Cystinosis, a rare autosomal recessive disorder, results from mutations in the CTNS gene on chromosome 17p13.2 (1–3). Through excessive accumulation and crystallization of cystine within lysosomes, CTNS mutations cause widespread damage to various tissues and organs, including the brain (4–8). Structural brain differences (9–13) and difficulties related to cognitive function (14–18) have been described in association with CTNS mutations.

Despite having shown larger visual evoked potentials (19) and some differences in sensory memory and attentional processes (20, 21), our prior electrophysiological (EEG) work in cystinosis revealed generally intact sensory processing and no extensive neural differences (20, 21). Though encouraging, this set of findings belies the poor performance on standardized neurocognitive assessments and the difficulties with school work that are commonly seen in individuals with cystinosis (22). Our previous work was, however, primarily focused on early sensory auditory and visual processing and did not explore downstream cognitive complex processes, more likely to explain such differences and difficulties. One set of skills of interest that could indeed relate to poor performance in academic and professional contexts falls under executive functioning.

Executive function describes the set of control processes that govern goal-directed behavior and serve to optimize performance on complex cognitive tasks, allowing one to behave flexibly and to adapt to novel, changing circumstances (23). Executive functions, which relate to a broad range of responses—from basic and immediate responses to slower ones, influenced by long-term planning (24)—undergo gradual and continuous development that extends into adulthood (24, 25). While only three behavioral studies have explored executive function in cystinosis, findings indicate some difficulties in this population (15, 16, 26), though not consistently. Using the Delis-Kaplan Executive Function System (D-KEFS) Trail Making Test, Verbal Fluency Test, Design Fluency Test, Color-Word Interference Test, and Sorting Test Ballantyne et al. demonstrated poorer performance across these executive functioning components among children and adolescents with cystinosis (15); Besouw et al. observed differences only in sustained attention, not in other executive functioning components (16).

Here, we focused on conflict monitoring, a process that detects conflict and initiates control processes to resolve said conflict. Conflict monitoring involves two key elements: a monitoring component that assesses the level of conflict and a control adaptation component that adjusts attentional filters based on current task demands. While the former appears to be associated with the dorsal regions of the anterior cingulate cortex, the latter is likely linked to activity in the dorsolateral prefrontal cortex (27–30). These components work in tandem—the conflict signal prompts control adaptation, indicating the need for attentional changes; subsequent adjustments in attention result in a reduction of conflict (29). Behaviorally, children living with cystinosis seem to present difficulties monitoring conflict (15). But the neural mechanisms of conflict monitoring have not yet been investigated in this population and thus it is unclear at what stage of the conflict monitoring process such difficulties arise.

Conflict monitoring is commonly measured with tasks in which ignoring distracting cues is crucial for adequate performance. In the current study, we employed a behavioral color-word interference task (from the Delis-Kaplan Executive Function System, D-KEFS; 31) and an EEG Flanker task (32). In the color-word interference task, one expects that the presence of conflict results in longer completion times and more errors. In the classic Flanker task, participants indicate the direction of a centrally presented target arrow that is flanked by arrows that are either directionally congruent (pointing in the same direction) or directionally incongruent (pointing in the opposite direction). The flanker congruency effect, i.e., slower and less accurate responses in incongruent versus congruent trials, is expected behaviorally and represents conflict related processing. Electrophysiologically, the presentation of a stimulus inducing conflict typically results in a more prominent negative event-related potential (ERP) occurring fronto-centrally 200–300 ms after stimulus presentation (N2) and a positive ERP roughly 150 ms later with a centro-parietal topography (P3) (33, 34). The response inhibition evoked N2, attributed to neural activity from anterior/mid cingulate cortex and inferior frontal cortices, is associated with conflict detection and resolution (35). Its amplitude correlates with task difficulty, being more negative for more challenging tasks (33). The P3 reflects stimulus evaluation and decision-making processes (36) and indexes cognitive control and attentional allocation in conflict resolution (37). P3 sources include a complex interplay of cortical regions specifically tailored to the demands of cognitive conflict resolution and attentional allocation of a given task and primarily involve the anterior cingulate, frontal, and parietal lobes (e.g., (38, 39)).

