A fuzzy clustering approach to study the auditory P50 component in schizophrenia

An overview of clustering methods with guidelines for application in mental health research

Cluster analyzes have been widely used in mental health research to decompose inter-individual heterogeneity by identifying more homogeneous subgroups of individuals. However, despite advances in new algorithms and increasing popularity, there is little guidance on model choice, analytical framework and reporting requirements. In this paper, we aimed to address this gap by introducing the philosophy, design, advantages/disadvantages and implementation of major algorithms that are particularly relevant in mental health research. Extensions of basic models, such as kernel methods, deep learning, semi-supervised clustering, and clustering ensembles are subsequently introduced. How to choose algorithms to address common issues as well as methods for pre-clustering data processing, clustering evaluation and validation are then discussed. Importantly, we also provide general guidance on clustering workflow and reporting requirements. To facilitate the implementation of different algorithms, we provide information on R functions and libraries.

Abnormal beta and gamma frequency neural oscillations mediate auditory sensory gating deficit in schizophrenia

BACKGROUND

Sensory gating is a process in which the brain's response to irrelevant and repetitive stimuli is inhibited. The sensory gating deficit in schizophrenia (SZ) is typically measured by the ratio or difference score of the P50 event-related potential (ERP) amplitudes in response to a paired click paradigm. While the P50 gating effect has usually been measured in relation to the peak amplitude of the S1 and S2 P50 ERPs, there is increasing evidence that inhibitory processes may be reflected by evoked or induced oscillatory activity during the inter-click interval in the beta (20-30 Hz) and gamma (30-50 Hz) frequency bands. We therefore examined the relationship between frequency specific activity in the inter-click interval with gating effects in the time and frequency domains.

METHOD

Paired-auditory stimuli were presented to 131 participants with schizophrenia and 196 healthy controls (HC). P50 ERP amplitudes to S1 and S2as well as averaged- and single-trial beta (20-30 Hz) and gamma (30-50 Hz) frequency power during the inter-click interval were measured from the CZ electrode site.

RESULTS

In the time domain, P50 gating deficits were apparent in both ratio and difference scores. This effect was mainly due to smaller S1 amplitudes in the patient group. SZ patients exhibited less evoked beta and gamma power, particularly at the 0-100 ms time point, in response to S1. Early (0-100 ms) evoked beta and gamma responses were critical in determining the S1 amplitude and extent of P50 gating across the delay interval for both HC and SZ.

CONCLUSION

Our findings support a disruption in initial sensory registration in those with SZ, and do not support an active mechanism throughout the delay interval. The degree of response to S1 and early beta and gamma frequency oscillations in the delay interval provides information about the mechanisms supporting auditory sensory gating, and may provide a framework for studying the mechanisms that support sensory inhibition.

Sensory processing deficiencies in patients with borderline personality disorder who experience auditory verbal hallucinations

Auditory verbal hallucinations (AVH) are common in patients with borderline personality disorder (BPD). We examined two candidate mechanisms of AVH in patients with BPD, suggested to underlie sensory processing systems that contribute to psychotic symptoms in patients with schizophrenia; sensory gating (P50 ratio and P50 difference) and change detection (mismatch negativity; MMN). Via electroencephalographic recordings P50 amplitude, P50 ratio, P50 difference and MMN amplitude were compared between 23 borderline patients with and 25 without AVH, and 26 healthy controls. Borderline patients with AVH had a significantly lower P50 difference compared with healthy controls, whereas no difference was found between borderline patients without AVH and healthy controls. The groups did not differ on MMN amplitude. The impaired sensory gating in patients with borderline personality disorder who experience AVH implies that P50 sensory gating deficiencies may underlie psychotic vulnerability in this specific patient group. Patients with borderline personality disorder with or without AVH did not have problems with auditory change detection. This may explain why they are spared from the poor outcome associated with negative symptoms and symptoms of disorganization in patients with chronic schizophrenia.

A Novel, Fast, Reliable, and Data-Driven Method for Simultaneous Single-Trial Mining and Amplitude-Latency Estimation Based on Proximity Graphs and Network Analysis

Both amplitude and latency of single-trial EEG/MEG recordings provide valuable information regarding functionality of the human brain. In this article, we provided a data-driven graph and network-based framework for mining information from multi-trial event-related brain recordings. In the first part, we provide the general outline of the proposed methodological approach. In the second part, we provide a more detailed illustration, and present the obtained results on every step of the algorithmic procedure. To justify the proposed framework instead of presenting the analytic data mining and graph-based steps, we address the problem of response variability, a prerequisite to reliable estimates for both the amplitude and latency on specific N/P components linked to the nature of the stimuli. The major question addressed in this study is the selection of representative single-trials with the aim of uncovering a less noisey averaged waveform elicited from the stimuli. This graph and network-based algorithmic procedure increases the signal-to-noise (SNR) of the brain response, a key pre-processing step to reveal significant and reliable amplitude and latency at a specific time after the onset of the stimulus and with the right polarity (N or P). We demonstrated the whole approach using electroencephalography (EEG) auditory mismatch negativity (MMN) recordings from 42 young healthy controls. The method is novel, fast and data-driven succeeding first to reveal the true waveform elicited by MMN on different conditions (frequency, intensity, duration, etc.). The proposed graph-oriented algorithmic pipeline increased the SNR of the characteristic waveforms and the reliability of amplitude and latency within the adopted cohort. We also demonstrated how different EEG reference schemes (REST vs. average) can influence amplitude-latency estimation. Simulation results revealed robust amplitude-latency estimations under different SNR and amplitude-latency variations with the proposed algorithm.

