RESEARCH ARTICLE Sensory and motor mechanisms of oculomotor inhibition of return Zhiguo Wang • Jason Satel • Raymond M. Klein Received: 5 October 2011 / Accepted: 4 February 2012 / Published online: 22 February 2012 Ó Springer-Verlag 2012 Abstract We propose two explicit mechanisms contrib- uting to oculomotor inhibition of return (IOR): sensory and motor. Sensory mechanism: repeated visual stimulation results in a reduction in visual input to the superior col- liculus (SC); consequently, saccades to targets that appear at previously stimulated retinotopic locations will have longer latencies than those that appear at unstimulated locations. Motor mechanism: the execution of a saccade results in asymmetric activation in the SC; as a result, saccades that reverse vectors will have longer latencies than those that repeat vectors. In the IOR literature, these two mechanisms correspond to IOR effects observed following covert exogenous orienting and overt endogenous orienting, respectively. We predict that these two independent mech- anisms will have additive effects, a prediction that is con- firmed in a behavioral experiment. We then discuss how our theory and findings relate to the oculomotor IOR literature. Keywords Inhibition of return  Spatial attention  Superior colliculus  Saccade  Dynamic neural field model Introduction Inhibition of return (IOR), first discovered by Posner and Cohen (1984) using a cue-target paradigm, is a behavioral effect originally characterized by slower response times (RTs) to cued than to uncued targets (for a review, see Klein 2000). In the model (cue-target) task pioneered by Posner and Cohen (1984; see Fig. 1a for an illustration), an uninformative peripheral cue is ignored by the participant. Subsequently, various investigators required responses (manual and/or saccadic) to both the cue and target (for a review, see Taylor and Klein 1998). In these target-target experiments, the cue and the target can control orienting exogenously (as when the target is a peripheral visual onset) or endogenously (as when the target is an arrow at fixation). Considering both the cue-target and target-target paradigms, and the use of exogenous and endogenous cues and targets, Taylor and Klein (1998) noted that there were 24 possible combinations [2 types of the cue (exogenous or endogenous) 9 3 response possibilities to the cue (no response, manual, or saccadic response) 9 2 types of target (exogenous or endogenous) 9 2 response possibilities to the target (manual or saccadic response)], all of which were tested by Taylor and Klein (2000; see Fig. 9 for a summary of their findings). Regardless of the underlying neural mechanisms elicited by these variations in experimental tasks, the behavioral effects (slower responses to cued targets) observed have been labeled as IOR by scholars. Partially because scholars have extended the functional significance of IOR to the area of visual search (Klein 1988; Tipper et al. 1994; Klein and Macinnes 1999; Snyder and Kingstone 2000), slower return (overtly or covertly) to previously inspected locations during search is also labeled as IOR (for a review, see Wang and Klein 2010). Because IOR is a term used loosely by scholars (see Berlucchi 2006, for a discussion), in the present paper we regard IOR as a behavioral effect in the context of orienting, both covert and overt. But despite the singular name, we will propose and then demonstrate two distinct and physiologically Z. Wang  R. M. Klein (&) Department of Psychology, Dalhousie University, Halifax, NS B3H 4R2, Canada e-mail: Ray.Klein@dal.ca Z. Wang Institute of Psychology, Chinese Academy of Sciences, Beijing, China J. Satel Faculty of Computer Science, Dalhousie University, Halifax, NS B3H 4R2, Canada 123 Exp Brain Res (2012) 218:441–453 DOI 10.1007/s00221-012-3033-8