9. Anexos
9.10 Anexo 10. Codificación del Hardware
The discovery of similar frontoparietal networks recruited during strategic spatial orienting to various modalities have led several researchers to postulate the existence of supramodal circuits in the dFP regions that biases relevant spatial location in a modality-independent way (Macaluso et al., 2002b; Macaluso, 2010; Smith et al., 2010; Green et al., 2011). We examined this hypothesis by exploiting novel MVPA techniques to discriminate attended modality from the distributed patterns of activity in the overlapping frontoparietal ROIs found by mass univariate analysis. The multivariate classification results revealed that information about attended modality were present in the majority of overlapping frontoparietal ROIs. When participants attended to the left hemifield, modality-specific information could be decoded in the right parietal cortex (that includes IPS), and most of the frontal regions including superior frontal cortex (location of FEF), right middle frontal cortex (DLPFC), and right inferior frontal cortex (IFG). Similarly, when participants attended to the right hemifield, modality-specific information was discovered in the bilateral inferior parietal and right superior parietal cortex, but comparatively less informative regions were found in the frontal cortex, and only the right middle frontal region could significantly classify above chance.
These findings cannot be explained by the selection bias in the analysis (Kriegeskorte et al., 2009; Vul et al., 2009; Vul and Kanwisher, 2010), because independent datasets for defining the ROIs and MVPA analysis were used. In particular, the first half of the dataset were used to functionally define the frontoparietal ROIs using univariate conjunction analysis, while the second half was later used for MVPA classification. Moreover, the functional ROIs were defined in individual brain space rather than normalised space to preserve subject-specific variance in the anatomical locations of the overlapping activations.
In addition to ROI-based MVPA classification, we also independently analysed the imaging data using searchlight analyses to map cortical regions that contain information about the attended modality and/or attended location. In the first searchlight analysis, we looked for
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modality-specific but location-nonspecific information, and found widespread activity spanning across bilateral posterior parietal cortex (PPC), including IPS and SMG/TPJ, extending to superior premotor areas (SMA, FEF) and the IFG. By contrast, searchlight analysis found no evidence for location-specific but modality-nonspecific information in the brain, suggesting the existence of distinct location-specific coding for different modalities.
These findings are inconsistent with the hypothesis of supramodal dFP networks that control the deployment of spatial attention in a modality independent manner (Macaluso, 2010). However, the results of our study are in line with several fMRI and TMS studies that reported domain- and modality- specificity of the PPC regions. Reversible disruptions of right SMG using TMS has been shown to impair strategic orienting of spatial attention in the visual modality but not in the somatosensory modality (Chambers et al., 2004b). Related evidence was reported in an MVPA study investigating the role of frontoparietal networks in controlling shifts of spatial and feature-based attention in vision (Greenberg et al., 2010). They discovered that despite the observation of overlapping activations in the frontoparietal regions, the classifier could still decode domain-specific transient signals in the PPC region. Furthermore, a recent multisensory integration study reported that functional connectivity between IPS and the lower-level sensory regions were weighted by the reliability of the sensory modality (Beauchamp et al., 2010). Using structural equation modelling, they found that connection weights between IPS and somatosensory cortex were significantly increased when tactile stimuli was more informative than visual stimuli, whereas the connection weight between IPS and visual cortex increased when visual stimuli was more informative than tactile stimuli. The findings of the present study further corroborate and complement the previous findings by providing evidence for modality-specific coding in the PPC region during multisensory spatial attention task.
In addition to the PPC region, modality-specific information was also found in the IFG. A growing number of evidence from previous neuroimaging studies have highlighted the involvement of the IFG in various domains, including cognitive control (Duncan and Owen, 2000; Derrfuss et al., 2005), response inhibition (Logan and Cowan, 1984; Aron et al., 2004), and natural language comprehension (Constable et al., 2004; Jung-Beeman, 2005). The findings of modality-specific information in the IFG complements these previous findings and provides additional evidence for the existence of adaptive coding in prefrontal cortex that integrates various cognitive functions like memory, attention, language, and control (Duncan and Owen, 2000; Duncan, 2001; Woolgar et al., 2011a).
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4.4.4 Summary
In the present study, we applied fMRI and MVPA classification to probe the generality and specificity of the frontoparietal attention networks during sustained spatial attention to visual and somatosensory modality. Consistent with previous studies, we found that visual and somatosensory attention elicited widespread overlapping neural activation, consistent with a supramodal selection mechanism. On closer inspection, however, most of these bimodal conjunctions contained fine-grained modality-specific information that cannot be explained by changes in sensory inputs, motor output, or eye position. Searchlight analyses further confirmed that strategic spatial attention can be accounted for by integrated modality-specific processes in the parietal cortex. We still, however, observed some commonly activated regions that contained no modality-specific information in the frontal cortex, implying the existence of potentially supramodal regions. Overall, our findings revealed evidence for modality-specific top-down selection in the parietal cortex, and support the view that spatial attention is driven by modality-specific systems that are anatomically intermingled yet functionally distinct.
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