Dose-Dependent Changes in Auditory Sensory Gating in the Prefrontal Cortex of the Cynomolgus Monkey

BACKGROUND Sensory gating, often described as the ability to filter out irrelevant information that is repeated in close temporal proximity, is essential for the selection, processing, and storage of more salient information. This study aimed to test the effect of sensory gating under anesthesia in the prefrontal cortex (PFC) of monkeys following injection of bromocriptine, haloperidol, and phencyclidine (PCP). MATERIAL AND METHODS We used an auditory evoked potential that can be elicited by sound to examine sensory gating during treatment with haloperidol, bromocriptine, and PCP in the PFC in the cynomolgus monkey. Scalp electrodes were located in the bilateral PFC and bilateral temporal, bilateral parietal, and occipital lobes. Administration of bromocriptine (0.313 mg/kg, 0.625 mg/kg, and 1.25 mg/kg), haloperidol (0.001 mg/kg, 0.01 mg/kg, and 0.05 mg/kg), and the N-methyl-D-aspartic acid receptor antagonist PCP (0.3 mg/kg) influenced sensory gating. RESULTS We demonstrated the following: (1) Administration of mid-dose bromocriptine disrupted sensory gating (N100) in the right temporal lobe, while neither low-dose nor high-dose bromocriptine impaired gating. (2) Low-dose haloperidol impaired gating in the right prefrontal cortex. Mid-dose haloperidol disrupted sensory gating in left occipital lobe. High-dose haloperidol had no obvious effect on sensory gating. (3) Gating was impaired by PCP in the left parietal lobe. CONCLUSIONS Our studies showed that information processing was regulated by the dopaminergic system, which might play an important role in the PFC. The dopaminergic system influenced sensory gating in a dose- and region-dependent pattern, which might modulate the different stages that receive further processing due to novel information.

State-dependent changes in auditory sensory gating in different cortical areas in rats

Sensory gating is a process in which the brain’s response to a repetitive stimulus is attenuated; it is thought to contribute to information processing by enabling organisms to filter extraneous sensory inputs from the environment. To date, sensory gating has typically been used to determine whether brain function is impaired, such as in individuals with schizophrenia or addiction. In healthy subjects, sensory gating is sensitive to a subject’s behavioral state, such as acute stress and attention. The cortical response to sensory stimulation significantly decreases during sleep; however, information processing continues throughout sleep, and an auditory evoked potential (AEP) can be elicited by sound. It is not known whether sensory gating changes during sleep. Sleep is a non-uniform process in the whole brain with regional differences in neural activities. Thus, another question arises concerning whether sensory gating changes are uniform in different brain areas from waking to sleep. To address these questions, we used the sound stimuli of a Conditioning-testing paradigm to examine sensory gating during waking, rapid eye movement (REM) sleep and Non-REM (NREM) sleep in different cortical areas in rats. We demonstrated the following: 1. Auditory sensory gating was affected by vigilant states in the frontal and parietal areas but not in the occipital areas. 2. Auditory sensory gating decreased in NREM sleep but not REM sleep from waking in the frontal and parietal areas. 3. The decreased sensory gating in the frontal and parietal areas during NREM sleep was the result of a significant increase in the test sound amplitude.

Fuzzy logic: A "simple" solution for complexities in neurosciences?

Background:

Fuzzy logic is a multi-valued logic which is similar to human thinking and interpretation. It has the potential of combining human heuristics into computer-assisted decision making, which is applicable to individual patients as it takes into account all the factors and complexities of individuals. Fuzzy logic has been applied in all disciplines of medicine in some form and recently its applicability in neurosciences has also gained momentum.

Methods:

This review focuses on the use of this concept in various branches of neurosciences including basic neuroscience, neurology, neurosurgery, psychiatry and psychology.

Results:

The applicability of fuzzy logic is not limited to research related to neuroanatomy, imaging nerve fibers and understanding neurophysiology, but it is also a sensitive and specific tool for interpretation of EEGs, EMGs and MRIs and an effective controller device in intensive care units. It has been used for risk stratification of stroke, diagnosis of different psychiatric illnesses and even planning neurosurgical procedures.

Conclusions:

In the future, fuzzy logic has the potential of becoming the basis of all clinical decision making and our understanding of neurosciences.

