Brain sequencing alters visual perception in Parkinson's patients with hallucinations |IM Medicine

A team from the Parkinson's Disease and Movement Disorders Group of the São Paulo Research Institute (IR Sant Pau) and the Department of Neurology at the Hospital de São Paulo has for the first time described in detail how visual perception is altered in patients with Parkinson's disease.

The Parkinson's Disease and Movement Disorders Group of the Sant Pau Research Institute (IR Sant Pau) and the Neurological Service of the Sant Pau Hospital have for the first time detailed how visual perception changes in people with Parkinson's disease who experience hallucinations.The study, led by researcher Laura Perez-Carasol and doctors Javier Pagonabaraga and Saul Martínez-Horta, has just been published in the Nature group npj Parkinson's disease and offers one of the most comprehensive descriptions of the neurocognitive mechanisms that explain the phenomenon.

Visual hallucinations are one of the most common and clinically significant non-motor symptoms of Parkinson's disease, even in the early stages.They usually appear as a brief sense of presence, figures that fade when viewed directly, or shadows that cross the edge of the field of vision.They can also involve very subtle illusions, such as attaching facial features to objects on a daily basis.Although many people admit that these ideas are not true, their appearance suggests that the systems that allow us to interpret the environment are starting to operate with less certainty.Several studies have shown that these initial events increase the risk of developing cognitive impairment and progression to more complex and persistent hallucinations.

Dr. Javier Pagnabaraga said, "The early manifestations suggest us that the brain has been difficulty coordinating the information obtained from the senses with the internal mechanisms that give meaning to the experience.Although the patient is completely aware that the perceptions are not real, they reflect an imbalance that is worse with time. "

How the mind constructs perception

To understand which processes are disrupted and cause hallucinations, the researchers studied 93 Parkinson's patients without dementia.Participants completed a visual classification task in which they had to quickly decide whether an image corresponded to a face, an object, or just a face that resembled an object, while their brain activity was recorded by electroencephalography.It allows us to observe with temporal precision how cognition progresses to the level of cognitive review.

Analyzes showed that patients with hallucinations did not present changes in the first stages of vision or visual processing.The P100 signal, which reflects this first phase, is normal in all groups, indicating that they perceive sharpness, contrast, or motion as well as any person without hallucinations.Differences appear at later stages, when the brain has to organize and assign meaning to visual information.

A detailed analysis of brain activity shows that the first disruption occurs at the level of systematic coding, the brain interprets the shape of the stimulus and forms a recognition image.This activity, shown in the N170 signal, appears to be significantly reduced in patients with hallucinations, even in those without cognitive impairment.This reduction indicates that the first visual images are made with less precision, internal definition and less structural stability, which causes the brain to function on a careless and weak basis.

According to Laura Pérez Carasol, "This first change is necessary to understand everything that follows. If the first visual representation is already weak or incomplete, the brain loses the stable reference that it normally creates meaning. It is as if the image is half-formed, and this opens the door to receive other large internal signals."

In addition to this weakening of the perceptual base, a second imbalance occurs.Normally, the brain uses prior knowledge (memories, learned associations, expectations) only when visual information is ambiguous.However, in patients with hallucinations, this mechanism is activated prematurely and with excessive intensity.An increase in the N300 signal indicates that the system completes perception too quickly, imposing internal meaning before visual information is fully available.This tendency is even more pronounced in patients with mild cognitive impairment, who rely more heavily on internal processes to interpret external stimuli.

The study also identifies a third modification of the stage aimed at reviewing and correcting the interpretation generated.This phase, associated with the P600 signal, appears to be impaired in patients with hallucinations, particularly in those with mild cognitive impairment.A reduction in the P600 indicates a poor capacity for cognitive monitoring, which makes it difficult to correct interpretations that do not match reality and allows false perceptions to gain greater persistence and confidence more easily.

This series of changes—weak structural coding, premature semantic activation, and an inadequate monitoring system—explains why perception loses stability and becomes more vulnerable to producing hallucinatory experiences.As Dr. Pigonabarraga summarizes, "What we see is a progressive loss of harmony between systems that must function correctly and in order. When this harmony is disrupted, conception ceases to be a stable process and begins to rely heavily on internal signals. This combination ends up being fertile ground for fertility."

Lack of coordination with important clinical consequences

In addition to describing how hallucinations occur, the authors emphasize that the pattern of lack of coordination between different stages of visual processing provides valuable information about the clinical evolution of the disease.The results suggest that these perceptual changes reflect progressive changes in cognitive dynamics, particularly in individuals who exhibit mild cognitive impairment.This combination – perceptual instability and cognitive instability – governs a particular vulnerability profile.does, which is important both for understanding current symptoms and for predicting future evolution.

As Pérez-Carasol explains, "These early signs allow us to see when the brain begins to lose tuning and regulation before more obvious symptoms appear. If we can understand this vulnerability early, we can identify patients who need close monitoring and adaptive interventions before more disabling symptoms occur."

Clinical interpretation of these results will help transform healthcare, said Dr.Sal Martínez-Horta who points out: "If we can detect these changes in perceptual and cognitive dynamics early, we can adapt treatments and support strategies earlier based on the profile of each patient."

In general, the study shows that false perception is part of the phase of serocognitive disintegration, where the combination between perception, meaning and judgment becomes stronger.This method, especially for patients with mild cognitive impairment, has been combined as a clinical indicator of great importance to identify risk groups and guide people's diagnosis and follow-up methods.

A breakthrough that opens new avenues for early detection and personalized care

The authors point out that a detailed model of how the different stages of perception change could change the way non-motor symptoms of Parkinson's disease are assessed.The identification of characteristic neurophysiological patterns—such as a decrease in the N170, an early increase in the N300, or a decrease in the P600—opens the door to the development of objective markers that allow the detection of subtle changes before hallucinations become apparent or more significant cognitive impairment occurs.gives

For Dr. Javier Pagonabarra;This approach is a significant development: "Understanding this mechanism not only explains hallucinations, but also allows us to identify early signs of risk and provide more specific intervention strategies. We can envision more detailed assessments combining clinical and neurophysiological measures."

Furthermore, the study results reinforce the role of IR Sant Pau as an international reference center in research on non-motor symptoms of Parkinson's disease.The work demonstrates that a detailed analysis of perceptual mechanisms can provide fundamental information on the progression of the disease and the possibilities for intervention in the early stages.This type of research, funded by the Fundació La Marató de TV3 and the Carlos III Health Institute, marks a line of work dedicated to the development of advanced diagnostic tools and new therapeutic strategies based on a precise understanding of the brain processes that support perception and cognition.