Investigating consciousness with psychophysical and electrophysiological means : Temporal aspects of feature integration
We presented a series of single lines giving the illusion of a single element moving across the screen. The lines were composed of two vertical segments separated by a small gap. One of the lines was offset, that is the lower segment was not aligned with the upper one. With this manipulation, interestingly, the whole sequence of lines (further referred to as a stream) appeared to be offset in the direction of this line because of motion grouping of the single lines. We varied the position of this offset line (further referred to as the vernier) within the stream, as well as the length of the stream. The task of the observers was to report the offset direction of the stream. Reaction time (RT) did not depend on the position of the vernier, at least for the earlier positions. For example, RT was roughly the same when the first or the third line was offset even though the third line was presented 80 ms after the first one. These results support the hypothesis that a decision cannot be made before the end of a temporal window of about 160 ms from the start of the stimulus. Moreover, there is no access to constituent features of the stream, i.e. the line offset.