The visual system has increasingly been recognized as an important site of pathology in patients with schizophrenia and other psychoses. Visual system impairments manifest as visual perceptual aberrations, deficits in visual processing tasks, and visual hallucinations (VH). In psychosis spectrum disorders, increased visual aberrations are strongly correlated with worse hallucinations and delusions. It is also recognized that poorer performance on visual spatial working memory, visual integration, and velocity discrimination tasks are associated with greater negative symptoms (a major contributor to disability). VH are common in psychotic disorders (30-70% prevalence) and can be refractory to existing treatments. VH have been understudied in psychosis with much of the literature focusing on auditory hallucinations. Despite the neuroscientific and clinical significance of VH, the brain regions responsible are less clear.

Functional neuroimaging studies have identified neural correlates of VH across multiple brain regions (lingual, fusiform, cuneus, lateral geniculate nucleus, and occipital cortex) and support hypotheses that increased visual cortex activity and sensory cortex over-stimulation generate VH. However, whether these neuroimaging findings represented a cause, consequence or incidental correlate of VH was unclear until recently. Using a newly validated technique termed lesion network mapping, researchers demonstrated that focal brain lesions having a causal role in the development of VH can occur in different brain locations, both inside and outside sensory pathways, and that these lesions are functionally connected to the lateral geniculate nucleus, a major relay center for the visual pathway. They also found that 98% of the subcortical and cortical lesions were connected to the exact same location in the extrastriate visual cortex. Therefore, the association between extrastriate visual cortex activation and VH would suggest this region may be optimal for modulation via brain stimulation.

One method by which cortical excitability can be altered is through the use of transcranial electrical stimulation (tES), a non-invasive brain stimulation technique. High definition tES (HD-tES) is a refined version of tES with improved spatial precision of cortical stimulation. This involves the application of a weak electrical current (1-2 mA) delivered to the brain via scalp electrodes. The effects of tES modulate cortical excitability where anodal stimulation tends to increase (i.e. the resting potential becomes less negative) and cathodal stimulation tends to decrease the underlying membrane potential (i.e. the resting potential becomes more negative) (14,15). Studies have shown that tES can modulate visual cortical function in a polarity-dependent manner, where anodal stimulation can increase and cathodal stimulation can decrease the amplitude of the N70 component from the visual-evoked potential. While tES is a promising adjunctive treatment of auditory hallucinations and negative symptoms in schizophrenia, less is known about its role in treating VH. To date, two cases have been described where cathodal tES (i.e., outward current flow) over the occipital area was applied to patients experiencing treatment refractory VH, and this resulted in symptomatic improvement. Taken together, the recent lesion network mapping identifying the extrastriate visual cortex as a major source of VH in schizophrenia combined with these two single-patient case studies suggest that it may be possible to alleviate VH by designing a tES protocol that targets the extrastriate visual cortex bilaterally. Technological advances in noninvasive neuromodulation and electrical field modeling further allow us to create a tES protocol specifically guided by the results of lesion network mapping studies (i.e., using the exact Montreal Neurological Institute coordinates) with high spatial resolution (i.e., using HD-tES).