Teppei Matsui

Functional Connectivity between Anatomically Unconnected Areas Is Shaped by Collective Network-Level Effects in the Macaque Cortex

Coherent spontaneous blood oxygen level-dependent (BOLD) fluctuations have been intensely investigated as a measure of functional connectivity (FC) in the primate neocortex. BOLD-FC is commonly assumed to be constrained by the underlying anatomical connectivity (AC); however, cortical area pairs with no direct AC can also have strong BOLD-FC. On the mechanism generating FC in the absence of direct AC, there are 2 possibilities: 1) FC is determined by signal flows via short connection patterns, such as serial relays and common afferents mediated by a third area; 2) FC is shaped by collective effects governed by network properties of the cortex. In this study, we conducted functional magnetic resonance imaging in anesthetized macaque monkeys and found that BOLD-FC between unconnected areas depends less on serial relays through a third area than on common afferents and, unexpectedly, common efferents, which does not match the first possibility. By utilizing a computational model for interareal BOLD-FC network, we show that the empirically detected AC-FC relationships reflect the configuration of network building blocks (motifs) in the cortical anatomical network, which supports the second possibility. Our findings indicate that FC is not determined solely by interareal short connection patterns but instead is substantially influenced by the network-level cortical architecture.

Direct Comparison of Spontaneous Functional Connectivity and Effective Connectivity Measured by Intracortical Microstimulation: An fMRI Study in Macaque Monkeys

Correlated spontaneous activity in the resting brain is increasingly recognized as a useful index for inferring underlying functional-anatomic architecture. However, despite efforts for comparison with anatomical connectivity, neuronal origin of intrinsic functional connectivity (inFC) remains unclear. Conceptually, the source of inFC could be decomposed into causal components that reflect the efficacy of synaptic interactions and other components mediated by collective network dynamics (e.g., synchronization). To dissociate these components, it is useful to introduce another connectivity measure such as effective connectivity, which is a quantitative measure of causal interactions. Here, we present a direct comparison of inFC against emEC (effective connectivity probed with electrical microstimulation [EM]) in the somatosensory system of macaque monkeys. Simultaneous EM and functional magnetic resonance imaging revealed strong emEC in several brain regions in a manner consistent with the anatomy of somatosensory system. Direct comparison of inFC and emEC revealed colocalization and overall positive correlation within the stimulated hemisphere. Interestingly, we found characteristic differences between inFC and emEC in their interhemispheric patterns. Our results suggest that intrahemispheric inFC reflects the efficacy of causal interactions, whereas interhemispheric inFC may arise from interactions akin to network-level synchronization that is not captured by emEC.

Transient neuronal coactivations embedded in globally propagating waves underlie resting-state functional connectivity.

Significance Resting-state functional connectivity (FC) is a commonly used method in neuroimaging to noninvasively study network organization of brains in humans and other animals. FC is reproducible across different institutions and sensitive enough to detect network changes due to psychiatric disorders. FC is thus a core tool for projects such as the Human Connectome Project. However, because hemodynamic signals are an indirect measure of neuronal activity, actual spatiotemporal neuronal activity underlying FC is still unknown. This study used simultaneous wide-field optical imaging of neuronal calcium signals and hemodynamic signals in transgenic mice to understand the spatiotemporal neuronal dynamics underlying FC. Transient spatial patterns of neuronal coactivations embedded within waves of activity propagating across neocortex were found to be particularly important for FC. Resting-state functional connectivity (FC), which measures the correlation of spontaneous hemodynamic signals (HemoS) between brain areas, is widely used to study brain networks noninvasively. It is commonly assumed that spatial patterns of HemoS-based FC (Hemo-FC) reflect large-scale dynamics of underlying neuronal activity. To date, studies of spontaneous neuronal activity cataloged heterogeneous types of events ranging from waves of activity spanning the entire neocortex to flash-like activations of a set of anatomically connected cortical areas. However, it remains unclear how these various types of large-scale dynamics are interrelated. More importantly, whether each type of large-scale dynamics contributes to Hemo-FC has not been explored. Here, we addressed these questions by simultaneously monitoring neuronal calcium signals (CaS) and HemoS in the entire neocortex of mice at high spatiotemporal resolution. We found a significant relationship between two seemingly different types of large-scale spontaneous neuronal activity—namely, global waves propagating across the neocortex and transient coactivations among cortical areas sharing high FC. Different sets of cortical areas, sharing high FC within each set, were coactivated at different timings of the propagating global waves, suggesting that spatial information of cortical network characterized by FC was embedded in the phase of the global waves. Furthermore, we confirmed that such transient coactivations in CaS were indeed converted into spatially similar coactivations in HemoS and were necessary to sustain the spatial structure of Hemo-FC. These results explain how global waves of spontaneous neuronal activity propagating across large-scale cortical network contribute to Hemo-FC in the resting state.

