Picture encoding task
This task comprised an encoding session and a retrieval session 210 min later. Four versions of this task were created for each of the sessions and the order was counterbalanced for the nap conditions. In the encoding session, participants were instructed to look at each picture and depending on the picture set chosen for the session, determine using a key press (1—yes, 2—no) whether or not a specific feature (e.g. building/ no building, indoor/ outdoor, body of water/ no body of water, and street/ no street) was present in the picture. They were told to be as accurate as possible, and to view the pictures carefully as their memory for the pictures would be tested later. To control for the influence of differing strategy use, participants were advised with the same strategy—to try to remember the pictures by looking out for distinct objects in each scene. In the retrieval session, participants were tested for their recognition of the previously shown images which were intermixed with new images. Each picture was shown for 4 sec and participants responded with a key press (1—definitely did not see, 2—probably did not see, 3—unsure, 4—probably saw, and 5—definitely saw).Responses were split into four outcome measures: (1) confidence ratings of four (probably saw) and five (definitely saw) to old images were classed as “hits,” (2) ratings of four and five to new images were “false positives,” (3) ratings of one (definitely did not see) and two (probably did not see) to old images were classed as “misses,” and (4) one and two ratings to new images were “correct rejections.” The non-parametric signal detection measure Aʹ [where For each version of the task, 240 gray-scale pictures of identical luminance and dimensions were prepared, with half containing a specific feature (e.g. a building, a body of water) and a half without the feature (e.g. no building, no body of water). Each picture set was distinct from the others. In total, 160 of the pictures were used for the encoding session, and the retrieval session contained these 160 old pictures as well as 80 new distractor pictures.