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Keywords:

  • conflict adaptation;
  • conflict frequency;
  • obstructive sleep apnea;
  • post-error slowing

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of Interest
  8. References

Obstructive sleep apnea syndrome is associated with executive cognitive impairment. An important question is whether impairment in executive functioning in obstructive sleep apnea syndrome is independent of dysfunction in attention. Attentional control is a subcomponent of executive functioning that is mediated by frontal lobe processing. In the current study, we investigated whether attentional control is deficient in obstructive sleep apnea syndrome. Attentional control processes were investigated through conflict adaptation and conflict frequency paradigms. These neuropsychological paradigms were assessed by using the Simon, Flanker and Stroop tasks. We additionally analysed post-error slowing data within these tasks. Error processing is another index of cognitive control that is mediated by frontal lobe functioning. Our sample consisted of 14 healthy adults and 24 patients with untreated moderate–severe obstructive sleep apnea syndrome. Results indicated that attentional control is partially dysfunctional among patients with obstructive sleep apnea syndrome. Attentional control processes were deficient when focal attention (Flanker task) processes were involved, but were intact when observed using the Simon and Stroop tasks. A non-significant trend in post-error slowing data suggested that error processing, assessed with the Flanker task, was diminished among patients with obstructive sleep apnea syndrome. These results support the view that obstructive sleep apnea syndrome leads to some amount of frontal lobe dysfunction, and that attentional control and error processing might be particularly affected by obstructive sleep apnea syndrome.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of Interest
  8. References

Obstructive sleep apnea syndrome (OSAS) is a respiratory disorder that is characterized by a complete (apnea) or partial (hypopnea) cessation of breathing as a result of the collapse of the upper air pathway during sleep. OSAS is associated with several neuropsychological impairments, including dysfunction in psychomotor abilities, memory, attention and executive functions (Mazza et al., 2005; Naegele et al., 1995; Saunamaki et al., 2009).

An important question is whether executive function impairment in OSAS is independent of dysfunctions in attentional processes. Executive functions are a group of neuropsychological processes that enable individuals to behave flexibly in a goal-directed manner as well as adapt to task and environmental demands (Lezak et al., 2004). Task switching, inhibition, attentional control, planning, flexibility, set shifting and categorization are partially independent executive functions that are mediated by frontal lobe functioning (Miyake et al., 2000).

Frontal lobe dysfunction has been associated with OSAS. Specifically, Beebe and Gozal (2002) proposed that sleep disruption and hypoxia during apnea–hypopnea events lead to frontal lobe pathology. In turn, this pathology leads to deficits in executive functions. Disruptions in executive functions within several domains, such as inhibition, planning, adaptation, working memory and verbal fluency, have been reported in patients with OSAS (Saunamaki et al., 2009).

Verstraeten and Cluydts (2004) reviewed the neurocognitive framework of executive function, attention and arousal, and concluded that neural structures responsible for executive functions were modulated by those that regulate arousal and alertness. Consequently, they argued that the decrease in vigilance – which can be defined as the ability to stay focused on a particular task for a prolonged period – due to sleepiness among patients with OSAS might be a key factor underlying detrimental performance on executive tasks. Accordingly, they argued that vigilance should be controlled for when investigating executive functions. Interestingly, Verstraeten et al. (2004) did not observe deficits in executive functions among patients with OSAS when appropriate measurement and analysis methods were administered, by controlling for vigilance.

Attentional control is one of the more important aspects of executive functioning. Attention is imperfect and is easily captured by distracters. Therefore, attentional control is necessary for the maintenance of task goals. Within the laboratory, attentional control functions have been investigated with selective attention tasks, such as the Stroop, Flanker and Simon tasks. Selective attention tasks require participants to attend to one (target) dimension of a stimulus while ignoring another (distracter) dimension. Typical results include faster and more accurate responses when the target and distracter dimensions are congruent rather than incongruent. The difference between responses to congruent and incongruent stimuli is referred to as the ‘congruency effect’. The size of the congruency effect reflects how well attention is controlled (Botvinick et al., 2001). Attentional control paradigms are used to investigate how participants govern their selective attention in order to adapt to internal (cognitive) or external (environmental) demands.

