Functional characterization of macaque insula using task-based and resting-state fMRI.

Neurophysiological investigations over the past decades have demonstrated the involvement of the primate insula in a wide array of sensory, cognitive, affective and regulatory functions, yet the complex functional organization of the insula remains unclear. Here we examined to what extent non-invasive task-based and resting-state fMRI provides support for functional specialization and integration of sensory and motor information in the macaque insula. Task-based fMRI experiments suggested a functional specialization related to processing of ingestive/taste/distaste information in anterior insula, grasping-related sensorimotor responses in middle insula and vestibular information in posterior insula. Visual stimuli depicting social information involving conspecific`s lip-smacking gestures yielded responses in middle and anterior portions of dorsal and ventral insula, overlapping partially with the sensorimotor and ingestive/taste/distaste fields. Functional specialization/integration of the insula was further corroborated by seed-based whole brain resting-state analyses, showing distinct functional connectivity gradients across the anterio-posterior extent of both dorsal and ventral insula. Posterior insula showed functional correlations in particular with vestibular/optic flow network regions, mid-dorsal insula with vestibular/optic flow as well as parieto-frontal regions of the sensorimotor grasping network, mid-ventral insula with social/affiliative network regions in temporal, cingulate and prefrontal cortices and anterior insula with taste and mouth motor networks including premotor and frontal opercular regions.

Categorization learning induced changes in action representations in the macaque STS.

Neuroimaging and single cell recordings have demonstrated the presence of STS body category-selective regions (body patches) containing neurons responding to presentation of static bodies and body parts. To date, it remains unclear if these body patches and additional STS regions respond during observation of different categories of dynamic actions and to what extent categorization learning influences representations of observed actions in the STS. In the present study, we trained monkeys to discriminate videos depicting three different actions categories (grasping, touching and reaching) with a forced-choice action categorization task. Before and after categorization training, we performed fMRI recordings while monkeys passively observed the same action videos. At the behavioral level, after categorization training, monkeys generalized to untrained action exemplars, in particular for grasping actions. Before training, uni- and/or multivariate fMRI analyses suggest a broad representation of dynamic action categories in particular in posterior and middle STS. Univariate analysis further suggested action category specific training effects in middle and anterior body patches, face patch ML and posterior STS region MT and FST. Overall, our fMRI experiments suggest a widespread representation of observed dynamic bodily actions in the STS that can be modulated by visual learning, supporting its proposed role in action recognition.

Intrinsic functional clustering of the macaque insular cortex.

The functional organization of the primate insula has been studied using a variety of techniques focussing on regional differences in either architecture, connectivity, or function. These complementary methods offered insights into the complex organization of the insula and proposed distinct parcellation schemes at varying levels of detail and complexity. The advent of imaging techniques that allow non-invasive assessment of structural and functional connectivity, has popularized data-driven connectivity-based parcellation methods to investigate the organization of the human insula. Yet, it remains unclear if the subdivisions derived from these data-driven clustering methods reflect meaningful descriptions of the functional specialization of the insula. In this study, we employed hierarchical clustering to examine the cluster parcellations of the macaque insula. As our aim was exploratory, we examined parcellations consisting of two up to ten clusters. Three different cluster validation methods (fingerprinting, silhouette, elbow) converged on a four-cluster solution as the most optimal representation of our data. Examining functional response properties of these clusters, in addition to their brain-wide functional connectivity suggested a functional specialization related to processing gustatory, somato-motor, vestibular and social visual cues. However, a more detailed functional differentiation aligning with previous functional investigations of insula subfields became evident at higher cluster numbers beyond the proposed optimal four clusters. Overall, our findings demonstrate that resting-state-based hierarchical clustering can provide a meaningful description of the insula's functional organization at some level of detail. Nonetheless, cluster parcellations derived from this method are best combined with data obtained through other modalities, to provide a more comprehensive and detailed account of the insula's complex functional organization.