An altered perception of time is often reported in ADHD.
ADHD is also characterized by an increased reaction time variance in reaction tests. The increased reaction time variance clearly distinguishes ADHD from other mental disorders such as anxiety, PTSD, ODD, CD and typical developmental disorders.
A slower reaction time and a larger standard deviation of the reaction time may be due to a lower decoding accuracy of the brain. Reaction time variance is particularly high in ADHD sufferers who make many commisson errors. In addition, increased cortisol secretion correlates with an increased variance in response time.
ADHD shows a later and reduced slowing of reaction time after errors compared to non-affected individuals. Increased reaction time and reaction time variance also appear to correlate with a high availability of dopamine receptors, which is associated with reduced dopamine levels.
There is evidence of a shorter reaction time in ADHD, and even more so in SCT. In one study, the reaction time to a stop signal, the percentage of failed response inhibitions and the standard deviation of reaction time to the “go” trial (SDRT) successfully differentiated ADHD sufferers from non-affected individuals. Ex-Gaussian decomposition of the reaction time distribution showed that both a larger tau and a larger sigma affected the results for the SDRT. However, the traditional measures of inhibitory control were equivalent, if not better, predictors of ADHD status than the ex-Gaussian parameters.
15.1. Increased reaction time variance in ADHD
ADHD is characterized by an increased variance in reaction time in reaction tests. The increased reaction time variability is said to correlate in particular with problems of sustained attention, but this is controversial. Slower reaction time and greater standard deviation of reaction time also appears 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 response time variance 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 an increased variance in response time. Elevated cortisol stress responses are very common in the ADHD-I subtype and atypical for the ADHD-HI subtype.
Furthermore, a later and reduced slowdown in reaction times after errors was reported than in those not affected.
Increased individual response variance is a sign of increased neural noise. MPH improves this. Neural noise is represented by arrhythmic signals in the cortex, which can be measured as “1/f noise” in the EEG. Dopamine deficiency worsens the signal-to-noise ratio. ADHD is characterized by reduced dopamine levels in the PFC and striatum. Stimulants such as MPH increase the dopamine level there. An increase in dopamine levels to the optimal level improves the signal-to-noise ratio.
Increased reaction time and increased reaction time variability seem to correlate with a high availability of dopamine receptors (“empty receptors”), which is associated with reduced tonic dopamine levels.
The symptom of increased reaction time variance also distinguishes ADHD significantly from other mental disorders such as
- Distress disorders (physical stress disorders, PTSD)
- Oppositional defiant behavior (ODD)
Disorder of social behavior (Conduct Disorder, CD)
- Typical developmental disorders
We are currently testing a reaction test to explore whether reaction time variability can be used to diagnose ADHD. The ADxS.org - ADHD reaction test starts here.
15.2. Reduced reaction time in ADHD?
Several studies and reports indicate a shorter reaction time in ADHD. According to Barkley, the reaction time is consistently reduced, especially with SCT (sluggish cognitive tempo)
According to another study, the reaction times of ADHD sufferers and non-affected people do not differ, but the level of diligence does.
A further study of ADHD sufferers found that 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 non-affected individuals, contrary to all expectations, but carriers of other DRD4 polymorphisms did. Others reported deviating audiovisual multisensory processing.
One study found a correlation of prolonged reaction times with ADHD-I.
15.3. Rhythm problems
One study reports that ADHD sufferers have significantly more problems tapping out a given rhythm with their fingers.
The test targets the function of the cerebellum, which is one of the brain regions involved in ADHD,