What happens in the brain a few seconds before the onset and offset of a hallucinatory episode?
Auditory verbal hallucinations (AVH), the sense of “hearing voices” in the absence of a corresponding auditory source, are a major symptom in schizophrenia. Hemodynamic and electrophysiology studies have shown aberrant activation in fronto-temporal speech and language areas (Curcic-Blake et al., 2017 for review) – indicating neurobiological mediators relating to onset and duration of AVH episodes. However, unanswered is the question of specific neuronal precursors to the spontaneous cessation of an AVH episode. We therefore studied what happens in the brain a few seconds before both the onset and offset of an AVH episode. Time-course analysis demonstrated a differential effect in the left paracingulate sulus, characterized by a significant dip in activation at the end-of-hallucination events, in contrast to a significant peak at the start-of-hallucination events. This activity preceded the motor response for the same events.
Dynamic switching between intrinsic and extrinsic mode networks as demands change from passive to active processing
In this study we report on the relationship between default and extrinsic mode networks across alternating brief periods of rest and active task processing. Three different visual tasks were used in a classic fMRI ON-OFF block design where task (ON) blocks alternated with equal periods of rest (OFF) blocks: mental rotation, working memory and mental arithmetic. We show the existence of a generalized task-positive network, labelled the extrinsic mode network (EMN) that is anti-correlated with the default mode network (DMN) as processing demands shifted from rest to active processing. The results showed a close to perfect correlation for the SMA and Precuneus/PCC time-courses for ON- and OFF-blocks. We suggest the existence of two large-scale networks, an extrinsic mode network and an intrinsic mode network, which are up- and down-regulated as environmental demands change from active to passive processing.
Glutamate- and GABA-modulated interhemispheric temporal lobe connectivity in auditory hallucinations – a combined fMRI and MR spectroscopy (MRS) study
Auditory hallucinations have been linked to aberrant functioning of the left superior temporal gyrus (STG), associated with impaired cognitive control regulated by areas in the prefrontal cortex. However, the mechanisms behind these dysfunctions are still unclear. The current study combined resting state connectivity fMRI with MR spectroscopy in a sample of 78 psychosis patients to explore how the neurochemical correlates of auditory hallucinations modulate left STG functioning. The analyses were focused on glutamate (Glu) and GABA, two neurotransmitters with excitatory and inhibitory functions, respectively. Glu and GABA showed differential effects on left STG connectivity in patients with and without hallucinations.