Task switching: a PDP model

When subjects switch between a pair of stimulus-response tasks, reaction time is slower on trial N if a different task was performed on trial N - 1. We present a parallel distributed processing (PDP) model that simulates this effect when subjects switch between word reading and color naming in response to Stroop stimuli. Reaction time on "switch trials" can be slowed by an extended response selection process which results from (a) persisting, inappropriate states of activation and inhibition of task-controlling representations; and (b) associative learning, which allows stimuli to evoke tasks sets with which they have recently been associated (as proposed by Allport & Wylie, 2000). The model provides a good fit to a large body of empirical data, including findings which have been seen as problematic for this explanation of switch costs, and shows similar behavior when the parameters are set to random values, supporting Allport and Wylie's proposal.     

Multiple causal pathways in attention-deficit/hyperactivity disorder – Do emerging executive and motivational deviations precede symptom development?

The multiple causal pathways model on the etiology of attention-deficit/hyperactivity disorder (ADHD) is well established. However, developmental implications of the model are not yet sufficiently analyzed. The model implies that critical neural and neuropsychological deviations from normative development precede secondarily developing ADHD symptoms. Cognitive, “cool” inhibitory control (CIC) and reward-related, “hot” functions (RRF) are regarded as neuropsychological basic deficits that indicate independent causal pathways. Both functions involve top-down control networks that undergo major normative developmental changes in the preschool period. We formalized the following assumptions in a path model: (a) CIC and RRF predict change in ADHD symptoms in the preschool period, (b) the reverse is not true, and (c) CIC and RRF independently contribute to this prediction. A community-based sample of 125 (71 boys) preschoolers was assessed at at age 4 and 5 years. At each assessment wave, CIC and RRF were measured by a battery of age-appropriate valid tasks. ADHD symptoms were measured by a clinical parent interview. Evaluation of model fit using manifest maximum likelihood parameter estimation clearly supported the hypothesized path model while controlling for gender of child and maternal education level. Thus, regarding the basic deficits of CIC and RRF, the results add evidence on the developmental implications of the multiple causal pathways model. Our findings point to the potential significance of these early emerging characteristics as indicators of risk and as targets for preventive interventions.