Relevance of attention in auditory sensory gating paradigms in schizophrenia A pilot study

The paired-click paradigm (PCP) is widely used to study sensory habituation or gating in a number of psychiatric and neurological conditions. The classic paradigm does not control for attentional factors. In order to assess the influences of incorporating attentional control measures we administered the auditory PCP (S1-S2) in three different attention (passive, auditory attention to S2, visual attention to a concurrent continuous performance task [CPT]) conditions to a group of chronic, medicated schizophrenia patients (N=12) and a group of healthy subjects (N=15) to evaluate the effects of attention on sensory gating measures. A significant effect of attention on S1 amplitudes was shown for P50 in both groups, and N100 or P200 in schizophrenia patients. Attention status had a significant effect on S2 amplitudes for N100 and P200, and N100 and P200 gating ratios. Despite the effect of attention on S1 P50 amplitudes there was no effect on the gating ratio. In terms of group differences, visual attention to the concurrent CPT during the paired-click sensory gating task significantly enhanced the detection of deficient gating of the N100 and P200 components in schizophrenia patients. The data support the continued utilization of the passive gating paradigm for examining P50 gating but strongly suggest that for studies examining gating of the N100 or P200 components, a visual distraction paradigm may enhance the detection of abnormal gating in schizophrenia patients.

Cognitive abilities and 50- and 100-msec paired-click processes in schizophrenia

OBJECTIVE

Abnormal 50- and 100-msec event-related brain activity derived from paired-click procedures are well established in schizophrenia. There is little agreement on whether group differences in the ratio score, i.e., the ratio of EEG amplitude after the second stimulus (S2) to the amplitude after the first stimulus (S1), reflect an encoding or gating abnormality. In addition, the functional implications remain unclear. In the present study, EEG and magnetoencephalography (MEG) were used to examine paired-click measures and cognitive correlates of paired-click activity.

METHOD

EEG and whole-cortex MEG data were acquired during the standard paired-click paradigm in 73 comparison subjects and 79 schizophrenia patients. Paired-click ratio scores were obtained at 50 msec (P50 evoked potential at Cz, M50 at left and right superior temporal gyrus [STG]) and 100 msec (N100 at Cz, M100 at left and right STG). A cognitive battery assessing attention, working memory, and long-delay memory was administered. IQ was also estimated.

RESULTS

Groups differed on ratio score and amplitude of S1 response. Ratio scores at 50 msec and 100 msec and S1 amplitude predicted variance in attention (primarily S1 amplitude), working memory, and long-delay memory. The attention findings remained after removal of variance associated with IQ.

CONCLUSIONS

Associations between paired-click measures and cognitive performance in patients support 50-msec and 100-msec ratio and amplitude scores as clinically significant biomarkers of schizophrenia. In general, cognitive performance was better predicted by the ability to encode auditory information than the ability to filter redundant information.

Auditory sensory gating in the neonatal ventral hippocampal lesion model of schizophrenia

BACKGROUND/AIMS

The neonatal ventral hippocampal lesion (NVHL) rat model shows biological and behavioral abnormalities similar to schizophrenia. Disturbed sensory gating reflects a consistent neurobiological abnormality in schizophrenia. Although of critical interest, sensory gating has not been evaluated in the NVHL model.

METHODS

The N40 rat analog of the human P50 was measured to assess sensory response and gating in NVHL and sham rats. Epidural electrodes recorded evoked potentials (EPs), from which amplitudes, latencies, difference scores (S1-S2) and gating ratios (S2/S1) were assessed. Power and phase locking were computed for evoked EEG activity, to test for frequency-specific abnormalities.

RESULTS

Prolonged S1 N40 latency was detected in the NVHL group, but amplitude and power measures did not differ. NVHL rats demonstrated disturbed phase-locked sensory gating at theta and beta frequencies, as well as reduced phase-locked gamma activity across stimuli, most robustly at S1.

CONCLUSIONS

While measures of sensory gating obtained from the EP were relatively insensitive to the NVHL model, phase locking across trials was affected. NVHL rats may have increased evoked response temporal variability, similar to patients with schizophrenia. This pattern of findings likely reflects core developmental NVHL disturbances in dorsal hippocampal circuits associated with temporal and frontal areas.

Contributions of spectral frequency analyses to the study of P50 ERP amplitude and suppression in bipolar disorder with or without a history of psychosis

OBJECTIVE

The present study investigated event-related brain potential (ERP) indices of auditory processing and sensory gating in bipolar disorder and subgroups of bipolar patients with or without a history of psychosis using the P50 dual-click procedure. Auditory-evoked activity in two discrete frequency bands also was explored to distinguish between sensory registration and selective attention deficits.

METHODS

Thirty-one individuals with bipolar disorder and 28 non-psychiatric controls were compared on ERP indices of auditory processing using a dual-click procedure. In addition to conventional P50 ERP peak-picking techniques, quantitative frequency analyses were applied to the ERP data to isolate stages of information processing associated with sensory registration (20-50 Hz; gamma band) and selective attention (0-20 Hz; low-frequency band).

RESULTS

Compared to the non-psychiatric control group, patients with bipolar disorder exhibited reduced S1 response magnitudes for the conventional P50 peak-picking and low-frequency response analyses. A bipolar subgroup effect suggested that the attenuated S1 magnitudes from the P50 peak-picking and low-frequency analyses were largely attributable to patients without a history of psychosis.