Two of the most common attentional control paradigms are conflict adaptation and conflict frequency. During conflict adaptation experiments, control of attentional processing following a conflict trial is observed: smaller congruency effects following incongruent trials compared with congruent trials reflect an intact conflict adaptation system (Botvinick et al., 2001). During conflict frequency experiments, attentional control is observed by manipulating the proportion of conflict trials. Smaller congruency effects when conflict is frequent compared with when it is infrequent reflect control of attentional processes according to the structure of the stimuli. Attentional control functions are associated with neural activity in frontal lobe areas, specifically the dorsolateral prefrontal cortex (DLPFC) and the anterior cingulate cortex (ACC; MacDonald et al., 2000).

The Stroop task has been extensively conducted among the OSAS population (Naegele et al., 1995; Verstraeten et al., 2004). However, to our knowledge, patients with OSAS have not been tested with the Flanker and Simon tasks, as well as conflict adaptation and conflict frequency effects have not been investigated in this population.

In the current study, we investigated attentional control among patients with OSAS. We predicted that if attentional processes were deficient in individuals with OSAS (Ayalon et al., 2009; Mazza et al., 2005), attentional control should also be impaired. Specifically, within the conflict adaptation paradigm, we hypothesized that patients with OSAS would not adjust their attentional processes as a function of the congruency from the previous trial; consequently, similar congruency effects would follow incongruent and congruent trials. Within the conflict frequency paradigm, we expected that patients with OSAS would not adjust their attentional processes as a function of the frequency of conflict in the present block; consequently, similar congruency effects would be observed in mostly congruent and mostly incongruent blocks. We also analysed post-error slowing (PES) data, which is another parameter of control associated with frontal lobe functioning (Li et al., 2008). Post-error slowing is an increase in reaction time (RT) following an erroneous response, which is an indication of cognitive control processing (Botvinick et al., 2001). This PES rate is calculated as the mean RT to a correct response following an error. We hypothesized that because error processing would be deficient among patients with OSAS, they would not show any slowing within trials following erroneous responses.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of Interest
  8. References

Participants

Fourteen healthy adults and 24 patients with untreated moderate–severe OSAS [apnea–hypopnea index (AHI) > 15] participated in the study. Their demographic characteristics are reported in Table 1. Patients were selected from applicants in the Sleep Laboratory of Selcuk University, Faculty of Medicine, Department of Chest Diseases who were diagnosed with OSAS for the first time, in a period of 6 months. Control participants were recruited from patients' relatives (matched for age, gender and education). Pregnant women; patients with known cardiovascular, neuropsychiatric, endocrine and/or pulmonary diseases; participants who scored below 25 on the Mini-Mental State Examination (Gungen et al., 2002); and patients on a drug therapy that might affect cognitive function (e.g. anti-psychotics, sedatives) were excluded from this study. Patients diagnosed with mild OSAS (AHI 5–15) were also excluded. Because the control participants were on average 49.6 ± 9.4 years old, some might have had mild OSAS (AHI 5–15) without reporting any OSAS symptoms. Therefore, excluding patients with mild OSAS from analyses ensured that the patient and control groups actually differed in AHI scores.

Table 1. Demographic variables
VariableControl participants (n = 14, 11 males)Moderate–severe OSAS Patients (n = 24, 23 males)P
  1. BMI, body mass index; OSAS, obstructive sleep apnea syndrome.

  2. Standard deviations and range are presented in parentheses. There were five missing values in Beck Depression Inventory and Trait Anxiety Inventory data.

Age (years)49.6 (9.4, 28–70)48.1 (10.2, 32–70)0.64
Education (years)12.4 (2.6, 8–15)10.9 (4.5, 5–19)0.26
BMI (kg m−2)26.7 (3.5, 19.9–32.3)32.0 (4.2, 25.7–41.9)<0.001
Smoking (pack years)14.3 (14.5, 0–50)18.8 (16.6, 0–60)0.41
Beck depression inventory2.8 (2.1, 0–8)4.8 (3.6, 0–12)0.09
Trait anxiety inventory27.3 (2.3, 22–31)30.6 (6.8, 21–49)0.10
Epworth sleepiness scale4.1 (4.7, 0–16)9.3 (5.5, 1–24)<0.001

The diagnosis of OSAS was established by a full overnight polysomnography (Alice 5 Diagnostic Sleep System; Philips Respironics, Amsterdam, The Netherlands), which included the recording of oronasal flow, chest and abdominal wall motion, electrocardiogram, submental and pretibial electromyography, electrooculography, electroencephalography (C3-A2 and O2-A1), pulse oximetry and body position. Results were scored using standard techniques and criteria (Iber et al., 2007). Patients' sleep data are reported in Table 2.

