| Summary: | In visual spatial-cueing experiments with uninformative peripheral cues, reaction time to cued-location targets is facilitated (relative to that for targets at uncued-locations) at short cue-target intervals and inhibited at longer intervals. Posner, Rafal, Choate and Vaughan (1985) labeled this inhibitory effect Inhibition of Return (IOR) and proposed that it serves to bias the visual system to orient towards new locations. This proposal remains a popular topic of debate and no consensus has yet been reached regarding the mechanisms responsible for producing IOR. The present project utilized event-related potential (ERP) measures of brain activity to investigate the processing changes underlying IOR. I examined the effect of uninformative visual cues both on the amplitude of early visual ERP peaks and on the motorrelated lateralized readiness potential (LRP). The LRP is a measure of motor cortex activity that is related to the preparation of responses. The onset of the LRP relative to the target and response provide relative measures of the duration of premotor and motor processes, respectively. Chapters 2 and 3 present the results of four experiments, in which observers were required to make manual responses to visual targets. In each experiment the target was preceded by an uninformative peripheral cue followed by a central re-orienting event. In separate experiments, subjects were required to make identity-based discrimination responses, localization responses, detection responses, or identity-based go-nogo responses. In each of these experiments IOR was associated with a slowing of premotor processes. The duration of motor processes were not affected, however, indicating that, when measured with manual keypresses, IOR does not arise from inhibition of motor processes. Consistent with a perceptual locus of IOR, the amplitudes of the occipital ERP peaks were reduced for cuedlocation targets relative to uncued-location targets. In a fifth experiment (Chapter 5) the reorienting event was omitted and subjects were required to discriminate the target identity. Compared to when a re-orienting event was present, the magnitude of both IOR and the ERP amplitude effects were reduced. This co-variation of behavioural and ERP effects suggests that that changes in the perceptual processes are, at least in part, responsible for generating IOR. === Arts, Faculty of === Psychology, Department of === Graduate
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