For each trial, the display sequence was preceded by an 800-1,200 ms fixation period. During this time, only the central fixation point was visible (Figure 4.1). In preparation for the presentation of the display sequence, participants were instructed to maintain fixation on the central point. The display sequence consisted of an initial 14-item RSVP stream comprised of 13 letters and a single digit. Letter stimuli (A, B, C, D, E, F, G, H, J, K, L, M, N, P, Q, R, S, T, U, V, W, X, Y, Z) were selected at random with the constraint that the same letter could not appear twice in the stream. A digit stimulus (1, 2, 3, 4, 5, 6, 7, 8) was selected at random with the constraint that an equal number of even and odd numbers would be presented within each block. The stimulus onset asynchrony (SOA) between successive items in the RSVP stream was 100 ms. Upon the completion of the RSVP stream, a search display was presented for 200 ms. The search array was subsequently masked for 200 ms.
The first target (T1) was the digit inserted into the RSVP stream that appeared at either the seventh (lag 8) or thirteenth (lag 2) position. The second target (T2) was the target singleton in the search array. Participants were instructed to first make a speeded response to T2 by identifying the orientation of the gray line inside the target singleton by pressing one of two response buttons as quickly as possible. Participants were then
probed to indicate whether T1 had been an even or an odd number. After a response was made to the probe, the next trial began.
The search display (T2) contained one target singleton and one highly salient distractor singleton. Target and distractor locations were varied to produce the following display configurations: lateral target, midline distractor (50%); midline target, lateral distractor (50%). The order of the display configurations was pseudo-randomly intermixed within each block of trials. Each experimental block was comprised of 36 trials. At the end of the block, participants were given a minimum 5 second rest period and were permitted to begin the next block whenever they decided. The experiment contained 24 blocks, for a total of 864 trials per participant. At least 36 practice trials were given to each participant prior to the start of the experiment.
4.2.5. Behavioural Analysis
Trials on which the participant responded incorrectly to either T1 or T2 were automatically excluded from the analysis. Trials with anticipatory RTs and excessively slow responses were excluded from analysis (less than 1% of all correct trials). Median reaction times to T2 were derived for search displays for each participant. The means of these median reaction times were then computed for both lag 2 and lag 8 trials.
Differences were statistically assessed using a paired t-tests. A repeated-measures ANOVA with two factors (target-distractor distance and lag) was then used to assess search performance as a function of target-distractor proximity. Next, the RT obtained on target-distractor distance 1 trials was compared with target-distractor distance 4 trials for both lag 2 and lag 8. This analysis sought to determine whether target-distractor proximity differentially affected search performance within versus outside of the attentional blink.
4.2.6. Electrophysiological Recording and Analysis
EEG and electrooculogram (EOG) were recorded from active sintered Ag-AgCl electrodes (Biosemi Active Two system) from 32 electrodes, using a modified montage that included electrode sites FP1, FP2, AF3, AFZ, AF4, F7, F3, FZ, F4, F8, FC5, FCZ, FC6, T7, C3, CZ, C4, T8, CP5, CPZ, CP6, P7, P3, PZ, P4, P8, PO7, POZ, PO8, O1, OZ,
O2. Horizontal EOGs were recorded using two electrodes positioned 1 cm lateral to the external canthi, and vertical EOGs were recorded using two electrodes positioned above and below the right eye. All EEG and EOG signals were digitized at 512 Hz, referenced in real time to an active common-mode electrode, and low-pass filtered using a fifth-order sinc filter with a -3 dB cutoff at 104 Hz. Electrode offsets were monitored to ensure the quality of the data. After the data acquisition, EEG data for each channel were high-pass filtered (-3 dB point at 0.05 Hz) and then converted from 24-bit to 12-bit integers.
EEG processing and ERP averaging were performed using event-related potential software system (ERPSS) (University of California, San Diego). A semi-automated procedure was used to discard epochs of EEG contaminated by blinks, eye movements, or excessive noise (Green et al., 2008). Any trial with an artifact within a 1-s interval (-200-800 ms post-stimulus) was rejected. Artifact-free epochs associated with the various display configurations of interest were then averaged separately to create ERP waveforms. The resulting ERPs were digitally low-pass filtered (-3 dB point at 32 Hz) and digitally re-referenced to the average of the left and right mastoids. All ERP amplitudes and baselines were computed using a 200 ms pre-stimulus window. The averaged event-related horizontal EOGs did not exceed 2 μV for any individual participant, indicating their gaze remained within 0.3° of the fixation point for a majority of the trials (McDonald &
Ward, 1999).
The primary analysis focused on ERPs elicited by search displays with following display configurations: (1) lag 2, lateral target, midline distractor; (2) lag 2, midline target, lateral distractor, (3) lag 8, lateral target, midline distractor; (4) lag 8, midline target, lateral distractor. On each trial, a search display contained a lateral singleton and a midline singleton. Biasing a singleton to the midline on each trial allowed for an equal number of trials where the N2pc and PD could be isolated.
For each participant, ERPs to the various search displays were collapsed across left and right visual hemi-fields, as well as left and right electrodes, to produce waveforms indexing the processing of a lateralized singleton. Lateralized ERP difference waveforms were then computed for the display configurations of interest by subtracting the ipsilateral waveform from the corresponding contralateral waveform at electrode sites P07 and P08.
All statistics were performed on lateralized ERP difference waveforms. For all ERPs shown here, negative voltages were plotted upward so that the N2pc would appear in these difference waveforms as an upward deflection and the PD as a downward deflection.
The mean-amplitude of the N2pc was first measured in a window 230-290 ms post stimulus onset. This window was determined a priori based on the findings of Hickey and colleagues (2009) and was identical to the N2pc window set in Chapter 3. As it was anticipated that the N2pc would be delayed during the AB, the window for lag 2 trials was shifted by 30 ms to match the ~30 ms latency shift observed by Lagroix et al., 2015.
The PD was computed in a window 290-330 ms post stimulus onset, approximately centered around the peak of the observed component. The 40 ms duration of the window used to measure the component was selected to mirror the size of the statistical windows used in the previous chapters but shifted later in time to account for later onset observed here. Because the component appeared later than it had in the previous chapters, an unbiased signed area measurement was also used to confirm the presence of the component. Identical to what was done in Chapter 3, the signed positive area of the PD
was computed using each individual participant’s difference waveforms in a wide 200-350 ms post-stimulus interval. The signed positive area of the PD was subtracted from the signed positive area of the baseline from -150-0 ms. This signal-minus-noise difference was then statistically assessed using standard parametric statistics.
On midline target, lateral distractor trials, the PPC was computed in a 50 ms window from 120-170 ms. This window is consistent with previous studies that have typically reported the PPC to occur between 120-190 ms (Fortier-Gauthier et al., 2012;
Jannati et al., 2014; Leblanc, Prime, & Jolicœur, 2008).
Latency onsets were defined as the 50%-of-peak-amplitude voltage in the 0-200 ms interval for the PPC and 200-400 ms interval for the N2pc and PD. Statistical tests were performed on jackknife-averaged ERPs and statistical thresholds were adjusted accordingly (Ulrich and Miller, 2001). All statistics were computed relative to a 100 ms pre-stimulus interval.