Inhibition of Return

A second important feature of Fig. 2, (seen only in the top), is that when SOA's following an exogenous cue are longer than 2-300 msec RT to detect a target is longer when a target appears at the valid location compared to the invalid location. That is, facilitation is transformed into inhibition. This is a standard outcome ofexperiments with nonpredictive cues, in which subjects have no reason to use the cue to guide attention and would prefer to keep attention focused at fixation or spread diffusely across the display.

Michael Posner and Yoav Cohen analyzed the effect of nonpredictive cues at the longer SOA durations, now known as the inhibition of return (IOR) phenomenon. Facilitation changes to inhibition (i.e., IOR) 200-300 msec after cue onset. IOR lasts for about 3 or 4 sec; it works in environmental rather than in retinal coordinates, it is not generated by endogenous cues unless the oculomotor system has been activated, and it declines as the distance from the original cued location increases.

What is the explanation for this counterintuitive transformation of facilitation into inhibition? Exogenous cueing commonly produces a reflexive shift of attention to the cued location or object (producing the early facilitation in Fig. 2). Even when people are asked not to pay attention to the cued location or it is in their favor to ignore the cued location, they find it difficult to avoid reacting to a peripheral luminance change and often cannot refrain from orienting to this kind of cue. As much as such efficient and rapid orienting is important for predatory and defensive behavior, voluntary control of reflexive orienting and the ability to strategically search the environment are also critical for survival. It appears that IOR is a mechanism that enables the organism to disengage from reflexive orienting and switch to the control of a more voluntary attentional system. How does it work? It seems that a location (or an object) that was recently cued or searched is tagged and IOR biases attention away from responding to events occurring at the tagged locations. Avoidance of tagged locations encourages search in new spatial locations. Accordingly, IOR seems to be a mechanism that supports efficient foraging behavior, which involves strategic search of the environment and use of knowledge about previous searched locations or objects.

Several lines of evidence point to the midbrain superior colliculus (SC) as the neural substrate for the implementation of IOR. IOR is abnormal in patients with damage to the SC. This has been shown in patients with midbrain degeneration due to progressive supranuclear palsy and in a patient with unilateral lesion to the SC. IOR is preserved in the presence of hemianopia, in which only the extrageniculate pathways are available to process visual information. It is present in newborn infants in whom the geniculostriate pathway is not yet developed. It is generated asymmetrically in the temporal and nasal visual fields. The temporal hemifield has stronger collicular representation than the nasal hemifield; accordingly, for monocular presentations of stimuli, IOR is larger for stimuli presented to the temporal than to the nasal hemifield. However, it seems that cortical structures play a role in the generation of IOR. In particular, the parietal lobe has been suggested as a structure that conveys the spatial coordinates of the tagged locations to the SC.

Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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