Abstract
Increased Intra-Subject Variability (ISV) is a candidate endophenotype of ADHD. ISV’s relationship with response speed is highly relevant for ADHD as patients are highly variable but typically no slower than controls. This brief report addresses the relationship between variability and speed by employing dimensional analyses for differentiated performance measures, with a particular focus on the ex-Gaussian measures, across relevant ADHD studies and in young healthy adults (N = 70). For both patients with ADHD and healthy adults, we found that reaction time standard deviation and mean reaction time were strongly correlated, thus failing to dissociate, but ex-Gaussian tau (τ) shared only little variance with Gaussian mu (μ), thus dissociating slow responses (τ) from response speed or—if given—slow responding (μ). Our results highlight the utility of employing the ex-Gaussian measures to disentangle ISV and speed, particularly for ADHD data as patients make more slow responses but are not overall slower than typical controls.
Highlights
Intra-Subject Variability (ISV) refers to short-term within-person variations in performance
Strong associations were found between RT standard deviations (RTSD) and mean reaction time (MRT), both across pertinent ADHD studies (Table 1) and in young healthy adults
The close relationship between the conventional measures of ISV and speed, as previously found in healthy adults [2, 17], seems to exist across ADHD studies. This finding is in line with a previous ADHD study involving multivariate familial factor analysis in which RTSD and MRT loaded highly on a single large factor, whereas omission and commission errors loaded highly on a separate factor [3]
Summary
Intra-Subject Variability (ISV) refers to short-term within-person variations in performance. Increased ISV of reaction times (RTs) is among the most robust findings and is a candidate endophenotype for Attention Deficit Hyperactivity Disorder [ADHD; [1]]. With regard to ISV’s relationship with response speed, strong correlations between RT standard deviations (RTSD), and mean reaction time (MRT) in healthy [2] and ADHD-related populations [3] suggest that these RT constructs may reflect the same underlying processes. By aggregating all deviations from the mean, RTSD reflects a convolution of all kinds of variability [e.g., (non-) linear trends, (non-) periodic fluctuations] and is a sensitive but non-specific indicator of ISV [10]. The same is true for MRT in the case of response speed.
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