2.1.1. Participants

Participants were 13 children (9 males, mean age = 5.12, Standard Deviation = 0.75 years) after data additional three children were discarded due to excessive EEG artefacts or inadequate response accuracy level (<75%). Children were recruited as part of an optional follow up to a separate large epidemiological early childhood developmental study [39]. Children were screened with a computerized adapted version of the Peabody Picture Vocabulary Test (PPVT; [40]). Parents completed two standardized age-normed screening assessments: Behavior Rating Inventory of Executive Function ®–Preschool Version (BRIEF ®-P; [41]), and the Child Behaviour Checklist for Ages 11 2 - 5(CBCL/1.5-5; [42]). All children lived in the same neighborhood and came from middle to high socio-economic status families. Participants had normal or corrected to normal vision and had no known, auditory, sensory or cognitive deficits. All participants were typically developing children with no history of medication or referral to disability assessment or services, as ascertained from parental reports and the day care center records. Participants' scores on all screening measures were within norm.

#### 2.1.2. Visual Sustained Selective-set Attention Task (VSSAT)

The VSSAT (see Figure 1) was originally selected because: (1) it was validated on samples of similar age and with similar background to the one we previously tested, a ffording direct comparisons; (2) differently than in most of the typical versions of go/no go tasks, it involves a continuous stream of picture stimuli in short blocks, which allows to determine whether participants are continuously attentive to the target throughout the trial (on correct trials) or when participants cease to attend to the target (on incorrect trials); (3) relative to older subject groups preschoolers can perform it with similar accuracy and engagemen<sup>t</sup> levels but show delay in responding to targets, therefore, reaction time measures embedded in this task can be modelled so as to reliably differentiate between the processing stage of visual target selection and the processing stage of the initiation of response to target; (4) this computerized paradigm can be easily used in combination with EEG recording.

**Figure 1.** Stimulus presentation and time intervals of each trial in the Visual Sustained Selective-set Attention Task (VSSAT). Adapted from D'Angiulli and Devenyi (2019) [41].

Stimuli were presented through the Neuroscan Stim software program (Neuroscan, North Carolina, USA) and displayed on a 19" flat screen monitor. Each trial in the VSSAT consisted of a white outline of a duck or a turtle presented in the center of the monitor on black background and remained on the screen for a duration of 500 ms, followed by a fixation cross for a duration of 500 ms (See Figure 2). Participants were instructed to press a button if the outline of a duck appeared and to refrain from pressing the button if any other image appeared.

**Figure 2.** Process flow of the present study. Steps 1–4 describe modeling of actual children's data; Steps 3–9 describe adult simulations.
