We have shown in the previous analysis (Section 5.3.2.1) that regions in the fronto-parietal attention network were both activated during orienting of attention to visual and tactile features. These findings suggests that some of these regions might contain populations of supramodal neurons necessary for controlling attention to both modalities, as has been previously proposed in other multisensory attention studies (Macaluso et al., 2002b; Eimer et al., 2002b; Green et al., 2011). However, due to the nature of an fMRI voxel which generally contain millions of neurons (Logothetis, 2008), it is possible that the specific underlying neural populations that were recruited might be unique for each condition. Activations of overlapping but functionally distinct neuronal populations could give rise to a small bias in each voxel, and by pooling these biases across adjacent voxels it may be possible to discriminate the task conditions that were previously inseparable (Kamitani and Tong, 2005; Tong and Pratte, 2012). This principle underlies the MVPA technique that we performed in this analysis for finding potential cortical regions that might contain population coding of modality- or dimension-specific information. A recently published study comparing the sources of spatial and feature-based attention also applied similar logic to investigate whether
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overlapping regions in the frontal-parietal ROIs contain dimension specific information (Greenberg et al., 2010). Using a linear-SVM classifier, they found that common activations in the PPC can significantly classify whether the subject was attending to a location or to a stimulus feature, but this was not the case with common activations in the frontal ROIs.
We applied MVPA searchlight analysis (Kriegeskorte et al., 2006; Kriegeskorte and Bandettini, 2007) to map brain regions that contained modality-specific information, separately for each feature dimension (i.e. shape or frequency). During the attend-shape condition (Figure 5.5a), above chance classification of attended modality was found in bilateral parietal cortex, with discriminative peaks observed in bilateral SMG, spreading posteriorly to the right IPS, and anteriorly to the primary and secondary somatosensory cortices (SI and SII). In frontal regions, a small cluster of informative voxels was also observed in the left IFG. By contrast, fewer distributed activations were found during the attend-frequency condition (Figure 5.5b). Disconnected clusters of discriminative voxels were observed in the right IPS, right SII, right premotor cortex, and around left STS, but almost no activations at all around the SMG in either hemisphere.
This searchlight result raises the interesting possibility that the degree of modality specificity of SMG might also depend on which type of feature requires attention. When attention was directed towards shape, searchlight MVPA revealed that SMG operated in a modality-specific manner. In contrast, even though attention to stimulus frequency also strongly activated bilateral SMG for both modalities (as shown in the result of conjunction analysis), no modality-specific information was found by searchlight analysis in these regions, suggesting possible existence of common supramodal neural mechanism for orienting towards stimulus frequency. The findings of modality-specific regions in SMG during attention to stimulus shape further corroborates previous TMS studies which provide evidence that SMG has a critical role in orienting of attention to visual, but not to somatosensory events (Chambers et al., 2004b). Likewise, in a related study investigating specificity and generality of spatial and feature-based attention in vision, it has been shown that disruption of the SMG only affected strategic orienting to a spatial location, with no impairment observed when attention was oriented to a stimulus colour (Schenkluhn et al., 2008). The present study further extends these TMS studies by suggesting that specific function of the SMG might also be modulated by competing intrinsic features that need to be attended. Although SMG was activated during both attention to shape and frequency, as discussed in the result of univariate analysis in Section 5.3.2.1, MVPA searchlight analyses revealed that this region contained modality-
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specific information only when shape was attended. This result suggests that SMG might be an important hub for top-down regulation of intermodal attention to shape-specific features but not to temporal-features like stimulus frequency.
In contrast, regions in the right IPS were able to classify the attended modality in shape and frequency conditions, showing modality specificity of this region across features dimensions. However, a closer observation revealed that the location of informative clusters in IPS during attention to shape were slightly more posterior than during attention to frequency (Section 5.3.2.3), indicating recruitment of anatomically distinct IPS sub-regions. Furthermore, cross- generalization searchlight analysis classifying the attended modality across both feature dimensions (modality-specific but feature non-specific information; Section 5.3.2.4) uncovered no significant voxels after correction for multiple comparisons. There was a trend towards significance in the right anterior IPS which overlaps with the location of informative clusters in IPS during the attend-shape condition. Together, these results indicate that although there was modality-specific information in the IPS during attention to shape and frequency, their specific anatomical locations were rather distinct, and thus cross- generalization searchlight analysis was unable to detect significant voxels that were both modality-specific and dimension-nonspecific.
Observation of modality-specific yet anatomically distinct region in IPS echoes a recent finding of resting state fMRI reported by Anderson et al. (2010). They investigated the internal architecture of IPS by tracing its connectivity to other brain regions using maximal connectivity clustering, and found that connectivity between IPS and other regions were topographically organized, with each connection specialized to different sensory modalities. The present results are also in line with similarly motivated MVPA study that investigated domain-specific information within overlapping activations in frontoparietal regions elicited by strategic spatial and feature-based attention (Greenberg et al., 2010). They found that region in the posterior parietal cortex (PPC; which includes IPS), but not in the frontal cortex, could reliably distinguished spatial and feature-based attention shifts, suggesting domain- specificity of the PPC regions.
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