Impact of Distractors on Sustained Attention and Inhibition in Children With ADHD

Abstract

Objective:

This study examines the impact of visual and auditory distractors on attention and inhibition in children with and without ADHD.

Method:

The study used the MOXO d-CPT child version. The sample consisted of 208 children aged 8 to 12 years, including 64 with ADHD and 144 controls.

Results:

Children with ADHD and controls differed in their reaction to distracting stimuli; visual distractors cause a higher decrease in sustained attention and inhibitory control in the ADHD group. Moreover, auditory distractors generate improved performance in the control group but not in the ADHD group. In addition, age-related effects were found in both sustained attention and inhibitory control in all children, regardless of whether the ADHD condition was present.

Conclusions:

The findings indicate that children with ADHD performed poorer compared to controls, and that distractors differently affected the performance of the two groups.

Keywords

ADHD sustained attention inhibitory control MOXO d-CPT

Introduction

Inattention, impulsivity, and restlessness, though common in typically developing children, become pathological and maladaptive in the case of ADHD (DSM-5; American Psychiatric Association [APA], 2013). Depending on the predominant symptoms, children can exhibit three subtypes of ADHD: predominantly inattentive, predominantly impulsive/hyperactive, and combined (APA, 2013). Inattentive children are easily distractible, struggle to focus attention for extended periods, and exhibit disorganization and poor concentration. Impulsive/hyperactive children have difficulty waiting and thinking before acting and show little concern for consequences and show excessive motor and verbal activity above and beyond what is necessary (e.g., difficulty staying sit or sitting still). Longitudinal clinical studies showed that these difficulties exhibit distinct developmental trajectories (Biederman et al., 2000; Fischer et al., 2005). Symptoms of inattention tend to persist from childhood to adolescence, whereas those of hyperactivity-impulsivity typically decrease (Larsson et al., 2011), suggesting that it is essential to consider individual variations throughout development (Biederman et al., 2000; Van Lier et al., 2007).

Some authors have emphasized poor behavioral inhibition as a central impairment in ADHD, with difficulties in sustained attention viewed as a cascade effect of the primary inhibition deficit (e.g., Barkley, 1997). However, subsequent studies have revealed that inhibition difficulties do not necessarily cascade to affect all other attentional or executive skills (Geurts et al., 2004).

Classic inhibitory tasks (e.g., go/no go tests) do not differentiate the performance of individuals with ADHD (Crippa et al., 2015). A reason for that may be found in the low ecological validity of neuropsychological tests, as they are administered in controlled circumstances, in a one-to-one condition, where the child is alone with the clinician, and the environmental distractions are avoided. Considering that, in everyday life, we constantly perform tasks while inhibiting environmental distractions, the aforementioned conditions make the test not always sensitive to the real-life attentional difficulties of children with ADHD (Neguț et al., 2016; Parsons et al., 2007).

A recent line of research investigated the effects of environmental distractors on ADHD symptoms using MOXO d-CPT, a continuous performance test (CPT) with visual and auditory distractors (Berger & Cassuto, 2014). Continuous performance tests (CPTs) are considered a gold standard for ADHD evaluation and are currently the most widely used objective laboratory measures to support the clinical diagnosis of the disorder because they can detect the patient’s typical behavior in the context of daily life.

The results of studies using MOXO d-CPT showed that children with ADHD had greater distractibility than controls when performing the CPT, measured by errors of omission in the presence of distracting visual, auditory, and a combination of distracting stimuli employing both sensory channels (Cassuto et al., 2013). In addition, results showed different effects of auditory and visual distractors on sustained attention and inhibitory control. Slobodin et al. (2018) showed that children and adolescents with ADHD have persistent distraction in response to visual distractors and reduced distraction in response to auditory distractors with increasing age (Slobodin et al., 2018). Similarly, Berger and Cassuto (2014) found that a group of adolescents with ADHD made significantly more omission errors with visual distractors, alone or in combination with auditory distractors, than in conditions without distractors. Differently, auditory distractors did not increase the omissions. Distracting stimuli did not affect performance in a group of control peers (Berger & Cassuto, 2014). Pelham et al. (2011) found the same results, investigating the effects of music and video on classroom behavior and performance of boys with and without ADHD. The study by Slobodin et al. (2018) also found that distractibility of adolescents with ADHD mirrors that of younger control children rather than that of healthy adolescents, suggesting a developmental delay in the clinical group (see also Berger et al., 2013). The authors suggested that in the case of MOXO d-CPT, visual distractors lead children to make more errors because they use the same cognitive modality as the main task (Wickens, 2008). The studies mentioned above found that sustained attention in children and adolescents with ADHD may be differently affected by the distractor’s modality. Less clear is the effect of the same distractors on inhibitory control. Inhibitory abilities are essential for success in both work and academics (Allan et al., 2014; Van Dooren & Inglis, 2015).. The ability to filter out distractions, resist impulsive behaviors, and regulate motor activity can significantly impact school performance (Colomer et al., 2017; DuPaul & Jimerson, 2014). In educational settings, distractions are ubiquitous, and students with high levels of impulsivity and hyperactivity may struggle to maintain focus and complete tasks. Investigating the impact of distractors can assist teachers in identifying effective teaching methods and creating classroom environments that meet the needs of these students and optimize their learning experiences. It can also guide the development of interventions and treatments that can help individuals cope with distractions more effectively and mitigate the challenges associated with impulsivity and hyperactivity (Daley & Birchwood, 2010).