The effective resolution of conflicting information, a crucial aspect of adaptive behavior and decision-making, requires extra executive function resources. We argue that this additional taxing of the system could be particularly impactful to those with cystinosis. Individuals with cystinosis may thus present, behaviorally, longer completion and reaction times and more errors and, electrophysiologically, a different modulation of the ERP components N2 and P3 when compared to their age-matched peers. A better characterization of the cognitive profile associated with cystinosis is critical to developing effective interventions to compensate for or improve areas of cognitive vulnerability.

Individuals living with cystinosis present difficulties in tasks that rely on executive functioning abilities (22). Utilizing standardized cognitive measures and EEG recordings, we asked whether individuals with cystinosis show, when compared to controls, differences in conflict monitoring—an important component of executive function and a crucial aspect of adaptive behavior and decision-making. We expected that, behaviorally, those with cystinosis would take longer to respond and complete tasks and make more errors. Additionally, we hypothesized that N2 and the P3 would be differently modulated in cystinosis.

The expected flanker congruency effect was observed in individuals with cystinosis and controls, that is, both groups presented slower and less accurate responses to incongruent versus congruent trials and larger N2 and P3 amplitudes in incongruent compared to congruent trials. One could thus argue that conflict monitoring appears to be largely maintained in cystinosis. This is consistent with intact color-word interference task performance compared to controls for this group of cystinosis participants. And, indeed, both behaviorally and electrophysiologically, the group differences are moderate.

Behaviorally, both groups were slower on incongruent trials compared to congruent trials. The difference in response time between incongruent and congruent trials was, however, larger in the cystinosis group than in the control group. Figure 1 suggests that although individuals with cystinosis were overall slower regardless of trial type (consistent with evidence of slower processing speed in this population (16, 49, 50), the incongruent trials distribution in the cystinosis group appears more skewed to longer reaction times. This enlarged difference is consistent with more effortful processing in cystinosis when monitoring conflict. Notably, incongruent trials imposed only a moderate additional reaction time burden (≈11 ms) beyond that observed in controls. Different from what was observed for reaction times, the difference in accuracy between congruent and incongruent trials was similar between the groups. Thus it appears that in dealing with increased cognitive demands under conflict conditions, accuracy was prioritized over reaction times in the cystinosis group. However, individuals with cystinosis showed large reductions in accuracy overall, regardless of type of trial (d ≈ 0.8), indicating a broad performance disadvantage that extends beyond conflict-specific demands. Similarly, in previous studies utilizing different paradigms, we reported overall lower accuracy rates in cystinosis (e.g., (49)). It is worth considering whether the overall differences in accuracy that we have observed in cystinosis relate to methodological choices. To guarantee that a specific response corresponds to one stimulus and not to the one that follows it, responses are only considered in a specific time window following stimulus presentation. As individuals with cystinosis show slower response times, it is possible that they are not responding less accurately, but that their answers are more often outside of the time window that is being considered. We think this is unlikely, however. As illustrated in Figure 1, the proportion of responses at the maximal end of the reaction-time window is very small, regardless of group, suggesting that most responses fell within the response window used for data analysis.

Similar to what was observed for reaction times, when compared to their age-matched peers, individuals with cystinosis showed larger differences between incongruent and congruent trials in the N2 and P3 brain responses. The pattern of brain responses in N2/P3 time windows appear, however, similar between individuals with cystinosis and their age-matched peers, suggesting, once again, that conflict monitoring is relatively maintained in cystinosis—even if individuals need more time and make more errors when detecting conflict. As for the enlarged difference between congruent and incongruent trials, consistent with the reaction time findings, conflict monitoring may be more effortful in cystinosis, and thus more cognitive resources need to be allocated. It is notable that for this stimulus configuration, in which congruent and incongruent trials were equally probable, P3 only modulated for the cystinosis group. The moderate-to-large group differences observed in our data indicate meaningfully amplified neural differentiation between incongruent and congruent trials in cystinosis relative to controls.