MRI-based localization of electrophysiological recording sites within the cerebral cortex at single-voxel accuracy.

The localization of microelectrode recording sites in the layers of primate cerebral cortex permits the analysis of relationships between recorded neuronal activities and underlying anatomical connections. We present a magnetic resonance imaging method for precise in vivo localization of cortical recording sites. In this method, the susceptibility-induced effect thickens the appearance of the microelectrode and enhances the detectability of the microelectrode tip, which usually occupies less than a few percent of the volume of an image voxel. In a phantom study, the optimized susceptibility-induced effect allowed tip detection with single-voxel accuracy (in-plane resolution, 50 μm). We applied this method to recording microelectrodes inserted into the brains of macaque monkeys, and localized the microelectrode tip at an in-plane resolution of 150 μm within the cortex of 2–3 mm in thickness. Subsequent histological analyses validated the single-voxel accuracy of the in vivo tip localization. This method opens up a way to investigate information flow during cognitive processes in the brain.

Functional differentiation of memory retrieval network in macaque posterior parietal cortex.

Human fMRI studies revealed involvement of the posterior parietal cortex (PPC) during memory retrieval. However, corresponding memory-related regions in macaque PPC have not been established. In this monkey fMRI study, comparisons of cortical activity during correct recognition of previously seen items and rejection of unseen items revealed two major PPC activation sites that were differentially characterized by a serial probe recognition paradigm: area PG/PGOp in inferior parietal lobule, along with the hippocampus, was more active for initial item retrieval, while area PEa/DIP in intraparietal sulcus was for the last item. Effective connectivity analyses revealed that connectivity from hippocampus to PG/PGOp, but not to PEa/DIP, increased during initial item retrieval. The two parietal areas with differential serial probe recognition profiles were embedded in two different subnetworks of the brain-wide retrieval-related regions. These functional dissociations in the macaque PPC imply the functional correspondence of retrieval-related PPC networks in macaques and humans.

Target dependence of orientation and direction selectivity of corticocortical projection neurons in the mouse V1

Higher order visual areas that receive input from the primary visual cortex (V1) are specialized for the processing of distinct features of visual information. However, it is still incompletely understood how this functional specialization is acquired. Here we used in vivo two photon calcium imaging in the mouse visual cortex to investigate whether this functional distinction exists at as early as the level of projections from V1 to two higher order visual areas, AL and LM. Specifically, we examined whether sharpness of orientation and direction selectivity and optimal spatial and temporal frequency of projection neurons from V1 to higher order visual areas match with that of target areas. We found that the V1 input to higher order visual areas were indeed functionally distinct: AL preferentially received inputs from V1 that were more orientation and direction selective and tuned for lower spatial frequency compared to projection of V1 to LM, consistent with functional differences between AL and LM. The present findings suggest that selective projections from V1 to higher order visual areas initiates parallel processing of sensory information in the visual cortical network.

Wide-field Ca2+ imaging reveals visually evoked activity in the retrosplenial area

Due to recent advances of genetic manipulation, mouse brain has become a useful model for studying brain function, which demands whole brain functional mapping techniques in the mouse brain. In the present study, to finely map visual responsive areas in the mouse brain, we combined high-resolution wide-field optical imaging with transgenic mice containing the genetically encoded Ca2+ indicator, GCaMP3. With the high signal amplitude of GCaMP3 expressing in excitatory neurons, this system allowed neural activity to be observed with relatively fine spatial resolution and cell-type specificity. To evaluate this system, we examined whether non-visual areas exhibited a visual response over the entire surface of the mouse hemisphere. We found that two association areas, the retrosplenial area (RS) and secondary motor/anterior cingulate area (M2/AC), were significantly responsive to drifting gratings. Examination using gratings with distinct spatiotemporal frequency parameters revealed that the RS strongly responded to high-spatial and low-temporal frequency gratings. The M2/AC exhibited a response property similar to that of the RS, though it was not statistically significant. Finally, we performed cellular imaging using two-photon microscopy to examine orientation and direction selectivity of individual neurons, and found that a minority of neurons in the RS clearly showed visual responses sharply selective for orientation and direction. These results suggest that neurons in RS encode visual information of fine spatial details in images. Thus, the present study shows the usefulness of the functional mapping method using a combination of wide-field and two-photon Ca2+ imaging, which allows for whole brain mapping with high spatiotemporal resolution and cell-type specificity.

