Cortical-hippocampal auditory processing identified by magnetoencephalography

Effects of white noise duration on somatosensory event-related potentials

Exposure to auditory white noise has been shown to facilitate cognitive function. This phenomenon is called stochastic resonance. The present study examined the effects of white noise duration on behavioral data [reaction time (RT), the SD of RT, and error rates] and the N140 and P300 components of event-related potentials in somatosensory Go/No-go paradigms. A Go or No-go stimulus was presented to the second or fifth digit of the left hand, respectively, at the same probability. Before recording event-related potentials, participants heard three different white noise durations (1, 3, and 5-min conditions), and performed somatosensory Go/No-go paradigms while listening to white noise. RT was significantly shorter under the 5 and 3-min conditions than under the 1-min condition, whereas SD of RT and error rates remained unchanged. The peak latency of Go-P300 was the shortest under the 5-min condition. White noise did not affect the peak latency of N140 or the peak amplitude of N140 or P300. Our results suggest that 5-min white noise exposure accelerated RT and the latency of Go-P300, reflecting changes in the neural activation of response execution processing.

Effects of stimulus intensity and auditory white noise on human somatosensory cognitive processing: a study using event-related potentials

Exposure to auditory white noise has been shown to facilitate cognitive function. This phenomenon is often called stochastic resonance, and a moderate amount of auditory noise has been suggested to benefit individuals in hypodopaminergic states. Previous studies using psychophysic methods reported that stochastic resonance was sensitive to stimulus intensity; however, the relationship between neural activities elicited by different stimulus intensities and auditory white noise has not yet been clarified Thus, the present study aimed to investigate the effects of stimulus intensity (Experiment 1) and auditory white noise (Experiment 2) on behavioral data (reaction time (RT), the standard deviation of RT, and error rates), and the N140 and P300 components of event-related potentials (ERPs) in somatosensory Go/No-go paradigms. The subjects had to respond to the somatosensory stimuli by pressing a button with their right thumb only after presentation of the Go stimulus. In Experiment 1 with four different stimulus intensity levels, the peak latencies of N140 and P300 became shorter, and the peak amplitudes of N140 and P300 were enhanced with increases in stimulus intensity. In Experiment 2 with weak and mild intensities under auditory white noise and control conditions, the amplitudes of Go-P300 and No-go-P300 were enhanced by white noise, irrespective of weak and mild intensities, during Go/No-go paradigms. Auditory white noise did not significantly affect the amplitude of N140 or the latencies of N140 and P300. These results suggest the presence of a characteristic cross-modal stochastic resonance in neural substrates utilizing somatosensory ERPs.

Non-invasive Investigation of Human Hippocampal Rhythms Using Magnetoencephalography: A Review

Hippocampal rhythms are believed to support crucial cognitive processes including memory, navigation, and language. Due to the location of the hippocampus deep in the brain, studying hippocampal rhythms using non-invasive magnetoencephalography (MEG) recordings has generally been assumed to be methodologically challenging. However, with the advent of whole-head MEG systems in the 1990s and development of advanced source localization techniques, simulation and empirical studies have provided evidence that human hippocampal signals can be sensed by MEG and reliably reconstructed by source localization algorithms. This paper systematically reviews simulation studies and empirical evidence of the current capacities and limitations of MEG “deep source imaging” of the human hippocampus. Overall, these studies confirm that MEG provides a unique avenue to investigate human hippocampal rhythms in cognition, and can bridge the gap between animal studies and human hippocampal research, as well as elucidate the functional role and the behavioral correlates of human hippocampal oscillations.

