The symbol-selection procedure used for free writing.

<p>Initially, there are eight groups of characters and control symbols (‘backspace’, ‘space’, and ‘accept’). When one group has been selected (here ‘abcd’), it unfolds into four individual symbols (here ‘a’, ‘b’, ‘c’, and ‘d’), after which a final selection is made (here ‘a’).</p

a, b) Schematic example trials for Exp. 1 and Exp. 2 (see text for details).

<p>c) Example target stimuli. The mean signal-to-noise ratios in Exp. 2 were 19.8% (on dark background) and 17.8% (on bright background).</p

Schematic paradigm and pupil-size traces for Exp. 1 (a) and Exp. 2 (b).

<p>Pupil size was larger on attend-dark than on attend-bright trials, which reflects attentional modulation of the PLR. This modulation was evident from 717 ms (in Exp 1) and 656 ms (in Exp. 2) after the presentation of the cue, as indicated by the gray shading. In Exp. 1 there was also an overall constriction in response to the (visual) cue, with a mean latency of 241 ms.</p

Behavioral results.

<p>a) In Exp. 1, responses were faster on validly cued trials, compared to invalidly cued trials. b) In Exp. 2, error rates were lower on validly cued trials, compared to invalidly cued trials.</p

The strength of the pupil-size modulation is related to the strength of the behavioral cueing effect in response times (a; Exp. 1) and accuracy (b; Exp. 2).

<p>a) Dots indicate participant bin means. Colors indicate different bins (see text for details). Regression line and slope CI are estimated with LMM and MCMC. b) Dots indicate participant means. Regression line and slope CI are estimated with linear regression. c) The strength of the pupil-size modulation as shown in a) and b) corresponds to the surface area between the attend-dark and attend-bright pupil-size traces in the 1000–2000 ms interval of the cue-target epoch. Intervals where the attend-bright trace fell above the attend-dark trace were counted as negative surface. The example shown here corresponds to a single data point in b), as indicated.</p

Horizontal gaze position over time for Exp. 1 (a) and Exp. 2 (b).

<p>Colored lines indicate the mean horizontal gaze deviation across participants. Central dotted lines indicate the display center. Peripheral dotted lines, respectively 3.76° and 2.82° from the display center, indicate the edges of the central gray shading (see Fig. 4). There was a slight tendency for the eyes to fixate leftwards of the display center, but no discernible tendency for gaze to gravitate towards the focus of attention. (In addition, for Exp. 2, any systematic gaze deviation would have been compensated for by the gaze-contingent algorithm, described in the main text.).</p

Pupil-size modulation (pupil size in attend-dark trials minus pupil size in attend-bright trials) during the cue-target and target-response epochs from Exp. 1 (a) and Exp. 2 (b).

<p>Gray lines correspond to individual-participant traces. Blue lines are grand-mean traces.</p

Pupillary responses during one brightness-transition cycle.

<p>a) Example data from one participant. Pupil size as a function of whether the target changes from bright to dark (blue line) or from dark to bright (orange line). Shadings indicate standard deviation. b) The pupil size difference (i.e. orange—blue) for all participants. The participant indicated in red did not reach our criteria for successful training. The participant indicated by the arrow corresponds to the example shown in (a). All data is from Phase 1, in which participants selected one out of two stimuli.</p

The selection procedure.

<p>a) Participants selected one of two (Phase 1), four (Phase 2), or eight (Phase 3) simultaneously presented stimuli. b) During each cycle, the brightness of the stimulus gradually changed in 0.5 s, and then remained constant for 0.75 s. Pupil size was measured during the last 0.25 s. c) The target stimulus was indicated by a cue. This example shows a correct selection, because the selected stimulus (‘a’) matches the cue. The size of the letters indicated how close they were to being selected. When a letter was selected, it smoothly moved toward the center. d) If there were more than two letters, letters were grouped by the brightness of their background. One group was eliminated on each selection, after which the remaining group was subdivided anew. This step-wise selection procedure repeated until a single winning stimulus remained.</p

Selection accuracy (top row) and speed (bottom row) for individual participants (gray bars) and across participants (blue bars).

<p>Horizontal dashed lines indicate chance level. a) Results for Phase 1. b) Results for Phase 2. c) Results for Phase 3. Error bars indicate 95% confidence intervals, within-subject where applicable [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0148805#pone.0148805.ref028" target="_blank">28</a>].</p