Behavioural Processes 89 (2012) 95–103 Contents lists available at SciVerse ScienceDirect Behavioural Processes journa l h omepa g e: www.elsevier.com/locate/behavproc Cognitive mechanisms of risky choice: Is there an evaluation cost? Justine Aw 1 , Tiago Monteiro 1 , Marco Vasconcelos 1 , Alex Kacelnik ∗,1 Department of Zoology, University of Oxford, South Parks Road, Oxford, OX1 3PS, UK a r t i c l e i n f o Article history: Received 7 April 2011 Received in revised form 27 September 2011 Accepted 27 September 2011 Keywords: Risky choice Sequential Choice Model Scalar Utility Theory Expectation of the ratios Starlings Food delay Food amount a b s t r a c t We contrast two classes of choice processes, those assuming time-consuming comparisons and those where stimuli for each option act independently, competing for expression by cross censorship. The Sequential Choice Model (SCM) belongs in the latter category, and has received empirical support in several procedures involving deterministic alternatives. Here we test this model in risky choices. In two treatments, each with five conditions, European starlings (Sturnus vulgaris) faced choices between options with unpredictable outcomes and risk-free alternatives. In the delay treatment the five conditions involved choices between a variable option offering two equiprobable delays to reward and a fixed option with delay differing between conditions. The amount treatment was structurally similar, but amount of reward rather than delay was manipulated. As assumed (and required) by the SCM, latency to respond in no-choice trials reflected each option’s richness with respect to the background alternatives, and, crucially, preferences in simultaneous choices were predictable from latencies to each option in forced trials. However, we did not detect reliable differences in response times between forced and choice trials, neither the lengthening expected from evaluation models nor the shortening expected from the SCM. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The present study deals with two aspects of foraging decisions: how do subjects attribute value to sources of reward, and what is the process by which they arrive at individual choices. With regard to the first issue, we consider propensities to under- or over-value food sources with respect to their mean rate of reward as a func- tion of the variability of their outcomes (see further discussion of this topic in Shapiro et al., 2011). With regards to the second, we examine how animals use valuations acquired and expressed in sequential encounters (i.e., when meeting one food source at a time) to make choices when more than one opportunity is simul- taneously present. Both topics have been discussed on their own elsewhere, but here we treat them jointly for the first time. Our basic question is: when animals make choices between stochas- tic sources of reward, do they, at the time of choice, evaluate the properties of each alternative, using time for the cognitive opera- tion, or do they treat each alternative independently? If the latter, a choice would simply reflect action towards the option eliciting the fastest response time. Risky choice should a priori be a better arena to compare the models because evaluating an option with ∗ Corresponding author. Tel.: +44 1865 271164. E-mail address: alex.kacelnik@zoo.ox.ac.uk (A. Kacelnik). 1 All authors contributed equally to this manuscript. variable outcomes may be more complex, and then involve longer evaluation times, if they exist at all. We focus on two choice models, labelled the Tug-of-War Model (ToWM) and the Sequential Choice Model (SCM; for a review see Kacelnik et al., 2011). Both assume that subjects value options as a function of factors experienced during learning, notably the prof- itability of each outcome (defined, following foraging theory, as the ratio of amount of food over delay to food), the profitability of alter- natives present in the same environment (e.g., Shapiro et al., 2008), and the state of the subject at the time of acquisition (e.g., Kacelnik and Marsh, 2002; Pompilio and Kacelnik, 2005; Vasconcelos and Urcuioli, 2008). However, the models differ in how these values control choice. According to the ToWM, when an agent faces an opportunity for reward it simply acts on it, but when it faces two or more simulta- neous opportunities it undergoes an evaluation to compare them, then acts on whichever is more favourable. This evaluation may be expected to have a time cost, so that acting towards each option may take longer when an alternative is present than when the same option is encountered alone (where no time-consuming compar- isons would be necessary). In the case of risky choice, the subject may know that each option yields a characteristic mean and vari- ance in outcomes, and to make a choice it may need to contrast both measures. For instance, a risk prone subject may be caricatured as thinking, “option A yields higher mean returns, but it has a lower chance of a jackpot, hence I will prefer B”. For normative consider- ations, the evaluation mechanism can be assumed to implement 0376-6357/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.beproc.2011.09.007