Click-train evoked steady state harmonic response as a novel pharmacodynamic biomarker of cortical oscillatory synchrony

Sensory networks naturally entrain to rhythmic stimuli like a click train delivered at a particular frequency. Such synchronization is integral to information processing, can be measured by electroencephalography (EEG) and is an accessible index of neural network function. Click trains evoke neural entrainment not only at the driving frequency (F), referred to as the auditory steady state response (ASSR), but also at its higher multiples called the steady state harmonic response (SSHR). Since harmonics play an important and non-redundant role in acoustic information processing, we hypothesized that SSHR may differ from ASSR in presentation and pharmacological sensitivity. In female SD rats, a 2 s-long train stimulus was used to evoke ASSR at 20 Hz and its SSHR at 40, 60 and 80 Hz, recorded from a prefrontal epidural electrode. Narrow band evoked responses were evident at all frequencies; signal power was strongest at 20 Hz while phase synchrony was strongest at 80 Hz. SSHR at 40 Hz took the longest time (∼180 ms from stimulus onset) to establish synchrony. The NMDA antagonist MK801 (0.025-0.1 mg/kg) did not consistently affect 20 Hz ASSR phase synchrony but robustly and dose-dependently attenuated synchrony of all SSHR. Evoked power was attenuated by MK801 at 20 Hz ASSR and 40 Hz SSHR only. Thus, presentation as well as pharmacological sensitivity distinguished SSHR from ASSR, making them non-redundant markers of cortical network function. SSHR is a novel and promising translational biomarker of cortical oscillatory dynamics that may have important applications in CNS drug development and personalized medicine.

Deviation from expected cognitive ability is a core cognitive feature of schizophrenia related to neurophysiologic, clinical and psychosocial functioning

Cognitive functioning in schizophrenia is characterized by a generalized impairment in current cognitive ability based on traditional population-based norms. However, these norms assume a normal cognitive trajectory and do not directly account for illness-related declines from expected cognitive potential. Indeed, schizophrenia patients exhibit even greater deviation between their observed and expected cognitive functioning based on expanded norms that leverage premorbid variables resistant to illness-related features. The current study further quantified the extent to which illness-related features account for this deviation from expectation and assessed its relationship to neurophysiologic (mismatch negativity, P3a, theta oscillations), clinical, and psychosocial functioning in schizophrenia patients. Expected cognitive ability (PENN-CNB global cognition) in patients (n = 684) was calculated using healthy comparison subject (n = 660) weighted regression based on premorbid variables resistant to illness-related decline (demographics, single-word reading, parental education). The magnitude of any deviation between current (observed) and regression-predicted (expected) cognitive ability was calculated. Results indicated that 24% (n = 164) of the total patient population exhibited significant (≥-1.96 SD) deviation between observed and expected global cognitive ability. Interestingly, 20% of the total patient population (n = 136) had "normal" range cognitive performance when using traditional population-based norms, but also had significant deviation from expected cognitive ability. The magnitude of this deviation was associated with more severe neurophysiologic abnormalities, longer illness duration, higher levels of negative symptoms, and worse psychosocial functioning. Assessment of cognitive deviation is thus a complementary metric for characterizing the severity of illness-related cognitive declines in patients, while also reflecting the expression and severity of key endophenotypes of schizophrenia.

Decomposing the constituent oscillatory dynamics underlying mismatch negativity generation in schizophrenia: Distinct relationships to clinical and cognitive functioning

