Inhibition of return (IOR) is the demonstration that subjects are sometimes slower to respond to a stimulus when it is presented at a previously stimulated location compared to when it is presented at a new location (Maylor, 1985; Maylor & Hockey, 1985; Posner & Cohen, 1984; Posner, Rafal, Choate, & Vaughan, 1985; Tassinari, Aglioti, Chelazzi, Marzi & Berlucchi, 1987). The IOR phenomenon indicates that inhibitory processes may play a critical role in attentional search and, more generally, in selection-for-action. It is therefore important to gain a better understanding of the processes that are involved in generating IOR as well as those that are influenced by IOR once it is established.

In a typical IOR study, a location "cue" is presented to one of two peripheral locations. After a variable time interval following the onset of the cue, a target is presented with equal probability at the cued location or at the uncued location in the opposite field. The subject is usually required to manually detect the onset of the target stimulus (e.g., Posner & Cohen, 1984) or to respond based on its location (e.g., Maylor, 1985). When the cue-target stimulus onset asynchrony (SOA) is shorter than 150-200 ms, responses are faster for targets appearing at the cued location than for targets appearing at the uncued location. This performance enhancement on valid-cue trials indicates that the cue summoned attention to its location. However, when the SOA is longer than 200 ms, responses are slower for targets appearing at the cued location than for targets appearing at the uncued location. The IOR effect is the performance decrement that occurs on longer SOAs for targets appearing at the cued location relative to targets appearing at the uncued location.

The popular interpretation of the IOR effect is that it reflects a bias against attention returning to the cued location (Gibson & Egeth, 1994; O’Donnell & Pratt, 1996; Pratt, Kingstone, & Khoe, in press; Reuter-Lorenz, Jha, & Rosenquist, 1996). The most common explanation for this bias is as follows. Although subjects maintain eye fixation throughout the experiment, the peripheral box brightening causes a momentary shift in attention to its location, thereby facilitating target detection at the cued location for a short period (Jonides, 1981; Posner & Cohen, 1984). However, subjects quickly return their attention to the centre of the display because the cue does not accurately predict the location of the target and/or because special incentive is provided to focus attention at fixation (e.g., by presenting most of the target at fixation or by presenting a second cue at fixation; Posner and Cohen, 1984). Some time after attention is directed away from the cued location, an inhibit ory process develops that biases attention from returning to the cued location. Thus, if the target is presented after attention has been re-directed to the centre of the display, more time is required to shift attention back to the cued location than is required to shift attention to a new location. On this basis, the IOR effect may be related to attention in two important, but separate, ways. First, orienting attention away from the cued location may produce IOR. This issue pertains to the mechanism of IOR. Second, subsequent attentional processes may be inhibited by IOR. This issue pertains to the consequence of IOR.

My research programme examines (1) whether IOR to a spatial location is produced by attentional or non-attentional (sensory, oculomotor) mechanisms, and (2) whether attentional processes are inhibited by IOR. With regards to the consequences of IOR, we have found that non-predictive visual cues influence the amplitude of several ERP components, including the sensory-evoked P1 (McDonald, Ward, & Kiehl, in press, Perception & Psychophysics). Consistent with an attentional account of IOR, these ERP findings indicate that IOR reflects inhibited sensory/perceptual processing at the cued location.