Altered integration of bodily and visual signals in visual snow syndrome

Background Visual snow syndrome (VSS) is characterized by persistent visual disturbances and is considered to involve altered sensory integration and salience processing. Neuroimaging studies suggest aberrant attention and salience network integration, yet the real-time interaction between internal bodily signals and external sensory processing remains unclear. Methods We conducted a case–control electroencephalogram study in 18 VSS patients and 16 matched controls performing a visual oddball task with standard and deviant stimuli. Heartbeat-evoked potentials (HEP) were extracted as an index of cortical interoceptive processing and visual mismatch negativity (MMN) as a marker of exteroceptive prediction error. Trial-wise analyses and source localization assessed interoceptive–exteroceptive interactions. Results VSS participants exhibited altered HEP amplitudes relative to controls, together with reduced condition-dependent differentiation between standard and deviant trials. The relationship between HEP and MMN amplitudes was significantly moderated by group and associated with symptom intensity in VSS. Source analysis localized these effects to a left frontotemporal network. Conclusions VSS is associated with altered interoceptive–exteroceptive interactions during visual perception, which may contribute to persistent visual symptoms.