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INTRODUCTION
Directing attention to a cued location facilitates target detection at that location, if the cue-to-target interval is less than 300 ms. However, if the cue-to-target delay is between 300 and 2000 ms, target detection is typically faster at an uncued location, compared to the cued location (Klein, 2000). This phenomenon has been termed Inhibition of Return (IOR), and was first demonstrated by Posner and Cohen (1984). IOR is thought to be a highly adaptive mechanism that assists visual search by biasing attention to novel, previously unattended areas (Klein, 2000; Tipper et al., 1996). For example, at a gross level, when searching for a lost object efficient search requires attention to be moved across the environment to novel locations, rather than returning to locations that have previously been searched. Therefore, deficits in IOR may result in perseverative behaviors, such as those seen in obsessive-compulsive disorder (OCD).
IOR is thought to involve effects of the ocular-motor system (Harman et al., 1994; Posner et al., 1985; Taylor & Klein, 1998), and therefore to be mediated by midbrain structures. Posner et al. (1985) investigated IOR in patients with progressive supranuclear palsy (PSP), a disorder in which the superior colliculus is compromised. They reported a loss of IOR in PSP patients, indicating that the superior colliculus may be necessary for IOR to operate. Further evidence implicating the superior colliculus in IOR comes from a recent study that recorded activity from neurons in the superior colliculus in monkeys during a cue-saccade paradigm (Dorris et al., 1998). Reduced responses were found for targets that appeared at the cued location, compared to targets presented at the uncued location. This finding suggests that the superior colliculus is involved in IOR (Klein, 2000).
Midbrain deficiencies, such as atypical eye movements, have been reported in OCD (Rosenberg et al., 1997). Specifically, patients with OCD have been reported to have increased latencies producing antisaccades where an eye movement away from a stimulus event is required (Maruff et al., 1999). Furthermore, problems with orienting attention, also thought to involve midbrain structures (Rafal et al., 1988), have similarly been reported (Christensen et al., 1992; Clayton et al., 1999; Head et al., 1989).
Nelson et al. (1993) used a Posner-type visual paradigm to determine the ability of OCD patients...