Before testing participants with psychosis we began by testing the scanning protocols and experimental task in healthy control participants. We recorded neurochemical levels from a region of medial frontal cortex non-invasively using an MRI technique called MR spectroscopy (MRS) while participants performed a working memory task and a simple visual (control) task.
In the memory task participants learned the colour, shape and location of four stimuli (the encoding phase) and then recalled the colour of one item from either a shape cue or a location cue (the recall phase). In the control task participants selected the colour of a single item from four colour options.
In our exciting results, we were able to see the change in glutamate (the brain’s main excitatory neurotransmitter) over time from the onset of the stimuli. We observed an increase in glutamate peaking at 1.1 seconds during the encoding phase and peaking at 1.4 seconds in the recall phase, compared to the during the control task in which there was no systematic change in glutamate over time. Ours study is the first MR spectroscopy study to show how the glutamate response changes over time after stimulus onset. This is really important as it means we can determine the best time to record glutamate from in future studies. Individuals with psychosis often report being slowed in their processing speed, so we can also assess if the glutamate time course is slower in individuals with psychosis.
We did not find any overall change in GABA (the main inhibitory neurotransmitter) across participants during the memory trials compared to the control trials, but individuals who had higher GABA levels during the memory trials compared to the control trials tended to be better at the working memory task than those who had lower GABA levels in the memory trials.