Inhibition of return in aging and Alzheimer's disease: performance as a function of task demands and stimulus timing.

Inhibition of return (IOR) is a phenomenon of spatial attention that biases attention toward novel events in the environment. Recent evidence suggests that the magnitude and timing of IOR varies as a function of task conditions (e.g., detection vs. discrimination tasks, short vs. long cue-target intervals, intrinsic vs. extrinsic cues). Although IOR appears relatively preserved with both normal aging and Alzheimer's disease (AD), it has been tested under relatively simple task conditions. To test whether IOR is resistant to age and / or AD when cognitive demands are increased, we employed a double-cue IOR paradigm that required categorization as well as detection responses. The stimulus onset asynchrony (SOA) between the cue and target events was varied to determine whether group differences existed in IOR effects over time. Younger normal adults and older normal adults exhibited significant IOR effects on both the detection task and the categorization task at a short cue-target SOA (950 ms). In contrast, AD patients exhibited significant IOR effects at the short SOA on the detection task but not on the categorization task. From the short to the long SOA (3500 ms), IOR effects exhibited by younger normal adults declined significantly during both the detection and the categorization tasks, suggesting that inhibition resolved over time. In contrast, neither older normal adults nor AD patients exhibited SOA-related IOR reductions on the detection task. These results suggest that IOR may show differential age- and AD-related vulnerabilities depending on task conditions and timing characteristics.