To our knowledge, only two studies considered the effect of distractors on inhibitory control. Uno et al. (2006) found that, in children with ADHD, inattention, and hyperactivity were reduced by auditory distractors more than in the control group, while visual distractors only reduced inattention, having no impact on inhibitory control. Borkowska (2016), using MOXO d-CPT, found different results, as children with ADHD showed an increase in impulsive responses with distracting visual stimuli, alone or combined with auditory distractors, while hyperactive responses were not significantly affected by the modality of the distractors.

The present study investigated the effects of different distractors on sustained attention and inhibitory control in children and early adolescents with ADHD compared to their healthy peers, using MOXO d-CPT to assess the attention profile through four separate indicators that evaluate sustained attention and inhibitory control (Kanaka et al., 2008). The test is based on a go/no-go task that requires the participants to keep their attention on a continuous stream of stimuli and respond to a predetermined target. A measure of sustained attention regardless of sluggishness is acquired through two indices of Attention and Timing. Traditional measurements of temporal deficits during CPT performance usually include response time and variability. MOXO d-CPT introduces a blank period after each stimulus with varying duration for each item presented, and the test can distinguish between accurate and good-timed responses (given during the stimulus presentation) and accurate but bad-timed responses (given during the void period).

Inhibitory control is assessed using two distinct indices of impulsivity and hyperactivity. Traditionally, impulsivity through CPT has been assessed with commission errors, representing instances where individuals fail to inhibit responses to non-targeted stimuli. In contrast, the MOXO d-CPT impulsivity index includes as impulsive responses only pressions on the keyboard spacebar in response to non-targeted stimuli. All other uninhibited responses, such as pressing keys more than once, are encoded as hyperactive responses. Unlike other CPTs, MOXO d-CPT also includes unique environmental stimuli that act as auditory and visual distractors that are ecologically valid. The presence of these two types of distractors allows the identification of individual differences. According to the literature, we hypothesized that visual distractors have a negative effect on both sustained attention and inhibitory control indicators. On the other hand, we hypothesized that auditory distractors may have a different impact on performance by producing a recovery of sustained attention and inhibitory control. We also expected that the negative effect of distractors is stable across ages in the sample with ADHD compared to the control group (Slobodin et al., 2018).

Method

Participants

A total sample of 286 children aged 8 to 12 years participated in the study. Of those, 207 children composed the control group that contributed to the Italian standardization of the MOXO d-CPT. They were recruited in public primary and middle schools located in an Italian North-western town including children of low and high socioeconomic status. Seventy-nine children composed the clinical group and were recruited from a Complex Operating Unit of Child Neuropsychiatry that performs the diagnosis of ADHD and from a private clinical center specialized in the diagnosis and treatment of specific learning disorders, ADHD, and other neurodevelopmental disorders.

The following inclusion criteria for the control group were considered: participants were not diagnosed with ADHD or other disorders commonly present in comorbidity with ADHD; furthermore, they should not have been at risk of ADHD, as revealed by a screening tool used as part of this study. According to these inclusion criteria, children at risk for ADHD (N = 44) and those with a comorbid diagnosis (N = 10) were excluded; in addition, children with outliers (>3 DS) in Moxo indices (N = 9) were excluded. A total of 63 children were excluded. Thus, the final control group consisted of 144, typically developing individuals (TD group; 46 females, M_age = 9.93, SD = 1.37). As for the clinical group, the following inclusion criteria were considered: children should have a diagnosis of ADHD (also in comorbidity with SLD or ODD). According to this inclusion criteria, 15 children were removed because they did not receive a diagnosis of ADHD. The final clinical group consisted of 64 children (ADHD group; 19 females, M_age = 9.80, SD = 1.18) with a diagnosis of ADHD released by multidisciplinary specialized teams of professionals based on the criteria provided by DSM-4/DSM-5 or the ICD10.

Parental consent was obtained beforehand for all children. The study was carried out according to the recommendations of the Ethics Code of the Italian National Council of Psychologists and the Ethics Guidelines of the Italian Association of Psychology.

Procedure

Children in the TD group were individually evaluated at school by trained operators; children in the ADHD group were individually assessed in clinical settings by a clinician. Parents of TD children were asked to complete a screening questionnaire to evaluate ADHD in their children.

Measures

ADHD Symptoms

The ADHD Rating Scale-5 for children and adolescents (Barkley, 2011) was administered to parents. It consists of 18 items that correspond to the 18 ADHD symptoms included in the DSM-5. Three symptom scores are derived: Inattention (items 1–9), Hyperactivity/Impulsivity (items 10–18), and Total score (sum of the scores for each of the 18 items). For each of the three scores, a higher score corresponds to greater difficulties. In this study, the total score was used.