Abnormalities in early auditory information processing (EAIP) contribute to higher-order deficits in cognition and psychosocial functioning in schizophrenia. A passive auditory oddball paradigm is commonly used to evoke event-related potential (ERP) measures of EAIP reflecting auditory sensory registration and deviance detection, including mismatch negativity (MMN) and P3a responses. MMN and P3a have been extensively studied in healthy subjects and neuropsychiatric patient populations and are increasingly used as translational biomarkers in the development of novel therapeutics. Despite widespread use, relatively few studies have examined the constituent oscillatory elements and the extent to which sensory registration and deviance detection represent distinct or intercorrelated processes. This study aimed to determine the factor structure and clinical correlates of these oscillatory measures in schizophrenia patients (n = 706) and healthy comparison subjects (n = 615) who underwent clinical, cognitive, and functional characterization and EEG testing via their participation in the Consortium of Genomics in Schizophrenia (COGS-2) study. Results revealed significant deficits in theta-band (4-7 Hz) evoked power and phase locking in patients. Exploratory factor analyses of both ERP and oscillatory measures revealed two dissociable factors reflecting sensory registration and deviance detection. While each factor shared a significant correlation with social cognition, the deviance detection factor had a unique relationship to multiple cognitive and clinical domains. Results support the continued advancement of functionally relevant oscillatory measures underlying EAIP in the development of precognitive therapeutics.

Measuring P50 suppression and prepulse inhibition in a single recording session

OBJECTIVE

Two distinct measures have been used to assess inhibitory gating deficits in schizophrenia patients: P50 suppression and prepulse inhibition of the startle response. It remains unclear whether both measures can be assessed in a single testing session.

METHOD

Twelve normal subjects underwent testing in a carefully designed combined P50/prepulse inhibition session using stimulus characteristics similar to those described in the existing literature.

RESULTS

The levels of both P50 suppression and prepulse inhibition obtained in the combined session were highly similar to those obtained in independent testing of previous cohorts of normal subjects. As in previous experiments, P50 suppression and prepulse inhibition were not significantly correlated.

CONCLUSIONS

Measuring P50 suppression and prepulse inhibition in a single session is feasible and offers a unique opportunity to assess these two distinct gating measures contemporaneously in cohorts of normal comparison subjects and schizophrenia patients, so that temporal shifts in one or both measures are minimized.

Impact of prepulse characteristics on the detection of sensorimotor gating deficits in schizophrenia

Schizophrenia patients have prominent deficits in information processing that can be detected by measures of prepulse inhibition (PPI) of the startle response. Deficient PPI in schizophrenia is thought to reflect a failure of brain-based information 'protective' mechanisms that normally inhibit responsivity for 30-500ms after a weak prepulse stimulus. The relationship between specific prepulse stimulus characteristics and PPI deficits in this study was examined in 31 schizophrenia patients and 34 normal comparison subjects. Schizophrenia patients had overall deficits in PPI across four conditions where the prepulse was either discrete (abrupt) or continuous (sustained) and consisted of either white noise or a pure tone. On inspection and analysis of the data, it appears that the white noise conditions, rather than tone conditions, account for the group differences. Thus, the discrete white noise prepulse was most effective in eliciting PPI deficits, resulting in a large effect size between groups (d=0.85; P<0.01). Deficits in information-protective mechanisms in schizophrenia may be differentially sensitive to specific stimulus characteristics; this observation may be relevant both to the neurobiology of information processing deficits in schizophrenia and to the methodologies for studying these deficits experimentally.

Schizophrenia patients demonstrate a dissociation on declarative and non-declarative memory tests

Declarative memory refers to the recall and recognition of factual information. In contrast, non-declarative memory entails a facilitation of memory based on prior exposure and is typically assessed with priming and perceptual-motor sequencing tasks. In this study, schizophrenia patients were compared to normal comparison subjects on two computerized memory tasks: the Word-stem Priming Test (n=30) and the Pattern Sequence Learning Test (n=20). Word-stem Priming includes recall, recognition (declarative) and priming (non-declarative) components of memory. The schizophrenia patients demonstrated an impaired performance on recall of words with relative improvement during the recognition portion of the test. Furthermore, they performed normally on the priming portion of the test. Thus, on tests of declarative memory, the patients had retrieval deficits with intact performance on the non-declarative memory component. The Pattern Sequence Learning Test utilizes a serial reaction time paradigm to assess non-declarative memory. The schizophrenia patients' serial reaction time was significantly slower than that of comparison subjects. However, the patients' rate of acquisition was not different from the normal comparison group. The data suggest that patients with schizophrenia process more slowly than normal, but have an intact non-declarative memory. The schizophrenia patients' dissociation on declarative vs. non-declarative memory tests is discussed in terms of possible underlying structural impairment.

