Chemogenetic activation of mammalian brain neurons expressing insect Ionotropic Receptors by systemic ligand precursor administration.

Chemogenetic approaches employing ligand-gated ion channels are advantageous regarding manipulation of target neuronal population functions independently of endogenous second messenger pathways. Among them, Ionotropic Receptor (IR)-mediated neuronal activation (IRNA) allows stimulation of mammalian neurons that heterologously express members of the insect chemosensory IR repertoire in response to their cognate ligands. In the original protocol, phenylacetic acid, a ligand of the IR84a/IR8a complex, was locally injected into a brain region due to its low permeability of the blood-brain barrier. To circumvent this invasive injection, we sought to develop a strategy of peripheral administration with a precursor of phenylacetic acid, phenylacetic acid methyl ester, which is efficiently transferred into the brain and converted to the mature ligand by endogenous esterase activities. This strategy was validated by electrophysiological, biochemical, brain-imaging, and behavioral analyses, demonstrating high utility of systemic IRNA technology in the remote activation of target neurons in the brain.

Enhanced Retrieval of Taste Associative Memory by Chemogenetic Activation of Locus Coeruleus Norepinephrine Neurons.

The ability of animals to retrieve memories stored in response to the environment is essential for behavioral adaptation. Norepinephrine (NE)-containing neurons in the brain play a key role in the modulation of synaptic plasticity underlying various processes of memory formation. However, the role of the central NE system in memory retrieval remains unclear. Here, we developed a novel chemogenetic activation strategy exploiting insect olfactory ionotropic receptors (IRs), termed "IR-mediated neuronal activation," and used it for selective stimulation of NE neurons in the locus coeruleus (LC). Drosophila melanogaster IR84a and IR8a subunits were expressed in LC NE neurons in transgenic mice. Application of phenylacetic acid (a specific ligand for the IR84a/IR8a complex) at appropriate doses induced excitatory responses of NE neurons expressing the receptors in both slice preparations and in vivo electrophysiological conditions, resulting in a marked increase of NE release in the LC nerve terminal regions (male and female). Ligand-induced activation of LC NE neurons enhanced the retrieval process of conditioned taste aversion without affecting taste sensitivity, general arousal state, and locomotor activity. This enhancing effect on taste memory retrieval was mediated, in part, through α1- and ß-adrenergic receptors in the basolateral nucleus of the amygdala (BLA; male). Pharmacological inhibition of LC NE neurons confirmed the facilitative role of these neurons in memory retrieval via adrenergic receptors in the BLA (male). Our findings indicate that the LC NE system, through projections to the BLA, controls the retrieval process of taste associative memory.SIGNIFICANCE STATEMENT Norepinephrine (NE)-containing neurons in the brain play a key role in the modulation of synaptic plasticity underlying various processes of memory formation, but the role of the NE system in memory retrieval remains unclear. We developed a chemogenetic activation system based on insect olfactory ionotropic receptors and used it for selective stimulation of NE neurons in the locus coeruleus (LC) in transgenic mice. Ligand-induced activation of LC NE neurons enhanced the retrieval of conditioned taste aversion, which was mediated, in part, through adrenoceptors in the basolateral amygdala. Pharmacological blockade of LC activity confirmed the facilitative role of these neurons in memory retrieval. Our findings indicate that the LC-amygdala pathway plays an important role in the recall of taste associative memory.

Semantic Segmentation of the Choroid in Swept Source Optical Coherence Tomography Images for Volumetrics.

The choroid is a complex vascular tissue that is covered with the retinal pigment epithelium. Ultra high speed swept source optical coherence tomography (SS-OCT) provides us with high-resolution cube scan images of the choroid. Robust segmentation techniques are required to reconstruct choroidal volume using SS-OCT images. For automated segmentation, the delineation of the choroidal-scleral (C-S) boundary is key to accurate segmentation. Low contrast of the boundary, scleral canals formed by the vessel and the nerve, and the posterior stromal layer, may cause segmentation errors. Semantic segmentation is one of the applications of deep learning used to classify the parts of images related to the meanings of the subjects. We applied semantic segmentation to choroidal segmentation and measured the volume of the choroid. The measurement results were validated through comparison with those of other segmentation methods. As a result, semantic segmentation was able to segment the C-S boundary and choroidal volume adequately.

Sound frequency dependence of duration mismatch negativity recorded from awake rats.