MOXO d-CPT

The MOXO d-CPT Kids version was used to assess sustained attention and inhibitory control (Berger et al., 2013). It consists of 424 trials organized in eight phases. In each trial, a stimulus (target or nontarget) is presented in the middle of the computer screen for a variable duration of presentation (500, 1,000, or 3,000 ms). It is followed by a “void” period of the same duration, during which no stimuli are presented. Participants are required to press the spacebar once as quickly as possible when a target stimulus appears, and to refrain from the response in case of nontargets. Both target and nontarget stimuli are cartoon images and no letters or numbers are included. Stimuli are simultaneously presented to a series of visual and auditory distractors, which appear at the periphery of the screen. To simulate the everyday environment, all distractors are typical elements of the child’s environment (e.g., animation of a flying airplane, a dog bark). The test consists of eight different phases: the first phase (Basic 1) is without any distractors; by the second phase (Visual 1) the first visual distractors are introduced but of less intensity than in the next phase 3 (Visual 2); in the fourth phase (Audio 1) auditory distractors are introduced in place of visual distractors, and as in the previous case the phase 4 distractors (audio 1) are less intense than in the subsequent phase 5 (Audio 2); the sixth (Combo 1) and seventh phases (Combo 2) contain both auditory and visual distractors, less and more intense, respectively; the last phase (Basic 2) is the final baseline without any distractors. The sequence of distractors and their exact location on the display are constant for each level. Visual distractors appear in one of four spatial locations at the sides of the screen: bottom, top, left, or right; in contrast to target and non-target stimuli that occur in the center of the screen. MOXO d-CPT provides information to assess key variables of ADHD, particularly attention (a), timing (t), impulsivity (i), and hyperactivity (h). Attention is the number of correct responses to a target stimulus given in the target + void period (i.e., both quick and slow correct responses). Timing measures correct responses given before the target disappears (i.e., only quick and correct responses). Impulsivity is the number of responses to nontarget stimuli (i.e., commission errors due to inhibitory difficulties). Hyperactivity is the total number of all types of errors that have not been coded as impulsive responses, such as pressing the space bar on the keyboard more than once in response to target or nontarget stimuli or random presses of any keyboard key other than the space bar (i.e., errors related to motor restlessness). In the case of attention and timing, better performance corresponds to higher test scores; conversely, for impulsivity and hyperactivity, better performance corresponds to lower test scores.

Statistical Analyses

A post hoc power analysis was conducted to verify that the sample size was adequate. Given an effect size of 0.25 and a power of 0.95, the sample required is 173, indicating that our sample was appropriate. Preliminary t-test and chi-squared test were performed to investigate the age and sex differences between groups. Descriptive analysis was performed on participants’ characteristics and MOXO d-CPT performance by group. To explore the effect of age, the sample was divided into two groups composed of 82 children (25 with ADHD) between 8 and 9 years of age and 126 children (39 with ADHD) between 10 and 12 years of age, respectively. A series of repeated measures ANOVAs were performed to examine the effect of Age (8–9 and 10–12 years, regardless of the diagnosis), Group (ADHD and TD), and type of distractor (Phase) on each MOXO d-CPT indicator. The corresponding post hoc tests were performed with the Bonferroni test. Considering that the interactions are more informative than the main effects, we described only these effects in the results section.

Results

No differences were found between the groups with ADHD and controls in terms of age (t = 0.72, p = .475) and sex distribution (χ² = 0.11, p = .746). The descriptive statistics are shown in Table 1. The repeated measures ANOVAs do not show significant interactions between Group, Age, and Phase for any of the indices. A summary of the results obtained for all indices is shown in Table 2.

Table 1.

Descriptive Statistics for All Indices by Group and Phase.

