Functional columns in the primate prefrontal cortex revealed by optical imaging in vitro

Topographic Mapping as a Basic Principle of Functional Organization for Visual and Prefrontal Functional Connectivity

The organization of region-to-region functional connectivity has major implications for understanding information transfer and transformation between brain regions. We extended connective field mapping methodology to 3-D anatomic space to derive estimates of corticocortical functional organization. Using multiple publicly available human (both male and female) resting-state fMRI data samples for model testing and replication analysis, we have three main findings. First, we found that the functional connectivity between early visual regions maintained a topographic relationship along the anterior-posterior dimension, which corroborates previous research. Higher order visual regions showed a pattern of connectivity that supports convergence and biased sampling, which has implications for their receptive field properties. Second, we demonstrated that topographic organization is a fundamental aspect of functional connectivity across the entire cortex, with higher topographic connectivity between regions within a functional network than across networks. The principle gradient of topographic connectivity across the cortex resembled whole-brain gradients found in previous work. Last but not least, we showed that the organization of higher order regions such as the lateral prefrontal cortex demonstrate functional gradients of topographic connectivity and convergence. These organizational features of the lateral prefrontal cortex predict task-based activation patterns, particularly visual specialization and higher order rules. In sum, these findings suggest that topographic input is a fundamental motif of functional connectivity between cortical regions for information processing and transfer, with maintenance of topography potentially important for preserving the integrity of information from one region to another.

Differences in task-phase-dependent time-frequency patterns of local field potentials in the dorsal and ventral regions of the monkey lateral prefrontal cortex

Although the ventral and dorsal regions of the lateral prefrontal cortex (lPFC) are anatomically distinct, their functional differentiation is still controversial. Local field potentials (LFPs) reflect synaptic input and are widely modulated by information from both the external world and the internal state of the brain. However, functional mapping using LFPs has not been fully tested and is expected to provide new insights into their differences. Thus, the present study analyzed the task-phase-dependent modulations of LFPs recorded from the lPFC of monkeys as they performed a shape manipulation task. Hierarchical cluster analyses of the LFP time-frequency spectra revealed characteristic patterns, especially in the theta and low gamma ranges. In particular, the theta range distinguished the ventral and dorsal parts of the lPFC well. The ventral part exhibited a block of similar LFP patterns whereas the dorsal part showed scattered patterns of small or single sites with different LFP patterns. These results suggest that functional segregation within the lPFC, especially between the ventral and dorsal regions, can be evaluated using task-phase-dependent time-frequency modulations in LFPs.

Individual mediodorsal thalamic neurons project to multiple areas of the rat prefrontal cortex: A single neuron-tracing study using virus vectors

The prefrontal cortex has an important role in a variety of cognitive and executive processes, and is generally defined by its reciprocal connections with the mediodorsal thalamic nucleus (MD). The rat MD is mainly subdivided into three segments, the medial (MDm), central (MDc), and lateral (MDl) divisions, on the basis of the cytoarchitecture and chemoarchitecture. The MD segments are known to topographically project to multiple prefrontal areas at the population level: the MDm mainly to the prelimbic, infralimbic, and agranular insular areas; the MDc to the orbital and agranular insular areas; and the MDl to the prelimbic and anterior cingulate areas. However, it is unknown whether individual MD neurons project to single or multiple prefrontal cortical areas. In the present study, we visualized individual MD neurons with Sindbis virus vectors, and reconstructed whole structures of MD neurons. While the main cortical projection targets of MDm, MDc, and MDl neurons were generally consistent with those of previous results, it was found that individual MD neurons sent their axon fibers to multiple prefrontal areas, and displayed various projection patterns in the target areas. Furthermore, the axons of single MD neurons were not homogeneously spread, but were rather distributed to form patchy axon arbors approximately 1 mm in diameter. The multiple-area projections and patchy axon arbors of single MD neurons might be able to coactivate cortical neuron groups in distant prefrontal areas simultaneously. Furthermore, considerable heterogeneity of the projection patterns is likely, to recruit the different sets of cortical neurons, and thus contributes to a variety of prefrontal functions. J. Comp. Neurol. 525:166-185, 2017. © 2016 Wiley Periodicals, Inc.

Distinct Spatiotemporal Activation Patterns of the Perirhinal-Entorhinal Network in Response to Cortical and Amygdala Input

The perirhinal (PER) and entorhinal cortex (EC) receive input from the agranular insular cortex (AiP) and the subcortical lateral amygdala (LA) and the main output area is the hippocampus. Information transfer through the PER/EC network however, is not always guaranteed. It is hypothesized that this network actively regulates the (sub)cortical activity transfer to the hippocampal network and that the inhibitory system is involved in this function. This study determined the recruitment by the AiP and LA afferents in PER/EC network with the use of voltage sensitive dye (VSD) imaging in horizontal mouse brain slices. Electrical stimulation (500 μA) of the AiP induced activity that gradually propagated predominantly in the rostro-caudal direction: from the PER to the lateral EC (LEC). In the presence of 1 μM of the competitive γ-aminobutyric acid (GABA_A) receptor antagonist bicuculline, AiP stimulation recruited the medial EC (MEC) as well. In contrast, LA stimulation (500 μA) only induced activity in the deep layers of the PER. In the presence of bicuculline, the initial population activity in the PER propagated further towards the superficial layers and the EC after a delay. The latency of evoked responses decreased with increasing stimulus intensities (50–500 μA) for both the AiP and LA stimuli. The stimulation threshold for evoking responses in the PER/EC network was higher for the LA than for the AiP. This study showed that the extent of the PER/EC network activation depends on release of inhibition. When GABA_A dependent inhibition is reduced, both the AiP and the LA activate spatially overlapping regions, although in a distinct spatiotemporal fashion. It is therefore hypothesized that the inhibitory network regulates excitatory activity from both cortical and subcortical areas that has to be transmitted through the PER/EC network.

