Abstract
In many species, rapid defensive reflexes are paramount to escaping acute danger. These reflexes are modulated by the state of the environment. This is exemplified in fear-potentiated startle, a more vigorous startle response during conditioned anticipation of an unrelated threatening event. Extant explanations of this phenomenon build on descriptive models of underlying psychological states, or neural processes. Yet, they fail to predict invigorated startle during reward anticipation and instructed attention, and do not explain why startle reflex modulation evolved. Here, we fill this lacuna by developing a normative cost minimisation model based on Bayesian optimality principles. This model predicts the observed pattern of startle modification by rewards, punishments, instructed attention, and several other states. Moreover, the mathematical formalism furnishes predictions that can be tested experimentally. Comparing the model with existing data suggests a specific neural implementation of the underlying computations which yields close approximations to the optimal solution under most circumstances. This analysis puts startle modification into the framework of Bayesian decision theory and predictive coding, and illustrates the importance of an adaptive perspective to interpret defensive behaviour across species.
Highlights
The mammalian startle reflex is a protective postural change and eye blink response
The organism needs to minimise the cost of the startle response, plus the expected cost of a blow: CTOT 1⁄4 CR þ PðBj XÞCB
We presented a normative approach to startle modulation, by formalising the consequences of a startle response as costs, and analysing the cost-minimising behaviour
Summary
The mammalian startle reflex is a protective postural change and eye blink response. It occurs in response to a suspected immediate blow to the head or upper body, as signalled by sudden noise, sharp movement, or touch (Yeomans et al, 2002). Their predictions are in contradistinction to the pattern of startle modulation by reward and instructed attention commonly observed in humans (Sabatinelli et al, 2001; Skolnick and Davidson, 2002; Lipp et al, 1997, 1998) We fill this lacuna with a formal account of the startle response in the framework of Bayesian decision theory (Körding, 2007). The organism infers from the likely presence of aversive events such as electric shocks used in experimental paradigms to the likely presence of other threats such as a blow to the head, because both are assumed to be manifestations of the same cause This non-specificity is normative in environments in which manifestations of physical danger are correlated. To account for more realistic biological scenarios, the appendix contains a generalisation for a continuous (scalar) blow magnitude which replicates all results from the discrete model
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