Photonics meets connectomics: case of diffuse, long-range horizontal projections in rat cortex

Long-Range, Border-Crossing, Horizontal Axon Radiations Are a Common Feature of Rat Neocortical Regions That Differ in Cytoarchitecture

Employing wide-field optical imaging techniques supported by electrophysiological recordings, previous studies have demonstrated that stimulation of a spatially restricted area (point) in the sensory periphery results in a large evoked neuronal activity spread in mammalian primary cortices. In rats' primary cortices, such large evoked spreads extend diffusely in multiple directions, cross cortical cytoarchitectural borders and can trespass into other unimodal sensory areas. These point spreads are supported by a spatially matching, diffuse set of long-range horizontal projections within gray matter that extend in multiple directions and cross borders to interconnect different cortical areas. This horizontal projection system is in addition to well-known area-to-area clustered projections to defined targets through white matter. Could similar two-projection cortical systems also be found in cortical regions that differ in their cytoarchitectural structure? To address this question, an adeno-associated viral vector expressing green fluorescent protein (GFP) was injected as an anterograde tract tracer into granular somatosensory cortex (trunk area), dysgranular cortex (somatosensory dysgranular zone and extrastriate cortex) and agranular motor cortex (MCx). Irrespective of the injection site the same two projection systems were found, and their quantification revealed a close similarity to findings in primary sensory cortices. Following detailed reconstruction, the diffuse horizontal axon radiation was found to possess numerous varicosities and to include short, medium and long axons, the latter extending up to 5.2 mm. These "proof of concept" findings suggest that the similarity of the two projection systems among different cortical areas could potentially constitute a canonical motif of neocortical organization.

Imaging Cajal's neuronal avalanche: how wide-field optical imaging of the point-spread advanced the understanding of neocortical structure-function relationship

This review brings together a collection of studies that specifically use wide-field high-resolution mesoscopic level imaging techniques (intrinsic signal optical imaging; voltage-sensitive dye optical imaging) to image the cortical point spread (PS): the total spread of cortical activation comprising a large neuronal ensemble evoked by spatially restricted (point) stimulation of the sensory periphery (e.g., whisker, pure tone, point visual stimulation). The collective imaging findings, combined with supporting anatomical and electrophysiological findings, revealed some key aspects about the PS including its very large (radius of several mm) and relatively symmetrical spatial extent capable of crossing cytoarchitectural borders and trespassing into other cortical areas; its relationship with underlying evoked subthreshold activity and underlying anatomical system of long-range horizontal projections within gray matter, both also crossing borders; its contextual modulation and plasticity; the ability of its relative spatiotemporal profile to remain invariant to major changes in stimulation parameters; its potential role as a building block for integrative cortical activity; and its ubiquitous presence across various cortical areas and across mammalian species. Together, these findings advance our understanding about the neocortex at the mesoscopic level by underscoring that the cortical PS constitutes a fundamental motif of neocortical structure-function relationship.

A methodological pipeline for serial-section imaging and tissue realignment for whole-brain functional and connectivity assessment

BACKGROUND

Understanding the neurobiological basis of cognition and behavior, and disruptions to these processes following injury and disease, requires a large-scale assessment of neural populations, and knowledge of their patterns of connectivity.

NEW METHOD

We present an analysis platform for large-scale investigation of functional and neuroanatomical connectivity in rodents. Retrograde tracers were injected and in a subset of animals behavioral tests to drive immediate-early gene expression were administered. This approach allows users to perform whole-brain assessment of function and connection in a semi-automated quantitative manner. Brains were cut in the coronal plane, and an image of the block face was acquired. Wide-field fluorescent scans of whole sections were acquired and analyzed using Matlab software.

RESULTS

The toolkit utilized open-source and custom platforms to accommodate a largely automated analysis pipeline in which neuronal boundaries are automatically segmented, the position of segmented neurons are co-registered with a corresponding image acquired during sectioning, and a 3-D representation of neural tracer (and other products) throughout the entire brain is generated.

COMPARISON WITH EXISTING METHODS

Current whole brain connectivity measures primarily target mice and use anterograde tracers. Our focus on segmented units of interest (e.g., NeuN labeled neurons) and restricting measures to these units produces a flexible platform for a variety of whole brain analyses (measuring activation, connectivity, markers of disease, etc.).

CONCLUSIONS

This open-source toolkit allows an investigator to visualize and quantify whole brain data in 3-D, and additionally provides a framework that can be rapidly integrated with user-specific analyses and methodologies.

Long, intrinsic horizontal axons radiating through and beyond rat barrel cortex have spatial distributions similar to horizontal spreads of activity evoked by whisker stimulation

Stimulation of a single whisker evokes a peak of activity that is centered over the associated barrel in rat primary somatosensory cortex, and yet the evoked local field potential and the intrinsic signal optical imaging response spread symmetrically away from this barrel for over 3.5 mm to cross cytoarchitectonic borders into other "unimodal" sensory cortical areas. To determine whether long horizontal axons have the spatial distribution necessary to underlie this activity spread, we injected adeno-associated viral vectors into barrel cortex and characterized labeled axons extending from the injection site in transverse sections of flattened cortex. Combined qualitative and quantitative analyses revealed labeled axons radiating diffusely in all directions for over 3.5 mm from supragranular injection sites, with density declining over distance. The projection pattern was similar at four different cortical depths, including infragranular laminae. Infragranular vector injections produced patterns similar to the supragranular injections. Long horizontal axons were detected both using a vector with a permissive cytomegalovirus promoter to label all neuronal subtypes and using a calcium/calmodulin-dependent protein kinase II α vector to restrict labeling to excitatory cortical pyramidal neurons. Individual axons were successfully reconstructed from series of supragranular sections, indicating that they traversed gray matter only. Reconstructed axons extended from the injection site, left the barrel field, branched, and sometimes crossed into other sensory cortices identified by cytochrome oxidase staining. Thus, radiations of long horizontal axons indeed have the spatial characteristics necessary to explain horizontal activity spreads. These axons may contribute to multimodal cortical responses and various forms of cortical neural plasticity.

Emergence of spatiotemporal invariance in large neuronal ensembles in rat barrel cortex

Invariant sensory coding is the robust coding of some sensory information (e.g., stimulus type) despite major changes in other sensory parameters (e.g., stimulus strength). The contribution of large populations of neurons (ensembles) to invariant sensory coding is not well understood, but could offer distinct advantages over invariance in single cell receptive fields. To test invariant sensory coding in neuronal ensembles evoked by single whisker stimulation as early as primary sensory cortex, we recorded detailed spatiotemporal movies of evoked ensemble activity through the depth of rat barrel cortex using microelectrode arrays. We found that an emergent property of whisker evoked ensemble activity, its spatiotemporal profile, was notably invariant across major changes in stimulus amplitude (up to >200-fold). Such ensemble-based invariance was found for single whisker stimulation as well as for the integrated profile of activity evoked by the more naturalistic stimulation of the entire whisker array. Further, the integrated profile of whisker array evoked ensemble activity and its invariance to stimulus amplitude shares striking similarities to “funneled” tactile perception in humans. We therefore suggest that ensemble-based invariance could provide a robust neurobiological substrate for invariant sensory coding and integration at an early stage of cortical sensory processing already in primary sensory cortex.