The Study Of Brain Mechanism On Inhibition Of Return In Spatial Orienting

The orienting of visual-spatial attention, which is fundamental to most organisms, plays an important role in rapid and efficient search of visual environments. This process is thought to include two mechanisms: endogenous and exogenous orienting. Endogenous orienting refers to the purposeful allocation of attentional resources to a predetermined location in space, whereas exogenous orienting is thought to be triggered reflexively and automatically by an abrupt onset event. The early benefit of exogenous orienting at the cued location is usually attributed to the capture of attention by the peripheral cue, and the subsequent decline in performance at the cued location has been referred to as inhibition of return (IOR).IOR is studied by using the typical spatial cue-target paradigm. In such an experiment, subjects respond faster to a target presented at a cued location than that at an uncued location when a stimulus onset asynchrony (SOA) between cue and target is shorter than 250 ms, whereas subjects respond slower to a target appearing at a cued location than that at an uncued location when SOA is longer than 250 ms. IOR is thought to represent a decline in salience in the vicinity of the cue that discourages re-orienting back to the cued location. The primary assumed mechanism is that attention is inhibited from returning to previously searched locations. Since Posner & Cohen's seminal study, IOR has become an actively investigated component of orienting, and has been a hot topic of many debates.Various technologies are employed to the studies related to IOR. These include behavioral studies, event-related potentials (ERPs), functional magnetic resonance imaging (fMRI), transcranial magnetic stimulation (TMS), signal-cell recording, positron emission computerized topography (PET) and so on.This dissertation focuses on the studies of brain mechanisms of IOR by high temporal resolution ERPs with the source localization of low resolution electromagnetic tomography (LORETA) and high spatial resolution fMRI respectively. A series of multilevel, novel and detailed researches on IOR have been carried out. Our major works and achievements are listed as below:1,A study on the dynamic brain mechanism of visual spatial-cueing effectWithin the cue-target paradigm, this study analyses the ERPs elicited by the peripheral uninformative cue and uses LORETA to localize the results of the different processing stages. Unlike previous ERP investigations of IOR, in this study we focus on the time characteristic of the neural activity (via EEG) elicited by the cue prior to the appearance of the target. The results show that after cue onset, activations may be approximately divided into three stages. In the early stage (110ms-240ms), activations are in the prefrontal cortex, the intraparietal cortex, and the contralateral occipito-temporal cortex. In the middle stage (240ms-350ms), activations are mainly in the frontal cortex and the parietal cortex. In the late stage (350ms-650ms), the main activations are in the occipito-parietal cortex. Unlike the early stage, the activation areas shift to the ipsilateral side of the cued location. These findings indicate that IOR is related to both attention and motor response, and also suggest that the time course of initial facilitation and IOR is concurrent and mediated by two neural networks. Finally, a time table is proposed, which extends the spatial mechanism of IOR to a spatio-temporal mechanism.2,A study on the brain mechanism of IOR based on the response inhibitionWith an uninformative peripheral cued Go/Nogo task experiment, this study aims to characterize the neural mechanism of IOR by both early (P1 and N1) and late (Go/Nogo N2 and P3) ERPs. The scalp topographies and LORETA show that the changes of the early ERPs, the cueing effects, are distributed mainly over the dorsal occipito-parietal areas. The changes of the late ERPs are distributed mainly over the frontocentral areas. The Nogo-N2 is earlier and smaller in valid trials than in invalid trials, which suggests that the late components related to IOR are modulated by response preparation inhibition. The Nogo-P3 is later and larger in valid trials than that in invalid trials, indicating that the frontal eye field(FEF)is free from an inhibitory marker in the cued locations. These evidences suggest that the mechanism of IOR consists of both the sensory inhibition and the response preparation inhibition. 3,A study on the brain mechanism of interaction between spatial orienting and response inhibitionThe previous studies have suggested that the prefrontal cortex (PFC), including dorsolateral prefrontal cortex (DLPFC), inferior frontal cortex (IFC), medial frontal cortex (MFC) and the anterior cingulate cortex (ACC), plays an important role in response inhibition in Go/Nogo tasks. However, it remains unclear how these"executive"brain regions will act when the conflict control process interacts with spatial orienting. Therefore, event-related fMRI is used to investigate the differential neural mechanisms underlying interactions between spatial orienting and response inhibition. Two types of figures (go stimulus and Nogo stimulus) are random presented either at the cued location or uncued location. Endogenous response inhibition activates widespread cortical regions including bilateral temporoparietal junctions, bilateral superior frontal gyrus, right superior temporal gyrus, and inferior frontal gyrus. Conversely, exogenous response inhibition activates only two areas including right superior frontal gyrus and right middle frontal gyrus. Meanwhile, the interaction of the timecourse in spatial orienting has been investigated in this study. The timecourse of endogenous orienting indicates activations in the medial frontal gyrus, whereas the timecourse of exogenous orienting indicates activations in the bilateral middle frontal gyrus. These results show that response inhibition in different orienting involves different brain areas, thus they are quite possibly two dissociable processes.4,A study on the brain mechanism of IOR in visual search tasksExogenous orienting is considered to be a reflexive and automatic process. It initiates facilitation, which is then replaced by a delayed response (i.e. IOR). However, it is still unclear whether the initial facilitation and the later IOR are two stages of one process or simply two independent processes. Some studies have observed IOR when endogenous attention is fixed and found that these processes are driven by separate mechanisms. To date, however, few studies have directly addressed the relationship between exogenous attention facilitation and IOR. Here, we investigate the different effects of exogenous facilitation and IOR in visual search tasks by recording ERPs with high temporal resolution. When exogenous attention remains to be fixed at the serial search locations, a delayed response is observed (i.e. IOR) with three ERP components: a Pd200 at the posterior parietal areas, an Nd240 at the middle tended to left prefrontal areas and an Nd280 at the bilateral temporoparietal areas. When exogenous attention stays at parallel search locations, a faster response is observed (exogenous facilitation). Here, one ERP component appears, an Nd280 at the middle occipito-parietal areas. These results suggest that the exogenous facilitation and IOR involve different brain areas and/or neural processes, and are therefore most likely two dissociable processes.