Table 2. Sleep data of patients with moderate–severe OSAS (n = 24)
  1. AHI, apnea–hypopnea index; TST, total sleep time.

  2. Standard deviations and range are presented in parentheses.

AHI42.1 (16.4, 16.9–84.7)
Average SaO292.3 (2.8, 82–95)
Lowest SaO276.0 (10.1, 45–89)
Time with SaO2 <90%14.0 (19.6, 0–76.9)
Desaturation index (%)38.3 (19.9, 6.3–93)
Maximum desaturation (%)17.4 (8.6, 8–47)
Arousal index (no.∙h−1)25.2 (13.5, 1.5–49.5)
TST (min)372.2 (59.9, 216.5–494.5)
Sleep stages
Stage N1 (%TST)10.1 (5.1, 1.8–24.9)
Stage N2 (%TST)66.8 (6.4, 51.7–78.4)
Stage N3 (%TST)9.4 (5.6, 0–19.7)
Stage R (%TST)13.7 (5, 0–21)

In order to exclude persons with possible OSAS from the control group, control participants and their bed partners were questioned on the participant's OSAS symptoms, such as snoring, daytime sleepiness, witnessed apneas, nocturnal urination and choking arousals. Ten control participants did not report any OSAS symptoms. Four control participants reported one OSAS symptom (either daytime sleepiness or snoring). Therefore, these control participants underwent an overnight polysomnography to exclude any potential OSAS (AHI < 5).

The study was approved by the Ethics Committee of Non-invasive Clinical Research, Selcuk University, Faculty of Medicine. Written informed consent was obtained from all participants. Control participants and patients with OSAS reported normal colour vision, and normal or corrected-to-normal visual acuity.

Materials and procedure

After giving their informed consent, participants were administered a questionnaire assessing demographic characteristics, and the Turkish versions of the Standardized Mini-Mental State Examination (Gungen et al., 2002), Epworth Sleepiness Scale (Izci et al., 2008), Beck Depression Inventory (Hisli Sahin, 1988) and Trait Anxiety Inventory (Öner and Le Compte, 1985).

Experiments were run automatically on a standard PC through E-prime 2.0 (Psychology Software Tools, Pittsburgh, PA, USA) in a dimly lit and silent room. All stimuli were presented on a 19-inch colour monitor with Arial font (font size: 60 points) against a grey background. Participants were comfortably seated 50–60 cm from the monitor. They were tested on the morning following the overnight polysomnography. The experiment session took about 30 min.

Conflict adaptation experiment

Simon and Flanker tasks were administered to investigate the conflict adaptation effect. During the Simon task, stimuli (the letters ‘H’ and ‘S’) were presented on the left or right side of the monitor. Participants were instructed to discriminate between the letters by pressing the left ALT key for ‘H’ and the right CTRL key for ‘S’ with their left and right index fingers, respectively. The response keys were tagged with stickers. During the congruent trials, the stimulus and the response were at the same location (i.e. ‘H’ presented on the left side of the screen and responded with the left index finger, ‘S’ presented on the right side of the screen and responded with the right index finger). During the incongruent trials, the stimulus and the response were at opposite locations (i.e. ‘H’ presented on the right side of the screen and responded with the left index finger, ‘S’ presented on the left side of the screen and responded with the right index finger). The task consisted of 48 congruent and 48 incongruent trials presented in random order. Before each trial, a blank screen was presented for 1000 ms, followed by a fixation cross (for 1000 ms) at the centre of the screen. No feedback was given following the responses. The next trial was presented immediately. The inter-stimulus interval and stimulus duration were kept constant across trials, that is, stimulus duration and inter-trial interval were identical. There were 12 practice trials, which were not included in the analyses, prior to the experimental trials. Participants were instructed to respond as fast as possible without compromising on accuracy.

The Flanker task was identical to the Simon task except for the stimuli used. The Flanker stimuli were a sequence of five letters presented at the centre of the screen. Participants were instructed to respond to the middle letter of the sequence while ignoring the remaining letters. The middle letter was flanked by identical letters during congruent trials (i.e. ‘HHHHH’ or ‘SSSSS’). The middle letter was flanked by different letters during incongruent trials (‘HHSHH’ or ‘SSHSS’). Participants completed both the Simon and Flanker tasks. The order of the Simon and Flanker tasks was counterbalanced across participants.

