Abstract
Increased intra-subject variability of reaction times (ISV-RT) is one of the most consistent findings in attention-deficit/hyperactivity disorder (ADHD). Although the nature of this phenomenon is still unclear, it has been hypothesised to reflect interference from the Default Mode Network (DMN). So far, ISV-RT has been operationally defined either as a frequency spectrum of the underlying RT time series, or as a measure of dispersion of the RT scores distribution. Here, we use a novel RT analysis framework to link these hitherto unconnected facets of ISV-RT by determining the sensitivity of different measures of RT dispersion to the frequency content of the underlying RT time series. N=27 patients with ADHD and N=26 healthy controls performed several visual N-back tasks. Different measures of RT dispersion were repeatedly modelled after individual frequency bands of the underlying RT time series had been either extracted or suppressed using frequency-domain filtering. We found that the intra-subject standard deviation of RT preserves the “1/f noise” characteristic typical of human RT data. Furthermore and most importantly, we found that the ex-Gaussian parameter τ is rather exclusively sensitive to frequencies below 0.025 Hz in the underlying RT time series and that the particularly slow RTs, which nourish τ, occur regularly as part of an quasi-periodic, ultra-slow RT fluctuation. Overall, our results are compatible with the idea that ISV-RT is modulated by an endogenous, slowly fluctuating process that may reflect DMN interference.
Highlights
Increased intra-subject variability of reaction times (ISV-RT) in attention-deficit hyperactivity disorder (ADHD) has turned from a neglected abnormality [1,2,3] to a field of productive research [4,5,6,7,8,9]
Part of this ambiguity stems from the fact that in the vast majority of ADHD studies ISV has been quantified through the intra-subject standard deviation of RT (RTSD [8])
The increased mean RT and RT standard deviation (RTSD) found in ADHD appear to reflect increased τ, with normal μ and σ [23], suggesting that increased ISV could be the consequence of increased density of slow responses
Summary
Increased intra-subject variability of reaction times (ISV-RT) in attention-deficit hyperactivity disorder (ADHD) has turned from a neglected abnormality [1,2,3] to a field of productive research [4,5,6,7,8,9]. Abnormally high σ has been found in most of these studies (with the exception of the adults in [27]), suggesting that whatever drives the slow responses reflected in τ is not the only “ingredient” of increased ISV in ADHD While measures such as τ identify increased ISV as an abnormality in the RT distribution, other approaches consider the same data as reaction time series [29,30] and search for abnormalities in patients in the temporal structure of the time series. Castellanos et al [3] were the first to analyse the periodic structure of RT series in patients with ADHD and reported increased power of RT oscillations around 0.05Hz, pointing to fluctuations with a cycle length of about 20 seconds This finding has been replicated conceptually in other studies [11,31,26,32]. Our characterisation of dispersion measures by their frequency specificity employs standard filters to extract or suppress certain frequency bands in the RT time series before re-modelling the measures of RT dispersion (see methods and Figure 1)
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