Supported by overwhelming evidence that, as a group, children with ADHD consistently exhibit deficits in multiple versions of the continuous performance task (Corkum and Siegel 1993
), early accounts of the neuropsychology of ADHD posited a central deficit of sustained attention or vigilance, which worsens as test sessions are prolonged (Sykes et al 1971
). The straightforward prediction in ADHD was that performance relative to comparison subjects should also worsen over time, but this was not observed in carefully performed experiments (e.g., Van der Meere and Sergeant 1988
), which instead detected roughly comparable between-group differences from the earliest testing blocks. This apparent falsification of a theoretic prediction led to alternative formulations, focusing primarily on deficits of inhibitory function or state regulation as central to the neuropsychology of ADHD (Barkley 1997
; Sergeant 2000
By contrast, we assert below that deficits in sustained attention are not simply synonymous with fatigue and that they can be straightforwardly detected with current approaches. To illustrate the process of time-based analysis, we are using data that were collected as part of the doctoral dissertation of one of the authors in The Netherlands (AS) (for extensive description of the sample and methods of this ethically approved and conducted study, see Scheres et al 2003
; note that these focused on the Flanker incongruency effect and did not report RT per se). Briefly, in an arrow version of an Eriksen Flanker task (Ridderinkhof et al 1997
), subjects responded by pressing the corresponding right or left response key, depending on the right or left direction of a target arrow. On neutral trials, the target arrow was flanked by two rectangles on either side; on congruent trials, five arrows were presented, all pointing in the same direction as the target; and on incongruent trials, the flanking arrows pointed in the direction opposite to that of the target arrow. The present secondary analyses were obtained from up to 24 boys with ADHD, ranging in age from 6 t o 1 2 years (mean ± SD, 8.7 ± 1.7 years), who were tested in up to five conditions (baseline n
= 22, double-blind crossover placebo n
= 23), and low (5 mg, n
= 24), medium (10 mg, n
= 24), and high (15 mg [20 mg if weight > 25 kg], n
= 23) doses of immediate-release methylphenidate, along with a healthy comparison group composed of 18 boys (aged 9.6 ± 1.8 years; p
> .10) as determined from parent and teacher ratings. These secondary analyses were conducted with data sets stripped of all protected health information, as approved by the institutional review board of New York University School of Medicine.
The Eriksen Flanker task was administered in six blocks of 180 sec each. A total of 803 blocks of RT data were analyzed (108 blocks from 18 control subjects and 695 blocks from 22–24 ADHD participants; data from one block were corrupted). Reaction times in each block were converted to evenly spaced time-series data, detrended to remove low-frequency (< .03 Hz) noise, and analyzed with FFT (with a 7-point Tukey-Hamming window). The specific FFT results analyzed were the frequency of the largest oscillations (i.e., the frequency of the highest peak in the FFT per 180-sec block) and the power of the oscillation (i.e., the area under the FFT curve for specific frequency bands).
This analysis addressed four questions, as follows.
Is RT Variability Random or Do RTs Oscillate at a Specific Frequency?
Mean RT and its SD were calculated for each block of 60 trials (one outlier was capped) for the two groups at baseline. Although the groups did not differ significantly in average RT [t
(38) = 1.39; p
= .17], boys with ADHD had significantly greater overall RT variability than comparison subjects [t
(38) = 2.65; p
= .01; two outliers (identified with SPSS Tukey Exploratory Data Analysis; SPSS, Chicago, Illinois) for values >3.3 SD] were “capped,” that is, set equal to the next highest or lowest value to minimize spurious correlations (Seguin et al 1995
; Tabachnick and Fidell 1996
, p. 69). (See for smoothed RT and the corresponding FFT and wavelet analyses for representative blocks of data from a control boy and a boy with ADHD.)
Figure 2 Top panels: Time-series (reaction time [RT] plotted against elapsed time) for two representative individuals during the second of six blocks of the Eriksen Flanker task. The control subject (left panels) shows less power in RT oscillations compared with (more ...)
The FFT analysis indicated that RT in the Eriksen Flanker task oscillated at similar specific frequencies in both groups. The most commonly encountered oscillations in the control group FFT were at .05 Hz and .075 Hz (44% and 33%, respectively), whereas 69% of the baseline or placebo blocks from the ADHD group manifested oscillations centered at .05 Hz. The mean frequencies in the two groups were indistinguishable (.069 ± .02 Hz and .067 ± .02 Hz for the control and ADHD groups, respectively, not significant). Within the patient group, the frequency of the main oscillation per block was statistically unaffected by medication.
On the basis of this confirmation of an underlying oscillation in RT in both groups centered between .05 and .075 Hz, the amplitude of oscillations in the .02–.07-Hz band (the area under the curve within this frequency band) was used as the dependent variable in subsequent analyses. Seven extreme outliers (2.9% of the entire data set) were capped.
Do ADHD Children Differ from Control Subjects in Specific RT Oscillations?
The power of RT oscillations centered at .05 Hz was 50% greater for the ADHD group compared with control subjects [t (38) = 2.54; p = .01; see ].
Figure 3 The area under the fast Fourier power spectra was integrated for the .02–.07-Hz band for each of the five conditions for the attention-deficit/hyperactivity disorder (ADHD) group and for the normal comparison boys during baseline. The mean spectral (more ...)
Does Methylphenidate Affect RT Variability and Oscillations?
Within the ADHD group, RT were significantly faster after any dose of medication [repeated-measures analysis of variance: F(23,1) = 66.94; p = .0005; two outliers were capped]. Reaction times were significantly less variable after any dose of medication [repeated-measures analysis of variance: F(23,1) = 31.98; p < .0005; one outlier was capped]. The power of oscillations in the .05-Hz band was also significantly reduced after any dose of methylphenidate [F(23,1) = 10.17; p = .004], as is shown in .
Does Time on Task Affect RT Oscillations?
Reaction time oscillations were not linearly affected by increasing time on task. Regression of power for the .05-Hz band against block yielded standardized β coefficients of .059 and .069 for the control and ADHD groups, respectively (not significant). Thus, the power of these oscillations in RT was relatively constant over all six blocks in both groups, as was the overall between-group difference.