Cortex during Encoding or Retrieval Modulates Episodic
5.3.2 Experimental Design
5.3.2 Experimental Design
This study was designed as a single‐blind, sham and cortical‐site controlled experiment.
Participants were recruited separately for the first and second experiment. In the first experiment participants were stimulated during the encoding phase, 4 sessions, and in the second experiment participants were stimulated during the recognition phase, 3 sessions. The experiment for the former group was conducted first. All participants attended the whole sessions. Each session contained a different type of stimulation; left DLPFC anodal; left DLPFC cathodal; M1; sham. Table 5‐1 summarises the stimulation types in the two experiments and Figure 5‐1 shows the schematic presentation of the electrode positions in different stimulation conditions. Order of conditions was randomised. To minimise carryover effects, the interval between sessions was at least 48 hours (Boggio, Ferrucci, et al., 2006).
Table 5‐1. Full list of conditions used in this experiment.
(a) (b) (c)
Figure 5‐1. Schematic drawing of electrode positions in the study. (a) Anodal and Sham stimulation of left dorsolateral prefrontal cortex (DLPFC) (b) cathodal stimulation of left DLPFC, (c) stimulation of primary motor cortex. Red rectangles show anode electrode and blue rectangle show cathode electrode.
Each session was composed of two phases, an encoding phase and a recognition phase. At the beginning of each phase participants were asked to complete a Stanford Sleepiness Scale (SSS) (Hoddes, et al., 1973), a standard measure of subjective alertness. In the encoding phase participants were shown words, one at a time, and they were asked to first judge the number of syllables of the word as quickly and as accurate as possible and then to memorise it.
Participants were instructed to imagine the words in order to memorise them. They were
asked to use imagery method in order to make the memorisation strategy consistence in between the participants. Further, it aimed to reduce the possibility of participants changing their memorisation strategy throughout the session. The instruction given to the participants was as follows:
“Words will be shown one by one on the screen. Your task is to first judge the number of syllables of the words. Words are either one syllable or two syllables. Please use the mouse with your left hand to specify the number of syllables. The left button of the mouse goes for words with one syllable and the right button of the mouse goes for words with two syllables. Please respond as quickly and as accurately as possible.
After responding to the number of syllables, try to imagine the word to memorise it. All the words are selected in such a way that they are easily imaginable. Words are either nouns such as ‘apple’ or verbs such as ‘walking’ or adjectives such as ‘happy’. Try to imagine the words to memorise them. Later you will be asked to discriminate words which you are going to memorise from new words.”
To exclude the possible influence of interference with the stimulation on motor cortex in the stimulated hemisphere, participants were instructed to perform the tasks during both encoding and recognition phases with their left hand while the left hemisphere was being stimulated. In addition to the above instructions they were debriefed about the procedure of each session and each phase separately, relative to their experiment number. Figure 5‐2 shows the procedure of each session.
Each session was composed of two phases, an encoding phase and a recognition phase. The encoding phase was composed of 4 blocks. The first block contained 35 words; the remaining 3 blocks contained 30 words. The first 5 words of the 1st block were considered as practice trials to ensure that participants were familiar with the procedure of each trial. These words were later discarded and were not used in the recognition phase. The timeline of a trial in the encoding phase is shown in Figure 5‐3. At the end of each block the percentage of each participant’s correct response to the number of syllables of the words was feedback to the subject for 3 seconds. There was a 15s rest interval in between the blocks.
(a) First experiment; stimulation during encoding.
(b) Second experiment; stimulation during retrieval.
Figure 5‐2. Procedure of each session; Participants were assigned to one of two experiments that differed in the period during which tDCS was applied. Stimulation was either active or sham. In sham stimulation type, stimulation was stopped after 30’ of stimulation. In the active stimulation type, stimulation was initiated after the 2nd block, either in encoding phase or recognition phase, and continued for 15’ until the beginning the 3rd block and continued for 5 more minutes to the end of the 4th block.
Figure 5‐3. Procedure during the encoding phase; Participants were asked to quickly respond to the number of syllables of the words and then try to imagine the words to memorise them. A question mark appeared on the screen after the word presentation. It changed into a cross after participants’ responded to the number of syllables. Participants were told that while the cross is on the screen they have time to memorise the word. An exclamation mark was shown to inform the participant that the next word is about to be presented. They were asked to use the mouse with their left hand using the left button for words with one syllable and the right button for words with two syllables.
The mean retention interval was 49.32’ (SD 3.40’) enough time for the brain to recover from the tDCS (Cerruti & Schlaug, 2008). In between the two encoding and retrieval phases and during the 15’ within encoding phase period, in order to engage the participants with a simple task and keep their activity consistence throughout the sessions and between participants, sketches of the Mr. Bean TV Series (Universal Pictures Ltd) were shown. This TV series was
selected to avoid any possible interference of the verbal communications as the amount of verbal communications in this TV series is very little.
The recognition phase was also composed of 4 blocks. Each block consisted of 30 pairs of words. In each trial two words were shown in which one was an old word previously presented in the encoding phase, and one was a new word. The order of the words in the recognition phase was not the same as the order of words in the encoding phase and they were again randomised. The procedure of each trial of the recognition phase is shown in Figure 5‐4. Participants were instructed to select the word they saw during the encoding phase, as quickly and as accurate as possible. Accuracy was stressed in the instruction. A question mark appeared on the screen after the words presentation. It changed into a cross after participants responded. An exclamation mark was shown to inform the participant that the next pair of words is about to be presented. For the selection of words appearing on the left hand side, they were asked to use the left button of the mouse with their left hand and for words on the right hand side they used the right button. Participants were not given any feedback of their performance. There was a 15s rest interval in between the blocks.
Figure 5‐4. Procedure of the recognition phase;
The experiment was performed using a PC computer. Stimuli were presented on a 17”
monitor, 75Hz refresh rate, subtending approximately 3‐6 degrees of horizontal visual angle.
Stimuli were presented on a black background with Arial font, white colour, in capital letters and 53cm from participants’ eyes. Stimulus presentation and timing of all stimuli and response events were achieved using MATLAB (MathWorks company) and the Psychtoolbox v3 (Brainard, 1997).