Modular activation and suppression of neocortical activity in the monkey revealed by optical imaging

Stimulation-Evoked Effective Connectivity (SEEC): An in-vivo approach for defining mesoscale corticocortical connectivity

BACKGROUND

Cortical electrical stimulation is a versatile technique for examining the structure and function of cortical regions and for implementing novel therapies. While electrical stimulation has been used to examine the local spread of neural activity, it may also enable longitudinal examination of mesoscale interregional connectivity.

NEW METHOD

Here, we sought to use intracortical microstimulation (ICMS) in conjunction with recordings of multi-unit action potentials to assess the mesoscale effective connectivity within sensorimotor cortex. Neural recordings were made from multielectrode arrays placed into sensory, motor, and premotor regions during surgical experiments in three squirrel monkeys. During each recording, single-pulse ICMS was repeatably delivered to a single region. Mesoscale effective connectivity was calculated from ICMS-evoked changes in multi-unit firing.

RESULTS

Multi-unit action potentials were able to be detected on the order of 1 ms after each ICMS pulse. Across sensorimotor regions, short-latency (< 2.5 ms) ICMS-evoked neural activity strongly correlated with known anatomical connections. Additionally, ICMS-evoked responses remained stable across the experimental period, despite small changes in electrode locations and anesthetic state.

COMPARISON WITH EXISTING METHODS

Previous imaging studies investigating cross-regional responses to stimulation are limited to utilizing indirect hemodynamic responses and thus lack the temporal specificity of ICMS-evoked responses.

CONCLUSIONS

These results show that monitoring ICMS-evoked neural activity, in a technique we refer to as Stimulation-Evoked Effective Connectivity (SEEC), is a viable way to longitudinally assess effective connectivity, enabling studies comparing the time course of connectivity changes with the time course of changes in behavioral function.

Mapping mesoscale cortical connectivity in monkey sensorimotor cortex with optical imaging and microstimulation

To map in vivo cortical circuitry at the mesoscale, we applied a novel approach to map interareal functional connectivity. Electrical intracortical microstimulation (ICMS) in conjunction with optical imaging of intrinsic signals (OIS) was used map functional connections in somatosensory cortical areas in anesthetized squirrel monkeys. ICMS produced activations that were focal and that displayed responses which were stimulation intensity dependent. ICMS in supragranular layers of Brodmann Areas 3b, 1, 2, 3a, and M1 evoked interareal activation patterns that were topographically appropriate and appeared consistent with known anatomical connectivity. Specifically, ICMS revealed Area 3b connections with Area 1; Area 1 connections with Areas 2 and 3a; Area 2 connections with Areas 1, 3a, and M1; Area 3a connections with Areas M1, 1, and 2; and M1 connections with Areas 3a, 1, and 2. These somatosensory connectivity patterns were reminiscent of feedforward patterns observed anatomically, although feedback contributions are also likely present. Further consistent with anatomical connectivity, intra-areal and intra-areal patterns of activation were patchy with patch sizes of 200-300 μm. In summary, ICMS with OIS is a novel approach for mapping interareal and intra-areal connections in vivo. Comparisons with feedforward and feedback anatomical connectivity are discussed.

Columnar connectome: toward a mathematics of brain function

Understanding brain networks is important for many fields, including neuroscience, psychology, medicine, and artificial intelligence. To address this fundamental need, there are multiple ongoing connectome projects in the United States, Europe, and Asia producing brain connection maps with resolutions at macro- and microscales. However, still lacking is a mesoscale connectome. This viewpoint (1) explains the need for a mesoscale connectome in the primate brain (the columnar connectome), (2) presents a new method for acquiring such data rapidly on a large scale, and (3) proposes how one might use such data to achieve a mathematics of brain function.

Plasticity of the Primate Prefrontal Cortex

Cortical plasticity refers to flexible and long-lasting changes in neuronal circuitry and information processing, which is caused by learning and experience. Although cortical plasticity can be observed in every cortex of the brain, the plasticity of the prefrontal cortex (PFC) is particularly important because the PFC is involved in various cognitive functions, and its plasticity could lead to adaptive changes in the use of other brain regions. Cortical plasticity occurs at several levels, from functional molecules to the organization of large areas of the brain. Here, the authors focus mainly on the development and remodeling of the functional and structural organization of the primate PFC. They discuss how the columnar modules of the PFC develop in the immature brain, how these modules form a “cognitive field” that is responsible for a specific cognitive function, how the cognitive field could be reorganized by training in the mature brain, and how monoaminergic systems contribute to these various levels of plasticity. They suggest that monoaminergic systems, especially the dopaminergic system, are involved in various levels of cortical plasticity, such as behavioral learning and learning-dependent cortical remodeling, thereby contributing to the reorganization of the cognitive field in the primate PFC. NEUROSCIENTIST 13(3):229—240, 2007.