Attention
Phase	Group	Age 8 years					Age 10 years
Phase	Group	N	M	SD	Min	Max	N	M	SD	Min	Max
Basic 1	0	57	31.61	1.85	26.00	33.00	87	32.71	0.63	30.00	33.00
Basic 1	1	25	30.76	3.19	17.00	33.00	39	32.05	2.65	17.00	33.00
Visual 1	0	57	30.72	2.78	19.00	33.00	87	32.20	1.35	26.00	33.00
Visual 1	1	25	29.16	3.88	19.00	33.00	39	30.38	4.90	3.00	33.00
Visual 2	0	57	30.93	3.50	10.00	33.00	87	32.41	1.01	28.00	33.00
Visual 2	1	25	29.24	3.13	22.00	33.00	39	30.28	5.30	8.00	33.00
Audio 1	0	57	31.61	2.17	23.00	33.00	87	32.40	0.93	29.00	33.00
Audio 1	1	25	29.56	2.72	22.00	33.00	39	30.05	5.32	4.00	33.00
Audio 2	0	57	31.40	2.35	24.00	33.00	87	32.17	1.36	27.00	33.00
Audio 2	1	25	29.68	3.08	23.00	33.00	39	30.77	3.51	14.00	33.00
Combo 1	0	57	29.91	3.61	16.00	33.00	87	31.59	1.86	22.00	33.00
Combo 1	1	25	27.64	4.34	19.00	33.00	39	29.90	5.20	4.00	33.00
Combo 2	0	57	30.54	3.58	9.00	33.00	87	32.02	1.28	27.00	33.00
Combo 2	1	25	28.16	4.10	16.00	33.00	39	29.72	3.85	15.00	33.00
Basic 2	0	57	30.35	2.32	23.00	33.00	87	31.46	1.92	19.00	33.00
Basic 2	1	25	27.00	5.77	6.00	32.00	39	30.13	2.56	22.00	33.00
Timing
Basic 1	0	57	22.33	4.27	13.00	31.00	87	26.94	3.60	16.00	33.00
Basic 1	1	25	21.64	4.22	10.00	30.00	39	24.13	4.57	11.00	32.00
Visual 1	0	57	21.68	4.07	13.00	31.00	87	25.28	3.59	14.00	33.00
Visual 1	1	25	19.16	4.80	10.00	28.00	39	21.49	5.38	2.00	29.00
Visual 2	0	57	22.12	4.74	7.00	30.00	87	26.01	3.24	17.00	32.00
Visual 2	1	25	20.12	4.15	11.00	28.00	39	21.46	5.40	5.00	29.00
Audio 1	0	57	24.16	4.28	12.00	31.00	87	27.21	3.42	16.00	33.00
Audio 1	1	25	21.12	5.08	11.00	30.00	39	22.62	5.78	4.00	31.00
Audio 2	0	57	23.86	4.29	12.00	32.00	87	27.07	2.90	18.00	33.00
Audio 2	1	25	21.32	4.38	13.00	28.00	39	23.13	5.19	10.00	33.00
Combo 1	0	57	21.53	5.35	6.00	32.00	87	25.16	3.58	16.00	32.00
Combo 1	1	25	18.16	5.18	11.00	29.00	39	20.74	5.23	2.00	29.00
Combo 2	0	57	21.79	4.55	5.00	31.00	87	25.32	3.40	15.00	31.00
Combo 2	1	25	20.04	4.77	10.00	28.00	39	21.13	4.57	10.00	30.00
Basic 2	0	57	22.84	4.48	12.00	30.00	87	26.32	3.84	15.00	33.00
Basic 2	1	25	20.08	5.87	4.00	30.00	39	22.77	4.34	14.00	30.00
Hyperactivity
Basic 1	0	57	1.26	1.45	0.00	6.00	87	0.53	1.13	0.00	6.00
Basic 1	1	25	1.08	1.35	0.00	5.00	39	0.79	1.40	0.00	5.00
Visual 1	0	57	2.35	2.39	0.00	11.00	87	1.41	1.91	0.00	8.00
Visual 1	1	25	5.16	6.00	0.00	30.00	39	3.85	5.89	0.00	27.00
Visual 2	0	57	4.02	3.89	0.00	19.00	87	1.62	1.92	0.00	9.00
Visual 2	1	25	9.20	8.52	0.00	31.00	39	5.18	7.85	0.00	42.00
Audio 1	0	57	3.35	4.45	0.00	21.00	87	1.23	1.78	0.00	11.00
Audio 1	1	25	7.44	6.39	0.00	27.00	39	3.51	7.22	0.00	43.00
Audio 2	0	57	4.19	5.09	0.00	24.00	87	1.36	1.73	0.00	9.00
Audio 2	1	25	10.80	10.82	0.00	44.00	39	6.00	15.56	0.00	78.00
Combo 1	0	57	3.79	4.53	0.00	19.00	87	1.37	1.79	0.00	7.00
Combo 1	1	25	8.56	11.41	0.00	54.00	39	8.74	20.30	0.00	121.00
Combo 2	0	57	4.16	4.91	0.00	23.00	87	1.80	3.37	0.00	26.00
Combo 2	1	25	8.56	9.91	0.00	47.00	39	6.59	12.16	0.00	72.00
Basic 2	0	57	3.26	5.51	0.00	32.00	87	1.22	1.86	0.00	8.00
Basic 2	1	25	8.96	12.93	0.00	47.00	39	6.38	12.50	0.00	69.00
Impulsivity
Basic 1	0	57	0.98	1.03	0.00	4.00	87	0.83	0.90	0.00	4.00
Basic 1	1	25	1.32	1.55	0.00	6.00	39	0.82	0.97	0.00	5.00
Visual 1	0	57	1.32	1.02	0.00	4.00	87	1.05	0.93	0.00	4.00
Visual 1	1	25	1.84	1.68	0.00	7.00	39	1.72	2.45	0.00	11.00
Visual 2	0	57	1.33	1.29	0.00	6.00	87	1.08	0.94	0.00	4.00
Visual 2	1	25	2.44	2.35	0.00	9.00	39	2.08	2.09	0.00	10.00
Audio 1	0	57	1.09	1.04	0.00	5.00	87	0.98	0.99	0.00	4.00
Audio 1	1	25	2.12	1.62	0.00	6.00	39	1.49	1.55	0.00	5.00
Audio 2	0	57	1.40	1.56	0.00	8.00	87	1.28	1.22	0.00	5.00
Audio 2	1	25	3.84	2.76	0.00	11.00	39	2.85	4.23	0.00	25.00
Combo 1	0	57	1.40	1.44	0.00	6.00	87	1.29	1.28	0.00	8.00
Combo 1	1	25	2.56	3.16	0.00	16.00	39	2.36	3.82	0.00	24.00
Combo 2	0	57	1.46	1.99	0.00	10.00	87	1.36	1.14	0.00	5.00
Combo 2	1	25	2.88	2.44	0.00	11.00	39	1.79	2.27	0.00	13.00
Basic 2	0	57	2.04	3.06	0.00	22.00	87	1.47	1.38	0.00	6.00

