Table 2.
Summary of brain areas that are important in understanding the effects of neurostimulation on AVHs, and their connectivity with other regions.
| Brain region | Role in AVHs | Relevance to neurostimulation treatment | Connectivity |
|---|---|---|---|
| Superior temporal gyrus (STG) | Includes PAC, Wernicke's area, and planum temporale. Structural abnormalities and functional activity consistently implicated in AVHs, and during monitoring tasks. | Posterior STG activity reduced after neurostimulation; this correlates with reduction in AVH severity. | Strong connectivity with TPJ, and effective connectivity with ACC. Also connected to IFG through arcuate fasciculus white matter tract. |
| Inferior frontal gyrus (IFG, Broca's area) | Crucial for production of speech (including inner speech), particularly in the left hemisphere. Role of right IFG still relatively unexplored. | rTMS of Broca's area does not lead to a reduction in AVH frequency. Reduction in activity in IFG following stimulation of left TPJ, though not correlated with reduction in AVH frequency. | Connected to STG through arcuate fasciculus white matter tract. Excessive functional connectivity with putamen in voice-hearers. |
| Anterior cingulate cortex (ACC) | Activation seen during AVHs may reflect conscious evaluation of stimuli, and in combination with STG, may be involved in monitoring processes. | Reduction in activity in ACC following stimulation of left TPJ, though not correlated with reduction in AVH frequency. | Connectivity with STG and TPJ may reflect verbal monitoring processes – effective connectivity during monitoring task is reduced in voice-hearers. |
| Inferior parietal lobe (IPL) | Often activated in symptom-capture studies of AVHs, and commonly linked to feelings of self-agency. | Data from neuroimaging has not implicated changes in activation post-neurostimulation; however, close proximity could mean activity is modulated by TPJ stimulation. | May be part of an alternative pathway that runs laterally to the arcuate fasciculus, between IFG and STG. |
| Putamen | Hoffman's corticostriatal loop model specifies that an overabundance of language representations initiated by the putamen may surface ‘unbidden thoughts’ as AVHs, due to hyperconnectivity with STG and IFG. | If Hoffman's model is supported, disruption of hyperconnectivity with this region may be related to the therapeutic effect of neurostimulation. | Excessive functional connectivity with IFG and STG in voice-hearers. |
STG, superior temporal gyrus; PAC, primary auditory cortex; TPJ, temporoparietal junction; ACC, anterior cingulate cortex; IFG, inferior frontal gyrus; IPL, inferior parietal lobe.