Microelectrode array stimulation combined with intrinsic optical imaging: A novel tool for functional brain mapping

BACKGROUND

Functional brain mapping via cortical microstimulation is a widely used clinical and experimental tool. However, data are traditionally collected point by point, making the technique very time consuming. Moreover, even in skilled hands, consistent penetration depths are difficult to achieve. Finally, the effects of microstimulation are assessed behaviorally, with no attempt to capture the activity of the local cortical circuits being stimulated.

NEW METHOD

We propose a novel method for functional brain mapping, which combines the use of a microelectrode array with intrinsic optical imaging. The precise spacing of electrodes allows for fast, accurate mapping of the area of interest in a regular grid. At the same time, the optical window allows for visualization of local neural connections when stimulation is combined with intrinsic optical imaging.

RESULTS

We demonstrate the efficacy of our technique using the primate motor cortex as a sample application, using a combination of microstimulation, imaging and electrophysiological recordings during wakefulness and under anesthesia. Comparison with current method: We find the data collected with our method is consistent with previous data published by others. We believe that our approach enables data to be collected faster and in a more consistent fashion and makes possible a number of studies that would be difficult to carry out with the traditional approach.

CONCLUSIONS

Our technique allows for simultaneous modulation and imaging of cortical sensorimotor networks in wakeful subjects over multiple sessions which is highly desirable for both the study of cortical organization and the design of brain machine interfaces.

Computational modeling of direct neuronal recruitment during intracortical microstimulation in somatosensory cortex

OBJECTIVE

Electrical stimulation of cortical tissue could be used to deliver sensory information as part of a neuroprosthetic device, but current control of the location, resolution, quality, and intensity of sensations elicited by intracortical microstimulation (ICMS) remains inadequate for this purpose. One major obstacle to resolving this problem is the poor understanding of the neural activity induced by ICMS. Even with new imaging methods, quantifying the activity of many individual neurons within cortex is difficult.

APPROACH

We used computational modeling to examine the response of somatosensory cortex to ICMS. We modeled the axonal arbors of eight distinct morphologies of interneurons and seven types of pyramidal neurons found in somatosensory cortex and identified their responses to extracellular stimulation. We then combined these axonal elements to form a multi-layered slab of simulated cortex and investigated the patterns of neural activity directly induced by ICMS. Specifically we estimated the number, location, and variety of neurons directly recruited by stimulation on a single penetrating microelectrode.

MAIN RESULTS

The population of neurons activated by ICMS was dependent on both stimulation strength and the depth of the electrode within cortex. Strikingly, stimulation recruited interneurons and pyramidal neurons in very different patterns. Interneurons are primarily recruited within a dense, continuous region around the electrode, while pyramidal neurons were recruited in a sparse fashion both near the electrode and up to several millimeters away. Thus ICMS can lead to an unexpectedly complex spatial distribution of firing neurons.

SIGNIFICANCE

These results lend new insights to the complexity and range of neural activity that can be induced by ICMS. This work also suggests mechanisms potentially responsible for the inconsistency and unnatural quality of sensations initiated by ICMS. Understanding these mechanisms will aid in the design of stimulation that can be used to generate effective sensory feedback for neuroprosthetic devices.

Optical imaging of cortical networks via intracortical microstimulation

Understanding cortical organization is key to understanding brain function. Distinct neural networks underlie the functional organization of the cerebral cortex; however, little is known about how different nodes in the cortical network interact during perceptual processing and motor behavior. To study cortical network function we examined whether the optical imaging of intrinsic signals (OIS) reveals the functional patterns of activity evoked by electrical cortical microstimulation. We examined the effects of current amplitude, train duration, and depth of cortical stimulation on the hemodynamic response to electrical microstimulation (250-Hz train, 0.4-ms pulse duration) in anesthetized New World monkey somatosensory cortex. Electrical stimulation elicited a restricted cortical response that varied according to stimulation parameters and electrode depth. Higher currents of stimulation recruited more areas of cortex than smaller currents. The largest cortical responses were seen when stimulation was delivered around cortical layer 4. Distinct local patches of activation, highly suggestive of local projections, around the site of stimulation were observed at different depths of stimulation. Thus we find that specific electrical stimulation parameters can elicit activation of single cortical columns and their associated columnar networks, reminiscent of anatomically labeled networks. This novel functional tract tracing method will open new avenues for investigating relationships of local cortical organization.