While N2 has been associated with conflict detection (35), the classic P3 has been argued to reflect stimulus evaluation and decision-making processes (36, 51) and index cognitive control and attentional allocation in conflict resolution (37). In previous studies, we argued for differences in attention (19, 20) and memory (21) in cystinosis. Consistent with this idea, the flanker effect is often attributed to insufficient attentional filtering of flankers, which leads to competing response activations in the case of incongruent trials (e.g., (52)). Attentional difficulties in cystinosis could thus call for additional resources when monitoring conflict, leading to increased flanker conflict effects. The contribution of potential attention and memory difficulties in cystinosis, which may have contributed to the lower nonverbal IQ and nonverbal working memory scores in the present data, should be considered when drawing conclusions about conflict monitoring. Executive functioning is not unitary. Rather, it comprises separable components (53, 54) such as task-switching, updating, and information monitoring. Each of these components may be recruited across a variety of tasks including selective attention, decision-making, working memory, error monitoring, conflict monitoring, and language processing (53, 55, 56). It is thus difficult to disentangle how such components interact and to design experimental paradigms that tap into the different processes in exclusive ways. The brain regions involved in solving executive functioning type tasks are, likewise, multiple and their associations complex. Here we only manipulate one type of executive function, conflict monitoring. Additional research is clearly needed to 1) understand the landscape of executive functioning in cystinosis, and 2) how this changes over development. Network analysis and graph theory applied to a broad set of executive functioning tasks could shed some light on how different components inform each other in cystinosis.

In summary, while some differences were found between individuals with cystinosis and a group of controls while responding to a traditional conflict monitoring task, our findings suggest that individuals with cystinosis are able to monitor conflict, even if slower, more effortfully, and less accurately than their age-matched peers. That individuals with cystinosis were generally slower and made more mistakes during this conflict monitoring is, nevertheless, relevant when defining strengths and devising best practices to support academic and professional development in this population. Of note, non-verbal and visual-spatial difficulties are often much more prevalent in this population than verbal differences, as our current findings confirm. As slower and/or less accurate monitoring of conflict could impact some of the academic difficulties observed in cystinosis, training the ability to monitor and manage response conflict (and filter out irrelevant information) or other related processes such as attentional filtering may be impactful and generalize to other related executive functioning components.

This study is not without its limitations. First, the groups were not matched for IQ. While we elected not to include IQ as a covariate in our statistical analyses given its close association with the diagnostic status, this decision may nonetheless have influenced the observed outcomes, as differences in cognitive ability could contribute to variability in performance across groups. Second, variables related to current health status (such as a measure of renal function) and compliance to treatment, which has been linked to better clinical outcomes (57), were not included in the present study but could be useful in understanding group- and individual-level differences in a larger sample study. Relatedly, however, most individuals who participated in the study were relatively healthy due to the requirement to travel relatively long distances (for most participants) to participate and thus, our sample is not representative of the full spectrum of individuals living with cystinosis. It is possible that individuals with greater cystinosis-related symptoms experience greater challenges in executive functions such as conflict monitoring. Third, whereas our data span a large age range that includes both children and adults, our participant numbers are not sufficient to power examination of the developmental trajectory of the inhibitory processes of interest. While developmental changes would be expected based on well-characterized shifts in connectivity across the neural networks engaged by executive functions (58), assessing if this developmental trajectory is altered in cystinosis is challenging due to the rarity of the condition. Nevertheless, it is important that future work will target this question. It is also important to consider the effect sizes and the sensitivity of the paradigm employed. While we chose to balance the proportion of congruent and incongruent trials per block, others have shown the visual flanker effect to be larger in blocks in which the proportion of congruent trials is high compared to low (see (59) for a review). Still, behavioral and electrophysiological conflict effects were present in both groups and differed between groups. Future work systematically manipulating congruent/incongruent stimulus probabilities could help to understand how increasing levels of conflict are managed by individuals with cystinosis. Lastly, although real-life situations often require engagement in more than one task at a time (60), here we focused on a single-item task. As we gather more evidence on the cognitive functioning profile characteristic of cystinosis, it is crucial to shed more light on whether specific systems have limits after which effectiveness is unmanageable or whether it is the accumulation of slight differences in separate components and their interactions that result in the cognitive difficulties described in cystinosis.