Inhibition of return (IOR) is the slowing of responses to targets appearing at previously attended (cued) locations, occurring approximately 300 ms or more after an exogenous spatial cue. First described by Michael Posner and Yoav Cohen (1984), IOR is thought to function as a "foraging facilitator" — biasing attention away from already-inspected locations to promote efficient visual search and exploration.
The Basic Phenomenon
In Posner's cueing paradigm, a brief peripheral cue initially facilitates responses to targets at the cued location (attentional capture, peaking around 100-150 ms). But at longer cue-target intervals (beyond approximately 300 ms), this facilitation reverses to inhibition: responses are now slower at the cued location than at uncued locations. This inhibition can persist for several seconds and occurs across multiple previously inspected locations.
~300+ ms: Inhibition at cued location (IOR)
IOR magnitude = RT(cued) − RT(uncued) [when positive = inhibition]
Functional Significance
The "foraging facilitator" hypothesis proposes that IOR evolved to support efficient visual search. By biasing attention away from previously inspected locations, IOR prevents repeated sampling of the same locations and encourages exploration of new areas. Consistent with this, IOR has been demonstrated during visual search tasks, and its magnitude correlates with search efficiency. Klein (2000) proposed that IOR reflects a tagging mechanism that marks inspected locations, with the tag discouraging return to those locations.
Whether IOR reflects inhibition of perceptual processing at the cued location or inhibition of motor responses directed toward it has been debated. Evidence suggests both components exist. Perceptual IOR is observed in discrimination tasks (where the response is not spatially directed), while motor IOR is observed in detection and localization tasks. The relative contribution of each may depend on the specific task demands and the type of response required.
Neural Mechanisms
The superior colliculus (SC) plays a critical role in generating IOR. Patients with damage to the SC do not show IOR, and neurons in the SC show reduced activity for stimuli at previously cued locations at the appropriate time course. Cortical contributions come from the posterior parietal cortex and the frontal eye fields, which are involved in the attentional control signals that produce IOR. The interaction between the SC and cortical attention networks appears to be essential for the full expression of the phenomenon.
Ecological and Applied Relevance
IOR has been observed in naturalistic tasks including reading (inhibition at previously fixated words), visual search in complex scenes, and foraging tasks. In applied settings, understanding IOR is relevant for designing visual displays and interfaces that support efficient search, as well as for understanding search failures in domains like medical imaging and airport security screening.