Table 2.

TD vs ADHD Groups. Summary of Results.

	Indices	Findings
Global performance	Attention	TD > ADHD, except for phase Basic 1
	Timing	TD > ADHD, except for phase Basic 1
	Hyperactivity	TD < ADHD, except for phase Basic 1
	Impulsivity	TD < ADHD, except for phases Basic 1 and Visual 1 and Combo
Visual distractors	Attention	Negatively impact performance in both groups, especially ADHD
	Timing	Negatively impact performance in both groups
	Hyperactivity	Negatively impact ADHD performance
	Impulsivity	Negatively impact ADHD performance
Auditory distractors	Attention	Do not impact the performance of both groups
	Timing	Positively impact the TD group.
	Hyperactivity	Negatively impact ADHD performance
	Impulsivity	Negatively impact ADHD performance
Combined distractors	Attention	Negatively impact performance similarly in both groups
	Timing	Negatively impact performance similarly in both groups
	Hyperactivity	Do not impact the performance of both groups
	Impulsivity	Reduced impulsivity in the ADHD group
Intensity of distractors	Attention	Does not impact performance
	Timing	Does not impact performance
	Hyperactivity	High-intensity Auditory distractors but not low-intensity ones negatively impact ADHD performance.
	Impulsivity	High-intensity Auditory distractors but not low-intensity ones negatively impact ADHD performance
Fatigue	Attention	Negatively impact performance similarly for both groups
	Timing	No fatigue effect emerges for both group
	Hyperactivity	Fatigue effect emerges only in the ADHD group
	Impulsivity	Negatively impact performance similarly for both groups

Attention

The repeated measures ANOVA on the attention index (a) shows three significant main effects: Phase (F_{7, 1,428} = 21.67, p < .001, η²_p = .10), Group (F_{1, 204} = 29.19, p = .001, η²_p = .13), and Age (F_{1, 204} = 16.10, p < .001, η²_p = .07). The interactions Phase × Group (F_{7, 1,428} = 3.11, p = .003, η²_p = .02) and Phase × Age (F_{7, 1,428} = 2.75, p = .008, η²_p = .01) are also significant.

Phase × Group Interaction

Post hoc tests for the interaction between Phase and Group reveal that the performance in the eight phases of the test was different for the two groups and that the presence of visual distractors negatively impacted especially the ADHD group (Figure 1): except for the first baseline (Basic 1), where the two groups did not differ (p = 1.00), the TD group performed better than the ADHD group in all phases (p = .044 or less). The introduction of visual distractors (phase Visual 1) resulted in a significant decrease in performance for both groups (p = .007 or less), but the decrease was larger for the ADHD group. Then, both groups remained stable until phase Audio 2, which means that the increase in the intensity of visual distraction and the introduction of acoustic distractors did not modify children’s performance. The performance of the two groups decreased significantly with the combined distractors (Combo 1; p = .004 or less) and did not show any other significant change until the end of the test. In addition, the ADHD and TD groups showed significant differences between phases Basic 1 and Basic 2 (p < .001), suggesting that fatigue affects the attention indicator, independently of the clinical condition.

Figure 1.

Interaction between phase and group in attention index.

Phase × Age Interaction

The significant interaction between Phase and Age suggests that the distractors impacted differently based on age group (8–9 years vs. 10–12 years; Figure 2). Compared to the older group, the younger group showed a lower initial and final baseline attention score (p = .007 or less) and poorer performance with the introduction of combined distractors (p = .041; Combo 1).

Figure 2.

Interaction between phase and age in attention index.

Considering the performance of each age group in all phases, the introduction of visual distractors in phase Visual 1 was similarly disruptive for both groups (p < .001). After that, the performance in the older group remained stable under the baseline values, with nonsignificant variations until the end of the task. In contrast, the younger group appeared to be more influenced by distracting events. Youngest children’s accuracy decreased with the introduction of combined distractors (p < .001; Combo 1). After that, performance remained stable and lower than the first baseline (Basic 1).

Timing

The repeated measures ANOVA on the timing index (t) shows three significant main effects: Phase (F_{7, 1,428} = 24.33, p < .001, η²_p = .11), Group (F_{1, 204} = 33.42, p < .001, η²_p = .14), and Age (F_{1, 204} = 26.29, p < .001, η²_p = .11). The Phase × Group interaction is significant (F_{7, 1,428} = 2.83, p = .006, η²_p = .01). No significant interaction emerged between Age and Phase.

