Quantitative analysis of phosphenes induced by navigation-guided repetitive transcranial magnetic stimulation

Though a cortical visual prosthesis is a promising method for treating severe visual disturbances, long-term blindness is known to depress visual cortex activity. We examined the use of repetitive transcranial magnetic stimulation (rTMS) with a navigation system as a direct functional assessment tool for the visual cortex.

We performed rTMS of the occipital cortex at three different stimulus frequencies (1 Hz, 5 Hz, and 20 Hz), on five stimulus targets around the calcarine fissure in 10 healthy subjects and 3 patients with visual impairment.

In the subjects with normal vision, phosphenes were mostly induced in the visual hemifield contralateral to the stimulation site, and stimulation on the targets inferior to the calcarine fissure predominantly induced phosphenes in the upper visual hemifield. High-frequency stimulation induced larger and complicated-shaped phosphenes at higher rates. The phosphenes induced rate and spatial distribution were altered in the blind patients.

The rTMS has the ability to directly assess the regional visual function of the occipital cortex both in normal-sighted subjects and blind patients. Precise targeting with a navigation system appropriately stimulated the lingual gyri, which contributed to consistently inducing the phosphenes in the upper visual fields. Atypical representation of the phosphenes in patients with visual impairment suggests the alteration of regional cortical excitations and spatial representation due to the cortical reorganization after the loss of visual inputs.