Sensory gating in schizophrenia

Sensory gating is the process by which the brain filters out irrelevant stimuli from the meaningful one. It prevents an overload of information in the higher cortical centres in the brain. The process is central to explaining the delusions and hallucinations in schizophrenia. Sensory gating can be measured through the P50 response, a test where the subject hears two uniform sounds with an interval of 500 milliseconds while connected to an EEG that monitors brain activity. In a normal subject, there’s a decrease in brain activity while hearing the second sound, whereas a subject that shows equal brain activity to the first and second sound is more likely to have schizophrenia. As there’s often an overload of stimuli in cases of schizophrenia, the P50 response illuminates how sensory gating works on a neurological level.

What’s interesting is that research indicates that smoking, which is significantly more common among schizophrenics, can affect sensory gating. Nicotine acutely improves P50 suppression and also stimulates the release of glutamate. In response to the increased levels of glutamate, GABA is released which inhibits certain hippocampal regions and pyramidal neurons, thus suppressing the subsequent response to the next stimuli. When nicotine receptors desensitize quickly, the effect is short-lived, but even in chronic smokers with schizophrenia, there’s an improvement in the P50 response. The cause of this is probably that smoking inhibits monoamine oxidase, which in turn affects dopamine metabolism. I’m in no way advocating smoking as a way to combat schizophrenia – I know from experience that a couple of cigarettes would not have fixed my delusions. However, it’s fascinating that there is a correlation between nicotine and P50 suppression.

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