15. Reaction time changes in ADHD
In one study, reaction time to a stop signal, percentage of failed response inhibitions, and standard deviation of reaction time to “go” trial (SDRT) successfully discriminated ADHD affected and unaffected individuals. Ex-Gaussian decomposition of the reaction time distribution showed that both greater tau and greater sigma affected the results for SDRT. Meanwhile, the traditional measures of inhibitory control were equivalent, if not better, predictors of ADHD status than the ex-Gaussian parameters.
15.1. Reaction time variance increased in ADHD
ADHD is characterized by increased variance in reaction time in response tests. The increased reaction time variability is thought to correlate particularly with problems of sustained attention, but this is controversial. Slower reaction time and greater standard deviation of reaction time also appear to be the result of lower parieto-occipital multivariate decoding accuracy, which occurred approximately 240-340 ms after the onset of visual search.
Apparently, the variance in response time is particularly high in the group of ADHD sufferers who make a particularly high number of commisson errors (false-positive errors). Increased cortisol responses to a stressor correlated with increased variance in response time. Increased cortisol stress responses are very common in the ADHD-I subtype and are atypical for the ADHD-HI subtype.
Further, a later and reduced slowing of reaction times after errors than in non-affected individuals was reported.
Increased individual response variance is a sign of increased neural noise. MPH improves this. Neural noise is represented by arhythmic signals in the cortex, measurable as “1/f noise” in the EEG. Dopamine deficiency worsens the signal-to-noise ratio. ADHD is characterized by decreased dopamine levels in the PFC and striatum. Stimulants such as MPH raise dopamine levels there. Increasing dopamine levels to the optimal level improves the signal-to-noise ratio.
Increased reaction time and increased reaction time variability appear to correlate with high dopamine receptor availability (“empty receptors”), which is associated with decreased tonic dopamine levels.
The symptom of increased reaction time variance also significantly distinguishes ADHD from other mental disorders such as
- Distress disorders (physical stress disorders, PTSD)
- Oppositional Defiant Behavior (ODD)
- Conduct Disorder (CD)
- Typical developmental disorders
We are currently testing a reaction test to explore whether reaction time variability can be used for ADHD diagnosis. The ADxS.org - ADHD reaction test starts here.
15.2. Reaction time reduction in ADHD?
Several studies and reports indicate a shorter reaction time in ADHD. According to Barkley, reaction time is consistently reduced in SCT (Sluggish Cognitive Tempo) in particular
According to another study, the reaction times of ADHD sufferers and non-affected people do not differ, but the care performance does.
Another study of ADHD sufferers found that, contrary to expectation, carriers of the DRD4-7R gene polymorphism, which is one of the main candidates for increased sensitivity and ADHD, did not have worse reaction times than unaffected individuals, but carriers of other DRD4 polymorphisms did. Others reported aberrant audiovisual multisensory processing.
One study found a correlation of prolonged reaction times with ADHD-I.