Dissociating Slow Responses From Slow Responding

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.

Intra-Subject Variability, Intelligence, and ADHD Traits in a Community-Based Sample

“…However, our results also revealed a dissociation between σ , predicting general ADHD traits, and τ , associated with specific attention problems unrelated to the general ADHD score. This result is in line with Salunkhe et al (2020), who reported σ and τ to load on different factors in young healthy adults. Regarding the proposed interpretation of the ex-Gaussian parameters, this dissociation may be raised by different neuropsychological processes involved in the ADHD core symptoms (e.g., σ reflecting neural noise (Salunkhe et al, 2020), τ— default mode interference (Feige et al, 2013)).…”

“…In addition, μ was a significant predictor of general ADHD traits with lower μ , that is, faster PS, being associated with more pronounced ADHD traits. Though at first sight surprising, this observation is in line with several studies, reporting lower μ in patients with ADHD (Salunkhe et al, 2020), in some cases alongside elevated mean RT. A possible explanation may be increased impulsivity in patients with ADHD mirrored specifically by μ .…”

“…In contrast, σ predicted ADHD symptoms significantly for long as well as short (1.5 seconds) ISIs. In another study, RTSD, across studies almost perfectly correlated with τ (Salunkhe et al, 2020), was observed to be related to ADHD symptoms only in tasks with slow event rates and exclusively in teacher but not in parent ratings (Kuntsi et al, 2005). Since the ISI of 2 seconds in the present study was closer to the short than to the long ISI condition used by Hwang-Gu et al (2019) and parent ratings on the SWAN scale were analyzed, these factors may explain the missing link between τ and general ADHD traits, partially responsible for the dissociation between σ and τ .…”

“…Moreover, increased ISV in ADHD is suggested as candidate endophenotype, an intermediate phenotype linking genetic vulnerability and behavioral symptoms (Castellanos & Tannock, 2002). Consistently, studies using the ex-Gaussian approach have demonstrated that while patients with ADHD often show comparable or even faster average RTs (μ) than healthy controls, the slow ex-Gaussian "tail" of the RT distribution, reflected by τ, best distinguishes between both groups (Salunkhe et al, 2020). Of the DDM parameters, v is reported to significantly separate patients with ADHD from healthy controls (Huang-Pollock et al, 2020;Metin et al, 2013).…”

Differences in Ex-Gaussian Parameters from Response Time Distributions Between Individuals with and Without Attention Deficit/Hyperactivity Disorder: A Meta-analysis

“…Unlike the mean and standard deviation, fitting ex-Gaussian distributions to empirical RTs allows RTs to be separated into a speed component, µ, and two variability components, σ and τ (Heathcote, 1996). These components are independent of each other, unlike RT mean and variance (Salunkhe et al, 2020). Furthermore, as discussed below, σ and τ have different meanings.…”