Effects of different transcranial magnetic stimulation coil types on phosphene thresholds and their association with motor cortex excitability

Phosphenes can be induced by applying transcranial magnetic stimulation (TMS) to the visual cortex. Since multiple factors influence phosphene perception, this study aimed to examine the effects of different TMS coil types on phosphene thresholds (PTs). Additionally, the relationship between PT and motor cortex excitability was explored. In Session 1, TMS was applied to the left visual cortex of 22 healthy individuals using a round coil and a figure-eight coil, and PT was recorded. Resting motor threshold (RMT), active motor threshold (AMT), and short-interval intracortical inhibition (SICI) were assessed by stimulating the left motor cortex. After 5–7 days, the visual cortex was stimulated again in Session 2 with the same healthy individuals. In Session 1, the mean PTs obtained with the round and figure-eight coils were 39.71 ± 9.67% (n = 17) and 53.93 ± 14.26% (n = 15), respectively (p = 0.001). In Session 2, the mean PTs were 37.83 ± 11.34% and 51.53 ± 14.03% (p = 0.001) for the round and figure-eight coils, respectively. The intraclass correlation coefficients for PTs across studies were 0.832 for the round coil and 0.591 for the figure-eight coil. No significant correlation was found between PT and AMT, RMT, or SICI. PTs were lower with the round coil than with the figure-eight coil. The reliability of PTs can be considered good for round coil and moderate for figure-eight coil. Additionally, the findings suggested that motor cortex and visual cortex excitability are different entities.