Overview

The purpose of this basic science study is to better understand human visual perception. Phosphenes are flashes of light that can be induced using Transcranial Magnetic Stimulation (TMS). The location of these phosphenes is known to change with eye movements, but the exact influence of these movements is not fully understood.

The main questions this study aims to answer are:

How does stimulating specific areas of the visual cortex relate to where a person perceives a phosphene? How do eye movements affect the accuracy of mapping these sensations? The study will enroll three groups of adults (aged 18-65): patients with Visual Snow Syndrome, color-blind participants, and healthy volunteers.

During the experiment, participants will receive single-pulse TMS to their visual cortex. Simultaneously, their eye movements will be monitored with an eye-tracker, and their brain activity will be recorded with EEG. Participants will be asked to report the location of the perceived light flashes. This will help create a map of the brain areas responsible for visual sensations and clarify how eye movements influence this process.

Eligibility

Inclusion Criteria:

For the "Patients with Visual Snow Syndrome" Arm:

Confirmed diagnosis of Visual Snow Syndrome according to the International Classification of Headache Disorders, 3rd edition (ICHD-3) criteria.

Presence of persistent, dynamic, tiny dots ("visual snow") across the entire visual field.

Presence of at least one of the following additional visual symptoms:

Palinopsia (prolonged afterimages). Photophobia (hypersensitivity to light). Nyctalopia (impaired night vision). Blurred vision. Patients whose symptoms appeared after a SARS-CoV-2 viral illness may be included.

For the "Color-blind Participants" Arm:

Presence of a color perception disorder. Must have a formal diagnosis of color blindness confirmed by specific ophthalmologic tests (e.g., Ishihara, Rabkin, Justova, or Stilling polychromatic plates, or the Farnsworth-Munsell test).

For the "Healthy Controls" Arm:

Absence of any active or past neurological, psychiatric, or ophthalmological disorders.

Exclusion Criteria:

General Criteria (Applicable to all participants):

Use of psychotropic drugs. Any contraindications to Transcranial Magnetic Stimulation (TMS), such as a history of epilepsy, seizures, or metallic implants in the head.

Pregnancy.

Specific to the "Patients with Visual Snow Syndrome" Arm:

Presence of any organic abnormalities in the eyes or visual system identified during a standard ophthalmologic examination.

Presence of comorbid psychiatric disorders or other medical conditions (e.g., migraine with aura) that could explain the symptoms.

Specific to the "Color-blind Participants" Arm:

Any non-congenital cases of color blindness. Presence of concomitant visual impairments that could affect visual perception independently of color blindness, such as cataracts, glaucoma, or macular degeneration.

Presence of concomitant neurological or psychiatric diseases, such as schizophrenia, epilepsy, or mood disorders, that could affect the study results.