Phase × Group Interaction

Considering the between-group differences, except in phase Basic 1 (p = .654), the children with ADHD were generally less timely in all phases than the TD group (Figure 3), so they gave fewer correct answers with appropriate timing (p < .001). Considering the within-group differences, the performance of both groups decreased between phases Basic 1 and Visual 1 (p = .002 or less) with the introduction of visual distractors, and between phases Audio 2 and Combo 1 (p < .001) with the introduction of combined distractors. Before phase Audio 2 and after phase Combo 1, a gradual increase in scores was observed for both groups, although it was greater and significant for the control group between phases Visual 2 and Audio 1 (p < .001), when acoustic distractors were introduced. Neither of the groups showed significant differences between phases Basic 1 and Basic 2 (p > .515).

Figure 3.

Interaction between phase and group in timing index.

Hyperactivity

The repeated measures ANOVA on the hyperactivity index (h) shows three significant main effects: Phase (F_{7, 1428} = 21.42, p < .001, η²_p = .10), Group (F_{1, 204} = 27.63, p < .001, η²_p = .12), and Age (F_{1, 204} = 8.08, p = .005, η²_p = .04). Moreover, two interactions are significant: Phase × Group (F_{7, 1,428} = 8.19, p < .001, η²_p = .04) and Phase × Age (F_{7, 1,428} = 2.81, p < .001, η²_p = .01).

Phase × Group Interaction

Considering the between-group differences, post-hoc tests show that the two groups started at the same level (p = 1.000), but by phase Visual 1, the two groups began to drift apart (p = .012 or less; Figure 4). Considering the within-group differences, children without a diagnosis of ADHD performed without significant fluctuations (p > .118), unlike the clinical group. Hyperactivity of children with ADHD increased significantly from phase Visual 1 (p < .001) and in phase Audio 2 (p = .032) and successively did not show any significant variations (p = 1.000). The difference between the two groups is particularly marked for this index. The effect of fatigue is shown only by the clinical sample, which at Basic 1 differed significantly from the final phase (Basic 2; p < .001).

Figure 4.

Interaction between phase and group in hyperactivity index.

Phase × Age Interaction

Considering the between-group differences, post hoc tests reveal that the two age groups differ significantly from each other in phases Visual 2 and Audio 1 (p = .007 or less). Considering the within-group differences, all children made more hyperactive errors after phase Basic 1 (p < .001). The increase in the intensity of the visual distractors between phases Visual 1 and Visual 2 significantly increased hyperactivity in the youngest group (p < .001) and not in the oldest group (p = 1.000). Performance did not present other significant variations in either age group (Figure 5).

Figure 5.

Interaction between phase and age in hyperactivity index.

Impulsivity

The repeated measures ANOVA on the impulsivity index (i) shows three significant main effects: Phase (F_{7, 1,428} = 22.33, p < .001, η²_p = .10), Group (F_{1, 204} = 25.17, p < .001, η²_p = .11), and Age (F_{1, 204} = 4.62, p = .033, η²_p = .02). Moreover, two interactions are significant: Phase × Group (F_{7, 1,428} = 7.13, p < .001, η²_p = .03). The interaction between Phase and Age for the Impulsivity index was not significant. In the case of impulsivity, a high score corresponds to a worse performance.

Phase × Group Interaction

Post hoc tests reveal that, from phase Visual 2 to Audio 2, and in Basic 2, impulsivity was higher in children with ADHD than in the TD group (p = .011 or less; Figure 6). Considering within-group performance, children with ADHD showed a gradual increase in impulsivity between Basic 1 and Visual 2 (p < .001), with the introduction of visual distractors, a nonsignificant recovery of performance in phase Audio 1, and a new increase in impulsive response in phase Audio 2 (p < .001), when auditory distractors of higher intensity were presented. Then, a significant reduction in impulsivity occurred in phase Combo 1 (p = .002), and a further increase in phase Basic 2 (p = .016). On the other hand, the TD group displayed a constantly low level of impulsive responses. Both groups showed a significant increase in impulsive responses between phases Basic 1 and Basic 2 (p = .009 or less), indicating an effect of fatigue.

Figure 6.

Interaction between phase and group in impulsivity index.

Discussion

Sustained attention and inhibitory control are different but interrelated abilities that are impaired in individuals with ADHD. This study evaluated the impact of visual and auditory distractors on different measures of sustained attention and inhibitory control obtained from children with ADHD and their typically developing peers performing the MOXO d-CPT task. Previous studies investigated the impact of distractors on sustained attention using MOXO (Berger & Cassuto, 2014; Slobodin et al., 2018), but only a few focused on the impulsivity and hyperactivity index (Borkowska, 2016; Uno et al., 2006). Studying the effects of distractors on impulsivity and hyperactivity is essential for a deeper understanding of ADHD traits, improving educational strategies, developing effective interventions, optimizing cognitive performance, and ultimately enhancing the quality of life for individuals affected by these difficulties.

