They describe how hallucinations originate in Parkinson’s

A team of Parkinson’s Disease and Movement Disorders Group of the Sant Pau Research Institute (IR Sant Pau) and the Neurology Service of the Sant Pau Hospital of Barcelona has described “for the first time in detail” how the visual perception of people with Parkinson’s disease who experience hallucinations is altered.

Posted in ‘NPJ Parkinson’s Disease‘, the study led by the researcher Laura Pérez-Carasol and the doctors Javier Pagonabarraga and Saül Martínez-Horta offers one of the “most complete characterizations carried out to date” on the neurocognitive mechanisms that explain this phenomenon, the hospital reported this Thursday in a statement.

The visual hallucinations They constitute one of the most frequent and clinically significant non-motor symptoms in Parkinson’s disease, even in the initial stages, and usually manifest as brief sensations of presence, figures that fade when looking directly or shadows that cross the periphery of the visual field and which can also include very subtle illusions, such as attributing facial features to everyday objects.

Although many people recognize that these perceptions are not real, their appearance signals that the systems that allow us to interpret the environment begin to function with less precision, and numerous studies have shown that these initial episodes increase the risk of developing cognitive impairment and to evolve towards more complex and persistent hallucinations.

The researchers studied 93 patients with Parkinson’s disease without dementia, and the participants performed a visual categorization task in which they had to quickly decide whether an image corresponded to a face, an object, or an object that simply resembled a face, while their brain activity was recorded using electroencephalography.

This methodology allowed us to observe, with great temporal precision, how the brain constructs a perception from its most basic stages to the interpretation and cognitive review phases.

Signs

The analyzes showed that patients with hallucinations do not present alterations in vision or in the first stages of visual processing: the P100 signal, which reflects this initial phase, is normal in all groups, indicating that they perceive sharpness, contrast or movement in the same way as any person without hallucinations.

Alterations appear in later stages in three signals, when the brain must organize visual information and give it meaning, and that is when the detailed analysis of brain activity shows that the first disruption appears in the structural coding phase, when the brain interprets the shape of the stimulus and generates a recognizable representation.

This activity, reflected in the signal N170, appears markedly reduced in patients with hallucinations, even in those without cognitive impairment.

The increase of the signal N300 indicates that the system tries to complete the perception too soon, imposing internal meanings before the visual information is fully available, a tendency even more marked in patients with mild cognitive impairment, who rely more on internal processes to interpret external stimuli.

The study also identifies a third alteration in the phase intended to review and correct the generated interpretation: this stage, associated with the P600 signalappears weakened in patients with hallucinations, especially in those who also have mild cognitive impairment.

The authors have pointed out that the identification of characteristic neurophysiological patterns opens the door to the development of objective markers that allow subtle changes to be detected before hallucinations become clearly manifest or before more marked cognitive deterioration occurs.