Coupling of hippocampal theta and ripples with pontogeniculooccipital waves.

The hippocampus has a major role in encoding and consolidating long-term memories, and undergoes plastic changes during sleep1. These changes require precise homeostatic control by subcortical neuromodulatory structures2. The underlying mechanisms of this phenomenon, however, remain unknown. Here, using multi-structure recordings in macaque monkeys, we show that the brainstem transiently modulates hippocampal network events through phasic pontine waves known as pontogeniculooccipital waves (PGO waves). Two physiologically distinct types of PGO wave appear to occur sequentially, selectively influencing high-frequency ripples and low-frequency theta events, respectively. The two types of PGO wave are associated with opposite hippocampal spike-field coupling, prompting periods of high neural synchrony of neural populations during periods of ripple and theta instances. The coupling between PGO waves and ripples, classically associated with distinct sleep stages, supports the notion that a global coordination mechanism of hippocampal sleep dynamics by cholinergic pontine transients may promote systems and synaptic memory consolidation as well as synaptic homeostasis.

Lifting the veil on the dynamics of neuronal activities evoked by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) is a widely used non-invasive tool to study and modulate human brain functions. However, TMS-evoked activity of individual neurons has remained largely inaccessible due to the large TMS-induced electromagnetic fields. Here, we present a general method providing direct in vivo electrophysiological access to TMS-evoked neuronal activity 0.8-1 ms after TMS onset. We translated human single-pulse TMS to rodents and unveiled time-grained evoked activities of motor cortex layer V neurons that show high-frequency spiking within the first 6 ms depending on TMS-induced current orientation and a multiphasic spike-rhythm alternating between excitation and inhibition in the 6-300 ms epoch, all of which can be linked to various human TMS responses recorded at the level of spinal cord and muscles. The advance here facilitates a new level of insight into the TMS-brain interaction that is vital for developing this non-invasive tool to purposefully explore and effectively treat the human brain.

Unravelling cerebellar pathways with high temporal precision targeting motor and extensive sensory and parietal networks.

Increasing evidence has implicated the cerebellum in providing forward models of motor plants predicting the sensory consequences of actions. Assuming that cerebellar input to the cerebral cortex contributes to the cerebro-cortical processing by adding forward model signals, we would expect to find projections emphasising motor and sensory cortical areas. However, this expectation is only partially met by studies of cerebello-cerebral connections. Here we show that by electrically stimulating the cerebellar output and imaging responses with functional magnetic resonance imaging, evoked blood oxygen level-dependant activity is observed not only in the classical cerebellar projection target, the primary motor cortex, but also in a number of additional areas in insular, parietal and occipital cortex, including sensory cortical representations. Further probing of the responses reveals a projection system that has been optimized to mediate fast and temporarily precise information. In conclusion, both the topography of the stimulation effects and its emphasis on temporal precision are in full accordance with the concept of cerebellar forward model information modulating cerebro-cortical processing.

The effects of electrical microstimulation on cortical signal propagation.

Electrical stimulation has been used in animals and humans to study potential causal links between neural activity and specific cognitive functions. Recently, it has found increasing use in electrotherapy and neural prostheses. However, the manner in which electrical stimulation-elicited signals propagate in brain tissues remains unclear. We used combined electrostimulation, neurophysiology, microinjection and functional magnetic resonance imaging (fMRI) to study the cortical activity patterns elicited during stimulation of cortical afferents in monkeys. We found that stimulation of a site in the lateral geniculate nucleus (LGN) increased the fMRI signal in the regions of primary visual cortex (V1) that received input from that site, but suppressed it in the retinotopically matched regions of extrastriate cortex. Consistent with previous observations, intracranial recordings indicated that a short excitatory response occurring immediately after a stimulation pulse was followed by a long-lasting inhibition. Following microinjections of GABA antagonists in V1, LGN stimulation induced positive fMRI signals in all of the cortical areas. Taken together, our findings suggest that electrical stimulation disrupts cortico-cortical signal propagation by silencing the output of any neocortical area whose afferents are electrically stimulated.

