Stimuli
The same stimuli as in Gundlach et al. (2020) were used. All stimuli
were created using the Psychtoolbox 3 (Kleiner et al. 2007) running on a
PC with a Windows 8 operating system (Microsoft Cooperation). All visual
stimuli were presented in a magnetically shielded room by a video
projector (Panasonic PT-D7700E, 60 Hz refresh rate) via a mirror system.
The distance between the projection screen (45 cm by 33.5 cm) and the
participants was kept constant at 133.5 cm. The stimuli display
consisted of two rings presented at 6.4º to the left and right of
central fixation cross (0.25º by 0.25º). The outer and inner diameter of
the two rings were 4.6º and 1.8º, respectively. The background color of
the screen was set to 30% of maximum white color (luminance 13 cd/
m2). In order to evoke steady state visual evoked
fields (SSVEFs) both rings flickered at 12 Hz and 15 Hz each, whereby
the assignment of each flicker frequency to the left or right visual
field was counter-balanced across subjects. The rings had a luminance of
70% of maximum white color (60 cd/ m2) during the
on-frames. For one half of the participants, the fixation cross target
stimuli consisted of small decreases of the either vertical or
horizontal length by 0.05º in visual angle depicted for 60 ms. Increases
of the same size served as distractor events. For the other half of
participants, the target and distractor stimuli were reversed.
For the pre-cue task, target events for the peripheral rings were
luminance decreases of a small segment of both rings (10º) ranging
between 55 % to 65 % of maximum white screen color (8 cd/
m2 to 23 cd/ m2) lasting for 150 ms.
The position of the luminance reduced segment was chosen randomly
between 0º and 360º around an arc spanning each of the two rings.
Changes of luminance of the segment in only one of the two rings were
defined as distractors. For the post-cue task (see below), luminance
changes of the peripheral ring in the visual hemifield that had to be
attended, defined a target event, whereas changes in the opposite
hemifield were considered as distractors.