Conflict frequency experiment

Following the Simon and Flanker tasks, participants completed the Stroop task (Macleod, 1991). The Stroop task was administered to measure the conflict frequency effect. During the Stroop task, the Turkish colour words were presented in different colours, and participants were instructed to respond to the ink colour but not to the word itself. During the congruent trials, the meaning of the word and the colour were the same. During the incongruent trials, the meaning of the word and the colour were different. There were two blocks: mostly congruent (MC) and mostly incongruent (MI). In both blocks, there were 18 congruent and 18 incongruent experimental trials. In the MC block, there were additional 36 congruent filler trials, which ensured that 75% of the trials were congruent. In the MI block, there were an additional 36 filler incongruent trials, which ensured that 75% of the trials were incongruent. An additional 18 neutral trials (a string of three percentage signs % % % printed in each colour) were added to both blocks to identify any group differences in the colour identification task itself. Trials were presented in random order. Before each trial, a blank screen was presented for 1000 ms, followed by a fixation cross (for 1000 ms) at the centre of the screen. Participants responded by pressing buttons on a response box (Psychology Software Tools, Pittsburgh, PA, USA), which were tagged with colour stickers. Participants were instructed to respond as fast as possible without compromising on accuracy. Feedback was given on the screen after each response. The next trial was presented immediately following feedback. The inter-stimulus interval and stimulus duration were kept constant across trials, that is, the stimulus duration and inter-trial interval were identical. The order of the MC and MI blocks was counterbalanced across participants. There were 36 practice trials (12 congruent, 12 incongruent and 12 neutral) at the beginning of the task. At the end of the experiment, the purpose and rationale of the study were explained to the participants.

Statistical analyses

Statistical analyses were conducted with spss 15.0 (SPSS Inc., Chicago, IL, USA). Statistical significance (P-values) was calculated by using general linear model procedures for normally distributed variables. Other variables were analysed with non-parametric Mann–Whitney U-tests. Only theoretically important significant main effects and interactions are reported.

The average error rate (ER) and RTs of correct responses for the Simon and Flanker tests were calculated for each participant for each condition. The RTs were calculated after excluding trials with incorrect responses, trials following incorrect responses, outliers (RTs > ± 3 SD) and trial repetitions. Data for two control participants were excluded due to procedural errors. The RT and ER data were analysed separately with a 2 × 2 × 2 × 2 mixed-design anova with participant group (control versus moderate–severe OSAS) as the between-subjects factor, and previous congruency (congruent versus incongruent), current congruency (congruent versus incongruent) and task (Simon versus Flanker) as the within-subject factors.

The conflict frequency effect was investigated with RT and ER data for the Stroop task. Statistical analyses were conducted only on predetermined experimental trials, and filler trials were excluded. Thirty-six predetermined experimental trials included 18 congruent and 18 incongruent Stroop trials, and E-prime tagged these trials automatically. The RT and ER data were submitted to separate 2 × 2 × 2 mixed-design anovas with participant group (control versus moderate–severe OSAS) as a between-subjects factor, and block (MC versus MI) and congruency (congruent versus incongruent) as within-subject factors.

Average PES was calculated for all tasks and for each participant. Post-error slowing was defined as the RT for a correct response following an incorrect response. Post-error slowing data were limited to participants who gave an erroneous response. Because several participants completed the tasks without any errors, PES results were analysed for each task separately with one-way anovas (control versus moderate–severe OSAS).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of Interest
  8. References

Demographic data are presented in Table 1. Sleep apnea patients and control participants were similar on all demographic variables except for body mass index (BMI). As expected, patients with OSAS had a higher BMI than the control participants. Data on sleep-related variables are presented in Table 2. Among the 24 patients with OSAS, six were diagnosed with moderate OSAS (AHI 15–30), and 18 were diagnosed with severe OSAS (AHI > 30).

Conflict adaptation with the Simon and Flanker tasks

For the Simon and Flanker tasks, average ERs were very low (0.9% for the Simon and 1.1% for the Flanker task). Sixteen (five control, 11 moderate–severe OSAS) participants finished the Simon task without any errors. Eighteen (seven control, 11 moderate–severe OSAS) participants finished the Flanker task without any errors. These results suggested that participants understood the task demands and followed them appropriately. The anova for ERs did not display a conflicting pattern of results with the RT data.