Relationship between neural and hemodynamic signals during spontaneous activity studied with temporal kernel CCA.

Functional magnetic resonance imaging (fMRI) based on the so-called blood oxygen level-dependent (BOLD) contrast is a powerful tool for studying brain function not only locally but also on the large scale. Most studies assume a simple relationship between neural and BOLD activity, in spite of the fact that it is important to elucidate how the "when" and "what" components of neural activity are correlated to the "where" of fMRI data. Here we conducted simultaneous recordings of neural and BOLD signal fluctuations in primary visual (V1) cortex of anesthetized monkeys. We explored the neurovascular relationship during periods of spontaneous activity by using temporal kernel canonical correlation analysis (tkCCA). tkCCA is a multivariate method that can take into account any features in the signals that univariate analysis cannot. The method detects filters in voxel space (for fMRI data) and in frequency-time space (for neural data) that maximize the neurovascular correlation without any assumption of a hemodynamic response function (HRF). Our results showed a positive neurovascular coupling with a lag of 4-5 s and a larger contribution from local field potentials (LFPs) in the γ range than from low-frequency LFPs or spiking activity. The method also detected a higher correlation around the recording site in the concurrent spatial map, even though the pattern covered most of the occipital part of V1. These results are consistent with those of previous studies and represent the first multivariate analysis of intracranial electrophysiology and high-resolution fMRI.

How not to study spontaneous activity.

Brains are restless. We have long known of the existence of a great deal of uninterrupted brain activity that maintains the body in a stable state--from an evolutionary standpoint one of the brain's most ancient tasks. But intrinsic, ongoing activity is not limited to subcortical, life-maintaining structures; cortex, too, is remarkably active even in the absence of a sensory stimulus or a specific behavioral task. This is evident both in its enormous energy consumption at rest and in the large, spontaneous but coherent fluctuations of neural activity that spread across different areas. Not surprisingly, a growing number of electrophysiological and functional magnetic resonance imaging (fMRI) studies are appearing that report on various aspects of the brain's spontaneous activity or "default mode" of operation. One recent study reports results from simultaneously combined electrophysiological and fMRI measurements in the monkey visual cortex (Shmuel, A., Leopold, D.A., 2008. Neuronal correlates of spontaneous fluctuations in fMRI signals in monkey visual cortex: implications for functional connectivity at rest. Hum. Brain Mapp. 29, 751-761). The authors claim to be able to demonstrate correlations between slow fluctuations in blood-oxygen-level-dependent (BOLD) signals and concurrent fluctuations in the underlying, locally measured neuronal activity. They even go on to speculate that the fluctuations display wave-like spatiotemporal patterns across cortex. In the present report, however, we re-analyze the data presented in that study and demonstrate that the measurements were not actually taken during rest. Visual cortex was subject to almost imperceptible but physiologically clearly detectable flicker induced by the visual stimulator. An examination of the power spectral density of the neural responses and the neurovascular impulse response function shows that such imperceptible flicker strongly suppresses the slow oscillations and changes the degree of covariance between neural and vascular signals. In addition, a careful analysis of the spatiotemporal patterns demonstrates that no slow waves of activity exist in visual cortex; instead, the presented wave data reflect differences in signal-to-noise ratio at various cortical sites due to local differences in vascularization. In this report, assuming that the term "spontaneous activity" refers to intrinsic physiological processes at the absence of sensory inputs or motor outputs, we discuss the need for careful selection of experimental protocols and of examining the degree to which the activation of sensory areas might influence the cortical or subcortical processes in other brain regions.

Pharmacological MRI combined with electrophysiology in non-human primates: effects of Lidocaine on primary visual cortex.

