Dictionary definitions of impulsiveness include terms such as ‘acting without thinking’, ‘on the spur of the moment’ and ‘in brief intervals of time’. Psychologists define impulsiveness as a lack of persistence, reduced decision time and increased threshold for boredom (Buss & Plomin 1975
), lack of patience, risk, sensation and seeking (Eysenck 1993
), lack of ‘futuring’/temporal foresight (Barrat 1994
) and resistance to delayed rewards (Logue 1995
). We have defined impulsiveness as a poorly controlled and inappropriately timed, usually premature, non-reflected, immediateness-bound and delay-aversed response style where actions are executed before all available information and the future consequences are being considered (Rubia 2002
). It becomes obvious from these definitions of impulsiveness that abnormalities in functions of timing are an essential component of it.
‘Prematurity’, ‘acting in brief intervals of time’, ‘reduced decision time’ and ‘acting on the spur of the moment, without thinking’, for example, refer to the execution of acts at an inappropriately early moment in time, reflecting poor motor timing and underuse of reflection time. ‘Present boundedness’ or ‘poor futuring’ indicates insufficient use of inter-temporal bridging/temporal foresight. ‘Risk taking and sensation seeking’ reflect temporal myopia or poor consideration of the negative future consequences of such behaviours. ‘Lack of persistence and resistance to delayed rewards’ refer to steeper temporal discounting, i.e. the subjective devaluation of reward in proportion to its delay in time. Steeper temporal discounting is presumed to reflect shorter tolerance of temporal delays and enlarged subjective perception of time. A reduced tolerance to the passage of time is also reflected in other definitions such as ‘lack of patience’ and ‘increased thresholds of boredom’.
In this paper, we argue that abnormalities in timing functions are fundamental to impulsiveness. Accordingly, impulsiveness manifests in abnormalities in different timing functions within different temporal domains that can be classified into the following neuropsychologically measurable subcomponents:
- Motor timing. Impulsive timing of behaviour is characterized by a premature and inconsistently paced response style. In the laboratory, motor timing is typically measured in tests of sensorimotor synchronization or anticipation in the milliseconds or seconds range.
- Time perception. The passage of time appears to be subjectively longer/more intolerable forimpulsive people, suggestive of an abnormal time sense. In neuropsychological settings, fine-temporal perception is measured in time discrimination of short intervals in the milliseconds or seconds range. Time estimation is measured in tasks of time production/reproduction or estimation of typically seconds' intervals.
- Temporal foresight and temporal discounting. Inter-temporal bridging seems diminished in impulsive people. Temporal foresight is the most difficult to measure timing function as it can only be measured indirectly; for example, in gambling tasks that require the subject to learn and consider the future consequences of immediate reward choices, in tasks of forward planning or in tasks of temporal discounting, all of which comprise larger temporal domains such as days, weeks, months and years. Temporal discounting measures the degree to which a reward is discounted in relation to its temporal delay, i.e. the subjective value of the temporal delay in terms of reward. It requires temporal foresight in order to assess the larger future gain against the smaller immediate gain. Poor temporal discounting reflects either a dislike of temporal delay, probably due to an enlarged time sense, or reduced temporal foresight, or both.
We do not claim that all
impulsiveness features can be accounted for by poor timing functions. We have previously argued that besides timing functions, problems with self-control functions, manifesting in poor inhibitory and attention functions are also key ingredients to impulsiveness (Rubia 2002
). Timing, attention and inhibitory functions are in fact closely interrelated in behavioural as well as neuropsychological datasets (Olson et al. 2007
; Rubia et al. 2007a
), and appear to be different aspects of a multifaceted construct of impulsiveness (Reynolds et al. 2008
). Most timing functions furthermore co-measure other cognitive basis functions depending on the temporal domains they cover. For example, attention to time is co-measured in many timing tasks, in particular estimation of longer intervals. Time reproduction relies on working memory functions and the ability to delay a response (inhibitory control), whereas psychophysical time discrimination tasks also tap into sensory processes.
Problems with reward and motivation have also been related to impulsiveness (Gray 1987
). However, the most consistent association is with reduced sensitivity to reward delays, i.e. the timing of the reward, not the reward itself, reflecting hypersensitivity to the passage of time, not reward. It is not within the scope of this paper to discuss the inter-relationship between timing and these other functions that contribute to the complex construct of impulsiveness. The aim of this paper is to shape out—in our opinion—the underrated and underinvestigated timing aspects of impulsiveness. However, this does not reflect an exclusively reductionistic viewpoint and we acknowledge the importance of other aspects to impulsiveness, in particular inhibitory and attention functions.