Embedding of cortical representations by the superficial patch system

Pyramidal cells in layers 2 and 3 of the neocortex of many species collectively form a clustered system of lateral axonal projections (the superficial patch system--Lund JS, Angelucci A, Bressloff PC. 2003. Anatomical substrates for functional columns in macaque monkey primary visual cortex. Cereb Cortex. 13:15-24. or daisy architecture--Douglas RJ, Martin KAC. 2004. Neuronal circuits of the neocortex. Annu Rev Neurosci. 27:419-451.), but the function performed by this general feature of the cortical architecture remains obscure. By comparing the spatial configuration of labeled patches with the configuration of responses to drifting grating stimuli, we found the spatial organizations both of the patch system and of the cortical response to be highly conserved between cat and monkey primary visual cortex. More importantly, the configuration of the superficial patch system is directly reflected in the arrangement of function across monkey primary visual cortex. Our results indicate a close relationship between the structure of the superficial patch system and cortical responses encoding a single value across the surface of visual cortex (self-consistent states). This relationship is consistent with the spontaneous emergence of orientation response-like activity patterns during ongoing cortical activity (Kenet T, Bibitchkov D, Tsodyks M, Grinvald A, Arieli A. 2003. Spontaneously emerging cortical representations of visual attributes. Nature. 425:954-956.). We conclude that the superficial patch system is the physical encoding of self-consistent cortical states, and that a set of concurrently labeled patches participate in a network of mutually consistent representations of cortical input.

Optical imaging of rat prefrontal neuronal activity evoked by stimulation of the ventral tegmental area

Using a voltage-sensitive dye, the spatiotemporal dynamics of prefrontal neuronal activity evoked by electrical stimulation of the ventral tegmental area were visualized through optical imaging in anaesthetized rats. Even single-pulse stimulation of the ventral tegmental area elicited a widespread wave of depolarization followed by hyperpolarization in the dorsomedial shoulder region of the prefrontal cortex. We also examined the contribution of dopaminergic transmission to the optical signals by comparing normal and 6-hydroxydopamine-lesioned rats. The 6-hydroxydopamine lesions of ventral tegmental area resulted in a complete absence of depolarization in the prefrontal cortex, although hyperpolarization was preserved. These results indicate that dopaminergic neurons are needed to generate excitatory responses in the prefrontal cortex.

The role of short-term depression in sustained neural activity in the prefrontal cortex: a simulation study

Recent experimental researches have suggested that sustained neural activity in the prefrontal cortex is a process of memory retention in decision making. Previous theoretical studies indicate that a balance between recurrent excitation and feedback inhibition is important for sustaining the activity. To investigate a plausible balancing mechanism, we simulated a biophysically realistic network model. Our model shows that short-term depression (STD) enables the network to sustain its activity despite the presence of long-term inhibition by GABA(B) receptors and that the sustained firing rates have a bell-shaped dependence on the degree of STD. By analyzing the neural network dynamics, we show that the bell-shaped dependence on STD is formed by destabilizing the balance with either excessive or insufficient STD. We also show that the optimal degree of STD has a linear relationship with the neural network size. These results suggest that STD provides a balancing mechanism and controls levels of sustained activities of various size networks.

Rule-dependent shifting of sensorimotor representation in the primate prefrontal cortex

When we react to the outer world, perceived sensory information is frequently memorized over a temporal interval then transformed into a motor command based on a behavioural rule. In this type of memory-based sensorimotor transformation, working memory is considered to play an important role. It has been suggested that the lateral prefrontal cortex is involved in the process of the working memory. However, the neuronal mechanism for guiding a motor command from the working memory has not been established. To examine how visuospatial working memory is linked with a forthcoming saccade direction, we used an antisaccade paradigm for monkeys in which a behavioural rule was presented in the middle of a delay period. In this task, the subjects were required to maintain cue location and to select a response based on a behavioural rule. We found that a subset of mnemonic neurons in the lateral prefrontal cortex changed their representation from cue to saccade direction. Furthermore, the discriminability for saccade direction of these neurons tended to appear soon after the behavioural rule presentation, indicating their significant dependency on the behavioural rule. These results suggest that a subset of mnemonic neurons in the lateral prefrontal cortex change their activity depending on a behavioural rule to guide a prospective motor command.