Pharmacological magnetic resonance imaging (phMRI) is a current direction in biomedical imaging, whose goal is the non-invasive monitoring of pharmacological manipulations on brain processes. We have developed techniques combining phMRI with simultaneous monitoring of electrophysiological activity during local injections of pharmacological agents into defined brain regions. We have studied effects of the local anesthetic Lidocaine on BOLD activity in primary visual cortex (V1) of non-human primates. Using independent component analysis (ICA), we describe and quantify the pharmacodynamics and spatial distribution of Lidocaine effects on visually evoked V1 BOLD signal in a dose-dependent manner. We relate these findings to effects of Lidocaine on neural activity as estimated by multi unit activity (MUA) and the local field potential (LFP). Our results open the way for specific fMRI-based investigations regarding the impact of pharmacological agents on the BOLD signal and its coupling to the underlying neuronal activity.

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation.

To combine insights obtained from electric field potentials (LFPs) and neuronal spiking activity (MUA) we need a better understanding of the relative spatial summation of these indices of neuronal activity. Compared to MUA, the LFP has greater spatial coherence, resulting in lower spatial specificity and lower stimulus selectivity. A differential propagation of low- and high-frequency electric signals supposedly underlies this phenomenon, which could result from cortical tissue specifically attenuating higher frequencies, i.e., from a frequency-dependent impedance spectrum. Here we directly measure the cortical impedance spectrum in vivo in monkey primary visual cortex. Our results show that impedance is independent of frequency, is homogeneous and tangentially isotropic within gray matter, and can be theoretically predicted assuming a pure-resistive conductor. We propose that the spatial summation of LFP and MUA is determined by the size of these signals' generators and the nature of neural events underlying them, rather than by biophysical properties of gray matter.

Robust controlled functional MRI in alert monkeys at high magnetic field: effects of jaw and body movements.

The use of functional magnetic resonance imaging (fMRI) in alert non-human primates is of great potential for research in systems neuroscience. It can be combined with invasive techniques and afford better understanding of non-invasively acquired brain imaging signals in humans. However, the difficulties in optimal application of alert monkey fMRI are multi-faceted, especially at high magnetic fields where the effects of motion and of changes in B0 are greatly amplified. To overcome these difficulties, strict behavioral controls and elaborate animal-training are needed. Here, we introduce a number of hardware developments, quantify the effect of movements on fMRI data, and present procedures for animal training and scanning for well-controlled and artifact-reduced alert monkey fMRI at high magnetic field. In particular, we describe systems for monitoring jaw and body movements, and for accurately tracking eye movements. A link between body and jaw movement and MRI image artifacts is established, showing that relying on the immobilization of an animal's head is not sufficient for high-quality imaging. Quantitative analysis showed that body and jaw movement events caused large instabilities in fMRI time series. On average, body movement events caused larger instabilities than jaw movement events. Residual baseline brain image position and signal amplitude shifts were observed after the jaw and body movement events ended. Based on these findings, we introduce a novel behavioral paradigm that relies on training the monkeys to stay still during long trials. A corresponding analysis method discards all data that were not obtained during the movement-free periods. The baseline position and amplitude shifts are overcome by motion correction and trial-by-trial signal normalization. The advantages of the presented method over conventional scanning and analysis are demonstrated with data obtained at 7 T. It is anticipated that the techniques presented here will prove useful for alert monkey fMRI at any magnetic field.

Simultaneous recording of neuronal signals and functional NMR imaging.

We recently directly examined the relationship between blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signals and neural activity by simultaneously acquiring electrophysiological and fMRI data from monkeys in a 4.7-T vertical scanner (Logothetis NK, Pauls J, Augath MA, Trinath T, Oeltermann A. Neurophysiological investigation of the basis of the fMRI signal. Nature 2001;412:150-157). Acquisition of electrical signals in the microvolt range required extensive development of new recording hardware, including electrodes, microdrives, signal conditioning and interference compensation devices. Here, we provide a detailed description of the interference compensation system that can be used to record field and action potentials intracortically within a high-field scanner.

A novel functional magnetic resonance imaging compatible search-coil eye-tracking system.

