Developmental Neuropsychology: ADHD

Examining a pilot study of methylphenidate, interstimulus interval, and reaction time performance of children with attention deficit/hyperactivity disorder

Attention deficit/hyperactivity disorder (ADHD) is one of the most common found disorders in children. Characterized by inattention and hyperactivity, ADHD is primarily an inherited disorder. Treatment usually involves methylphenidate (MPH), which works by regulating the dopamine system. This often results in a decrease in impulsivity, hyperactivity, and inattention. Despite this, children on MPH still experience deficits in cognitive faculties. Other studies have shown that children with ADHD perform differently depending on the length of a stimulus they are exposed to. If the stimulus is a short interstimulus interval (ISI), the children experience a more positive performance in comparison to a long ISI. A short ISI has been shown to improve memory recognition, vigilance, motor timing, and more. It is thought that both MPH and a short ISI act upon the dopamine levels in the brain. It is proposed that having both of these working at the same time may over-activate the system and lead to detrimental effects. The goal of this study was to measure the isolated and combined effect of MPH and ISI on mean response times and errors of commission.

Procedure

For this study the sample consisted of 13 children (10 boys; 3 girls) diagnosed with ADHD. During the time of the study, MPH was gradually introduced for a two week period, either adjusting to a higher or lower dosage. After the children had been on MPH for 4 weeks, they were tested twice, once after the administration of MPH, and once after a placebo. The task which the participants engaged in was a computer based reaction time test known as Go/No-Go test. This involved pressing a button when the letter Q appeared onscreen and press nothing when the letter O appeared. If the participant responded to the letter O, this was recorded as an error of commission.

Looking at results

The results from the study indicate:

1. Children respond faster on MPH than not on MPH

2. Children respond faster in a condition with a short ISI rather than a long ISI

3. The interaction of ISI and MPH was not significant

4. Children on MPH made more errors of commission during the condition with a short ISI

5. Children on the placebo made fewer errors of commission during the condition with a short ISI

For discussion

This study set out to ascertain whether children with ADHD were able to inhibit responses when they are in a state of over-activation. The findings of the study indicate that children perform better with either MPH or in the presence of a short ISI in comparison with a long ISI. When MPH and a short ISI are combined, they have detrimental effects. The stimulus shift hypothesis is supported by this outcome. This outcome calls into question the optimal dosage levels when treating ADHD.

State regulation and motivation in ADHD

Throughout past research, weak motivation has been shown to be a factor involved in ADHD. Two models exist to try and explain this deficit. The response inhibition model hypothesizes that the majority of problems related to ADHD can be explained by impaired response inhibition. The state regulation model, in contrast, associates ADHD with a motivation deficit which then results in negative outcomes for social functioning and cognition. In the more recent years, studies have investigated the effects of incentives, noise, and the rates of the presentation of stimuli.

Presence of incentives

Studies on the effect of incentives on response inhibition have produced mixed results. Some studies have shown that incentives increase response inhibition, others found the opposite. The authors suggest the following as possible reasons for these mixed results:

1. Variation in the presentation of the rewards

2. Effort allocation will only take place when arousal deviates from its basic state.

3. Incentives act as signals; an increase in signals means an increase in motor activation

Noise

In accordance with the state regulation model, noise could activate the arousal system. One study showed that white noise was beneficial for the cognitive performance of children with ADHD, but was detrimental to control groups. The explanation for this result is the under-arousal found in ADHD.

Changing the presentation rate of stimuli

The state regulation model dictates that motor activation is effected by the presentation rate of a stimulus. Research has shown that children presented with a fast stimulus presentation rate perform better than when presented with a slow presentation rate. Very few studies have disputed this.

Top-down or bottom-up

From the studies conducted on reaction time and stimuli presentation rate, we see an interaction between motor activation and its control by effort. The researchers question whether this is due to a bottom up deficit in motor activation which in turn is hard to control by the top down effort mechanism, or actually the motor activation mechanism is functional and it is the effort mechanism which is not sufficiently in control. A suggestion is presented to examine the possibilities: to investigate the combined and individual effects of stimuli presentation rates and incentives on reaction time performance.

Looking at psychophysiological studies, dopamine, metabolic energy, and stimuli presentation rates

The following are findings from research conducted:

1. Psychophysiological indices, under slow presentation rates of stimuli, support the hypothesis that ADHD is associated with an effort deficit

2. Response impairments have been associated with either too high or too low levels of dopamine.

3. Results of fMRI research show that ADHD participants use widespread frontostriatal activity, including the thalamus and the cingulate, when involved in accurate response inhibition

4. Cortical excitability in adults is optimized by self chosen stimulation

5. Effort allocation and boredom are negatively correlated

In conclusion

There is a growing accumulation of evidence supporting that cognitive performance of children with ADHD is associated with weak state regulation. The state regulation model is far from perfect. The plausibility of either a top-down or bottom-up deficit must be examined further.

Investigating the dysfunctional modulation of default mode network activity in attention-deficit/hyperactivity disorder

In accordance with the state regulation deficit (SRD) model, individuals with ADHD have difficulty applying mental effort in situations which are suboptimal i.e. when there are either very fast or very slow event rates (ERs). In addition, ADHD has been shown to be associated with a deficit in suppression of the default mode network (DMN) and detriments to performance in tasks requiring effortful engagement. The authors of the study hypothesize that individuals with ADHD would show reduced deactivation in DMN when compared to controls in the tasks. They also predict a failure to modulate DMN activity as a part of ER by the ADHD participants.

Procedure

The sample consisted of 20 adults with ADHD and a control sample of 20 age and gender matched adults. The task to be performed was a simple computer based response task. The task was kept simple to maximize the amount of trials which could be used for the characterization of DMN analysis. During the experiment, fMRI was used to monitor brain activity.

Results and discussion

This study presents evidence for a disparity in the modulation of activity in specific DMN areas by event response in ADHD. Over-activity of DMN was observed only at ER extremes, no group differences were observed at a moderate level. This is in line with the SRD model. Further studies are necessary to test the hypothesis of whether or not individuals with ADHD have problems focusing effort on suppressing DMN interference when in less than ideal or energetically orientated conditions and also to investigate the role of effort allocation impairment on inattention and mind wandering.