Sample Size Limit

The small size of the sample here used introduced a risk of underpowered statistical computations in the study. However, we kept this risk under thorough control estimating the effect size for the statistically significant factors by means of partial eta squared values, i.e., η2*p*. Additionally, the alpha inflation due to multiple comparisons was also controlled by means of Greenhouse–Geisser epsilon (ε) correction.

#### *2.2. Stimulus Materials and Experimental Conditions*

Stimulus materials consisted of strings of five (5) contiguous arrows, serving as targets. Arrows were of two types: so-called "standard" and "star" arrows. While the former had a tip with a vertically linear rear side, the latter showed a slightly inward-bound and oblique rear-side at the starting of their shaft (see Figure 1a). The central arrow of each string consisted of the true target while the flanking two arrows on each side of the latter posed as potential distracters. Overall, each target-and-flankers-string subtended 8.7 degrees of visual angle along the horizontal meridian and 1.3 degrees along the vertical meridian. Regardless of the arrow-type, the tip of the central target arrow could point to the left or to the right side, whereas the flanking arrows could point toward the same (Congruent flankers) or the opposite direction (Incongruent flankers) as the target. All in all, then, there were eight (8) di fferent target strings combinations (see Figure 1b for examples of the latter).

Prior the presentation of arrow-strings, stimulation also included the administration or the omission of white asterisks in di fferent points in space of the stimulation PC monitor so to produce four di fferent cue-target conditions. All these stimulus materials were presented on the blackened background of a 17"- cathode-ray tube (CRT) screen in front of the volunteers. The luminance of both the asterisk-cue and the two types of target-arrow-strings were measured in candela/m2. The luminance of the asterisk amounted to 7.3 candela/m2. Conversely, the luminance assessments for the standard-arrow- and star-arrow-strings were 27.81 and 26.96 candela/m2, respectively, and were matched across arrow-pointing directions and target-flankers congruency.

Depending on the cue presentation positions (above or below the fixation cross (FC) or centered over it) or omission, as well as to target-related motor tasks to be performed, four (4) di fferent attention alerting or valid spatial attention orienting sets were induced in the participants (Figure 1c). More in details, the latter had to deal with: (1) the sequential presentation at random of vertically eccentric (above or below the FC) target strings without being preceded by any cue aimed at eliciting a possibly tonic and unspecific alerting or sustained attention condition over time (No Cue, NC) as well as a target-related exogenous spatial orienting of attention to the point in space where the arrow-strings were contingently presented from trial-to-trial; (2) the presentation of a cue overimposed on the FC aimed at eliciting a phasic attention alerting but not an attention orienting response followed by the presentation of a target string above or below the FC so to elicit a target-related exogenous spatial orienting of attention to the point in space where the target-string was delivered (Central cue, CC); (3) the presentation of a vertically eccentric (above or below the FC) cue aimed at eliciting both a cue-related phasic attention alerting and an exogenous orienting of attention to the point in space indicated by the cue, later on followed by a further endogenous orienting onto the target at that same point because the focus of attention was already centered there (Local or Spatial cue condition, LC).

**Figure 1.** Graphical depiction of the stimulus materials, of cue-target conditions, and of trial events used in the present study (ANT-SR (Attention Network Test-Slightly Redesigned); modified and redrawn from Fan et al. [40] and Posner [44]). (**a**) Two kinds of white arrow-target strings were presented: "standard" and "star" arrows. (**b**) Congruent and incongruent central targets vs. flanker strings were presented. Based on the direction of the arrow tip (left or right), eight target-flanker conditions resulted. (**c**) Four different cue-target and motor-task experimental conditions were administered in randomized order: a no cue (NC) condition; a center, alerting but spatially unpredictable cue (Central Cue, CC), and a valid spatially informative cue (Local Cue, LC). In these three conditions, participants' motor response to targets depended on a single-choice reaction time (RT) button-press. In a fourth cueing condition, LCmot, volunteers had to discriminate both the target-arrow type and orientation in order to make a double-choice RTs button-press. (**d**) Schematic exemplification of cue–target stimulus-events and inter-stimulus interval (ISI) duration during a single trial of the LC condition as well as of the random inter-trial interval (ITI) length. See text for further details.

For each of these three cueing conditions of the original ANT, on each trial the participants had to discriminate the direction towards which the central target-arrow-tip of the five-arrows-string pointed, independent of the arrow type presented (i.e., standard or star arrow), and of the direction of the flanking-arrows (Congruent and Incongruent), and to perform a single-choice-RTs button-press

with the index finger of the corresponding hand (right or left). Participants had also to deal with a fourth, newly introduced, cueing condition of the ANT-SR, defined LCmot. The latter was like LC but demanded a double-choice-RTs button-press according to the target-arrow type presented. Indeed, during the LCmot condition participants had to discriminate the type of target-arrows (i.e., standardor star-target-arrow), besides the direction to which the latter were pointing to, in order to perform a double-choice button-press with the index or middle fingers, respectively, of the corresponding hand (right or left, respectively; See Figure 1b,c again).

No matter the cue-target condition considered, on each trial the cue was presented for 100 ms followed after 400 ms by the delivery of an arrows-string, which remained on the screen for 1000 ms before its offset in order to avoid any possibly baffling stimulus-offset, besides stimulus-onset, related ERP recordings. Inter-trial interval (ITI) randomly varied between 330 and 1000 ms (Figure 1d).