The anova for RTs revealed a significant two-way interaction between previous congruency and current congruency (F1,34 = 5.89, P < 0.05). This result suggested that the overall conflict adaptation effect was significant. In other words, current congruency was reduced after incongruent trials as compared with congruent trials. Importantly, the four-way interaction between participant group, previous congruency, current congruency and task was significant (F1,34 = 5.97, < 0.05). This interaction revealed that conflict adaptation was not identical across groups or across tasks (Fig. 1). We investigated the conflict adaptation effect in the Flanker and Simon tasks separately. For the Flanker task, the three-way interaction between group, previous congruency and current congruency was significant (F1,34 = 5.55, < 0.05). This result suggested that conflict adaptation was deficient among the patients with OSAS compared with controls (Fig. 1a). On the other hand, for the Simon task, this three-way interaction was not significant (F1,34 = 1.33, = 0.25), while the two-way interaction between previous congruency and current congruency was significant (F1,34 = 10.05, < 0.05). Patients with OSAS and control participants performed similarly on the Simon task (Fig. 1b).

image

Figure 1. Average RT as a function of previous and current congruency during the Flanker (a, left) and Simon (b, right) tasks. OSAS, obstructive sleep apnea syndrome.

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Conflict frequency effect with the Stroop task

For the Stroop task, average ERs were very low (1.7%). Ten (three control, seven moderate–severe OSAS) participants finished the Stroop task without a single error. These results indicate that participants understood the task demands and followed them appropriately. The anova for ERs did not display a conflicting pattern of results with the RT data.

The anova for RTs revealed a significant main effect of group (F1,36 = 5.83, < 0.05). Control participants responded faster than patients with moderate–severe OSAS. There was a significant two-way interaction between block and congruency (F1,36 = 22.49, < 0.001), which revealed that the Stroop effect was smaller during the MI block than the MC block. This indicated that the conflict frequency effect was significant. The three-way interaction between participant group, block and congruency was not significant (F1,36 = 0.28, = 0.60), which suggested that patients with OSAS and control participants showed similar adaptation of control processes as a function of stimulus structure (Fig. 2).

image

Figure 2. Average RT as a function of block and congruency during the Stroop task. OSAS, obstructive sleep apnea syndrome.

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Post-error slowing

Post-error slowing analyses revealed that none of the between-group comparisons was significant. However, there was a marginally significant trend with the Flanker task (F1,17 = 3.87, = 0.067) in that patients with OSAS responded slower than did control participants following an error (Fig. 3).

image

Figure 3. PES for the Flanker, Simon and Stroop tasks. OSAS, obstructive sleep apnea syndrome.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of Interest
  8. References

The primary aim of the current study was to investigate attentional control among patients with OSAS. Attentional control is an executive function that is mediated by frontal lobe functioning (Botvinick et al., 2001; Miyake et al., 2000). We observed that attentional control processes were partially impaired in our OSAS sample. Conflict adaptation was deficient among these patients as compared with control participants, but only when attentional control was assessed with the Flanker task. Surprisingly, a similar deficiency was not observed when the Simon task was used for assessment. Moreover, we did not observe a significant difference in the conflict frequency effect between patients with OSAS and control participants. Our results do suggest that patients with OSAS were not able to adapt their attentional processing as a function of conflict in the previous trial during the Flanker task. However, attentional control processes were functionally normal when tested with the Simon and Stroop tasks. Furthermore, we observed that overall RTs were significantly higher for patients with OSAS as compared with healthy controls during the Stroop task but not the other tasks.

Therefore, our results demonstrate that attentional control is partially impaired among patients with OSAS. Interestingly, attentional control impairment appeared to be specific to the Flanker task. The Flanker task differs from the Simon and the Stroop tasks in terms of how selective attention is oriented to relevant stimulus feature and which stimulus features are deemed irrelevant. In the Flanker task, participants engage in focal attention, which is defined as orienting attention to a particular space. During this task, participants must focus their attention on the central letter and ignore the adjacent letters. Therefore, when patients with OSAS had to focus their attention on a specific stimulus and ignore other stimuli in their visual field, attentional control processes were impaired.

This deficiency in conflict adaption cannot be explained by impairment in error processing, because post-error data were excluded in our conflict adaptation analyses. Importantly, conflict adaption deficiency cannot be explained by a deficiency in selective attention. First, patients with OSAS showed a congruency effect during the Flanker task, a finding suggesting intact selective attention processes. Second, we carefully followed methodological and analytical recommendations in order to account for potential problems with interpretation (Verstraeten and Cluydts, 2004). Third, ERs were rather low, which suggests that our participants understood the task demands and followed them appropriately.

