2.2 EEG task and procedures
All participants completed the SORT task which has been described in prior publications (Chiang et al., 2014, 2015). SORT consists of word pairs presented simultaneously representing two conditions: retrieval condition and non-retrieval condition. The retrieval condition includes 56 word pairs which facilitate retrieval of a particular object from memory (e.g., the pair ‘humps’ and ‘desert’ facilitate retrieval of the concept ‘camel’). The non-retrieval condition contains 56 word pairs which come from the same set of words, but which are randomly paired and thus do not lead to memory retrieval of any object (e.g., ‘humps’ and ‘monitor’ do not facilitate retrieval of a specific item). Retrieval and non-retrieval word pairs were randomly sequenced to create two different versions of the task, which were randomly assigned and counterbalanced across participants. Word pairs appeared simultaneously on the screen with one word above the other for a total of three seconds. A fixation target (+) was presented at the center of the screen for three seconds between each word pair. Participants were asked to decide if each pair led to object retrieval by pushing either a ‘yes’ or ’no’ button on a response pad using their right index and middle finger, respectively.
Participants received the following instructions prior to the EEG task: “You are going to see two words. These represent features that are related to objects. Push the button under your index finger if the two words combine to bring to mind some particular object. If the two words do not combine to bring to mind a particular object, push the button under your middle finger.” Participants were encouraged to respond as quickly and accurately as possible. Response time (RT) and accuracy were recorded for each trial. Responses recorded outside the window of 300-3000 ms were labeled as incorrect. The entire task took approximately 11 minutes. The stimuli were presented on an LCD screen using Stim software (Compumedics Neuroscan, USA) and placed about 46 inches from the participant. The two words in each pair spanned approximately 5° of both vertical and horizontal visual angles, with the horizontal measure varying slightly by word length. All the words were presented against a white background in black, lower-case, Times New Roman font (size 72).