AIMS: The brain function that detects deviations in the acoustic environment can be evaluated with mismatch negativity (MMN). MMN to sound duration deviance has recently drawn attention as a biomarker for schizophrenia. Nonhuman animals, including rats, also exhibit MMN-like potentials. Therefore, MMN research in nonhuman animals can help to clarify the neural mechanisms underlying MMN production. However, results from preclinical MMN studies on duration deviance have been conflicting. We investigated the effect of sound frequency on MMN-like potentials to duration deviance in rats. METHODS: Event-related potentials were recorded from an electrode placed on the primary auditory cortex of free-moving rats using an oddball paradigm consisting of 50-ms duration tones (standards) and 150-ms duration tones (deviants) at a 500-ms stimulus onset asynchrony. The sound frequency was set to three conditions: 3, 12, and 50 kHz. RESULTS: MMN-like potentials that depended on the short-term stimulus history of background regularity were only observed in the 12-kHz tone frequency condition. CONCLUSIONS: MMN-like potentials to duration deviance are subject to tone frequency of the oddball paradigm in rats, suggesting that rats have distinct sound duration recognition ability.

Hybrid Three-Dimensional Visualization of Choroidal Vasculature Imaged by Swept-Source Optical Coherence Tomography.

PURPOSE: To create hybrid three-dimensional (3D) models of the choroidal vasculature from swept-source optical coherence tomography (SS-OCT) angiography images and to evaluate the model's characteristics. METHODS: This study used prospective, noncomparative case series, including 21 eyes of 21 healthy individuals. The 6 × 6-mm macular area was imaged repeatedly to obtain two cube image sets. Images from structural OCT (OCT-S) and OCT angiography (OCT-A) were exported. After vessel-like structures segmentation from the inverted black and white OCT-S images and the OCT-A images, both types of images were reconstructed in a 3D model. The volumes of the outer choroid and the choroidal vessels were measured after thresholding. The similarities of the segmented choroidal vessels (between OCT-S and OCT-A) and between repeatedly acquired images were measured. RESULTS: Mean vessel volume was 2.227 mm3 (29% of the outer choroidal volume) in OCT-S and 0.848 (11%) in OCT-A when measured after removal of the choriocapillaris equivalent volume. Three percent of the vessel volume in OCT-S and 8.4% of that in OCT-A overlapped. The Dice similarity coefficient of vessel volumes in repeated images from the same individual was 0.863 in OCT-S and 0.485 in OCT-A. The ratio of vessel volume to the outer choroidal volume was invariant in OCT-S but increased in OCT-A in the eyes with long axial length. CONCLUSIONS: Hybrid 3D vascular models of the choroidal vasculature were reconstructed from OCT-S and OCT-A. The new models should prove useful for volumetric analysis of the choroid. TRANSLATIONAL RELEVANCE: Hybrid 3D models of the choroidal vasculature enable volumetric analysis and facilitate morphologic evaluation.

Neonatal exposure to an inflammatory cytokine, epidermal growth factor, results in the deficits of mismatch negativity in rats.

Perinatal exposure to epidermal growth factor (EGF) induces various cognitive and behavioral abnormalities after maturation in non-human animals, and is used for animal models of schizophrenia. Patients with schizophrenia often display a reduction of mismatch negativity (MMN), which is a stimulus-change specific event-related brain potential. Do the EGF model animals also exhibit the MMN reduction as schizophrenic patients do? This study addressed this question to verify the pathophysiological validity of this model. Neonatal rats received repeated administration of EGF or saline and were grown until adulthood. Employing the odd-ball paradigm of distinct tone pitches, tone-evoked electroencephalogram (EEG) components were recorded from electrodes on the auditory and frontal cortices of awake rats, referencing an electrode on the frontal sinus. The amplitude of the MMN-like potential was significantly reduced in EGF-treated rats compared with saline-injected control rats. The wavelet analysis of the EEG during a near period of tone stimulation revealed that synchronization of EEG activity, especially with beta and gamma bands, was reduced in EGF-treated rats. Results suggest that animals exposed to EGF during a perinatal period serve as a promising neurodevelopmental model of schizophrenia.

Effects of storytelling on the childhood brain: near-infrared spectroscopic comparison with the effects of picture-book reading.

