Mean response times, variability, and skew in the responding of ADHD children: a response time distributional approach
Introduction
Cognitive psychologists have now studied the response performance of children with attention-deficit/hyperactivity disorder (ADHD) on a wide variety of information processing tasks (for a comprehensive review see Douglas, 1999). In conjunction with the fact that ADHD children often make more errors than control children, the most consistent finding in the ADHD cognitive literature is that the overall response times of ADHD children are typically both slower and more variable than those of control children. However, as noted by Douglas (1999), because much of the theoretical and empirical research involving ADHD children has focussed on more specific task manipulations, these two pervasive phenomena of slow and highly variable ADHD response times have still not been adequately addressed.
Section snippets
A response time distributional approach
This study will directly address these phenomena through a detailed statistical examination of the actual distributions of response times obtained from ADHD and control children within a four-choice warned reaction time (4C-WRT) task conducted at McGill University (King Elbaz, Douglas, Ditto & Van der Molen, 1999). In this task, groups of ADHD and age-matched control boys provided spatially compatible responses indicating which one of four, highly discriminable, stimuli had been presented in a
The ex-Gaussian distributional model
The ex-Gaussian distributional model can be used to provide useful quantitative measures of the distributional properties of a set of response times Heathcote, 1996, Luce, 1986, Ratcliff, 1979, Ratcliff and Murdock, 1976. This model assumes that each response time can be represented as the sum of a normally distributed random variable and an independent exponentially distributed random variable, and therefore, that the full distribution of response times can be characterized as a convolution of
Method
The data for the three experimental conditions of concern here were collected as part of a larger study conducted by King Elbaz et al. (1999). For details concerning the full set of experimental task manipulations used by those researchers see King Elbaz et al. (1999).
Results
The dependent variables for the following statistical analyses were the mean response times, the standard deviations, and the estimates of the three ex-Gaussian parameters (μ, σ, and τ) for the set of trial observations obtained from each individual participant at each foreperiod condition. Only correct response times were used in the calculation of these measures, and the data for each condition were collapsed over the responses to each of the four possible choice stimuli. All of the following
General discussion
The results of the statistical and ROC analyses of the ex-Gaussian distributional measures performed here are very clear: The ADHD responding can be differentiated from the age-matched control responding almost exclusively in terms of the size of the tails of their respective response time distributions (i.e., in the value of τ). In essence, the faster correct responses of the ADHD boys are almost as fast as those of the age-matched control boys, whereas the slower ADHD responses are much
Acknowledgements
This research was conducted while the first and second authors were graduate students at McGill University, and completed while the first author was a postdoctoral trainee in the Quantitative Methods Program of the Department of Psychology, University of Illinois at Urbana-Champaign. The data were collected by the second author as part of her doctoral dissertation research. Previously, this work has been presented at the 39th Annual Meeting of the Psychonomics Society, Dallas, TX, November,
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