Dynamic depolarization fields in the cerebral cortex

Recent physiological evidence shows that in response to stimuli and preceding motor activity, large fields of the upper layers of the cerebral cortex depolarize. It is argued that this finding is a general one and that these dynamic depolarization fields represent the computational elements of the cerebral cortex. Each depolarization field engages many more neurons than do columns and hyper-columns. These fields can be explained by cooperative neuronal computing in layers I-III of the cortex. In these layers, the computing modes might be general for all parts of the cerebral cortex and be sufficiently flexible to handle all sorts of cortical computations, including perception, memory storage, memory retrieval, thought and the production of behavior.

Real and illusory contour processing in area V1 of the primate: a cortical balancing act

It is known that neurons in area V2 (the second visual area) can signal the orientation of illusory contours in the primate. Whether area V1 (primary visual cortex) can signal illusory contour orientation is more controversial. While some electrophysiology studies have ruled out illusory signaling in V1, other reports suggest that V1 shows some illusory-specific response. Here, using optical imaging and single unit electrophysiology, we report that primate V1 does show an orientation-specific response to the 'abutting line grating' illusory contour. However, this response does not signal an illusory contour in the conventional sense. Rather, we find that illusory contour stimulation leads to an activation map that, after appropriate subtraction of real line signal, is inversely related to the real orientation map. The illusory contour orientation is thus negatively signaled or de-emphasized in V1. This 'activation reversal' is robust, is not due merely to presence of line ends, is not dependent on inducer orientation, and is not due to precise position of line end stimulation of V1 cells. These data suggest a resolution for previous apparently contradictory experimental findings. We propose that the de-emphasis of illusory contour orientation in V1 may be an important signal of contour identity and may, together with illusory signal from V2, provide a unique signature for illusory contour representation.

Unmasking of silent "task-related" neuronal activity in the monkey prefrontal cortex by a GABA(A) antagonist

To examine the role of GABA on prefrontal neuronal activity in the control of behavior, a GABA(A) receptor antagonist, bicuculline methiodite (BMI), was iontophoretically applied to prefrontal neurons while monkeys performed a visual reaction time task. Iontophoretic application of BMI uncovered "task-related" activity of silent neurons (n=40), which did not show any activity during performance of the task. The distribution, by type, of these silent "task-related" neurons differed from that of standard (i.e. active) task-related neurons (N=95), and a particular type of silent "task-related" neuron was found most frequently. These findings suggest that GABA continuously and preferentially suppresses neuronal activity via GABA(A) receptors to limit the population of prefrontal neurons related to behavior, thereby organizing neuronal activities for behavior mediated by the prefrontal cortex.

In vivo spectrometric calcium flux recordings of intrinsic Caudate-Putamen cells and transplanted IMR-32 neuroblastoma cells using miniature fiber optrodes in anesthetized and awake rats and monkeys

A method is described to enable the recording of transient intracellular calcium changes in deep brain structures in anesthetized and awake animals using a fluorescent indicator combined with in vivo optical detection methods. Optrodes were fabricated using a bifurcated fiber-optic cable with an attached infusion guide cannula. After intracranial implantation of an optrode, animals were prepared in the following manner, (1) rats (intra-striatal) and monkeys (intra-putamen) were infused with the fluorescent calcium indicator, Oregon Green, to load intrinsic cells; or (2) rats were intra-striatally transplanted with a slurry of dye-loaded IMR-32 neuroblastoma cells via pipette ejection. Excitation light from an argon-ion laser was launched through the optrode and passed into the tissue. The resulting calcium-induced fluorescence signals were captured by the optrode, then detected and processed by externalized photomultiplier- and CCD-based spectrometer electronics. In approximately 25% of all intrinsic cell recordings, the baseline fluorescence intensity was relatively stable over time whereas in the remainder, large amplitude oscillations were observed with a frequency in the range of 0.5-2 Hz. These Ca(2+) transients were inhibited by local infusion of 10 microM omega-conotoxin MVIIC and 1 microM TTX. Extracellular electrophysiological recordings that were made adjacent to the optrode tip revealed that the Ca(2+) oscillations were in phase with the burst firing of striatal neurons. This suggested that the optical signals had a neuronal origin, most likely from medium spiny neurons. Baseline fluorescence intensity increased during infusion of high [K(+)](o), the calcium ionophore, A-23187, or during temporary bilateral carotid artery occlusion. Monkey (Saimiri sciureus) putamen recordings also affirmed the presence of similar calcium-related transients in a non-human primate. In the transplant preparations, the IMR-32 cells displayed a stable, non-oscillating baseline fluorescence. They were similarly responsive to high [K(+)](o) challenge and appeared viable for at least several hours. Similar optical recording approaches might be applied to monitor other fluorescent, chemiluminescent or bioluminescent events from almost any brain structure. Moreover, transplanted transfected cells expressing a single specific receptor or ion-channel protein may effectively serve as biosensing elements for the measurement of extracellular neurochemical signaling.

