FMRI indices of auditory attention in schizophrenia.

The present study sought to identify abnormalities in activation in several brain regions in response to an auditory attention task in patients with schizophrenia. Ten patients and twenty healthy control participants were examined using Functional Magnetic Resonance Imaging (FMRI) measures acquired during an auditory attention task. Region of interest analyses of activation of targeted regions implicated in attention included: anterior cingulate cortex (ACC), dorsolateral prefrontal cortex (DLPFC), hippocampus, parahippocampal gyrus (PHG), and superior temporal gyrus (STG). The results indicated over-activation in patients with schizophrenia. While the control group showed notable coherence in activation within and across hemispheres the schizophrenia group showed relatively less coherence overall that was only present in the right hemisphere. These findings suggest that patients with schizophrenia show both an over-engagement of brain regions during attention task as well as a lack of communication among neural regions involved.

Neural networks underlying endogenous and exogenous visual-spatial orienting.

The orienting of visual-spatial attention is fundamental to most organisms and is controlled through external (exogenous) or internal (endogenous) processes. Exogenous orienting is considered to be reflexive and automatic, whereas endogenous orienting refers to the purposeful allocation of attentional resources to a predetermined location in space. Although behavioral, electrophysiological and lesion research in both primates and humans suggests that separate neural systems control these different modes of orienting, previous human neuroimaging studies have largely reported common neuronal substrates. Therefore, event-related FMRI (ER-FMRI) was used to independently examine different components of the orienting response including endogenous facilitation, exogenous facilitation and inhibition of return (IOR). In contrast to previous studies, endogenous versus exogenous facilitation resulted in widespread cortical activation including bilateral temporoparietal junction, bilateral superior temporal gyrus, right middle temporal gyrus, right frontal eye field and left intraparietal sulcus. Conversely, IOR compared to endogenous facilitation resulted in only a single focus of activation in the left superior temporal gyrus. These findings suggest that endogenous orienting activates a large cortical network to achieve internally generated shifts of attentional resources versus the automatic orienting that occurs with exogenous cues. However, similar networks may mediate endogenous orienting and IOR. The activation of the temporoparietal junction suggests that it is involved in more effortful processes, such as endogenous orienting, as well as in attentional reorienting and locating targets. Current results are discussed in terms of the functional development of the visual-spatial attentional system.

An event-related fMRI study of exogenous orienting: supporting evidence for the cortical basis of inhibition of return?

This event-related fMRI experiment examined the neural substrates of exogenous visuospatial attention. Exogenous attention produces a biphasic response pattern denoted by facilitation at short cue-target intervals and inhibition of return (IOR) at longer intervals. Whereas the volitional orienting of attention has been well described in the literature, the neural systems that support exogenous facilitation and IOR in humans are relatively unknown. In direct comparisons to valid facilitation trials, valid IOR trials produced unique foci of activation in the right posterior parietal, superior temporal, middle temporal, middle occipital, anterior cingulate, and dorsal medial thalamic areas. Valid IOR trials also resulted in activation of motor exploratory and frontal areas previously associated with inhibition and oculomotor control. In contrast, invalid IOR compared to facilitation trials only activated anterior cortical structures. These results provide support for both attentional and oculomotor theories of IOR and suggest that IOR may be mediated by two networks. One network may mediate the inhibitory bias following an exogenous cue, whereas a separate network may be activated when a response must be made to stimuli that appear in inhibited locations of space.