Increased variability, that is hypothesized to reflect underlying unstable attentional resources, is seen in tasks requiring continual responses to rapid stimuli as well as basic reaction time (RT) tasks [3].
Increased variability is seen in tasks requiring continual responses to rapid, externally-paced stimuli as well as in "free-operant" tasks without such a requirement.
In tasks requiring continual responses to rapid, externally-paced stimuli, however, children with ADHD display greater intra-individual variability in response times and are often found to respond more slowly and less accurately than their typically developing peers [ 6].
The reason for this is probably that reinforcers work on free-operant behaviour, but not on instructed behaviour typical in tasks requiring continual responses to rapid, externally-paced stimuli (cf., [ 68]).
We propose that increased intra-individual behavioural variability in continual responses to rapid, externally-paced stimuli are related to other factors than those causing increased variability in free-operant, non-externally paced tasks [ 8, 9, 20].
However, these explanations do not account for the ubiquitous findings in ADHD of intra-individual performance variability, particularly on tasks that require continual responses to rapid, externally-paced stimuli.
The reason for this may be that reinforcers may be less effective for tasks requiring continual responses to rapid, externally-paced stimuli than for subject-paced (free-operant) responding [ 68].
However, neither the dynamic developmental theory of ADHD, nor other theories and explanations account for the intra-individual performance variability of ADHD on tasks that require continual responses to rapid, externally-paced stimuli.
Research suggests constant psychological and emotional stress also induce a low-grade, continual cortisol response.
The objectives of this study were to develop and simulate the implementation of several strategies for repeated application of quantitative trait loci (QTL) detection and marker-assisted selection (MAS) and to compare the short-term and continual genetic responses.
