Methods

After consenting to procedures approved by Wilfrid Laurier University Research Ethics Board, 27 healthy participants (12 F, 14M, 1 other) participated voluntarily in the study. All participants were right-handed and had normal or corrected to normal vision. No participant reported having any psychiatric illnesses or brain trauma. None of the participants were on psychotropic medications. Analyses were conducted on all 27 participants who were between 19 to 31 years of age (mean age = 21 years). Participants signed informed consent forms prior to taking part in the study. They were granted course credits for their participation.

2.2.2 Stimuli and procedures

Stimuli were adapted from a published set of photographs (Brady et al., 2008). A MATLAB script was used to randomize the process of selection of these stimuli and stimuli were included from all categories. There were two phases in the experiment; the encoding task and the recollection or recognition task. During the encoding task, participants were sequentially presented with 800 pictures from a published set of photographs (Brady et al., 2008). There were short breaks every 8-minutes (every 100 pictures), which divided the encoding session into 8 blocks of 100 pictures each. Participants initiated each block by

pressing a button on a response pad. However, for the rest of the trials within the block, participants were not in control. Each trial started with a 1 s pre-encoding period, where participants focused on a blank screen with a central fixation dot. Each picture was presented for 250 ms, followed by a 1s-encoding period, during which the computer screen remained blank (see Figure 2.1). Participants were told in advance that they would be tested for their memory of the photographs and to watch each item carefully in order to perform the recognition memory test.

After a short break, participants performed the recognition test during which 500 pictures (300 old, 200 new taken from the same database) were randomly presented. The ratio of old to new stimuli was a bit higher for two reasons: first, the performance on recognition test was retrospectively used to categorize the encoding trials and more old trials gave us more SR and SNR trials. Second, the number of CR trials is always higher than FA and a smaller number of new trials gave us almost an equal number of trials for Hits and Misses. During the recognition test, pictures were presented on the screen until a response was recorded. Participants were instructed to press ‘1’ on the response pad if they remembered seeing the picture or press ‘2’ if they didn’t remember seeing the picture.

Figure 2.1. Schematic diagram of the timeline of a trial in the encoding and recognition test phases.

2.2.3 EEG data and Acquisition

EEG data were recorded from a 32-channel NeuroScan Quik-Cap (Compumedics, Charlotte, SC, USA) in a sound-attenuated, electrically shielded booth (Raymond EMC, Ottawa, ON, Canada) while participants performed the encoding and recognition tasks.

Impedances were kept below 5kΩ.

An EEG cap was placed onto the participant’s head such that electrodes fixed over the frontal lobes (Fp1, Fp2, F7, F3, Fz, F4, F8, FC3, FCz, FC4), temporal lobes (FT7, FT8, T7, T8, TP7, TP8), parietal lobes (CP3, CPz, CP4, P7, Pz, P4, P8), occipital lobes (O1, O2), and at the central position of the scalp (C3, Cz, C4). Electrode Cz was visually centered above the central vertex found halfway between the glabella and the external occipital protuberance medially and the preauricular points laterally. Electro-gel was used to improve conduction between the skin and the electrode surface. Surface electromyographic electrodes were positioned at the outer canthii of both eyes and above and below the left eye. EEG signals were initially referenced to mastoid electrodes (M1, M2), which were placed on the mastoid process behind each ear.

2.2.4 Behavioural Analyses


Performance in the recognition task was analyzed. Specifically, we investigated participants’ task accuracy and response time. Each participant’s hit, miss, correct-rejection and false alarm rates were calculated.

ERPs based on subsequently-remembered (SR) trials during the encoding task were compared to subsequently-not-remembered (SNR) trials based on the performance in the

analyzed using a one-way ANOVA with the within-subject factor as the subsequent memory effect (subsequently remembered/SR, not-remembered/SNR).

The perceptual sensitivity of the recognition task based on signal detection theory (SDT) was calculated by computing A′ for each participant. We used A′ as our main index of performance (Stanislaw & Todorov, 1999) because A′ is capable of distinguishing response bias from sensitivity. Usually in memory studies, where responses are mainly based on a yes/no paradigm, participants are highly responsive to suggestion or direction.

In that case, recall or recognition is improved but the false alarm rate also increases.

Therefore, it is important to measure response bias and sensitivity separately. A′ was computed in the following manner:

A^′= {

0.5 +(𝐻 − 𝐹)(1 + 𝐻 − 𝐹)

4𝐻(1 − 𝐹) 𝑤ℎ𝑒𝑛 𝐻 ≥ 𝐹 0.5 −(𝐹 − 𝐻)(1 + 𝐹 − 𝐻)

4𝐹(1 − 𝐻) 𝑤ℎ𝑒𝑛 𝐻 < 𝐹

𝐻 =

𝐻𝑖𝑡𝑠 + 𝑀𝑖𝑠𝑠𝑒𝑠

𝐹 = 𝐹𝑎𝑙𝑠𝑒𝐴𝑙𝑎𝑟𝑚𝑠

(𝐹𝑎𝑙𝑠𝑒𝐴𝑙𝑎𝑟𝑚𝑠 + 𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑅𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛𝑠)

A′ can take any values between 0.5 and 1, with 1 being perfect performance, while 0.5 occurs when a participant completely failed to distinguish old from new stimuli.

2.2.5 Controlling for Fatigue

Our experiment was almost two hours long and could cause fatigue. We ran an analysis to establish that the spectral measures from our data came from brain activity that resulted from memory processes rather than fatigue. Most importantly, studies report that alpha oscillations (9-12 Hz) increase with fatigue and drowsiness (Tanaka et al., 2012;

1999). Therefore, it was important to ensure that the alpha we observed in our results was not a spurious effect stemming from fatigue. To explore that, we divided our data from the encoding task into 8 blocks and extracted and averaged the oscillatory measures for each block for all participants. Our results revealed that there was no significant difference in the oscillatory activity across time (Hits across time: F (7, 192) = 1.8, p = n.s; Oscillatory activity: F (7, 192) = .52, p = n.s).

Figure 2.2. A schematic diagram of the encoding and recognition phases of the experiment. Based on the recognition performance, ERPs of subsequently remembered and subsequently not remembered stimuli were chosen for the encoding analysis.