As expected, the results show that children with ADHD performed lower in attention and timing, showing at the same time greater impulsivity and hyperactivity (Berger et al., 2017). However, distractors play a significant role in reducing their performance, regardless of age. The introduction of low-intensity visual distractors, alone or in combination with auditory distractors, resulted in a significant decrease in sustained attention and inhibitory control in the ADHD group, while in the control group these distractors only impacted sustained attention, and to a lesser extent than observed for the clinical group. Several explanations may account for this effect on sustained attention, such as visual distractors sharing the same cognitive modality as the main task (Wickens, 2008) and reduced visual attention in ADHD (Kofler et al., 2008). Together, these conditions interfere with target detection due to visual information overload. The explanation for the increase in hyperactive and impulsive responses is less obvious. Being easily distracted is a common feature of inattentive and hyperactive/impulsive symptom domains, regardless of age (Martel et al., 2016). The abilities to control interference from external stimuli and to refrain from an impulse can be considered parts of the inhibitory construct in humans (Diamond, 2013; Friedman & Miyake, 2004) and animal cognition (Loyant et al., 2022). Therefore, difficulties in controlling interference from distracting stimuli may have a common origin with those related to impulsive or hyperactive behaviors. Thus, distractors that interfere with sustained attention also produce more impulsive and hyperactive responses.

Present results confirm that visual distractors have a stronger impact both on sustained attention and inhibitory control of children and preadolescents with ADHD, while auditory distractors showed a greater negative impact on inhibitory control than on sustained attention measures (Pelham et al., 2011; Slobodin et al., 2018). The presence of auditory distractors resulted in a recovery of response timing during the task in the control group, but not in the group with ADHD. Consistently with previous findings, this result confirms the lack of a positive effect of auditory distractors on sustained attention in children with ADHD (Berger & Cassuto, 2014; Cassuto et al., 2013; Slobodin et al., 2018). The increase in arousal associated with a novel signal (Uno et al., 2006; van Mourik et al., 2007), may have been reduced by the fact that, differently from other tasks, the distractors were not synchronized with the targets in MOXO d-CPT (Berger & Cassuto, 2014).

The effects of auditory distractors must be considered jointly with those associated with the intensity level of the distractors. Unexpectedly, the level of auditory distractors had a different effect on sustained attention and inhibitory control in the clinical sample. Unlike the findings of Slobodin et al. (2018), who observed that children with ADHD make more omission errors in the low-intensity condition than in the high-intensity condition, in our study no intensity effect emerges on the sustained attention indices. Instead, for inhibitory control, more intense auditory distractors entailed more difficulties in controlling both hyperactive and impulsive responses in the clinical group, but not in the control group. Previous studies highlighted the different impact of auditory distractors on impulsivity for the ADHD population compared to healthy peers. The perceptual load model of attention (e.g., Lavie & Tsal, 1994) states that task-relevant processing load determines irrelevant distractor processing in such a way that increasing processing load prevents distractor processing. However, Tellinghuisen and Nowak (2003) showed that visual and auditory distractors work differently in a visual search task. While visual distractors negatively impact only easy tasks, and not high-demanding tasks (where the task load makes it impossible to process the distractor), auditory distractors impact both low- and high-demanding tasks. The authors suggested that auditory distractors are processed regardless of visual perceptual load. Our results are in line with this evidence. It could be possible that the increase of visual distractors was not processed, which did not affect performance on the MOXO task. In contrast, the increase in auditory distractors was processed, causing a decline in task performance. Such results are particularly relevant given the lack of literature on inhibitory control and distractors, particularly considering MOXO-d CPT. Furthermore, none of the existing studies offers an in-depth explanation of the possible processes underlying the results obtained.

MOXO d-CPT also allows for investigating the cumulative effect of task events on performance. The results showed a general decline in performance associated with the effect of fatigue for both groups. However, considering hyperactivity, the fatigue effect appears only in the ADHD group. Such a result is in line with the literature showing that hyperactivity is a specific factor characterizing ADHD, more than inattention and impulsivity, at least in young children (Bramham et al., 2012; Halperin et al., 1992). In this vein and in line with previous findings, hyperactivity emerges as the index that most differentiates the two groups in all phases of the task (Borkowska, 2016), but even more when the impact of fatigue is considered. In addition, hyperactive symptoms generally tend to reduce with increasing age, while attention deficits are more persistent.

As for age-related effects, they were found in both sustained attention (Attention index) and inhibitory control (Hyperactivity index) in all children, regardless of whether the ADHD condition was present. As expected, the younger group showed a lower level of sustained attention and inhibitory control and was more susceptible to task events, presenting more unstable performance. In this subsample, as found in the clinical sample, sustained attention appears to be more affected by the introduction of visual or combined distractors, while inhibitory control was more affected by increased intensity distractions. Taken together, the results are consistent with the literature showing that the development of inhibitory control precedes that of sustained attention (Betts et al., 2006; Kanaka et al., 2008; Rebok et al., 1997). The absence of a significant interaction between age and group with the different distractors suggests that, although a difference in sustained attention and inhibitory control between children with and without ADHD, the two groups show similar age-related differences. Other studies support the hypothesis of a developmental delay in sustained attention that characterizes children and adolescents with ADHD (Berger et al., 2013; Slobodin et al., 2018). Considering that the youngest participants and children with ADHD shared characteristics such as lower sustained attention and inhibitory control together with higher reactivity to visual and/or higher intensity distractions, we cannot exclude that the delay hypothesis may explain these similarities in both sustained attention and inhibitory control.

