Parkinson's disease (PD) is the second most common neurodegenerative condition worldwide, characterised by motor symptoms, but with other symptoms such as visual impairment.

The aim is to compare visual function between PD patients and healthy subjects in order to adequately characterise the visual capabilities of the PD population and perform oculomotor or perceptual therapy to find optometric solutions to slow down the visual impairment they suffer from or minimise their visual symptoms.

In the first phase, non-invasive tests will be carried out, such as measuring visual acuity, refraction, pupil diameter in different lighting conditions, sensory dominance, contrast sensitivity, colour vision, stereopsis, reading speed, binocular vision, eye movements and influence on quality of life.

In the second, visual oculomotor or perceptual exercises will be performed in a group of PD patients to assess whether there is stabilisation of impairment or improvement of these visual skills. These will be performed in a non-invasive way using simple and easy-to-use instruments or an application on an electronic device could be used.

Finally, in the third phase, those visual skills that have been treated will be re-evaluated to assess possible changes, compared with a group of PD patients who have not undergone the visual exercises.

Parkinson's disease (PD) is the second most common neurodegenerative condition worldwide. It is a movement disorder characterised by tremor, rigidity, bradykinesia and postural instability. However, 78% of PD patients report visual problems such as dry eye, reduced visual acuity, visual field defects, impaired contrast sensitivity (CS), impaired eye movements, diplopia, convergence insufficiency, impaired colour vision, visual hallucinations and problems in visual-spatial orientation. In fact, visual dysfunction is one of the initial symptoms of PD. A recent study has found that dopaminergic neurons affected in PD show physiological dysfunctions, but do not die. Currently, there is no cure for PD; treatments consist of controlling symptoms, and the earlier the disease is detected and administered, the more effective they are.

It is hypothesised that a programme of visual oculomotor therapy and/or visual perceptual learning (VPL) could slow or halt the deterioration of oculomotor ability and/or CS in these patients. Visual perceptual learning or VPL is a long-term improvement in performance on a visual task, any relatively permanent change in perception that arises from visual experience. Contrast Sensitivity or CS is the ability of the visual system to differentiate an object from the background in which it is located by the difference in contrast. It is the ability to detect differences in luminance between adjacent areas in an image.

If the results confirm the hypothesis, this study will have a great impact on the PD population because it could slow down the deterioration of oculomotor ability that 75% of them suffer from. It could also slow down the decline in CS that they suffer, as demonstrated by several studies. Consequently, patients would also experience an improvement in their quality of life, as they would be able to continue to perform activities of daily living that are affected by visual impairment such as walking, reading, driving and cooking. This would have a major impact on society as PD affects many people worldwide. In Europe, the prevalence and incidence rates of PD are estimated to be approximately 108-257/100 000 and 11-19/100 000 per year, respectively. Furthermore, this project would expand scientific knowledge because to date there is no literature that has evaluated VPL in patients with PD. However, there is research on amblyopia, Stargardt's disease and other neural pathologies, such as Huntington's disease. Likewise, with regard to oculomotor therapy, there is still a lot of research to be done, as there are few studies on this subject.

This is an analytical, longitudinal, prospective and observational study. In the first phase, to characterise the vision of the population with Parkinson's disease, non-invasive tests would be performed, such as measuring visual acuity, refraction, contrast sensitivity, colour vision, depth vision, the state of binocular vision and visual pathways and eye movements. Simultaneously, a control population will be measured to compare the data obtained in PD patients.

In a second phase, visual oculomotor and/or perceptual exercises would be performed in a group of PD patients to assess whether there is stabilisation of the impairment and/or improvement of these visual skills.

Finally, in the third phase, those visual skills that have been treated would be re-evaluated to assess possible changes, compared with a group of PD patients who have not undergone the visual exercises.

The research will be carried out at the Laboratory of Vision and Colour Sciences of the Department of Optics, Optometry and Vision Sciences of the University of Valencia, at the Arnau de Vilanova Hospital and at the Parkinson's Association of Valencia. These are the institutions from which the participants who take part in the study will come. They will do so voluntarily, will not be coerced to participate in any way and will be free to choose whether they want to participate or not, after having been explained to them what this project consists of. Furthermore, they will be able to revoke their signed consent at any time, so that they will automatically be excluded from the study and their data will not be used in the study if the patient so wishes. Furthermore, this project does not harm the patient's health and does not bring any financial benefit to the patient.

Only the members of the project will have access to the original study data. These data will be collected on paper, and the original source will be kept under lock and key in the laboratory of Vision and Colour Sciences of the Department of Optics, Optometry and Vision Sciences of the University of Valencia. For data analysis, the principal investigator will extract the data into a computer document in which each patient will be identified by a two-letter code (GP: control group; PD: Parkinson's patient) and three numbers. Not even the initials of the name will appear. Under this code, the data of each patient will be entered. This is the data that the rest of the group will see and have access to. Adverse event reporting and change management will be carried out through the Reservio platform, which will be used for appointment planning and online booking.

Data analysis will be carried out using the SPSS (Statistical Package for Social Sciences) statistical software and the Matlab numerical calculation system. The statistical analysis plan is as follows: the mean, standard deviation and normality of the samples will be performed with the Shapiro-Wilk test. Statistical comparison between groups will be performed with Student's t-test if the data follow a normal distribution or with the U-Mann-Whitney test otherwise. The statistical test used for qualitative variables will be Pearson's chi-square test.

Inclusion Criteria:

• Subjects without neurodegenerative diseases or systemic illnesses or ocular pathologies.
• Subjects not taking medication with visual side effects.
• Subjects able to perform the tests.
• Subjects of similar age and sex as their corresponding Parkinson's patient so that both samples are age and gender matched.
• Subjects diagnosed with Parkinson's disease.
• Parkinson's patients classified according to the Hoehn & Yahr Scale.

Exclusion Criteria:

• Subjects with ocular pathologies (such as Glaucoma, Age-Related Macular Degeneration, Retinopathies, Corneal Opacifications, Senile Cataracts, Severe Palpebral Ptosis) or systemic illnesses that may affect the visual system and alter the results (such as Severe Cardiopathies, Diabetes Mellitus, oncological diseases, systemic tissue disorders, chronic infectious diseases or conditions after organ or tissue transplantation).
• Subjects with neurodegenerative or neural diseases other than the study itself, such as Alzheimer's disease, Devic's disease, Huntington's disease, Creutzfeldt-Jakob disease, epilepsy, ataxia, multiple sclerosis or amyotrophic lateral sclerosis.
• Subjects taking medication that may alter any visual ability such as anxiolytics, antidepressants, sleeping pills.
• Subjects with problems in understanding and following the tests.
• Patients who have been previously treated under a vision therapy program.

Patients with Parkinson's Disease.