In children, storytelling provides many psychological and educational benefits, such as enhanced imagination to help visualize spoken words, improved vocabulary, and more refined communication skills. However, the brain mechanisms underlying the effects of storytelling on children are not clear. In this study, the effects of storytelling on the brains of children were assessed by using near-infrared spectroscopy (NIRS). Results indicated significant decreases of the blood flow in the bilateral prefrontal areas during picture-book reading when the subjects were familiarized in comparison to the cases of the subject naïve to the stories. However, no significant differences in the blood flow were found during storytelling between the subjects naïve and familiarized to the stories. The results indicated more sustained brain activation to storytelling in comparison with picture-book reading, suggesting possible advantages of storytelling as a psychological and educational medium in children.

The Motor Network Reduces Multisensory Illusory Perception.

Observing mouth movements has strikingly effects on the perception of speech. Any mismatch between sound and mouth movements will result in listeners perceiving illusory consonants (McGurk effect), whereas matching mouth movements assist with the correct recognition of speech sounds. Recent neuroimaging studies have yielded evidence that the motor areas are involved in speech processing, yet their contributions to multisensory illusion remain unclear. Using functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) in an event-related design, we aimed to identify the functional roles of the motor network in the occurrence of multisensory illusion in female and male brains. fMRI showed bilateral activation of the inferior frontal gyrus (IFG) in audiovisually incongruent trials. Activity in the left IFG was negatively correlated with occurrence of the McGurk effect. The effective connectivity between the left IFG and the bilateral precentral gyri was stronger in incongruent than in congruent trials. The McGurk effect was reduced in incongruent trials by applying single-pulse TMS to motor cortex (M1) lip areas, indicating that TMS facilitates the left IFG-precentral motor network to reduce the McGurk effect. TMS of the M1 lip areas was effective in reducing the McGurk effect within the specific temporal range from 100 ms before to 200 ms after the auditory onset, and TMS of the M1 foot area did not influence the McGurk effect, suggesting topographical specificity. These results provide direct evidence that the motor network makes specific temporal and topographical contributions to the processing of multisensory integration of speech to avoid illusion.SIGNIFICANCE STATEMENT The human motor network, including the inferior frontal gyrus and primary motor cortex lip area, appears to be involved in speech perception, but the functional contribution to the McGurk effect is unknown. Functional magnetic resonance imaging revealed that activity in these areas of the motor network increased when the audiovisual stimuli were incongruent, and that the increased activity was negatively correlated with perception of the McGurk effect. Furthermore, applying transcranial magnetic stimulation to the motor areas reduced the McGurk effect. These two observations provide evidence that the motor network contributes to the avoidance of multisensory illusory perception.

Action Selection and Flexible Switching Controlled by the Intralaminar Thalamic Neurons.

Learning processes contributing to appropriate selection and flexible switching of behaviors are mediated through the dorsal striatum, a key structure of the basal ganglia circuit. The major inputs to striatal subdivisions are provided from the intralaminar thalamic nuclei, including the central lateral nucleus (CL) and parafascicular nucleus (PF). Thalamostriatal neurons in the PF modulate the acquisition and performance of stimulus-response learning. Here, we address the roles of the CL thalamostriatal neurons in learning processes by using a selective neural pathway targeting technique. We show that the CL neurons are essential for the performance of stimulus-response learning and for behavioral flexibility, including reversal and attentional set-shifting of learned responses. In addition, chemogenetic suppression of neural activity supports the requirements of these neurons for behavioral flexibility. Our results suggest that the main contribution of the CL thalamostriatal neurons is functional control of the basal ganglia circuit linked to the prefrontal cortex.

Crucial information for efficient face searching by humans and Japanese macaques.

Humans can efficiently detect a face among non-face objects, but few studies of this ability have been conducted in animals. Here, in Japanese macaques and humans, we examined visual searching for a face and explored what factors contribute to efficient facial information processing. Subjects were asked to search for an odd target among the different numbers of distracters. Faces of the subjects' own species, the backs of the head of the subjects' own species, faces of the subjects' closely related species or race, and faces of species that are clearly different from the subjects' own species were used as the target. Both the macaques and humans detected a face of their own species more efficiently than a face from a clearly different species. Similar efficient detections were confirmed for the faces of the subjects' closely related species or race. These results suggest that conspecific faces and faces that share morphological similarity with conspecific faces can be detected efficiently among non-face objects by both humans and Japanese macaques. In another experiment, facial recognition efficiency was observed when the subjects searched for own-species faces that had lower-spatial-frequency components compared to faces with higher-spatial-frequency components. It seems reasonable that the ability to search efficiently for faces by using holistic face processing is derived from fundamental social cognition abilities that are broadly shared among species.

