Combining backward masking and transcranial magnetic stimulation in human observers

Both backward masking and transcranial magnetic stimulation (TMS) are capable of hindering perception of a visual target. To study the relationship between these two methods we applied TMS over the occipital pole in combination with the visual backward masking technique shine-through. The recently discovered weak shine-through mask consists of a horizontal grating with 25 vernier elements. In three subjects we determined discrimination thresholds for vernier acuity without and with the shine-through mask. Modulation of the vernier discrimination threshold was determined in both conditions with TMS at various stimulus onset asynchronies (SOAs). In the unmasked condition TMS deteriorates discrimination moderately around 120 ms TMS SOA from 25" (arc seconds) to about 130". If in addition to TMS the vernier is backward-masked, discrimination of the vernier offset is completely abolished (>300"). Therefore, TMS and backward masking can interact in a non-linear manner, strongly interfering with early visual processing.