5. Limitations and Future Research
The first limitation of the present study could have been the lack of eye-gaze verification. Central fixation was encouraged and participants were instructed to make eye saccades after key pressing. Nevertheless, it is possible that participants' gaze was not directed to the central fixation cross on every trial. We have to admit that these variations in gaze could have influenced performance. However, given the low error percentage together with fast stimulus presentation (150 ms), it seems unlikely that variations in gaze confounded our results.
A second limitation is that the university students are not representative with the adult population in general. In addition, the present sample includes more females than males. As a result, the present outcome needs replication for adults in general and should also examine samples with more equal sex distribution. Having this said, the CAARS scores are corrected for gender and the reaction time data showed no difference between male and female students. It is well-recognized that the validity of self-reports from students may be questioned. For instance, one should be suspicious if students rate themselves as being significantly symptomatic yet have managed to achieve well in school and in other life activities, especially, if their T-scores were above 80 [64]. Please note that in our sample the majority responded in a valid and reliable way with scores within the population mean.
A third limitation is that our data apply only to right-handers. The relation between ADHD symptoms and interhemispheric interaction remains unexplored in inconsistent- and left-handers; this calls for future studies on inconsistent-handers specifically as it has been reported that inconsistent handedness is associated with increased interhemispheric interaction [65] and ADHD [16].
The present study focused on integrating visual information processing (subserved by the posterior part of the corpus callosum). It is recommended to direct future research towards the anterior part of the corpus callosum connecting the frontal and prefrontal cortex (i.e., genu). This structure subserves higher order cognition such as executive function and its possible interactions with self-regulation and effort which are assumed to be compromised in ADHD [66–68].