Multimodal EEG Recordings, Psychometrics and Behavioural Analysis

High spatial and temporal resolution measurements of neuronal activity are preferably combined. In an overview on how this approach can take shape, multimodal electroencephalography (EEG) is treated in 2 main parts: by experiments without a task and in the experimentally cued working brain. It concentrates first on the alpha rhythm properties and next on data-driven search for patterns such as the default mode network. The high-resolution volumic distributions of neuronal metabolic indices result in distributed cortical regions and possibly relate to numerous nuclei, observable in a non-invasive manner in the central nervous system of humans. The second part deals with paradigms in which nowadays assessment of target-related networks can align level-dependent blood oxygenation, electrical responses and behaviour, taking the temporal resolution advantages of event-related potentials. Evidence-based electrical propagation in serial tasks during performance is now to a large extent attributed to interconnected pathways, particularly chronometry-dependent ones, throughout a chain including a dorsal stream, next ventral cortical areas taking the flow of information towards inferior temporal domains. The influence of aging is documented, and results of the first multimodal studies in neuropharmacology are consistent. Finally a scope on implementation of advanced clinical applications and personalized marker strategies in neuropsychiatry is indicated.

Developmental cigarette smoke exposure II: Hippocampus proteome and metabolome profiles in adult offspring

Exposure to cigarette smoke during development is linked to neurodevelopmental delays and cognitive impairment including impulsivity, attention deficit disorder, and lower IQ. Utilizing a murine experimental model of "active" inhalation exposure to cigarette smoke spanning the entirety of gestation and through human third trimester equivalent hippocampal development [gestation day 1 (GD1) through postnatal day 21 (PD21)], we examined hippocampus proteome and metabolome alterations present at a time during which developmental cigarette smoke exposure (CSE)-induced behavioral and cognitive impairments are evident in adult animals from this model system. At six month of age, carbohydrate metabolism and lipid content in the hippocampus of adult offspring remained impacted by prior exposure to cigarette smoke during the critical period of hippocampal ontogenesis indicating limited glycolysis. These findings indicate developmental CSE-induced systemic glucose availability may limit both organism growth and developmental trajectory, including the capacity for learning and memory.

The effect of elevations in internal temperature on event-related potentials during a simple cognitive task in humans

The effect of hyperthermia on cognitive function remains equivocal, perhaps because of methodological discrepancy. Using electroencephalographic event-related potentials (ERPs), we tested the hypothesis that a passive heat stress impairs cognitive processing. Thirteen volunteers performed repeated auditory oddball paradigms under two thermal conditions, normothermic time control and heat stress, on different days. For the heat stress trial, these paradigms were performed at preheat stress (i.e., normothermic) baseline, when esophageal temperature had increased by ∼0.8°C, when esophageal temperature had increased by ∼2.0°C, and during cooling following the heat stress. The reaction time and ERPs were recorded in each session. For the time control trial, subjects performed the auditory oddball paradigms at approximately the same time interval as they did in the heat stress trial. The peak latency and amplitude of an indicator of auditory processing (N100) were not altered regardless of thermal conditions. An indicator of stimulus classification/evaluation time (latency of P300) and the reaction time were shortened during heat stress; moreover an indicator of cognitive processing (the amplitude of P300) was significantly reduced during severe heat stress (8.3 ± 1.3 μV) relative to the baseline (12.2 ± 1.0 μV, P < 0.01). No changes in these indexes occurred during the time control trial. During subsequent whole body cooling, the amplitude of P300 remained reduced, and the reaction time and latency of P300 remained shortened. These results suggest that excessive elevations in internal temperature reduce cognitive processing but promote classification time.

Resting-state functional connectivity and pitch identification ability in non-musicians

Previous studies have used task-related fMRI to investigate the neural basis of pitch identification (PI), but no study has examined the associations between resting-state functional connectivity (RSFC) and PI ability. Using a large sample of Chinese non-musicians (N = 320, with 56 having prior musical training), the current study examined the associations among musical training, PI ability, and RSFC. Results showed that musical training was associated with increased RSFC within the networks for multiple cognitive functions (such as vision, phonology, semantics, auditory encoding, and executive functions). PI ability was associated with RSFC with regions for perceptual and auditory encoding for participants with musical training, and with RSFC with regions for short-term memory, semantics, and phonology for participants without musical training.