Ventrolateral Prefrontal Cortex Updates Chosen Value According to Choice Set Size.

Having chosen an item typically increases the subjective value of the chosen item, and people generally enjoy making choices from larger choice sets. However, having too many items to choose from can reduce the value of chosen items-for example, because of conflict or choice difficulty. In this study, we investigated the effects of choice set size on behavioral and neural value updating (revaluation) of the chosen item. In the scanner, participants selected items from choice sets of various sizes (one, two, four, or eight items). After they chose an item, participants rerated the chosen item, and we quantified revaluation by taking the difference of postchoice minus prechoice ratings. Revaluation of chosen items increased up to choice sets of four alternatives but then decreased again for items chosen from choice sets of eight alternatives, revealing both a linear and a quadratic effect of choice set size. At the time of postchoice rating, activation of the ventrolateral pFC (VLPFC) reflected the influence of choice set size on parametric revaluation, without significant relation to either prechoice or postchoice ratings tested separately. Additional analyses revealed relations of choice set size to anterior cingulate and insula activity during actual choice and increased coupling of both regions to revaluation-related VLPFC during postchoice rating. These data suggest that the VLPFC plays a central role in a network that relates choice set size to updating the value of chosen items and integrates choice overload with value-enhancing effects of larger choice sets.

Epidermal growth factor signals attenuate phenotypic and functional development of neocortical GABA neurons.

Phenotypic development of neocortical GABA neurons is highly plastic and promoted by various neurotrophic factors such as neuregulin-1. A subpopulation of GABA neurons expresses not only neuregulin receptor (ErbB4) but also epidermal growth factor (EGF) receptor (ErbB1) during development, but the neurobiological action of EGF on this cell population is less understood than that of neuregulin-1. Here, we examined the effects of exogenous EGF on immature GABA neurons both in culture and in vivo and also explored physiological consequences in adults. We prepared low density cultures from the neocortex of rat embryos and treated neocortical neurons with EGF. EGF decreased protein levels of glutamic acid decarboxylases (GAD65 and GAD67), and EGF influences on neuronal survival and glial proliferation were negligible or limited. The EGF treatment also diminished the frequency of miniature inhibitory postsynaptic currents (mIPSCs). In vivo administration of EGF to mouse pups reproduced the above GABAergic phenomena in neocortical culture. In EGF-injected postnatal mice, GAD- and parvalbumin-immunoreactivities were reduced in the frontal cortex. In addition, postnatal EGF treatment decreased mIPSC frequency in, and the density of, GABAergic terminals on pyramidal cells. Although these phenotypic influences on GABA neurons became less marked during development, it later resulted in the reduced ß- and γ-powers of sound-evoked electroencephalogram in adults, which is regulated by parvalbumin-positive GABA neurons and implicated in the schizophrenia pathophysiology. These findings suggest that, in contrast to the ErbB4 ligand of neuregulin-1, the ErbB1 ligand of EGF exerts unique maturation-attenuating influences on developing cortical GABAergic neurons.

[Brodmann Areas 27, 28, 36 and 37: The Parahippocampal and the Fusiform Gyri].

First, Brodmann areas 27, 28, 36 and 37, were anatomically defined in the beginning of this review. These areas exist in the parahippocampal or fusiform gyrus of the ventral temporal lobe in humans. Subsequently, the current understanding of their functions was summarized on the basis of recent findings mainly through human functional neuroimaging studies and animal studies. Rodent studies have shown the existence of neuronal activities for representing space, such as those involving head-direction cells or grid cells, in areas 27 (the parasubicular cortex) and 28 (the ventral entorhinal cortex). Recent human neuroimaging studies have provided support for the idea that grid cells may also exist in the human entorhinal cortex. Many previous animal studies have shown that area 36 (the lateral perirhinal cortex) is crucial for various types of associative learning. Earlier human neuroimaging studies have also indicated that faces, bodies and visual word forms are represented in different regions of area 37 in the posterior fusiform gyrus. Recent neuroimaging studies in humans have shown substantial functional differentiation between face-related regions in areas 37 and 36, which is similar to that seen in macaque monkeys, as shown through their face patches. This implies the crucial involvement of both areas in face processing.

Effects of tilted orientations and face-like configurations on visual search asymmetry in macaques.

