dipoles up from cerebellum to fusiform gyrus mainly due to the removal of cardiac activity. The group mean dipole locations were also found in posterior fusiform gyri, and did not differ significantly between EEG and MEG. The result was replicated in a repeated measurement 3 months later.

Correlates of implicit memory for words and faces in event-related brain potentials

Prior research has suggested an ERP correlate of implicit memory for words consisting of a centro-parietal positivity around 400 ms. We attempted (1) to replicate this ERP modulation in a different task, involving only trials with correct responses, and (2) to compare the findings to the domain of faces. Two experiments were conducted with a modified Sternberg task, in which both targets and nontargets were presented repeatedly. In Experiment 1, positive ERP differences between repeated and new nontargets were observed, which were domain-specific in topography and, for words, replicated the previously reported findings. In Experiment 2, the amplitude of the modulation for words, but not for faces, was unaffected by a variation of the level of processing during encoding, supporting the implicitness of the processes underlying the ERP modulation to nontarget words.

Looking for faces: Attention modulates early occipitotemporal object processing

Looking for somebody's face in a crowd is one of the most important examples of visual search. For this goal, attention has to be directed to a well-defined perceptual category. When this categorically selective process starts is, however, still unknown. To this end, we used magnetoencephalography (MEG) recorded over right human occipitotemporal cortex to investigate the time course of attentional modulation of perceptual processes elicited by faces and by houses. The first face-distinctive MEG response was observed at 160-170 ms (M170). Nevertheless, attention did not start to modulate face processing before 190 ms. The first house-distinctive MEG activity was also found around 160-170 ms. However, house processing was not modulated by attention before 280 ms (90 ms later than face processing). Further analysis revealed that the attentional modulation of face processing is not due to later, for example, back-propagated activation of the M170 generator. Rather, subsequent stages of occipitotemporal object processing were modulated in a category-specific manner and with preferential access to face processing.

Time-delayed decorrelation for the identification of cardiac artifact components in MEG data

The time-delayed decorrelation (TDD) Independent Component Analysis (ICA) applied to continuous multichannel magnetoencephalographic (MEG) recordings is a straightforward calculation in the time domain: Its main ingredient is a calculation of the correlation between signals having different time shifts with respect to each other. The equivalent mathematical representation in the frequency domain and a simple simulation show that TDD exploits different complex frequency spectra of the sources. Applying ICA to 80-channel whole head MEG data obtained under visual stimulation we can assign five of the 80 source components to the cardiac artifact. Their complex spectra exhibit a randomly distributed phase showing that TDD is capable of isolating quasi-periodic signals even for the case of an unspecified phase.

MEG-analysis using the Hilbert transform

Event-related fields (ERFs) measured by magnetoencephalography (MEG) upon visual stimulation are analysed by Hilbert transformation. The Hilbert transform of real-valued measured ERF is an analytic complex signal, represented by phase and amplitude. The temporal behaviour of the derivative of the phase, i.e. the instantaneous frequency, allows to distinguish time intervals containing meaningful signal from noise. On the basis of both phase and amplitude, energies and latencies of single event ERFs can be determined.

Cardiac artifact subspace identification and elimination in cognitive MEG data using time-delayed decorrelation

To reduce physiological artifacts in magnetoencephalographic (MEG) and electroencephalographic recordings, a number of methods have been applied in the past such as principal component analysis, signal-space projection, regression using secondary information, and independent component analysis. This method has become popular as it does not have constraints such as orthogonality between artifact and signal or the need for a priori information. Applying the time-delayed decorrelation algorithm to raw data from a visual stimulation MEG experiment, we show that several of the independent components can be attributed to the cardiac artifact. Calculating an average cardiac activity shows that physiologically different excitation states of the heart produce similar field distributions in the MEG sensor system. This is equivalent to differing spectral properties of cardiac field distributions in the raw data. As a consequence, the algorithm combines, e.g., the R peak and the T wave of the cardiac cycle into a single component and the one-to-one assignment of each independent component with a physiological source is not justified in this case. To improve the signal quality of visually evoked fields, the multidimensional cardiac artifact subspace is suppressed from the data. To assess the preservation of the evoked signal after artifact suppression, a geometrical and a temporal measure are introduced. The suppression of cardiac and alpha wave artifacts allows, in our experimental setting, the reduction of the number of epochs to one half while preserving the visually evoked signal.

Inferior temporal mechanisms for invariant object recognition

The specific size and retinal location of an object are readily perceived, yet recognition of an object's identity is hardly affected by transformations of its size or location. To explore how such stimulus transformations are treated by known mechanisms for visual short-term memory in inferior temporal (IT) cortex, IT cells were recorded in monkeys performing a delayed matching-to-sample task. The stimuli were pictures of complex objects, and the monkeys ignored differences in size and retinal location when matching the test items to the sample held in memory. The sensory information communicated by cells was assessed in their responses to the sample stimuli, and mnemonic information was assessed in their responses to the test stimuli. In the sensory domain, the ordering of relative stimulus preferences for nearly all cells was invariant over changes in size or location; however, some cells nonetheless preferred stimuli of a given size or location. In the mnemonic domain, the responses of many cells were modulated according to whether the test stimulus matched the sample held in memory, and these memory effects were invariant over the relative sizes and locations of the stimuli. Thus, IT neuronal populations may mediate not only the recognition and memory of object identity, which are invariant over size and location, but also the perception of the transformations themselves.