Monitoring Population Membrane Potential Signals from Neocortex

Voltage-sensitive dyes (VSDs) and optical imaging are useful tools for studying spatiotemporal patterns of population neuronal activity in cortex. Because fast VSDs respond to membrane potential changes with microsecond temporal resolution, these are better suited than calcium indicators for recording rapid neural signals. Here we describe methods for using a 464 element photodiode array and fast VSDs to record signals ranging from large scale network activity in brain slices and in vivo mammalian preparations with sensitivity comparable to local field potential (LFP) recordings. With careful control of dye bleaching and phototoxicity, long recording times can be achieved. Absorption dyes have less photo-toxicity than fluorescent dyes. In brain slices, the total recording time in each slice can be 1,000-2,000 s, which can be divided into hundreds of short recording trials over several hours. In intact brains when fluorescent dyes are used, reduced light intensity can also increase recording time. In this chapter, we will discuss technical details for the methods to achieve reliable VSD imaging with high sensitivity and long recording time.

Prefrontal cortical minicolumn: from executive control to disrupted cognitive processing

The prefrontal cortex of the primate brain has a modular architecture based on the aggregation of neurons in minicolumnar arrangements having afferent and efferent connections distributed across many brain regions to represent, select and/or maintain behavioural goals and executive commands. Prefrontal cortical microcircuits are assumed to play a key role in the perception to action cycle that integrates relevant information about environment, and then selects and enacts behavioural responses. Thus, neurons within the interlaminar microcircuits participate in various functional states requiring the integration of signals across cortical layers and the selection of executive variables. Recent research suggests that executive abilities emerge from cortico-cortical interactions between interlaminar prefrontal cortical microcircuits, whereas their disruption is involved in a broad spectrum of neurologic and psychiatric disorders such as autism, schizophrenia, Alzheimer's and drug addiction. The focus of this review is on the structural, functional and pathological approaches involving cortical minicolumns. Based on recent technological progress it has been demonstrated that microstimulation of infragranular cortical layers with patterns of microcurrents derived from supragranular layers led to an increase in cognitive performance. This suggests that interlaminar prefrontal cortical microcircuits are playing a causal role in improving cognitive performance. An important reason for the new interest in cortical modularity comes from both the impressive progress in understanding anatomical, physiological and pathological facets of cortical microcircuits and the promise of neural prosthetics for patients with neurological and psychiatric disorders.

In vitro intrinsic optical imaging can be used for source determination in cortical slices

In the last decades intrinsic optical imaging has become a widely used technique for monitoring activity in vivo and in vitro. It is assumed that in brain slices the source of intrinsic optical signals (IOSs) is the change in light scattering caused by cell swelling or shrinkage. The aim of the present study was to find a correlation between electrical activity and parallel optical characteristics, elicited by 4-aminopyridine-containing or Mg(2+) -free medium in rat cortical brain slices. Electrophysiological signals and reflected light alterations were recorded during spontaneous seizure activity. Current source density (CSD) analysis was performed on the electrophysiological records. Direct correlation analysis of IOS to CSD was made, and source distribution provided by IOS and CSD methods was compared by determining Matthews correlation coefficient. The gradual development of seizure-like activity elicited the reduction of light reflectance. The main findings of our experiments are that long-term epileptiform activity resulted in persistent alteration in IOSs of brain slices. The observed IOS pattern remained stable after 1 h incubation in convulsants. The pattern of IOS shows good correlation with the data obtained from the CSD analysis. Persistent IOS changes provide information about the area-specific changes of basic excitability, which can serve as a background for ictal and interictal-like epileptiform activity. We can conclude that changes in IOSs correlate well with electrophysiological recordings under different conditions. Our experiments provide evidence that underlying synchronised neuronal processes produce parallel alterations in IOSs and electrophysiological activity.

Structure of spike count correlations reveals functional interactions between neurons in dorsolateral prefrontal cortex area 8a of behaving primates

Neurons within the primate dorsolateral prefrontal cortex (dlPFC) are clustered in microcolumns according to their visuospatial tuning. One issue that remains poorly investigated is how this anatomical arrangement influences functional interactions between neurons during behavior. To investigate this question we implanted 4 mm×4 mm multielectrode arrays in two macaques' dlPFC area 8a and measured spike count correlations (r_sc) between responses of simultaneously recorded neurons when animals maintained stationary gaze. Positive and negative r_sc were significantly higher than predicted by chance across a wide range of inter-neuron distances (from 0.4 to 4 mm). Positive r_sc were stronger between neurons with receptive fields (RFs) separated by ≤90° of angular distance and progressively decreased as a function of inter-neuron physical distance. Negative r_sc were stronger between neurons with RFs separated by >90° and increased as a function of inter-neuron distance. Our results show that short- and long-range functional interactions between dlPFC neurons depend on the physical distance between them and the relationship between their visuospatial tuning preferences. Neurons with similar visuospatial tuning show positive r_sc that decay with inter-neuron distance, suggestive of excitatory interactions within and between adjacent microcolumns. Neurons with dissimilar tuning from spatially segregated microcolumns show negative r_sc that increase with inter-neuron distance, suggestive of inhibitory interactions. This pattern of results shows that functional interactions between prefrontal neurons closely follow the pattern of connectivity reported in anatomical studies. Such interactions may be important for the role of the prefrontal cortex in the allocation of attention to targets in the presence of competing distracters.