Reply to: Testing of ‘Executive Function’

We thank Dr. Jim Horne for his comments on our recent article published in Chronobiology International (Bratzke et al., 2012). He correctly points out that the Stroop and the Simon taskdonot completely index the concept of executive functions, which is certainlymultifaceted. However, we neither have claimed that these tasks fully index this concept nor was it the aim of our study to investigate the whole range of executive functions (Miyake et al., 2000). To our minds, a comprehensive assessment of these functions would exceed the scope of any study in this field. As clearly stated in our article, we used the Stroop and the Simon tasks to assess the influence of circadian rhythm and sleep loss on one specific executive function, namely, the inhibition of prepotent responses. The Stroop task is the prototypical task to evaluate changes in this specific executive function (e.g., Botvinick et al., 2001; Miyake et al., 2000). Dr. Horne argues that our study “was not a study of executive functions” for essentially two reasons. First, he claims that with repeated administration, the Stroop task becomes routine and no longer measures executive functions. It should be stressed that we included delta plot analyses (Ridderinkhoff, 2002; Schwarz & Miller, 2012) of Stroop and Simon performance to examine this issue. Specifically, if participants had learned a “technique”, as Dr. Horne believes, this should have most probably changed the shape of the delta plot function for the two tasks. This was, however, not the case and the functions revealed the typical shapes that are usually observed for the Stroop and the Simon tasks. Furthermore, neither the Stroop nor the Simon effect diminished over the course of the study. Therefore, it is highly unlikely that the repeated administration of these tasks influenced theway our participants processed the test stimuli. For these reasons, we clearly do not agree with Dr. Horne that the Stroop and the Simon tasks became “routine” and thus were no longer measuring executive functions. Second, Dr. Horne criticizes that we did not use the classical card version of the Stroop test that is typically used in neuropsychological testing. As already stated in our article (p. 56), the single-trial version of the Stroop task employed in our study has several advantages over the classical card version. In contrast to Dr. Horne’s concern, the single-trial version allows not only the recording of response speed but also of response accuracy. Consequently, this Stroop version is especially suited to assess possible changes in the trade-off between speed and accuracy in our tasks across sessions. It is important to note that our data did not reveal any indication of a speed-accuracy trade-off (see Figure 1, upper panel). In fact, higher error rates tended to be associated with slower rather than faster responses. Most importantly, the statistical analysis of error rates confirmed the conclusion obtained from reaction time analysis. Therefore, it is unclear why Dr. Horne claims that we dismissed to consider this trade-off, an issue that we have explicitly addressed in our data analysis. Finally, we acknowledge that many studies — several of them cited in our article — have shown that executive functions are vulnerable to sleep loss. Nevertheless, we think it is important to report contradicting results, as for example the null effect in our study. Not taking into account such null effects would bias research and create the exaggerated view that executive functions are always and inevitably affected by sleep loss (and circadian rhythm).