All participants provided written informed consent and received a modest fee. The stimulus configuration is shown in Fig. 1. It consisted Dabrafenib cost of two checkerboard
stimuli located 2° above and on either side of a fixation spot at horizontal eccentricities of 2.5° and 7.9°, respectively. The size of the inner checkerboards was 3.5° × 3.5°, with a spatial frequency of 0.7 cycles per degree; the size of the outer checkerboards was 4.7° × 4.7°, with a spatial frequency of 0.5 cycles per degree (Fig. 1). The larger size of the outer stimuli was chosen to adjust visual stimuli for the reduction in visual cortical area devoted to peripheral space (Adams & Horton, 2003; Frey et al., 2013). Dark checks had a luminance of 0.1 cd/m2, and white checks had a luminance of 118.2 cd/m2. The refresh rate of the monitor (model VP2655; ViewSonic, Walnut,
CA, USA) was set to 60 Hz, and on every refresh the checkerboard pattern of each stimulus either remained constant or was inverted as determined by a binary m-sequence of order 7 (e.g. (Sutter, 2000; Schmid et al., 2009). The binary m-sequence technique controls the inversion of the checkerboards displayed in each stimulus location by using AG-014699 manufacturer a pseudo-random sequence, which ensures that inversions in one location are statistically independent from the inversions in all other stimulus locations. Cortical evoked responses are then obtained by cross-correlation of the continuous EEG data around stimulus reversals with the checkerboard reversal sequence. An order of 7 indicates that each sequence was 27 = 128 monitor refresh cycles (i.e. 2.1 s) long. This duration is sufficient to fit four evoked responses of duration 500 ms. In half of the trials, we used this sequence, and in the other half we usedits inverse. Each trial was 2.95 s in length; however, the m-sequence used for estimating the evoked cortical response was only 2.1 s in length. In order to minimise stimulus onset
artefacts, Olopatadine we used another random sequence for the first 850 ms of each trial, and this time-frame was excluded from further analysis. For the experimental task, we overlaid each checkerboard with a central red ‘X’ (task stimulus). At the beginning of each block of 20 trials, participants were instructed to simultaneously attend to two of the checkerboards, and count how many times their task stimuli disappeared at the same time. This ensured that participants did not have to switch attention on each trial. Before each experimental trial, the two attended checkerboards were cued again, and, after a random interstimulus interval of 800–1200 ms, the experimental trial was started. Participants were instructed to ignore the uncued checkerboards, as task stimuli could also disappear in the uncued locations.