Two “Inhibitions of Return” Bias Orienting Differently

Inhibition of return (IOR) is usually viewed as an inhibitory aftermath of visual orienting typically seen in the form of slower responses to targets presented in a previously attended location or object (Posner & Cohen, 1984; Posner et al., 1985). Using the diagnostic patterns obtained when peripheral onset or central arrow targets are used, we have seen that there are two forms of inhibitory aftereffect: one caused by a peripheral stimulus whereby the effect is to decrease the efficiency of subsequent visual processing in the proximity of this stimulus (input effect); the second caused by oculomotor activation whereby the effect is a motor bias (output effect). These are distinguished clearly by whether the effect can only be measured by peripheral targets (input form when the reflexive oculomotor system is suppressed) or by whether there are roughly equivalent delays in response whether the targets are central or peripheral (output form when the reflexive oculomotor system is not suppressed). When performance is represented in speed-accuracy space the input form is manifest as a shift from one speed-accuracy tradeoff function to a less efficient one representing degraded or delayed processing of cued targets while the output form entails no shift in the function, but instead a movement along it (a response bias). Both forms bias orienting and hence can perform the novelty seeking function attributed to the inhibitions in the seminal papers: the input form does so by biasing perception, whereas the output form does so by biasing action.