Analysing functional connectivity in brain slices by a combination of infrared video microscopy, flash photolysis of caged compounds and scanning methods

We evaluate a novel set-up for scanning functional connectivity in brain slices from the somatosensory cortex of the rat. Upright infrared video microscopy for targeted placement of electrodes is combined with rapid photolysis of bath-applied caged neurotransmitter induced by a xenon flash lamp. Flash photolysis of caged glutamate and electrical stimulation produce comparable field potential responses and demonstrate that the viability of the submerged slices exceeds several hours. Glutamate release leads to field potential responses whose two phases are differentially affected by selective blockade of N-methyl-D-aspartate- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptors with DL-2-amino-5-phosphonovaleric acid and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulphonamide, respectively. Rapid computer-controlled scanning of hundreds of distinct stimulation sites with simultaneous recordings at a fixed reference site allows construction of functional input maps from peak amplitudes and delays to peak of field potential responses. Selective laminar expansion of the functional input maps after bicuculline application demonstrates that the combination of this conveniently assembled set-up with pharmacological and physical manipulations can provide insights into the determinants of functional connectivity in brain slices.

Structural divisions and functional fields in the human cerebral cortex

The question of what is a cortical area needs a thorough definition of borders both in the microstructural and the functional domains. Microstructural parcellation of the human cerebral cortex should be made on multiple criteria based on quantitative measurements of microstructural variables, such as neuron densities, neurotransmitter receptor densities, enzyme densities, etc. Because of the inter-individual variations of extent and topography of microstructurally defined areas, the final microstructurally defined areas appear as population maps. In the functional domain, columns, patches and blobs signifying synaptically active parts of the cortex appear as cortical functional fields. These fields are the largest functional entities of the cerebral cortex according to the cortical field hypothesis. In its strong version, the cortical field hypothesis postulates that all neurons and synapses within the fields perform a co-operative computation. A number of such fields together provide the functional contribution of the cerebral cortex. The functional parcellation of the human cerebral cortex must be based on field population maps, which after intersection analysis appear as functional domains. The major structural-functional hypothesis to be examined is whether these functional domains are equi-territorial to the microstructurally defined meta-maps. The cortical hypothesis predicts that, if two brain tasks make use of one or several identical or largely overlapping fields, they cannot be performed simultaneously without errors or increases in latency. Evidence for such interference is presented. This evidence represents a restriction in the parallel processing of the human brain. In the posterior part of the brain not only visual cortical areas may qualify for parallel processing, but also the somatosensory cortices appear to have separate functional streams for the detection of microgeometry and macrogeometry.

Patterns of intrinsic and associational circuitry in monkey prefrontal cortex

Both local and long-range connections are critical mediators of information processing in the cerebral cortex, but little is known about the relationships among these types of connections, especially in higher-order cortical regions. We used quantitative reconstructions of the label arising from discrete (approximately 350 microns diameter) injections of biotinylated dextran amine and cholera toxin B to determine the spatial organization of the axon collaterals and principal axon projections furnished by pyramidal neurons in the supragranular layers of monkey prefrontal cortex (areas 9 and 46). Both terminals and cell bodies labeled by transport along axon collaterals in the gray matter formed intrinsic clusters which were arrayed as a series of discontinuous stripes of similar size and shape. The co-registration of anterograde and retrograde transport confirmed that these convergent and divergent intrinsic connections also were reciprocal. Transport from the same injection sites along principal axons through the white matter formed associational clusters which were also arrayed as a series of discontinuous stripes. The dimensions of the anterogradely- and retrogradely-labeled associational stripes were very similar to each other and to the intrinsic stripes. These findings demonstrate that divergence, convergence, and reciprocity characterize both the intrinsic and associational excitatory connections in the prefrontal cortex. These patterns of connections provide an anatomical substrate by which activation of a discrete group of neurons would lead to the recruitment of a specific neuronal network comprised of both local and distant groups of cells. Furthermore, the consistent size of the intrinsic and associational stripes (approximately 275 by 1,800 microns) suggests that they may represent basic functional units in the primate prefrontal cortex.