The present results should be considered in light of some limitations. First, the two age groups are arbitrarily composed to balance the number of participants with ADHD between them. Furthermore, the cross-sectional design used in this study does not allow firm conclusions about individual development, for which a longitudinal study is needed. Although the power analysis confirms that the total sample size is adequate, the subsample of children with ADHD may be limited. In addition, the ADHD group is considered without differentiating between subtypes. Some differences in the impact of distractors on attention and inhibitory control may emerge if inattentive and impulsive-hyperactive subtypes were considered separately. More research is needed to better investigate this aspect.

In conclusion, this study allows us to shed light on the impact of distractors on CPT performance in children with ADHD. Although previous studies focused only on the attention index (Berger & Cassuto, 2014; Slobodin et al., 2018), we considered the attention, impulsivity, and hyperactivity indices, highlighting how distractors have a different impact on each of them. Different from previous studies, this allowed us to understand the possible processes underlying this impact. Globally, results showed that visual distractors have a negative impact on attention and inhibitory control that is greater for ADHD children, independently of age. In contrast, auditory distractors only affect inhibitory control in children with ADHD. They show higher inhibitory control when low-intensity auditory distractors appear. These results have important clinical relevance, as can help the clinician to understand the mechanism underpinning ADHD difficulties. In addition, such results can help parents and teachers, indicating how to prevent or minimize attentional difficulties in children with ADHD. Moreover, our result offers some evidence of the clinical relevance of MOXO-d-CPT, since the duration of the task, both in the presence or absence of distractors, makes the difference between the two groups (with and without ADHD) more prominent. In addition, the presence of different scores for each task phase allows for obtaining an in-depth description of the performance trend over time. Our results show that while TD children show globally a linear trend, those with ADHD show a more fluctuating performance.

Footnotes

Declaration of Conflicting Interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Carlotta Rivella, Paola Viterbori and Maria Carmen Usai are the authors of the MOXO-d-CPT Italian Standardization published by the publishing house Hogrefe. Maria Carmen Usai also received funding from Hogrefe publishing house to carry out standardization.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: In 2019, Maria Carmen Usai was scientifically responsible for a starting research scholarship at the University of Genoa funded by Hogrefe Publishing House.

ORCID iDs

Alice Bazzurro

Valeria Olla

Author Biographies

Carlotta Rivella, PhD, is a post-doctoral researcher in the Department of Education Sciences at the University of Genoa (Genoa, Italy). Her research interests mainly concern executive functions in typical and atypical development and in acquired brain injuries. She collaborates in the development of software for the assessment and intervention of executive functions in school-age children.

Alice Bazzurro psychologist and PhD student in Social Sciences, Psychology and Cognitive Science curriculum at the Department of Education Sciences, University of Genoa. She conducts research on regulatory and executive function skills, with a focus on the relationship between these skills and Neurodevelopmental Disorders.

Valeria Olla, psychologist and PhD, expert in Neuropsychology, specialized in Developmental and Learning Psychopathologies and in the treatment of Disruptive Behavior Disorders. Authorized team representative for the release of the first diagnostic certification on Specific Learning Disabilities for the Lombardy region. She carries out clinical activity on Neurodevelopmental Disorders at her private center, carries out training on developmental psychopathologies, collaborates with the Child Neuropsychiatry Operative Units. Her research interests Neurodevelopmental Disorders.

Cristina Potente is a Psychologist - Psychotherapist, director of a clinical center for Neurodevelopmental Disorders. She is responsible for the professional training of a cognitive training focused on attention and working memory. She is consultant for inclusion and special educational needs for italian school publishers.

Claudio Vio, is an Psychologist – Psychotherapist former head of the developmental age service at AULSS 4 - Veneto Contract professor at the Department of Developmental Psychology and Socialisation, University of Padua.

Paola Viterbori, PhD, is an associate professor of Developmental and Educational Psychology at the Department of Education Sciences, University of Genoa. She currently teaches Developmental psychology, Psychology of school learning and Psychology of developmental disabilities in early childhood. Her research interests mainly focus on executive function and disability.

Maria Carmen Usai, PhD, is a full professor in the Department of Education Sciences at the University of Genoa (Genoa, Italy). Her research interests mainly concern executive functions in typical and atypical development. She teaches developmental psychology in bachelor’s and master’s degree courses and psychology of disability in a master’s course. She was the Director of postgraduate courses on Learning Disabilities and the founding partner of a start-up for research and intervention in language and learning disorders.

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