Abstract: Inattentional blindness is a phenomenon that, when people focus on a visual stimulus, they often neglect other stimuli appearing in the visual field. It is influenced by many factors, such as the position, size, color of irrelevant stimuli, the relationship between unexpected stimulus and attended stimulus, and so on. The main purpose of this study is to examine the influence of the speed of unexpected stimulus on inattentional blindness. Two experiments were conducted to investigate the perception or awareness of the unexpected irrelevant stimulus with different moving speeds under equi-interval and equi-distance conditions with the revised Mack and Rock’s paradigm. Each experiment corresponds to a variation of Mack & Rock condition, the vigilant condition and the full attention condition respectively. Subjects were asked to count the number of a central fixation flashing within a 10s interval. After that, the screen was masked with a random line pattern for 2000 msec. Then subjects were required to report the number as accurately as possible. This procedure was followed on the first three trials. In the fourth trial (unexpected trial), a critical stimulus – an unexpected dark gray circle, with different speeds moving across one of the quadrant either vertically or horizontally. Immediately after the trial in which the critical stimulus was presented, subjects were asked whether they had seen anything on the screen other than the flashed cross, which was anything that had not been presented on previous trials. If subjects reported that they had seen something, they were asked to identify it by describing it and indicating the quadrant where the critical stimulus appeared and the moving direction of the critical stimulus. The fifth trial (divided-attention trial) was the same as the fourth, then one may expect that another shape of a critical stimulus was added in order to increase the confusion of the critical stimulus. The participants were asked to to report the properties related to the critical stimulus on the last trial (full-attention trial), but not to perform the counting task. Under the equi-interval condition, the moving critical stimulus was presented for 200ms; but under the equi-distance condition, the presentation time of the moving critical stimulus varied with the speed of the critical stimulus in order to keep the distance equal.
We found (1) The inattentional blindness was significantly influenced by the moving speed and the moving interval of the critical stimulus. (2) Within a certain range of moving interval, the faster was the critical stimulus, the smaller was the proportion of inattentional blindness. There was no significant difference when the moving distances of different moving speeds were the same. (3) When the presentation time was beyond some range, the influences of the moving speed to inattentional blindness decreased dramatically, and an inverted-U relationship was shown between the influences of moving speed to inattentional blindness and the presentation time. Detailed analyses and discussion of the results are presented at the end of the paper.

Key words: inattentional blindness, unexpected stimulus, moving speed, inverted-U curve