CONCLUSIONS

The analysis of distinct frequency bands of the auditory-evoked response elicited during the dual-click procedure allowed further specification of the nature of auditory sensory processing and gating deficits in bipolar disorder with or without a history of psychosis. The observed S1 effects in the low-frequency band suggest selective attention deficits in bipolar patients, especially those patients without a history of psychosis, which may reflect a diminished capacity to selectively attend to salient stimuli as opposed to impairments of inhibitory sensory processes.

Superior temporal gyrus spectral abnormalities in schizophrenia

Considerable evidence indicates early auditory stimulus processing abnormalities in schizophrenia, but the mechanisms are unclear. The present study examined oscillatory phenomena during a paired-click paradigm in the superior temporal gyrus (STG) as a possible core problem. The primary question addressed is whether first click and/or second click group differences in the time-domain evoked response in patients with schizophrenia are due to (1) group differences in the magnitude of poststimulus oscillatory activity, (2) group differences in poststimulus phase-locking, and/or (3) group differences in the magnitude of ongoing background oscillatory activity. Dense-array magnetoencephalography from 45 controls and 45 patients with schizophrenia produced left- and right-hemisphere STG 50- and 100-ms time-frequency evoked, phase-locking, and total power measures. Whereas first click 100-ms evoked theta and alpha abnormalities were observed bilaterally, evoked low beta-band differences were specific to the left hemisphere. Compared to controls, patients with schizophrenia showed more low-frequency phase variability, and the decreased 100-ms S1 evoked response observed in patients was best predicted by the STG phase-locking measure.

Evidence of a posterior cingulate involvement (Brodmann area 31) in dyslexia: a study based on source localization algorithm of event-related potentials

The study investigates the differences regarding the position of intracranial generators of P50 component of ERPs in 38 dyslexic children aged 11.47+/-2.12 years compared with their 19 healthy siblings aged 12.21+/-2.25. The dipoles were extracted by solving the inverse electromagnetic problem according to the recursively applied and projected multiple signal classification (RAP-MUSIC) algorithm approach. For improved localization of the main dipole the solutions were optimized using genetic algorithms. The statistical analysis revealed differences regarding the position of intracranial generators of low frequency of P50. Particularly, dyslexics showed main activity being located at posterior cingulate cortex (Brodmann's area 31) while controls exhibited main activity being located at retrosplenial cortex (Brodmann's area 30). These results may indicate a role for the posterior cingulate cortex in the pre-attentive processing operation of dyslexia beyond of its traditional function in terms of spatial attention and motor intention.

Sensory gating revisited: relation between brain oscillations and auditory evoked potentials in schizophrenia

Disturbances of auditory information processing have repeatedly been shown in schizophrenia. To contribute to a better understanding of the neurophysiological underpinnings of habituation in auditory processing and its disturbance in schizophrenia we used three different approaches to analyze auditory evoked responses, namely phase-locking (PL) analyses, single trial amplitudes, and averaged event-related potentials (P50 and N100). Given that brain oscillations reflect the neuronal correlates of information processing we hypothesized that PL and amplitudes reflect even more essential parts of auditory processing than the averaged ERP responses. In 32 schizophrenia patients and 32 matched controls EEG was continuously recorded using an auditory paired click paradigm. PL of the lower frequency bands (alpha and theta) was significantly reduced in patients whereas no significant differences were present in higher frequencies (gamma and beta). Alpha and theta PL and amplitudes showed a marked increase after the first click and to a minor degree after the second one. This habituation was more prominent in controls whereas in schizophrenia patients the response to both clicks differed only slightly. N100 suppression was significantly reduced in schizophrenia patients whereas no group differences were present with respect to the P50. This corresponded to the finding that gamma mostly contributed to the prediction of the P50 response and theta mostly to the N100 response. Our data showed that analyzing phase and amplitude in single trials provides more information on auditory information processing and reflects differences between schizophrenia patients and controls better than analyzing the averaged ERP responses.

Estimation of individual evoked potential components using iterative independent component analysis

Independent component analysis (ICA) has been successfully employed in the study of single-trial evoked potentials (EPs). In this paper, we present an iterative temporal ICA methodology that processes multielectrode single-trial EPs, one channel at a time, in contrast to most existing methodologies which are spatial and analyze EPs from all recording channels simultaneously. The proposed algorithm aims at enhancing individual components in an EP waveform in each single trial, and relies on a dynamic template to guide EP estimation. To quantify the performance of this method, we carried out extensive analyses with artificial EPs, using different models for EP generation, including the phase-resetting and the classical additive-signal models, and several signal-to-noise ratios and EP component latency jitters. Furthermore, to validate the technique, we employed actual recordings of the auditory N100 component obtained from normal subjects. Our results with artificial data show that the proposed procedure can provide significantly better estimates of the embedded EP signals compared to plain averaging, while with actual EP recordings, the procedure can consistently enhance individual components in single trials, in all subjects, which in turn results in enhanced average EPs. This procedure is well suited for fast analysis of very large multielectrode recordings in parallel architectures, as individual channels can be processed simultaneously on different processors. We conclude that this method can be used to study the spatiotemporal evolution of specific EP components and may have a significant impact as a clinical tool in the analysis of single-trial EPs.

