Abstract
Impulsivity, in the sense of the extent rewards are devalued as the time until their realization increases, is linked to various negative outcomes in humans, yet understanding of the cognitive mechanisms underlying it is limited. Variation in the imprecision of interval timing is a possible contributor to variation in impulsivity. We use a numerical model to generate predictions concerning the effect of timing imprecision on impulsivity. We distinguish between fixed imprecision (the imprecision that applies even when timing the very shortest time intervals) and proportional imprecision (the rate at which imprecision increases as the interval becomes longer). The model predicts that impulsivity should increase with increasing fixed imprecision, but decrease with increasing proportional imprecision. We present data from a cohort of European starlings (Sturnus vulgaris, n = 28) in which impulsivity had previously been measured through an intertemporal choice paradigm. We tested interval timing imprecision in the same individuals using a tri-peak temporal reproduction procedure. We found repeatable individual differences in both fixed and proportional imprecision. As predicted, birds with greater proportional imprecision in interval timing made fewer impulsive choices, whilst those with greater fixed imprecision tended to make more. Contradictory observations in the literature regarding the direction of association between timing imprecision and impulsivity might be clarified by distinguishing between fixed and proportional components of imprecision.
Highlights
The term impulsivity is used in a variety of senses (Evenden 1999)
Where the proportional imprecision is small, increasing the fixed imprecision α produces greater impulsivity. This is intuitive: if there is a constant increment in imprecision that applies to the estimation of both rewards, its over-valuation impact will be greater on the smaller sooner reward, because the discount function is steepest at this point
For any given α, increasing the proportional imprecision reduces impulsivity. This is because an excess of proportional imprecision will lead to larger error in the estimation of the delay to the larger later reward in particular, producing a greater over-valuation of that reward, and less impulsivity (Fig. 2c)
Summary
The term impulsivity is used in a variety of senses (Evenden 1999) We use it to mean the extent by which a reward is devalued as the time to its realization increases, known as delay discounting. Impulsivity of this kind varies between individuals. In neither humans nor non-human animals do we fully understand the cognitive mechanisms that underlie individual differences in impulsivity. These mechanisms are likely to involve the processing of reward magnitude, or the processing of time intervals, or both
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