Visual search asymmetry has been used as an important tool for exploring cognitive mechanisms in humans. Here, we examined visual search asymmetry in two macaques toward two types of stimulus: the orientation of line stimuli and face-like stimuli. In the experiment, the monkeys were required to detect an odd target among numerous uniform distracters. The monkeys detected a tilted-lines target among horizontal- or vertical-lined distracters significantly faster than a horizontal- or vertical-lined target among tilted-lined distracters, regardless of the display size. However, unlike the situation in which inverted-face stimuli were introduced as distracters, this effect was diminished if upright-face stimuli were used as distracters. Additionally, monkeys detected an upright-face target among inverted-face distracters significantly faster than an inverted-face target among upright-face distracters, regardless of the display size. These results demonstrate that macaques can search a target efficiently to detect both tilted lines among non-tilted lines and upright faces among inverted faces. This clarifies that there are several types of visual search asymmetry in macaques.

Sleep-stage correlates of hippocampal electroencephalogram in primates.

It has been demonstrated in the rodent hippocampus that rhythmic slow activity (theta) predominantly occurs during rapid eye movement (REM) sleep, while sharp waves and associated ripples occur mainly during non-REM sleep. However, evidence is lacking for correlates of sleep stages with electroencephalogram (EEG) in the hippocampus of monkeys. In the present study, we recorded hippocampal EEG from the dentate gyrus in monkeys overnight under conditions of polysomnographical monitoring. As result, the hippocampal EEG changed in a manner similar to that of the surface EEG: during wakefulness, the hippocampal EEG showed fast, desynchronized waves, which were partly replaced with slower waves of intermediate amplitudes during the shallow stages of non-REM sleep. During the deep stages of non-REM sleep, continuous, slower oscillations (0.5-8 Hz) with high amplitudes were predominant. During REM sleep, the hippocampal EEG again showed fast, desynchronized waves similar to those found during wakefulness. These results indicate that in the monkey, hippocampal rhythmic slow activity rarely occurs during REM sleep, which is in clear contrast to that of rodents. In addition, the increase in the slower oscillations of hippocampal EEG during non-REM sleep, which resembled that of the surface EEG, may at least partly reflect cortical inputs to the dentate gyrus during this behavioral state.

[Neural representations of facial identity and its associative meaning].

Since the discovery of "face cells" in the early 1980s, single-cell recording experiments in non-human primates have made significant contributions toward the elucidation of neural mechanisms underlying face perception and recognition. In this paper, we review the recent progress in face cell studies, including the recent remarkable findings of the face patches that are scattered around the anterior temporal cortical areas of monkeys. In particular, we focus on the neural representations of facial identity within these areas. The identification of faces requires both discrimination of facial identities and generalization across facial views. It has been indicated by some laboratories that the population of face cells found in the anterior ventral inferior temporal cortex of monkeys represent facial identity in a manner which is facial view-invariant. These findings suggest a relatively distributed representation that operates for facial identification. It has also been shown that certain individual neurons in the medial temporal lobe of humans represent view-invariant facial identity. This finding suggests a relatively sparse representation that may be employed for memory formation. Finally, we summarize our recent study, showing that the population of face cells in the anterior ventral inferior temporal cortex of monkeys that represent view-invariant facial identity, can also represent learned paired associations between an abstract picture and a particular facial identity, extending our understanding of the function of the anterior ventral inferior temporal cortex in the recognition of associative meanings of faces.

Distinct human face representations in the perirhinal cortex and fusiform gyrus.

Face representation, which is believed to be processed in the temporal visual pathway, has been extensively investigated in humans and monkeys through neuroimaging and electroneurophysiology. Lesion studies in monkeys indicate that simple facial features are processed in the caudal regions, and that the combined and integrated features of the face are stored in the perirhinal cortex (PRC). However, this hypothesis still lacks experimental evidence in normal human subjects; therefore, we conducted 2 functional magnetic resonance imaging experiments to investigate whether the function of the PRC differs from that of conventional face-related areas during face recognition tests. In experiment 1, normal subjects learned 6 facial identity-figure associations before scanning, and their brain activity was measured during recognition testing of correct and incorrect face-figure pairs in 3 different angles. The degree of activation in the PRC differed among the facial angles, and activation in response to frontal views was greater than that to other views. In experiment 2, where face angle, but not identity, was paired with an abstract figure, activation was significantly greater in response to the frontal view than that to other views. In contrast, the degree of activation in conventional face-related areas, i.e., the fusiform gyrus, did not differ among viewing angles in both experiments. The results indicate that the function of face representation in the PRC differs from that in the conventional face-related areas, and that a frontal view of the face plays a role in the activation of face representation stored in the PRC.

