Inhibition of return

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Inhibition of return (IOR) refers to an orientation mechanism that briefly enhances (for approximately 100-300 milliseconds (ms)) the speed and accuracy with which an object is detected after the object is attended, but then impairs detection speed and accuracy (for approximately 500-3000 milliseconds). IOR is usually measured with a cue-response paradigm, in which a person presses a button when he or she detects a target stimulus following the presentation of a cue that indicates the location in which the target will appear. Often, the cue is exogenous (or peripheral),[1] as opposed to endogenous because endogenous cues do not tend to activate IOR.[2] Although IOR occurs for both visual and auditory stimuli, IOR is greater for visual stimuli,[3] and is studied more often than auditory stimuli.


IOR was first described in depth by Michael Posner and Yoav Cohen,[1] who discovered that, contrary to their expectations, reaction times (RT) to detect objects appearing in previously cued locations were initially faster to validly cued location (known as the validity effect), but then after a period of around 300 ms, response times to a previously cued location were longer than to uncued locations. Specifically, IOR was described as "an inhibitory effect produced by a peripheral (or exogenous) cue or target."

In the experiment that demonstrated the paradigm, participants were instructed to fixate on a center box that was flanked with a box on its right and left sides. Each trial began with the brightening of the outline of one of the peripheral boxes that was randomly selected for 150 ms. During the trial, a target (a bright filled square) occurs in the center box at either 0, 50, 100, 200, 300, or 500 ms after the initial brightening. Participants had to respond to the target as quickly as possible by pressing a specified key. Participants' performance in RT on the cued side increased the first 150 ms; however, they then experienced inhibition of target RT on the cued side compared to the uncued side after 300 ms.

In order to explain the IOR mechanism, Anne Treisman and Gary Gelade's theory of visual search was expounded. This theory, known as the Feature integration theory proposes that there are two types of visual searches: parallel searches and serial searches.[4] According to Treisman and Gelade, attention is only required for serial searches. IOR is a mechanism that is specific for serial searches.


Posner and Cohen proposed three explanations for inhibition: 1) inhibition results from having two alternative positions, 2) inhibition could result from moving attention away from a cued stimulus back to the fixation point, and 3) inhibition may occur because the efficiency in some part of the pathway from the cued location is reduced by the cuing.

An alternative explanation of IOR is that IOR occurs after attention has been disengaged from the cued stimulus, resulting in a delayed response back to that cued stimulus. This occurs because it inhibits an individual from reorienting back to a stimulus they previously attended to.[5]

Although most subscribe to the attentional view of IOR, it has also been suggested that IOR is activated by midbrain oculomotor pathways.[2] Specifically, it was hypothesized that IOR may be activated either by an exogenous sensory signal presented in the visual periphery when the eyes are fixed to a location or by the endogenous activation of a saccadic eye movement.[2] However, this conclusion has been questioned by researchers who have found in their studies that endogenous saccade activation is not efficient to produce IOR.[6]

Others believe that IOR is caused by both a delay in activation of attentional and motor processes.[7]

Functional significance[edit]

It has been suggested that IOR promotes exploration of new, previously unattended objects during visual search or foraging by preventing attention from returning to already-attended objects.[5]

Klein hypothesized that in a parallel visual search, the difference between RTs at probe targets and empty locations should be less than in a serial visual search.[5] He suggested that this occurs because in serial searches, "inhibitory tags" are left at each location that has been attended to. Thus, IOR is a mechanism that allows a person not to re-search in previously searched visual fields as a function of "inhibitory tags." This is known as the foraging facilitator proposal.

Researchers (including Klein himself) initially challenged the foraging facilitator proposal. Pratt and Abrams[8] suggested that IOR was not a foraging assistant because inhibition only occurred at the most recently attended stimulus. Earlier, Klein and Taylor[9] found that it could not be concluded that attention was inhibited in IOR because at that time, inhibition had not been examined utilizing non-spatial discrimination tasks. Additionally, questions arose after it had been difficult to replicate Klein's findings, however, similar finding were reported eventually.

Although researchers had proposed these challenges initially, more recent empirical studies have not only replicated Klein's findings, but have also rebutted the challenges posed initially.


  1. ^ a b Posner, M.I.; Cohen, Y. (1984). "Components of visual orienting". In Bouma, H.; Bouwhuis, D. Attention and performance X: Control of language processes. Hillsdale, NJ: Erlbaum. pp. 531–56. 
  2. ^ a b c Rafal, Robert; Calabresi, Peter; Brennan, Cameron; Sciolto, Toni (1989). "Saccade preparation inhibits reorienting to recently attended locations". Journal of experimental psychology 15 (4): 673–685. PMID 2531204. 
  3. ^ Reuter-Lorenz, P.; Jha, A.; Rosenquist, J.N. (1996). "What is inhibited in inhibition of return". Journal of experimental psychology 22 (2): 367–378. PMID 8934850. 
  4. ^ Treisman, A.; Gelade, G. (1980). "A feature-integration theory of attention". Cognitive psychology 12 (1): 97–136. doi:10.1016/0010-0285(80)90005-5. PMID 7351125. 
  5. ^ a b c Klein, R.M. (1 April 2000). "Inhibition of return". Trends in Cognitive Sciences 4 (4): 138–147. doi:10.1016/S1364-6613(00)01452-2. ISSN 1364-6613. PMID 10740278. (subscription required (help)).  (URL also gives abstract)
  6. ^ Chica, A.; Klein, R.; Rafal, R.; Hopfinger, J. (2010). "Endogenous saccade preparation does not produce inhibiton of return: Failure to replicate Rafal, Calabresi, Brennan & Sciolto (1989)". Journal of experimental psychology: Human perception and performance 36 (5): 1193–1206. doi:10.1037/a0019951. 
  7. ^ Johnson, Addie; Proctor, Robert W. (2004). "Chapter 5: Attention and inhibition". Attention: Theory and Practice. Sage Publications. pp. 127–162. ISBN 978-0-7619-2761-7. 
  8. ^ Pratt, J.; Abrams, R. (1995). "Inhibition of Return to Successively Cued Spatial Locations". Journal of experimental psychology: Human perception and performance 21 (6): 1343–1353. doi:10.1037/0096-1523.21.6.1343. 
  9. ^ Dagenbach, Dale (1994). Dagenbach, Dale; Carr, Thomas H., eds. Inhibitory Processes in Attention, Memory, and Language (6th (illustrated) ed.). Academic Press. pp. 113–150. ISBN 978-0-12-200410-0. 

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