From another perspective, one might argue that the non-significant results observed in the Simon and Stroop tasks did not reflect normal functioning of attentional control within our OSAS sample but were due to low statistical power. Even though Quan and colleagues observed that the relationship between AHI and neuropsychological functioning is weak or absent (Quan et al., 2011), a meta-analysis conducted by Beebe et al. (2003) revealed that the effect size of executive impairment in OSAS is rather large. Moreover, a power analysis conducted with g*power 3.1 software (Faul et al., 2007) showed that our power to detect a significant difference with the Stroop and Simon tests was 1.0. Finally, the reliability of the Simon, Flanker and Stroop tasks is high; therefore, the observed dissociation between tasks probably did not stem from issues of reliability across the three tasks. Nonetheless, we can safely assume that attentional control is at least partially impaired in OSAS, and control processes are particularly deficient during the Flanker task. Our results support the view that OSAS leads to some impairment in executive functioning (Beebe and Gozal, 2002).

Attentional control has been associated with the DLPFC and ACC (Macleod, 1991). The conflict monitoring framework (Botvinick et al., 2001) holds that the DLPFC implements top-down control by enhancing/diminishing task-related/-unrelated neural representations, while the ACC triggers this process by monitoring the amount of conflict within neural representations (MacDonald et al., 2000). Accordingly, conflict on the previous trial is detected by the ACC, and the ACC triggers the DLPFC, which implements top-down control processes. Attentional control is higher (i.e. the congruency effect is smaller) following incongruent trials compared with congruent trials (Botvinick et al., 2001). Our results suggest that top-down control processes of focal attention and/or the trigger mechanism are deficient among patients with OSAS. This deficiency, being specific to the Flanker task, is in line with recent results showing that conflict-driven attentional control is governed by multiple mechanisms in the human brain that operate in a domain-specific manner (Kim et al., 2012).

Recent studies have examined the brain processes engaged by patients with OSAS [using functional magnetic resonance imaging (MRI)] during attention tasks (Ayalon et al., 2009; Thomas et al., 2005; Zhang et al., 2011). All the studies report decreased levels of activation in the cingulate and prefrontal cortices during attention-demanding tasks. However, to our knowledge, none of these studies has investigated conflict-driven attentional control mechanisms. Our results extend those suggesting that the ACC- and DLPFC-mediated functions are deficient among patients with OSAS, specifically when focal attention (Flanker task) is involved. Future functional MRI research should investigate abnormalities in the ACC–DLPFC loop to determine which parts of the conflict monitoring–attention control mechanism is dysfunctional in OSAS. Findings of decreases in frontal grey matter in patients with OSAS, and recovery of grey matter after continuous positive airway pressure (CPAP) therapy (Canessa et al., 2011) suggest that the detection and specificity of cognitive decline in OSAS is essential.

We focused on a single cognitive function (attentional control) in the current study. Most previous studies have investigated a broad range of cognitive functions by administering several neurocognitive tests within one study. Even though screening several cognitive domains highlighted the specificity of neurocognitive deficiency, our results imply that focusing on a single function can also provide important insights as to the nature of neurocognitive deficiency in OSAS. However, generalization from a single task to OSAS-related deficiency in other cognitive domains should be done with caution.

We also investigated PES data, which, to our knowledge, have not been analysed before in OSAS studies. Error processing is an index of control, and it is mediated by frontal lobe functioning (Li et al., 2008). We observed a non-significant trend in our PES data for the Flanker task, which suggests a deficiency in error processing in OSAS. We did not design our study to investigate PES, but we calculated PES from a small number of errors. Nevertheless, our results suggest that investigating PES in future studies might bring forth important information regarding cognitive impairment in OSAS.

We did not employ a task that measures participants' vigilance directly. Following Verstraeten and Cluydts (2004), we did not consider only the incongruent conditions in our analyses. Our conclusions were based on a significant interaction between group and Stroop, Flanker or Simon conditions. However, using an independent measure of vigilance and alertness, or using different trial windows in our tasks would have allowed us to have better control over these effects. This limitation should be addressed in future studies.

In conclusion, we provide evidence that attentional control is partially impaired among patients with OSAS. Conflict-driven attentional control and error processing were deficient, particularly when focal attention (Flanker task) processes were involved. Our results underline the importance of assessing attentional control functions in clinical contexts. Future studies are necessary to understand the nature of this cognitive impairment, as well as the effects of CPAP treatment on recovery.

Conflict of Interest

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of Interest
  8. References

Baykal Tulek, Nart Bedin Atalay Fikret Kanat and Mecit Suerdem have no conflicts of interest.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conflict of Interest
  8. References
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