Keep in mind that putting together your application materials is both a goal and a means to a goal. The primary goal is discovering if your interests and abilities match those of the Cognitive Science Program, and if you seem likely to benefit from an education here.
You have the option of choosing to supplement the required application materials with other data that shed light on your capabilities, but this is not required. Some materials (the transcript of your bachelor's degree, for example) are required by the university and cannot be waived by the program.
Items that are important in the evaluation process, for both domestic and international students, include: