The Research On The Mechanism Of Low Prevalence Effect

Previous studies have indicated that in the visual search task, the probability of target could affect the searching accuracy. Likelihood that targets will be missed varies with target prevalence, the frequency with which targets are presented across trials. Miss error rates are much higher at low target prevalence(≤10%) than at high prevalence(≥50%).This phenomenon is called low prevalence effect. There are four different explanations for this phenomenon: the weighing of speed and accuracy account,bottom-up processing theory, signal detection theory, two-threshold account. This study with three experiments uses experimental materials of Melina A.(2010).To balance action and accuracy, all experiments use reaction detection to control motion error. This study try to differentiate the rest of the three theories and find out the fundamental reason of low prevalence effect. In addition, experiment 3 uses event- related potential technology to show the difference of electrical components under the condition of low prevalence. Experiment 1 uses red and green frame hint to distinguish the top-down control and bottom-up processing, the results show that under the condition of effective frame there still exists low prevalence effect. When the dependent variable is missing rate and judgment standard, the low prevalence effect is significant. So we conclude that the low prevalence effect is decided by the judgment criterion, rather than bottom-up processing. Based on the study of Fan Yang, Experiment 2 try to distinguish the fundamental cause of low prevalence effect in both high and low visual complexity conditions, the result follows up: first, under the condition of high and low visual complexity, there is low prevalence effect. Second, in the condition of low visual complexity, the judgment criterions in the high and low prevalence conditions have no significant difference. However, in the condition of high visual complexity, the judgment criterions in the high and low prevalence conditions have significant difference. When the dependent variable is reaction time, in the low visual complexity and without targets case, reaction in low prevalence condition is significantly shorter than reaction in high prevalence condition. In the high visual complexity and without targets case, the same results were obtained. Based on two-threshold account, we can come to a conclusion that under the condition of low visual complexity, low prevalence effect is determined by the end value. And under the condition of high visual complexity, low prevalence effect is determined by the judgment criterion and the end value. In conclusion, the first two experiments sum up all four theory about low prevalence effect. Experiment 3 uses vent related potential technology and provides the electrophysiological evidence of low prevalence effect. It discovers that in the early stage of low prevalence effect, there are two components P1 and N1 of which amplitude is more noticeable. Results show that under the condition of low prevalence, the incubation period of P1 is shorter, the peak of N1 is larger. Luck and Hillyard(1995) found that in visual search P1 reflects the inhibition to the location of distractions and N1 represents the attention to the target location. We can draw the conclusion that it is earlier to appear the inhibition to distractions in the condition of low prevalence and cognitive processing for targets requires more attention resources. In addition, from the aspects of the brain, we can see P1 at top and occipital lobe and at occipital lobe the incubation period of P1 is longer. P1 wave amplitude at parietal and occipital lobe has no significant difference. At top and occipital lobe, the incubation period of N1 is longer, the peak is bigger. In the frontal lobe, the incubation period of N1 is shorter, the peak is smaller. So this research concludes that in low prevalence effect the inhibition to distractions first starts in the top lobe, followed by occipital lobe. And the numbers of neurons which participate in the activities of cognitive processing are similar. Attention to target starts in frontal lobe, followed by top and occipital lobe. At the same time, top and occipital lobe pay more attention to targets processing.