Orienting of attention and Parkinson's disease: tactile inhibition of return and response inhibition.

There is growing evidence for cognitive impairments in Parkinson's disease (PD), including in the orienting of attention and inhibition of return (IOR). IOR refers to the slowing of a response to a target stimulus presented in the same location as a previous stimulus. While some researchers have reported normal levels of visual IOR in PD patients using cue-target tasks, others have reported significant reductions in IOR in this patient group. However, the inhibitory effects observed in cue-target tasks may reflect non-ocular response inhibition associated with withholding a response from the cue stimulus, rather than attentional or oculomotor processes. Many researchers working with normal participants have circumvented this confound by using a target-target task, in which a response is made to all peripheral stimuli. Here, we compared IOR measured in cue-target and target-target tasks, using tactile rather than visual stimuli. Both the PD and the control groups exhibited significant inhibitory effects in the cue-target task, but only the control group exhibited significant IOR in the target-target task. Our results demonstrate a reduction, or elimination, of IOR in PD and this change may have been underestimated in previous studies, in which methodologically flawed cue-target tasks were used. This reduction in IOR may reflect impaired inhibitory processes or hyper-reflexive orienting in parkinsonian patients.