Abstract
When a right offset vernier is immediately followed by a left offset vernier (or vice versa), only one vernier is perceived. Feature fusion has occurred. In feature fusion, observers cannot resolve the two verniers individually. Subjects perform at chance level, if asked whether the first or second vernier is offset to the right. The perceived offset of the fused vernier is a combination of the offsets of the two presented verniers. To our surprise, the perceived offset can be systematically modulated by transcranial magnetic stimulation (TMS) applied over the occipital cortex. For an astonishing long period of 400ms, TMS enhances the contribution of the first or second vernier depending when TMS is applied. Thus, TMS can modulate feature integration for a long time but does not render the individual verniers visible. However, when the two verniers are separated by an interstimulus interval (ISI) of only 10ms, the verniers do become visible as single entities. Observers can discriminate whether the first or second vernier is offset to the right. Hence, feature fusion is interrupted. We postulate that the transient off/on signals due to the ISI render the verniers visible individually. On the other hand, single TMS pulses modulate but do not interrupt feature fusion.
This research was supported by the SNF.