How long does stimulus processing last?

How a stimulus is processed is at the very heart of all vision research. However, there is only little research about how long the processing of a stimulus lasts. One reason is that visual processing is often explicitly or implicitly thought to be feedforward, an assumption that is however only met in few paradigms. The other reason is that it is easy to know when stimulus processing begins but it is very hard to determine when it terminates. Here, we show psychophysically that stimulus features are unconsciously integrated for about 400ms. We presented a vernier in the center of the screen followed by a series of pairs of lines, creating the percept of an expanding stream. The vernier was offset either to the left or right, the lines had no offset. Even though the vernier was rendered invisible by sequential metacontrast masking of the lines, the subsequent lines appeared as offset. If one of the subsequent lines was also offset, the offsets integrated mandatorily, i.e., observers had no access to the individual offsets. This mandatory integration lasted for about 400ms. When the line offset appeared after 500ms both offsets could be reported separately. Such windows of integration start with the sequence onset. Importantly, integration is a sophisticated and constructive process. If for example some intermediate lines are removed, the streams are no longer perceived as continuous and no integration occurs. Observers can report the offsets separately. However, when the same lines are occluded instead, integration occur. Our results offer a new view on vision because they allow substantial processing time in any type of model.