Measuring eye movements (EMs) using the search-coil eye-tracking technique is superior to video-based infrared methods [Collewijn H, van der Mark F, Jansen TC. Precise recording of human eye movements. Vision Res 1975;15(3):447-50], which suffer from the instability of pupil size, blinking behavior and lower temporal resolution. However, no conventional functional magnetic resonance imaging (fMRI)-compatible search-coil eye tracker exists. The main problems for such a technique are the interaction between the transmitter coils and the magnetic gradients used for imaging as well as the limited amount of space in a scanner. Here we present an approach to overcome these problems and we demonstrate a method to record EMs in an MRI scanner using a search coil. The system described has a spatial resolution of 0.07 degrees (visual angle) and a high temporal resolution (22 kHz). The transmitter coils are integrated into the visual presentation system and the control/analysis unit is portable, which enables us to integrate the eye tracker with an MRI scanner. Our tests demonstrate low noise in the recorded eye traces and scanning with minimal artifact. Furthermore, the induced current in the search coil caused by the RF pulses does not lead to measurable heating. Altogether, this MR-compatible search-coil eye tracker can be used to precisely monitor EMs with high spatial and temporal resolution during fMRI. It can therefore be of great importance for studies requiring accurate fixation of a target, or measurement and study of the subject's oculomotor system.

Simultaneous EEG and fMRI in the macaque monkey at 4.7 Tesla.

Simultaneous electroencephalography (EEG)/functional magnetic resonance imaging (fMRI) acquisition can identify the brain networks involved in generating specific EEG patterns. Yet, the combination of these methodologies is hampered by strong artifacts that arise due to electromagnetic interference during magnetic resonance (MR) image acquisition. Here, we report corrections of the gradient-induced artifact in phantom measurements and in experiments with an awake behaving macaque monkey during fMRI acquisition at a magnetic field strength of 4.7 T. Ninety-one percent of the amplitude of a 10 microV, 10 Hz phantom signal could successfully be recovered without phase distortions. Using this method, we were able to extract the monkey EEG from scalp recordings obtained during MR image acquisition. Visual evoked potentials could also be reliably identified. In conclusion, simultaneous EEG/fMRI acquisition is feasible in the macaque monkey preparation at 4.7 T and holds promise for investigating the neural processes that give rise to particular EEG patterns.

Negative functional MRI response correlates with decreases in neuronal activity in monkey visual area V1.

Most functional brain imaging studies use task-induced hemodynamic responses to infer underlying changes in neuronal activity. In addition to increases in cerebral blood flow and blood oxygenation level-dependent (BOLD) signals, sustained negative responses are pervasive in functional imaging. The origin of negative responses and their relationship to neural activity remain poorly understood. Through simultaneous functional magnetic resonance imaging and electrophysiological recording, we demonstrate a negative BOLD response (NBR) beyond the stimulated regions of visual cortex, associated with local decreases in neuronal activity below spontaneous activity, detected 7.15 +/- 3.14 mm away from the closest positively responding region in V1. Trial-by-trial amplitude fluctuations revealed tight coupling between the NBR and neuronal activity decreases. The NBR was associated with comparable decreases in local field potentials and multiunit activity. Our findings indicate that a significant component of the NBR originates in neuronal activity decreases.

Mapping cortical activity elicited with electrical microstimulation using FMRI in the macaque.

Over the last two centuries, electrical microstimulation has been used to demonstrate causal links between neural activity and specific behaviors and cognitive functions. However, to establish these links it is imperative to characterize the cortical activity patterns that are elicited by stimulation locally around the electrode and in other functionally connected areas. We have developed a technique to record brain activity using the blood oxygen level dependent (BOLD) signal while applying electrical microstimulation to the primate brain. We find that the spread of activity around the electrode tip in macaque area V1 was larger than expected from calculations based on passive spread of current and therefore may reflect functional spread by way of horizontal connections. Consistent with this functional transynaptic spread we also obtained activation in expected projection sites in extrastriate visual areas, demonstrating the utility of our technique in uncovering in vivo functional connectivity maps.