Dysregulation of thalamic sensory "transmission" in schizophrenia: neurochemical vulnerability to hallucinations

Cholinergic arousal mechanisms predispose thalamic and cortical neurons to fire action potentials at gamma rhythms, which have a tendency to resonate in thalamocortical networks, thereby forming coherent assemblies under constraints of sensory input to specific thalamic nuclei, on the one hand, and prefrontal and limbic attentional mechanisms, on the other. Perception may be based on sustained assemblies of coherent gamma oscillations in thalamocortical circuits. In schizophrenia, the impact of sensory input on self-organization of thalamocortical activity may be generally reduced. As a result, processes underlying perception can become uncoupled from sensory input, particularly at times of hyperarousal, leading to domination of attentional mechanisms and the emergence of hallucinations. Evidence is reviewed that implicates excessive neuronal noise in specific thalamic nuclei in the generation of hallucinations in schizophrenia. Nicotinic receptor abnormalities, dopaminergic hyperactivity and glutamate-receptor hypofunction are reconciled within a model of psychotic symptom generation that places crucial emphasis on dysfunction of the reticular thalamic nucleus.

Acute mobile phone effects on pre-attentive operation

There is a debate whether electromagnetic field (EMF) emitted by mobile phones (MP) have an effect on cognitive functions. Since the auditory P50 component of event-related potentials (ERPs) reflects pre-attentive processing and working memory (WM) operation, the present study was designed to investigate whether the exposure to MP-EMF affects the patterns of the P50 component of ERPs elicited during a WM test. The P50 elicited during a WM task and evoked by two warning stimuli low and high frequency (500 and 3000 Hz) has been assessed in 19 normal subjects (10 women and 9 men) both without and with exposure to a 900 MHz signal, emitted by a dipole antenna placed near the subjects. Results showed that the presence of MP-EMFs induced statistically significant increase in the amplitude of P50 evoked by the low frequency stimuli, at Fp1 and O1 electrode leads as compared to themselves without MP-EMF exposure. In contrast the exposure to MP-EMFs revealed statistically significant decrease of the amplitude of P50 evoked by the high frequency stimuli, at Fp1 electrode lead as compared to themselves without MP-EMF exposure. These findings provide evidence that the MP-EMF emitted by mobile phone affect pre-attentive information processing as it is reflected in P50 evoked potential. The basis of such an effect is unclear, although several possibilities exist and call for potential directions of future research.

Fuzzy logic in medicine and bioinformatics

The purpose of this paper is to present a general view of the current applications of fuzzy logic in medicine and bioinformatics. We particularly review the medical literature using fuzzy logic. We then recall the geometrical interpretation of fuzzy sets as points in a fuzzy hypercube and present two concrete illustrations in medicine (drug addictions) and in bioinformatics (comparison of genomes).

Preattentive deficits in developmental disorders of scholastic skills

Working memory deficiency has been implicated in developmental disorders of scholastic skills. The auditory P50 component of event-related potentials reflecting preattentive processing was investigated in 38 children with developmental disorders of scholastic skills and 19 sibling control children, as elicited during a working memory test. The P50 was evoked by two tones of low and high frequency (500 Hz and 3000 Hz). The group with developmental disorders of scholastic skills showed prolonged P50 latency induced by the low tone, located at the frontal area. The amplitude of P50 induced by the low tone exhibited significantly negative associations with both age and memory performance, whereas age and memory performance were associated positively. These findings indicate that preattentive processing deficits may be implicated not only in auditory cognition but also in developmental disorders of scholastic skills.

Contributions of subtype and spectral frequency analyses to the study of P50 ERP amplitude and suppression in schizophrenia

BACKGROUND

Poor suppression of P50 event-related potential (ERP) amplitudes to paired-click stimuli may indicate genetic liability for schizophrenia and weak "sensory gating." Evidence suggests, however, that P50 amplitude is selectively impaired in nonparanoid, but not paranoid, schizophrenia subtypes. Furthermore, paired-click suppression can appear deficient in schizophrenia due to smaller evoked responses to the first stimulus (S1), rather than larger, less effectively "gated" responses to the second (S2). Finally, the P50 ERP is comprised of activity from at least two frequency components that may be distinctly impaired: the gamma band, associated with sensory registration, and the low frequency response, associated with attention/encoding processes. P50 and related frequency subcomponents were examined as a function of illness subtype to further integrate these concepts.

METHOD

The standard paired-click paradigm was administered to 38 schizophrenia (27 paranoid, 11 nonparanoid) and 38 age-matched healthy control participants. P50 amplitudes and spectral power of gamma band (GBR; 20-50 Hz) and low frequency (LFR; 1-20 Hz) responses were analyzed.