fMRI Activity in the Macaque Cerebellum Evoked by Intracortical Microstimulation of the Primary Somatosensory Cortex: Evidence for Polysynaptic Propagation

Simultaneous electrical microstimulation (EM) and functional magnetic resonance imaging (fMRI) is a useful tool for probing connectivity across brain areas in vivo. However, it is not clear whether intracortical EM can evoke blood-oxygenation-level-dependent (BOLD) signal in areas connected polysynaptically to the stimulated site. To test for the presence of the BOLD activity evoked by polysynaptic propagation of the EM signal, we conducted simultaneous fMRI and EM in the primary somatosensory cortex (S1) of macaque monkeys. We in fact observed BOLD activations in the contralateral cerebellum which is connected to the stimulation site (i.e. S1) only through polysynaptic pathways. Furthermore, the magnitude of cerebellar activations was dependent on the current amplitude of the EM, confirming the EM is the cause of the cerebellar activations. These results suggest the importance of considering polysynaptic signal propagation, particularly via pathways including subcortical structures, for correctly interpreting ‘functional connectivity’ as assessed by simultaneous EM and fMRI.

Functional segregation and development of mouse higher visual areas

Recent studies suggest that higher visual areas (HVAs) in the mouse visual cortex are segregated anatomically into two visual streams, likely analogous to the ventral and dorsal streams in primates. However, HVAs in mice have yet to be characterized functionally. Moreover, it is unknown when the functional segregation of HVAs occurs during development. Here, we investigated spatiotemporal selectivity of HVAs and their development using wide-field calcium imaging. We found that lateral HVAs in the anatomical ventral stream shared similar spatiotemporal selectivity, whereas the spatiotemporal selectivity of anterior and medial HVAs in the anatomical dorsal stream was not uniform and these areas were segregated functionally into multiple groups. This functional segregation of HVAs developed and reached an adult-like pattern ∼10 d after eye opening (EO). These results suggest, not only the functional segregation of ventral and dorsal streams, but also the presence of multiple substreams in the dorsal stream, and indicate that the functional segregation of visual streams occurs gradually after EO. SIGNIFICANCE STATEMENT Investigation of the spatiotemporal selectivity of nine higher visual areas (HVAs) in adult and developing mice revealed that lateral HVAs belonging to the putative ventral stream shared similar spatiotemporal selectivity, whereas the spatiotemporal selectivity of anterior and medial HVAs belonging to the putative dorsal stream was not uniform and these areas were segregated functionally into multiple groups. These results suggest the presence of multiple substreams within the putative dorsal stream for visuospatial processing. Furthermore, we found that initially immature functional segregation among HVAs developed to an adult-like pattern ∼10 d after eye opening. These results provide a foundation for using mouse HVAs as a model to understand parallel processing and its developmental mechanism.

Mouse optical imaging for understanding resting-state functional connectivity in human fMRI

ABSTRACT Resting-state functional connectivity (FC), which measures the temporal correlation of spontaneous hemodynamic activity between distant brain areas, is a widely accepted method in functional magnetic resonance imaging (fMRI) to assess the connectome of healthy and diseased human brains. A common assumption underlying FC is that it reflects the temporal structure of large-scale neuronal activity that is converted into large-scale hemodynamic activity. However, direct observation of such relationship has been difficult. In this commentary, we describe our recent progress regarding this topic. Recently, transgenic mice that express a genetically encoded calcium indicator (GCaMP) in neocortical neurons are enabling the optical recording of neuronal activity in large-scale with high spatiotemporal resolution. Using these mice, we devised a method to simultaneously monitor neuronal and hemodynamic activity and addressed some key issues related to the neuronal basis of FC. We propose that many important questions about human resting-state fMRI can be answered using GCaMP expressing transgenic mice as a model system.