Although previous studies have demonstrated visual processing in detection and discrimination task in human, how detection and discrimination task alter visual processing has not yet been completely elucidated. To investigate this issue, we design performing two classical perceptual tasks: 1) grating detection experiment, 2) grating orientation discrimination experiment. In grating detection experiment, participants were instructed to identify stimulus, if they saw a stimulus, by pressing right button as quickly and accurately as possible. In grating orientation discrimination experiment, participants were told to identify the orientation of the stimulus and pressed relevant button as quickly and accurately as possible. We assessed the diversity by measuring the magnitude of sensitivity and intercepts through reaction times (RT). The results showed that RT strongly depended on experimental task. The response to discrimination task is significantly slower than that for detection task (p < 0.05). However, visual detectability was not depended on experimental task, and no significant difference of sensitivity was found among detection task and discrimination task (p > 0.05). Our results provide unique insight into how the brain processes visual signal of different experimental task, and will provide some basic data for human-robot signal processing.