Hallucinations and cognitive impairments associated with psychosis may be underpinned by N-methyl D- aspartate (NMDA) receptor hypofunction. Within the Research Domain Criteria (RDoC) developed by the National Institute of Mental Health, features of psychosis illness, which can also be transiently produced in healthy normal subjects given NMDA receptor antagonists, fall under the Cognitive System domain. Within this domain, auditory perception and processing speed are most relevant to the proposed work. Research focused on the abnormal functioning of this cognitive system in both psychosis and psychosis vulnerability subject groups may increase understanding of these symptoms. Electroencephalography (EEG) has great potential to elucidate the potential link between NMDA receptor hypofunction and symptoms of illness. Gamma (30-80Hz) oscillations are known to be generated by fast- spiking interneuron and pyramidal neuron microcircuits, and NMDA receptors play an important role in fast- spiking interneuron activity. Using spectral decomposition to analyze EEG, frequency-specific oscillations, including the gamma band, can be isolated and extracted on a millisecond basis. The auditory steady-state response (ASSR) is often used to study impaired EEG gamma oscillations in schizophrenia. These tasks use short-duration trains of repetitive auditory stimulation at a fixed frequency (e.g., every 25ms or 40-Hz) to evoke an ASSR at that same frequency. In particular, the 40-Hz ASSR is typically reduced in evoked response power and exhibits more oscillation phase variability, or reduced phase synchrony, across trials in schizophrenia. These deficits have also been observed in first-degree relatives and individuals at clinical-high risk (CHR) for psychosis. More recently, two additional measures derived from ASSR have demonstrated sensitivity to psychosis pathophysiology. These measures are (1) spontaneous gamma band power in the pre- stimulus baseline period, which is abnormally elevated in schizophrenia, and (2) the 40-Hz ASSR oscillation phase angle, which is significantly delayed in schizophrenia. In a series of previously conducted studies across 9 laboratories, we collected 40-Hz ASSRs from chronically ill (i.e. > 5 years duration) psychosis patients, early illness (i.e. <= 5 years duration) psychosis patients, CHR subjects, and a wide age range of healthy comparison controls. We propose to align these existing EEG data from over one thousand males and females (12-61 years old), across diagnoses and the wellness spectrum. This project will extend prior work by using ASSR phase delay and spontaneous (baseline) gamma power to compare stages of psychosis illness. Any association between these gamma measures and hallucination symptom ratings, neuropsychological speed of processing tests, age, or sex will be determined. Longitudinal effects on 40-Hz ASSR will be assessed in CHR.
Slowed perception has upstream effects on cognition and may contribute to clinical features of psychosis across the wellness spectrum. Aligning existing EEG-based assays of auditory processing, across diagnoses and the wellness spectrum, we will address the association of 40-Hz oscillation phase delay and baseline gamma power with perceptual abnormalities and cognitive impairments, including processing speed.
