Excitation-inhibition balance (E/I balance) is a fundamental property of cortical microcircuitry. Disruption of E/I balance in prefrontal cortex is hypothesized to underlie cognitive deficits observed in neuropsychiatric illnesses such as schizophrenia. To elucidate the link between these phenomena, we incorporated synaptic disinhibition, via N-methyl-D-aspartate receptor perturbation on interneurons, into a network model of spatial working memory (WM). At the neural level, disinhibition broadens the tuning of WM-related, stimulus-selective persistent activity patterns. The model predicts that this change at the neural level leads to 2 primary behavioral deficits: 1) increased behavioral variability that degrades the precision of stored information and 2) decreased ability to filter out distractors during WM maintenance. We specifically tested the main model prediction, broadened WM representation under disinhibition, using behavioral data from human subjects performing a spatial WM task combined with ketamine infusion, a pharmacological model of schizophrenia hypothesized to induce disinhibition. Ketamine increased errors in a pattern predicted by the model. Finally, as proof-of-principle, we demonstrate that WM deteriorations in the model can be ameliorated by compensations that restore E/I balance. Our findings identify specific ways by which cortical disinhibition affects WM, suggesting new experimental designs for probing the brain mechanisms of WM deficits in schizophrenia.

Glutamatergic neurotransmission mediated by N-methyl-D-aspartate (NMDA) receptors is vital for the cortical computations underlying cognition and might be disrupted in severe neuropsychiatric illnesses such as schizophrenia. Studies on this topic have been limited to processes in local circuits; however, cognition involves large-scale brain systems with multiple interacting regions. A prominent feature of the human brain's global architecture is the anticorrelation of default-mode vs. task-positive systems. Here, we show that administration of an NMDA glutamate receptor antagonist, ketamine, disrupted the reciprocal relationship between these systems in terms of task-dependent activation and connectivity during performance of delayed working memory. Furthermore, the degree of this disruption predicted task performance and transiently evoked symptoms characteristic of schizophrenia. We offer a parsimonious hypothesis for this disruption via biophysically realistic computational modeling, namely cortical disinhibition. Together, the present findings establish links between glutamate's role in the organization of large-scale anticorrelated neural systems, cognition, and symptoms associated with schizophrenia in humans.default-mode network | task-based activation | task-based deactivation | pharmacological manipulation | fMRI D rug treatments for serious mental illnesses and investigations of the neurochemical bases of healthy cognition have, for the most part, targeted the slow neuromodulatory neurotransmitters, dopamine and serotonin (1). However, rapid excitatory glutamatergic and inhibitory γ-aminobutyric acid (GABA) signals mediate local and long-range cortical computations (2) and play a critical role in cognition and severe psychiatric illnesses such as schizophrenia (3-5). We investigated how disrupting the N-methyl-D-aspartate (NMDA) receptor component of fast glutamatergic neurotransmission via the administration of the NMDA receptor antagonist ketamine altered cognitive performance and systemslevel neural activity and connectivity in healthy volunteers. Furthermore, we related these system-level neural changes to behavior and transiently evoked psychotic symptoms associated with schizophrenia.Studies investigating glutamate's role in cognition have largely focused on local circuits (5-7); however, cognition involves largescale brain systems with multiple interacting regions. Recent neuroimaging work highlights the competitive relationships between two large-scale neural systems: a set of brain regions preferentially engaged during tasks that require goal-directed cognition and attention (task-positive) and the regions associated with resting conditions [default-mode network (DMN)] (8-10). The neurotransmitter mechanisms behind this inverse relationship remain unexplored, as does the role of this phenomenon in serious mental illness (11). Thus far, functional neuroimaging (fMRI) investigations of these large-scale neural systems have mostly been correlational (12), and the synaptic mechanisms for these eff...

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