Normal P50 suppression in schizophrenia patients treated with atypical antipsychotic medications

OBJECTIVE

Patients with schizophrenia have deficits in attention, cognition, and information processing. Measures such as P50 suppression are used to study cognitive and attentional dysfunction among these patients. P50 suppression is an operational measure of sensory gating that can be assessed by averaging electroencephalographic responses to multiple pairs of auditory clicks separated by 500 msec. Normally, the P50 response to the second click is smaller than the response to the first click. Many studies have demonstrated that schizophrenia patients have deficient P50 suppression, meaning that the difference between the first and second clicks is not as large as normal. Atypical antipsychotic medications may have superior clinical efficacy for negative symptoms and cognitive deficits. It is important, therefore, to evaluate the effects of atypical antipsychotic medications on measures such as P50 suppression.

METHOD

P50 suppression of 13 patients with schizophrenia receiving clinically effective doses of clozapine, olanzapine, or risperidone (classified as atypical antipsychotic medications) was compared to that of 13 patients receiving conventional antipsychotic medications.

RESULTS

The patient groups did not differ on clinical or demographic measures. The patients receiving atypical antipsychotic medications had normal-range P50 suppression (mean=72%). In contrast, the patients receiving typical antipsychotic medications had dramatically lower P50 suppression (mean=27%).

CONCLUSIONS

The results support the hypothesis that patients treated with atypical antipsychotic medications have normal P50 measures of sensory gating. Longitudinal within-subjects studies are warranted to clarify the mechanisms mediating this effect.

Do self-reports of perceptual anomalies reflect gating deficits in schizophrenia patients?

Jin and colleagues presented an innovative study examining P50 suppression and patients' self-reported perceptual anomalies as two related operational measures of sensory gating deficits in schizophrenia patients. They found that those schizophrenia patients who endorsed experiences of sensory inundation had normal levels of P50 suppression, whereas patients who tended to endorse fewer complaints of perceptual anomalies had P50 suppression deficits. Jin et al's finding challenges the common belief that P50 suppression deficits are associated with cognitive and sensory anomalies reflecting poor gating in schizophrenia patients. This article comments on how the dissociation between phenomenological experiences of gating disturbances and P50 suppression might be explained by the limits of self-report in schizophrenia patients who have deficient insight and self-awareness. We hypothesize that the self-reported inability to screen out irrelevant stimuli reflects a voluntary, controlled process that is different from the involuntary, automatic process measured by P50 suppression.

Sensory gating deficits assessed by the P50 event-related potential in subjects with schizotypal personality disorder

OBJECTIVE

The schizophrenia spectrum includes individuals with schizophrenia, their relatives, and individuals with schizotypal personality disorder. Subjects in the schizophrenia spectrum have disorders of attention, cognition, and information processing. Attention and information processing can be assessed by testing suppression of the P50 event-related potential; the amplitude of the P50 wave is measured in response to each of two auditory clicks. In normal subjects, the P50 wave following the second click is suppressed, or "gated." Schizophrenic patients and their relatives show less suppression of the second P50 wave. Deficits in P50 suppression have high heritability and show linkage to the alpha-7 subunit of the nicotinic cholinergic receptor gene in families with schizophrenia, suggesting that deficits in P50 suppression are trait markers for gating abnormalities in schizophrenia spectrum subjects. Although schizotypal subjects have been shown to have deficits in sensorimotor gating as measured by prepulse inhibition, to the authors' knowledge P50 sensory gating in schizotypal personality disorder has yet to be reported.

