Visual Hallucinations in Parkinson’s Disease Linked to Disrupted Visual-to-Semantic Brain Dynamics
Parkinson’s disease (PD) is widely recognized for its hallmark motor symptoms, including tremors, rigidity, and bradykinesia. However, non-motor symptoms such as cognitive impairment and neuropsychiatric disturbances often profoundly affect patients’ quality of life. Among these, visual hallucinations stand out as particularly disturbing and challenging to manage clinical phenomena. Researchers have long sought to unravel the neural mechanisms underpinning these hallucinations, which, despite their prevalence, remain poorly understood. A groundbreaking study published in npj Parkinson’s Disease in 2025 by Pérez-Carasol, Martinez-Horta, Horta-Barba, and colleagues sheds new light on the neural dynamics contributing to this perplexing symptom.
Visual hallucinations in PD patients range from simple flashes of light to vivid, complex scenes featuring people or animals. These hallucinations not only cause distress but also herald faster cognitive decline and increased risk of dementia. Despite extensive investigation, the precise pathophysiology has eluded consensus, with hypotheses implicating neurotransmitter imbalances, aberrant visual processing, and disrupted higher-order cognition. The new research integrates advanced neuroimaging, electrophysiological recording, and computational modeling to reveal that the critical disruption lies in the dynamic interplay between visual perception and semantic processing centers in the brain.
At the heart of this discovery is the concept of visual-to-semantic transformation—a complex neural process where raw visual inputs are translated into meaningful objects and concepts. In healthy individuals, incoming sensory signals from the retina are initially processed in early visual cortices before ascending via the ventral visual stream through progressively higher-order areas that assign semantic context. This flow allows us to interpret blurred, ambiguous, or incomplete images rapidly and reliably. The researchers hypothesized that aberrancies in this cascade could lead to misinterpretations of visual stimuli, potentially fueling hallucinatory experiences.
Using state-of-the-art magnetoencephalography (MEG) to measure brain activity with millisecond precision, the team conducted experiments comparing PD patients with and without visual hallucinations to healthy controls. Participants were presented with visually challenging stimuli designed to probe the efficiency of visual-to-semantic processing. The neurophysiological data unveiled that patients experiencing hallucinations exhibited marked delays and dyscoordination in the transmission of information from the visual cortex to regions responsible for semantic analysis, primarily situated in the anterior temporal lobe and prefrontal cortex.
Further depth was added through functional magnetic resonance imaging (fMRI), which revealed diminished connectivity between visual and semantic processing hubs during resting state and task-based conditions in hallucinating patients. These functional disconnects were coupled with altered neurotransmitter signatures detected through positron emission tomography (PET), providing biochemical substrate to the observed functional impairments. Critically, the severity of connectivity disruption correlated with hallucination frequency and intensity, indicating a causal relationship.
The study also leveraged computational models simulating neural network dynamics. These models demonstrated that introducing delays or noise within the visual-to-semantic pathway induced unstable representations, akin to the false percepts characteristic of hallucinations. This instability manifests as the brain’s semantic circuits attempting to ‘fill in gaps’ from ambiguous or degraded sensory input with internally generated imagery. Such insights align with emerging frameworks in cognitive neuroscience postulating that hallucinations may arise from predictive coding errors, where top-down expectations overpower bottom-up sensory signals.
Moreover, the research integrated genetic profiling, uncovering that certain PD patients with polymorphisms affecting synaptic transmission and neural plasticity showed heightened vulnerability to the breakdown of visual-to-semantic integration. This finding suggests that genetic predisposition may modulate the risk and phenomenology of hallucinations, offering avenues for personalized interventions. The implications are profound, emphasizing that hallucinations are not merely by-products of clinical progression but reflect specific dysfunction in brain circuit dynamics and molecular pathways.
Therapeutically, these revelations herald potential innovations in managing PD hallucinations. Current pharmacological treatments, often reliant on antipsychotics, are limited by side effects and inconsistent efficacy. Targeting the neural circuits implicated in visual-to-semantic transformation, possibly through neuromodulation techniques such as transcranial magnetic stimulation or novel drugs enhancing synaptic integration, offers a more focused approach. The study encourages future trials to adopt biomarkers identifying patients with disrupted visual-to-semantic connectivity for tailored therapies.
This research also enhances our understanding of perception in general. Visual hallucinations in PD, when viewed through the lens of disrupted brain dynamics, exemplify how complex cognitive functions depend on fluid communication between sensory input and higher-order semantic networks. It underscores the brain’s remarkable yet vulnerable capacity to generate coherent experience, and how subtle imbalances can give rise to profound perceptual anomalies. Such mechanistic insights are likely valuable beyond PD, extending to other neuropsychiatric conditions involving hallucinations, including schizophrenia and dementia with Lewy bodies.
Intriguingly, the study’s findings dovetail with recent advances in artificial intelligence and machine learning, where models emulate hierarchical sensory processing to interpret vast visual datasets. Understanding human brain dysfunction offers clues for refining AI architectures capable of resilient perception even under ambiguous conditions. Conversely, AI tools may accelerate deciphering pathological brain states, creating symbiotic progress in neuroscience and technology.
Additionally, the team’s multidisciplinary approach set a new benchmark for hallucination research, blending neuroimaging, electrophysiology, computational neuroscience, and molecular genetics. This integrative framework exemplifies how dissecting complex brain phenomena necessitates crossing traditional disciplinary boundaries. As researchers expand on these findings, collaborations across neurology, psychiatry, bioengineering, and computational modeling will be pivotal in unlocking further mysteries of the brain’s perceptual machinery.
The socio-clinical impact of this work cannot be overstated. Visual hallucinations erode patient autonomy, complicate caregiving, and increase healthcare burdens. By pinpointing concrete neural substrates and pathways, this study potentially accelerates the development of early diagnostic tools, preemptive interventions, and novel therapeutics. Such advancements promise to improve life quality for millions affected by Parkinson’s worldwide.
Looking forward, the authors emphasize the need to explore longitudinal changes in visual-to-semantic dynamics throughout the PD disease course. Determining how these disruptions evolve and interact with other neuropathological processes like dopaminergic loss or cortical atrophy may clarify whether interventions can restore normal perception or merely mitigate hallucination severity. Furthermore, extending investigations into other sensory modalities could reveal whether analogous mechanisms underlie different hallucination types.
In summation, the pioneering work of Pérez-Carasol and colleagues ushers in a new era in understanding visual hallucinations in Parkinson’s disease. By unraveling the disrupted neural dialogue linking visual perception to semantic cognition, the study transforms a longstanding clinical puzzle into a tangible target for innovative research and therapeutic strategies. As Parkinson’s patients continue to confront the challenges of their disease, these insights offer hope for clarity amid the hallucinated shadows.
Subject of Research: Neural mechanisms underlying visual hallucinations in Parkinson’s disease focusing on disrupted dynamics between visual and semantic brain regions.
Article Title: Disrupted visual-to-semantic dynamics promote visual hallucinations in Parkinson’s disease
Article References:
Pérez-Carasol, L., Martinez-Horta, S., Horta-Barba, A. et al. Disrupted visual-to-semantic dynamics promote visual hallucinations in Parkinson’s disease. npj Parkinsons Dis. (2025). https://doi.org/10.1038/s41531-025-01235-1
Image Credits: AI Generated
