Two-alternative forced choice tasks are often used in object detection, which regards detecting an object as a 'yes' response and detecting no object as a 'no' response. Previous studies have suggested that the processing of yes/no responses arises from identical or similar processing. In this study, we investigated the difference of processing between detecting an object ('yes' response) and not detecting any object ('no' response) by controlling the task difficulty in terms of fragment length and stimulus duration. The results indicated that a 'yes' response depends on accurate and stable decisions through grouping processing, and a 'no' response might involve two distinct processing, including accurate decisions and intuitive decisions. Accurate decisions of 'no' may arise after the rejection of a 'yes' response with grouping processing, which is an accurate but slow response in an easy task. Intuitive decisions of 'no' arise as the result of breaking down the decision process when the received information was insufficient for grouping processing in a difficult task. Therefore, intuitive decisions of 'no' arise quickly but are inaccurate. The different processes associated with yes/no responses were discussed in terms of the hierarchal structure of object recognition, especially with respect to receiving information and grouping.
Keywords: contour detection; fragmented contour; yes/no response.