Short-term synaptic plasticity in the dentate gyrus of monkeys.

The hippocampus plays an important role in learning and memory. Synaptic plasticity in the hippocampus, short-term and long-term, is postulated to be a neural substrate of memory trace. Paired-pulse stimulation is a standard technique for evaluating a form of short-term synaptic plasticity in rodents. However, evidence is lacking for paired-pulse responses in the primate hippocampus. In the present study, we recorded paired-pulse responses in the dentate gyrus of monkeys while stimulating to the medial part of the perforant path at several inter-pulse intervals (IPIs) using low and high stimulus intensities. When the stimulus intensity was low, the first pulse produced early strong depression (at IPIs of 10-30 ms) and late slight depression (at IPIs of 100-1000 ms) of field excitatory postsynaptic potentials (fEPSPs) generated by the second pulse, interposing no depression IPIs (50-70 ms). When the stimulus intensity was high, fEPSPs generated by the second pulse were depressed by the first pulse at all IPIs except for the longest one (2000 ms). Population spikes (PSs) generated by the second pulse were completely blocked or strongly depressed at shorter IPIs (10-100 or 200 ms, respectively), while no depression or slight facilitation occurred at longer IPIs (500-2000 ms). Administration of diazepam slightly increased fEPSPs, while it decreased PSs produced by the first pulse. It also enhanced the facilitation of PSs produced by the second stimulation at longer IPIs. The present results, in comparison with previous studies using rodents, indicate that paired-pulse responses of fEPSPs in the monkey are basically similar to those of rodents, although paired-pulse responses of PSs in the monkey are more delayed than those in rodents and have a different sensitivity to diazepam.

Neural representations of personally familiar and unfamiliar faces in the anterior inferior temporal cortex of monkeys.

To investigate the neural representations of faces in primates, particularly in relation to their personal familiarity or unfamiliarity, neuronal activities were chronically recorded from the ventral portion of the anterior inferior temporal cortex (AITv) of macaque monkeys during the performance of a facial identification task using either personally familiar or unfamiliar faces as stimuli. By calculating the correlation coefficients between neuronal responses to the faces for all possible pairs of faces given in the task and then using the coefficients as neuronal population-based similarity measures between the faces in pairs, we analyzed the similarity/dissimilarity relationship between the faces, which were potentially represented by the activities of a population of the face-responsive neurons recorded in the area AITv. The results showed that, for personally familiar faces, different identities were represented by different patterns of activities of the population of AITv neurons irrespective of the view (e.g., front, 90° left, etc.), while different views were not represented independently of their facial identities, which was consistent with our previous report. In the case of personally unfamiliar faces, the faces possessing different identities but presented in the same frontal view were represented as similar, which contrasts with the results for personally familiar faces. These results, taken together, outline the neuronal representations of personally familiar and unfamiliar faces in the AITv neuronal population.

A method for recording evoked local field potentials in the primate dentate gyrus in vivo.

Recording evoked local field potentials (LFPs) in the hippocampus in vivo has yielded us useful information about the neural mechanisms of learning and memory. Although this technique has been used in studies of the hippocampus of rodents, lagomorphs, and felines, it has not yet been applied to the primate hippocampus. Here, we report a method for recording evoked LFPs in the hippocampus of monkeys. A stimulation electrode and a recording electrode were implanted in the perforant pathway and dentate gyrus, respectively, under the guidance of electrophysiological recording. With a low stimulus intensity just above the threshold, the potential appeared as a slow positive-wave component, which was regarded as field excitatory postsynaptic potential (putative fEPSP); as stimulation intensity increased, the fEPSP amplitude increased, followed by a sharp negative component which was regarded as putative population spike. When the coordinates of the recording or stimulation electrode were moved stepwise, we observed a systematic change in the waveforms of evoked LFPs; this change corresponded to the structural arrangement through which the electrode passed. In a test for short-term synaptic plasticity by paired-pulse stimulation, potentials evoked by the second pulse were influenced by the first one in a manner dependent on interpulse intervals. In a test for long-term synaptic plasticity by high-frequency stimulation, the slopes of the fEPSPs and the area of population spikes were increased for more than 1 h. These results indicate that the method developed in the present study is useful for testing theories of hippocampal functions in primates.