RESULTS

P50 analyses revealed smaller S1 amplitude and normal S2 in schizophrenia participants collectively, but no differentiation of schizophrenia subtypes. Spectral analyses revealed smaller magnitude S1 and normal S2 responses in schizophrenia across both the GBR and LFR. The LFR, but not GBR, was found to distinguish nonparanoid from control groups, while paranoid participants evidenced no impairment in either frequency domain. LFR amplitude values correlated with clinical ratings of cognitive symptomatology.

CONCLUSIONS

ERP deficits in the dual-click paradigm were specific to S1 amplitudes and most prominent in the low frequency response. These results replicate previous findings and extend their relevance to schizophrenia subtype distinctions. Implications for the recurrent inhibition model of sensory gating are discussed.

Contribution of different EEG frequencies to auditory evoked potential abnormalities in schizophrenia

OBJECTIVE

We have shown previously [Clin Neurophysiol 2003;114:79] that phase reorganization of the ongoing electroencephalogram (EEG) plays an important role in the generation of auditory evoked potential (EP) components with a latency between 50 and 200 ms. In the present study, we investigate whether schizophrenia patients suffer from phase synchronization deficits as compared to normal subjects.

METHODS

The auditory EPs from 20 normal subjects and 19 schizophrenia patients were analyzed. EPs were obtained using a double stimulus paradigm, in which two identical tone bursts (S1 and S2) were delivered with an average inter-stimulus interval of 500 ms and an inter-pair interval of 8 s. The Piecewise Prony Method (PPM) was used to decompose single trial auditory evoked potentials into different frequency bands. Pre- and post-stimulus phase histograms were compared for each frequency band to determine the degree of phase synchronization produced by auditory stimulation in the two populations.

RESULTS

The S1 stimulus produced significantly less (P < 0.05) phase synchronization in schizophrenia patients than in normal subjects in the 2-12 Hz frequency range. Far fewer and smaller inter-population phase synchronization differences were seen for the S2 stimulus. Both populations showed more phase synchronization for S1 than S2. A significant correlation (P < 0.01) between N100 amplitude and phase synchronization 100 ms post S1 was observed for the normal population but not for the schizophrenia group. The correlation between P200 amplitude and phase synchronization 200 ms post S1 was significant for the normal group (P < 0.01) and the schizophrenia group (P < 0.03).

CONCLUSIONS

Schizophrenia patients have a phase synchronization deficiency, as compared to a normal control group, especially for the first stimulus, in the 2-12 Hz frequency range. This deficiency explains the lower EP amplitudes and may be a significant factor contributing to reduced sensory gating reported in schizophrenic subjects.

SIGNIFICANCE

The research presented here contributes to the understanding of the mechanism underlying sensory gating in health and gating deficiencies in schizophrenia.

Hallucinations: synchronisation of thalamocortical gamma oscillations underconstrained by sensory input

What we perceive is the product of an intrinsic process and not part of external physical reality. This notion is consistent with the philosophical position of transcendental idealism but also agrees with physiological findings on the thalamocortical system. gamma-Frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in thalamocortical networks, thereby transiently forming assemblies of coherent gamma oscillations under constraints of sensory input and prefrontal attentional mechanisms. Perception and conscious experience may be based on such assemblies and sensory input to thalamic nuclei plays merely a constraining role in their formation. In schizophrenia, the ability of sensory input to modulate self-organisation of thalamocortical gamma activity may be generally reduced. If during arousal thalamocortical self-organisation is underconstrained by sensory input, then attentional mechanisms alone may determine the content of perception and hallucinations may arise.

Predicting EEG responses using MEG sources in superior temporal gyrus reveals source asynchrony in patients with schizophrenia

OBJECTIVE

An integrated analysis using Electroencephalography (EEG) and magnetoencephalography (MEG) is introduced to study abnormalities in early cortical responses to auditory stimuli in schizophrenia.

METHODS

Auditory responses were recorded simultaneously using EEG and MEG from 20 patients with schizophrenia and 19 control subjects. Bilateral superior temporal gyrus (STG) sources and their time courses were obtained using MEG for the 30-100 ms post-stimulus interval. The MEG STG source time courses were used to predict the EEG signal at electrode Cz.

RESULTS

In control subjects, the STG sources predicted the EEG Cz recording very well (97% variance explained). In schizophrenia patients, the STG sources accounted for substantially (86%) and significantly (P<0.0002) less variance. After MEG-derived STG activity was removed from the EEG Cz signal, the residual signal was dominated by 40 Hz activity, an indication that the remaining variance in EEG is probably contributed by other brain generators, rather than by random noise.

CONCLUSIONS

Integrated MEG and EEG analysis can differentiate patients and controls, and suggests a basis for a well established abnormality in the cortical auditory response in schizophrenia, implicating a disorder of functional connectivity in the relationship between STG sources and other brain generators.

Semantic geodesic maps: a unifying geometrical approach for studying the structure and dynamics of single trial evoked responses

OBJECTIVES

A general framework for identifying and describing structure in a given sample of evoked response single-trial signals (STs) is introduced. The approach is based on conceptually simple geometrical ideas and enables the convergence of pattern analysis and non-linear time series analysis.

