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).