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It could be argued that depression relates not to a single decision but to a series of decisions. In this case, the number of time steps of not trying (in either environment) could be used as a measure of depression. However, from an omniscient perspective, resting is the optimal behaviour in the bad environment, so would be unlikely to be regarded as maladaptive (or labelled as depression) by others. As a refinement, the number of time steps of resting whilst in the good environment could be used as a measure of depression. However, because such behaviour is adaptive in the bad environment, the possibility of the environment switching types makes such a definition questionable. As an extension to (3), it could be imposed [http://www.selleckchem.com/products/ldn193189.html selleck inhibitor] that the individual has already tried (at least once) since entering the good environment. But as they may then have just been unlucky and had an experience which happened to be bad, it would again seem dubious to call such behaviour ��depressed��. To avoid the earlier problems, it makes sense to define depression (from a behavioural perspective) as something that can only be identified in the good environment, and where the individual has already experienced success since the environment has become good. Our most stringent definition, therefore, is to characterize depression as resting when in the good environment (i.e. when it would be better to try), despite having tried and succeeded since having entered the good environment. This final, most stringent, definition captures two key features of interest. First, from the perspective [http://www.selleckchem.com/products/a-1210477.html A-1210477 manufacturer] of an omniscient observer, the depressive [http://en.wikipedia.org/wiki/Diglyceride diglyceride] behaviour itself is not beneficial, since in good conditions it is always better to try than to rest. Second, the individual behaves as though conditions are bad despite recent evidence to the contrary��namely a success, which always increases the probability that conditions are good. Later, we show that the stringent definition of depression can be met even when individuals follow the optimal strategy. RESULTS We assume that the individual starts with no information about the current state of the environment and thus all four states are equally likely (i.e. P(Eg) = 0.5, P(��f) = 0.5). The decision to rest (rather than try) for numerous time steps can be induced by a sequence of failures (Fig. 2a). Following the first decision, the perceived probability that trying is likely to be rewarded declines because the individual experiences failure. Nothing is learned about the switching rate on this first update, but subsequent failures increase the perceived probability that the switching rate is low. After the first try, the individual sometimes chooses to rest. During these periods of resting, P(Eg) increases due to the possibility of conditions having switched during that period. P(��f) alters very little when resting, due to the small meta-switching probability, ��.