METHODS

Classical steps for analyzing the STs by waveform are first employed and the ST-analysis is transferred to a multidimensional space, the feature space, the geometry of which is systematically studied via multidimensional scaling (MDS) techniques giving rise to semantic maps. The structure in the feature space characterizes the trial-to-trial variability and this is utilized to probe functional connectivity between two brain areas. The underlying dynamic process responsible for the emerged structure can be described by a multidimensional trajectory in the feature space. This in turn enables the detection of dynamical interareal coupling as similarity between the corresponding trajectories.

RESULTS AND CONCLUSIONS

The utility of semantic maps was demonstrated using magnetoencephalographic data from a simple auditory paradigm. The coupling of ongoing activity and evoked response is vividly demonstrated and contrasted with the apparent deflection from zero baseline that survives averaging. Prototypes are easily identified as the end points of distinct paths in the semantic map representation, and their neighborhood is populated by STs with distinct properties not only in the latencies where the evoked response is expected to be strong, but also and very significantly in the prestimulus period. Finally our results provide evidence for interhemispheric binding in the (4-8 Hz) range and dynamical coupling at faster time scales.

Hippocampal and cortical sensory gating in rats: effects of quinpirole microinjections in nucleus accumbens core and shell

Sensory processing disturbances, as measured in the P50/sensory gating paradigm, have been linked to aberrant auditory information processing and sensory overload in schizophrenic patients. In this paradigm, the response to the second of paired-click stimuli is attenuated by an inhibitory effect of the first stimulus. Sensory gating has been observed in most healthy human subjects and normal laboratory rats. Because mesolimbic dopamine has been implicated in other filtering disturbances such as prepulse inhibition of the acoustic startle response and given the fact that amphetamine and apomorphine have been shown to disrupt gating, this study was performed to investigate the role of mesolimbic dopamine in sensory gating. The dopamine D2 receptor agonist quinpirole (10 microg/0.5 microl) was injected bilaterally in nucleus accumbens core and shell and effects on cortical and hippocampal sensory gating were investigated. Also, effects of the dopamine D2 receptor antagonist haloperidol (0.1 mg/kg, subcutaneously) as pretreatment were studied. First, quinpirole significantly reduced both the amplitude to the first click and gating as measured in the cortex and in the hippocampus. There was a tendency for the quinpirole effects on hippocampal gating to be more pronounced in rats injected in the shell. Secondly, haloperidol did not antagonize effects of quinpirole on hippocampal parameters, whereas haloperidol pretreatment fully antagonized quinpirole effects on cortical parameters. In conclusion, gating can be significantly reduced when a dopamine agonist is specifically targeted at mesolimbic dopamine D2 receptors. However, an important consideration is that the dopaminergic effects in the present study on gating are predominantly mediated by the effects on the amplitude to the first click. This has also been suggested for systemic amphetamine injections in rats and schizophrenic patients. This casts doubt on whether dopamine receptor activation affects the putative inhibitory process between the first and the second stimulus.

Event-related potentials and genetic risk for schizophrenia

BACKGROUND

Event-related potentials (ERPs) during an auditory oddball task were investigated in patients with schizophrenia and in their healthy siblings to explore the question of whether abnormalities of two-dimensional topographic scalp-distribution of P300 amplitude and latency relate to genetic risk for schizophrenia. We also examined the P50, N100, and P200-waves, elicited during the same task.

METHODS

We investigated 42 schizophrenic patients, 62 of their healthy siblings, and 34 unrelated normal control subjects with a standard auditory oddball paradigm and 16 electroencephalogram electrodes. Amplitudes and latencies of the ERPs P50, N100, P200, and P300 were topographically analyzed.

RESULTS

In the patients, P300 amplitude was significantly decreased in the range of 54%-58% over the left parietotemporal area. Siblings did not show decreased P300 amplitudes when compared with normal subjects. P300 latencies were unchanged in both groups. No significant group differences were observed for the other event-related potentials.

CONCLUSIONS

In line with previous studies, the P300 amplitude in schizophrenic patients was decreased over the left temporoparietal area; however, we found no evidence for a genetic trait effect in the event-related potential abnormality. Possible reasons for these largely negative findings are discussed.

Exploratory data analysis of evoked response single trials based on minimal spanning tree

OBJECTIVE

An exploratory data analysis framework, based on minimal spanning tree, is proposed as a means to support the analysis of single trial (ST) electrophysiological signals. The core of this framework is the compact description of the input ST sample in a form of content-dependent ordered lists. Based on the established hierarchies, efficient ways to increase the SNR, extract prototypical responses, visualize possible self-organization trends in the sample and track the course of evoked response along the trial-to-trial dimension, are proposed.

METHOD

Magnetoencephalographic auditory evoked responses were used for demonstrating and validating the introduced framework.

RESULTS AND CONCLUSION

The results demonstrate the benefits, from this intelligent manipulation of STs, in understanding and enhancing the actual evoked signal. Specifically we find support for stimulus-induced phase-resetting hypothesis in the 3-20 Hz band, the existence of trials void of the prototypical evoked response, and an order across the single trial set hinting at an underlying process with long time scale.

