Dynamic behavioural changes in the Spontaneously Hyperactive Rat: 2. Control by novelty.

An important aspect of attention deficit hyperactivity disorder (ADHD) is its temporary amelioration by novelty and by stimulant medication. Extant learning-based accounts of ADHD are unable to account for the temporary amelioration effect. One possible mechanism is a drive for novelty. Computational simulations have previously shown that such a drive can result from a "dopamine appetite". Empirical demonstration of such a process requires, as a first step, the development of standard criteria for identifying drive-based, versus learning-based, changes in behaviour. Using a variable-interval reinforcement schedule, we looked over a range of timescales for behavioural changes showing putative characteristics of drives, namely: low information content, unidirectionality, saturability, spontaneous reversibility in less than a day, and cycle stability. SHR lacks normal down-regulation of responding when the schedule becomes sparser. SHR appears to be re-learning the schedule length during the days of each calendar week. SHR hyperactivity is specific to the operant and develops gradually over the first five minutes of each session. Empirical within-session results were replicated by a simple simulation containing two interacting reward systems, one for water and the other for stimulation (including novelty). To summarise, enhanced sensation-seeking (or a subtype of it, novelty-seeking) provides the best available account of changes in SHR activity within sessions, though not of changes over longer timecourses. Sensation-seeking appears to be associated with low anxiety in the SHR. SHR propensity to display multiple influences on their behaviour makes them ideal for further pharmacological, genetic, and behavioural investigation.