Thalamocortical circuits for the formation of hierarchical pathways in the mammalian visual cortex

External sensory inputs propagate from lower-order to higher-order brain areas, and the hierarchical neural network supporting this information flow is a fundamental structure of the mammalian brain. In the visual system, multiple hierarchical pathways process different features of the visual information in parallel. The brain can form this hierarchical structure during development with few individual differences. A complete understanding of this formation mechanism is one of the major goals of neuroscience. For this purpose, it is necessary to clarify the anatomical formation process of connections between individual brain regions and to elucidate the molecular and activity-dependent mechanisms that instruct these connections in each areal pair. Over the years, researchers have unveiled developmental mechanisms of the lower-order pathway from the retina to the primary visual cortex. The anatomical formation of the entire visual network from the retina to the higher visual cortex has recently been clarified, and higher-order thalamic nuclei are gaining attention as key players in this process. In this review, we summarize the network formation process in the mouse visual system, focusing on projections from the thalamic nuclei to the primary and higher visual cortices, which are formed during the early stages of development. Then, we discuss how spontaneous retinal activity that propagates through thalamocortical pathways is essential for the formation of corticocortical connections. Finally, we discuss the possible role of higher-order thalamocortical projections as template structures in the functional maturation of visual pathways that process different visual features in parallel

Cell Type Specific Representation of Vibro-tactile Stimuli in the Mouse Primary Somatosensory Cortex

Although the processing of whisker deflections in the barrel area of the rodent primary somatosensory cortex (S1) has been studied extensively, how cutaneous vibro-tactile stimuli are processed in the rodent S1 outside the barrel area has not been fully examined. Particularly, the cell-type specific representation of multiple vibration frequencies in genetically identified inhibitory cells in the S1 has not been examined. Using two-photon calcium imaging, we examined the responses to vibration stimuli of excitatory and inhibitory neurons in the S1 hind limb area of male and female mice. The excitatory cells showed relatively sharp selectivity to vibration stimuli, whereas the inhibitory cells exhibited less selectivity. The excitatory and inhibitory cells with different preferred stimuli were intermingled in a “salt and pepper” manner. Furthermore, the noise correlation tended to be especially strong in excitatory-inhibitory and inhibitory-inhibitory cell pairs that have similar stimulus selectivity. These results suggest that excitatory cells tend to represent specific stimulus information and work together with similarly tuned inhibitory cells as a functionally connected network

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

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

SHORT COMMUNICATION Temporal characterization of memory retrieval processes: an fMRI study of the`tip of the tongue' phenomenon

Abstract Tip of the tongue' (TOT) is a natural phenomenon in which people cannot retrieve a target word immediately, even though they feel con®dent that they know the target. This provides us an opportunity to understand the human memory system, because cognitive components of memory retrieval such as retrieval effort and successful retrieval are temporally dissociated from each other during the TOT states. The purpose of the present study was to reveal the neural correlates of the cognitive components of the retrieval process by separating cognitive phases of the TOT phenomenon using event-related functional magnetic resonance imaging with multiple regression analysis. We demonstrated that the left dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex were activated at the time of successful retrieval, and the left DLPFC also showed activation when the subjects successfully retrieved the target names as compared to when they gave up. This result suggests that the left DLPFC is speci®c to the successful retrieval process. During the TOT state, a number of regions were activated, and this suggests that widely distributed brain regions are engaged when people make a hard effort to retrieve a proper name in the TOT state. Our new approach employing temporal resolution of the TOT phenomenon may contribute to the understanding of the mechanisms of the human memory system

大脳皮質の機能と局所回路

過去50年にわたり, 視覚野は, 大脳皮質の神経回路のモデル系として研究されてきた. 近年の技術の進歩により, 神経回路の機能構築を細胞レベルで研究することが可能になった. この総説では, 大脳皮質の局所回路について, 現在何が問題になっているかを説明するとともに, 技術の進歩によりもたらされた最新の研究結果について解説する

Cell-Type-Specific Thalamocortical Inputs Constrain Direction Map Formation in Visual Cortex

Summary: Finding the relationship between individual cognitive functions and cell-type-specific neuronal circuits is a central topic in neuroscience. In cats, the lateral geniculate nucleus (LGN) contains several cell types carrying spatially and temporally precise visual information. Whereas LGN cell types lack selectivity for motion direction, neurons in the primary visual cortex (area 17) exhibit sharp direction selectivity. Whether and how such de novo formation of direction selectivity depends on LGN cell types remains unknown. Here, we addressed this question using in vivo two-photon calcium imaging in cat area 17, which consists of two compartments receiving different combinations of inputs from the LGN cell types. The direction map in area 17 showed unique fragmented organization and was present only in small and distributed cortical domains. Moreover, direction-selective domains preferentially localized in specific compartments receiving Y and W inputs carrying low spatial frequency visual information, indicating that cell-type-specific thalamocortical projections constrain the formation of direction selectivity. : Nishiyama et al. find fragmented organization of direction columns in the cat primary visual cortex. The direction columns are predominantly located in cortical domains preferring low spatial frequency. The results suggest that cell-type-specific thalamocortical projections from LGN may constrain direction column formation in the cat visual cortex. Keywords: brain mapping, in vivo two-photon imaging, functional columns, direction selectivity, thalamus, visual cortex, thalamocortical circui