[The p300 cognitive event-related potential. II. Individual variability and clinical application in psychopathology]

The P300 wave is one of the cognitive components of the event-related potential (ERP) that is used to investigate the cognitive processes, and which can be used to study patient populations with a variety of psychiatric disorders. Its clinical utility has been increased by the identification of factors that contribute to the variability in its amplitude and latency. However, its value as a diagnostic index has not been entirely established. It can provide a useful recording of patients' information processing, and indicate the severity of the clinical state and its possible evolution. It can also assist in determining what therapeutic approach to adopt. In the present review, the findings in the literature concerning interindividual variation in the P300 wave are first described; several variables significantly influence the amplitude and latency of this wave, such as age, gender, intelligence and personality. Following this, the relevance of the data in the literature on the clinical applications of P300 in psychopathology is examined, including the studies undertaken to obtain an objective diagnostic index for mental disorders and also those carried out to assess the problems concerning the interpretation of information connected with the mental pathologies examined. P300-associated findings on dementia, schizophrenia, depression, alcoholism, drug addiction, anxiety disorders (panic disorder, obsessive-compulsive disorder, and post-traumatic stress syndrome) and on personality disorders (schizoid, antisocial or borderline personality disorder) have been examined in detail.

Schizophrenia: reduced signal-to-noise ratio and impaired phase-locking during information processing

OBJECTIVE

This study was performed in order to clarify the mechanisms which underlie the reduced signal-to-noise of event-related potentials in schizophrenic patients. Specifically, we wanted to find out, whether it is reduced activation and/or synchronization (phase-locking) in specific frequency bands of the ongoing EEG which is related to the decreased signal amplitude and signal-to-noise ratio in schizophrenics.

METHODS

We investigated 41 unmedicated schizophrenics (10 of them drug-naïve) and compared them with healthy control subjects (n = 233) as well as unmedicated subjects with schizotypal personality (n = 21), who were considered to be high-risk subjects for schizophrenia, and unmedicated depressive patients (n = 71). We measured event-related activity during an acoustical choice reaction paradigm and calculated the signal-to-noise ratio, signal power and noise for a time interval of 50-200 ms after stimulus presentation. Signal-to-noise ratio was calculated from the power of the averaged trials (signal power) divided by the mean power of the single trials minus the power of the average (noise power). Also, we performed a frequency analysis of the pre- and poststimulus EEG based on a factor analytical approach. Group comparisons were performed with ANCOVA.

RESULTS

As expected, a decreased signal-to-noise ratio of evoked activity was found in the schizophrenic and a non-significant trend in the schizotypal subjects and the depressive patients. We were able to show that the observed decrease is due to a reduced signal power and an increase of absolute noise power. Frequency analysis of the evoked activity revealed that normals, schizophrenics schizotypal subjects and depressive patients increased theta/delta activity between pre- and poststimulus interval to a similar extend. However, this theta/delta-augmentation does not correlate with signal power in schizophrenics. Also, normals and depressive subjects augment coherence between both temporal lobes during information processing, which is not found in schizophrenics and schizotypal subjects. In contrast, these two groups augment frontal lobe coherence, which goes along with an increase of noise.

CONCLUSIONS

Reduced stimulus-induced phase-locking and bitemporal coherence of cortically evoked activity but not a failure to activate the cortex may be responsible for the observed low signal-to-noise ratio during information processing in schizophrenics. Accordingly, schizophrenics increase noise after stimulus presentation instead of building up a signal. This is discussed in the framework of the theory of stochastic resonance.

Identification of reliable spike templates in multi-unit extracellular recordings using fuzzy clustering

A method for extracting single-unit spike trains from extracellular recordings containing the activity of several simultaneously active cells is presented. The technique is particularly effective when spikes overlap temporally. It is capable of identifying the exact number of neurons contributing to a recording and of creating reliable spike templates. The procedure is based on fuzzy clustering and its performance is controlled by minimizing a cluster-validity index which optimizes the compactness and separation of the identified clusters. Application examples with synthetic spike trains generated from real spikes and segments of background noise show the advantage of the fuzzy method over conventional template-creation approaches in a wide range of signal-to-noise ratios.

Elementary phenotypes in the neurobiological and genetic study of schizophrenia

This review describes the strategy of using elementary phenotypes for neurobiological and genetic linkage studies of schizophrenia. The review concentrates on practical aspects of selecting the phenotype and then understanding the confounds in its measurement and interpretation. Examples from the authors' studies of deficits in P50 inhibition and smooth pursuit eye movement dysfunction are presented. These two phenotypes share considerable similarity in their neurobiology, including a similar response to nicotine. They also appear to co-segregate with the genetic risk for schizophrenia as autosomal co-dominant phenotypes. Although most schizophrenic patients inherit these abnormalities unilinealy, i.e., from one parent, apparent bilineal inheritance produces a more severe illness, observed clinically as childhood-onset schizophrenia. The initial study showing linkage of the P50 deficit to the chromosome 15q14 locus of the alpha 7-nicotinic acetylcholine receptor is an example of the potential usefulness of these phenotypes for combined genetic and neurobiological study of schizophrenia.