Phosphene thresholds correlate with paired-pulse suppression of visually evoked potentials.

BACKGROUND

Phosphene thresholds (PT) induced by transcranial magnetic stimulation (TMS) as well as paired-pulse suppression (PPS) of visually evoked potentials (VEP) are used to characterize visual cortex excitability, however, their relation remains unknown.

METHODS

We measured PT after single TMS over the occipital lobe, and recorded VEPs after paired-pulse stimulation at short stimulus-onset-asynchronies in the same subject. PPS was expressed by the ratio second to first response.

RESULTS

We found a negative correlation between PT and PPS (r = -0.36, P = 0.039) indicating that higher PT were associated with smaller ratios indicative of low excitability, and vice versa. There was no difference in PPS between subjects who perceived phosphenes and those who did not.

CONCLUSIONS

Although both approaches target different mechanisms, PT and PPS seem to reflect common characteristics of visual cortex excitability. The lack of differences in PPS in subjects not perceiving phosphenes suggests that they might not have higher excitability levels.