METHOD

P50 suppression in 26 subjects with schizotypal personality disorder and 23 normal subjects was assessed through auditory conditioning and testing.

RESULTS

The subjects with schizotypal personality had significantly less P50 suppression than did the normal subjects.

CONCLUSIONS

Subjects with schizotypal personality disorder may have trait-linked sensory gating deficits similar to those in patients with schizophrenia and their relatives. Because these subjects may manifest sensory gating deficits without overt psychotic symptoms, it is likely that these deficits represent a core cognitive dysfunction of the schizophrenia spectrum.

Amphetamine disrupts P50 suppression in normal subjects

BACKGROUND

P50 suppression is viewed as an operational measure of sensory "gating" that is reduced in patients with schizophrenia and their family members. Previous reports have demonstrated that neural gating is regulated by monoaminergic tone in rodent models of P50 suppression.

METHODS

In this study, 11 healthy subjects participated in P50 event-related potential recordings at baseline and after either oral administration of dextroamphetamine (.3 mg/kg) or placebo, to determine if the administration of a monoaminergic agonist produces P50 suppression deficits similar to those observed in patients with schizophrenia.

RESULTS

As hypothesized, amphetamine disrupted the suppression of the P50 event-related potential. There was a statistically significant decrement in P50 suppression during the amphetamine challenge condition (t10 = 3.15, p < .01, mean difference = -44.1%, d = -2.5) relative to the baseline P50 condition. A comparison of P50 suppression in the placebo and amphetamine conditions (both after a baseline recording session) revealed a significant amphetamine-induced disruption of P50 suppression (t6 = 3.71, p < .01, mean difference = -54.4%, d = -3.14).

CONCLUSIONS

The biochemical alterations associated with an amphetamine-induced disruption of P50 suppression in this study may be related to the pathophysiology of P50 suppression deficits in schizophrenia. The findings are consistent with several careful examinations of suppression deficits in rodent models that have identified the monoaminergic regulation of P50 suppression. These data indicate that amphetamine induces a disruption of P50 suppression in normal subjects.

Human and animal studies of schizophrenia-related gating deficits

Prepulse Inhibition (PPI) of the startle response and the P50 auditory-evoked potential suppression are used to assess impairments in the regulation of the neural substrates and to determine the clinical significance of inhibitory deficits in schizophrenia. The study of gating deficits in schizophrenia and in related animal model studies have already advanced our understanding of the neural substrates of information processing abnormalities in patients with schizophrenia. Individuals with schizotypal personality disorder as well as clinically unaffected family members of patients with schizophrenia show PPI and P50 suppression deficits. These "schizophrenic spectrum" populations are not grossly psychotic, nor are they receiving antipsychotic medications. Therefore, the gating deficits are presumed to reflect core (eg, intermediate phenotypic) schizophrenia-linked information processing abnormalities. Several studies have reported that gating deficits are associated with clinical ratings of psychiatric symptoms, thought disorder, and neuropsychologic deficits in patients with schizophrenia. In addition, recent human pharmacologic studies have indicated that gating deficits can be reversed by rationally-selected compounds. Animal model studies have generally shown convergence with the human studies and may lead to improved identification of efficacious new antipsychotic medications for patients with schizophrenia.

The "incredible shrinking" P50 event-related potential

The P50 component of the auditory evoked response has been utilized in studies of sensory gating in schizophrenia for over 15 years. As P50 gating studies have had a greater impact in neuroscience research, investigators have refined several key variables (e.g., filtering) to enhance signal-to-noise ratios. A comprehensive review of P50 reports suggests P50 amplitude has been steadily decreasing over the years. Certain methodological "advances" are suggested as key reasons for this apparent reduction in P50 amplitude. Gating studies continue to yield interesting findings in neuropsychiatric research, especially when ratio vs. absolute difference scores are used.