Developmental cigarette smoke exposure: hippocampus proteome and metabolome profiles in low birth weight pups

Exposure to cigarette smoke during development is linked to neurodevelopmental delays and cognitive impairment including impulsivity, attention deficit disorder, and lower IQ. However, brain region specific biomolecular alterations induced by developmental cigarette smoke exposure (CSE) remain largely unexplored. In the current molecular phenotyping study, a mouse model of 'active' developmental CSE (serum cotinine > 50 ng/mL) spanning pre-implantation through third trimester-equivalent brain development (gestational day (GD) 1 through postnatal day (PD) 21) was utilized. Hippocampus tissue collected at the time of cessation of exposure was processed for gel-based proteomic and non-targeted metabolomic profiling with partial least squares-discriminant analysis (PLS-DA) for selection of features of interest. Ingenuity pathway analysis was utilized to identify candidate molecular and metabolic pathways impacted within the hippocampus. CSE impacted glycolysis, oxidative phosphorylation, fatty acid metabolism, and neurodevelopment pathways within the developing hippocampus.

Bilateral hippocampal dysfunction in schizophrenia

The hippocampus has long been known to be important for memory, with the right hippocampus particularly implicated in nonverbal/visuo-spatial memory and the left in verbal/narrative or episodic memory. Despite this hypothesized lateralized functional difference, there has not been a single task that has been shown to activate both the right and left hippocampi differentially, dissociating the two, using neuroimaging. The transverse patterning (TP) task is a strong candidate for this purpose, as it has been shown in human and nonhuman animal studies to theoretically and empirically depend on the hippocampus. In TP, participants choose between stimuli presented in pairs, with the correct choice being a function of the specific pairing. In this project, TP was used to assess lateralized hippocampal function by varying its dependence on verbal material, with the goal of dissociating the two hippocampi. Magnetoencephalographic (MEG) data were collected while controls performed verbal and nonverbal versions of TP in order to verify and validate lateralized activation within the hippocampi. Schizophrenia patients were evaluated to determine whether they exhibited a lateralized hippocampal deficit. As hypothesized, patients' mean level of behavioral performance was poorer than controls' on both verbal and nonverbal TP. In contrast, patients had no decrement in performance on a verbal and nonverbal non-hippocampal-dependent matched control task. Also, controls but not patients showed more right hippocampal activation during nonverbal TP and more left hippocampal activation during verbal TP. These data demonstrate the capacity to assess lateralized hippocampal function and suggest a bilateral hippocampal behavioral and activation deficit in schizophrenia.

Hippocampal activity during the transverse patterning task declines with cognitive competence but not with age

Background

The hippocampus is a brain region that is particularly affected by age-related morphological changes. It is generally assumed that a loss in hippocampal volume results in functional deficits that contribute to age-related cognitive decline. In a combined cross-sectional behavioural and magnetoencephalography (MEG) study we investigated whether hippocampal-associated neural current flow during a transverse patterning task - which requires learning relational associations between stimuli - correlates with age and whether it is modulated by cognitive competence.

Results

Better performance in several tests of verbal memory, verbal fluency and executive function was indeed associated with higher hippocampal neural activity. Age, however, was not related to the strength of hippocampal neural activity: elderly participants responded slower than younger individuals but on average produced the same neural mass activity.

Conclusions

Our results suggest that in non-pathological aging, hippocampal neural activity does not decrease with age but is rather related to cognitive competence.

Distinct neural generators of sensory gating in schizophrenia

Although malfunctioning of inhibitory processes is proposed as a pathophysiological mechanism in schizophrenia and has been studied extensively with the P50 gating paradigm, the brain regions involved in generating and suppressing the P50 remain unclear. The current investigation used EEG source analysis and the standard S1-S2 paradigm to clarify the neural structures associated with P50 gating in 16 schizophrenia patients and 14 healthy subjects. Based on prior research, the superior temporal gyrus, hippocampus, dorsolateral prefrontal cortex, thalamus, and their dipole moments were evaluated. In modeling the P50, a neural network involving all four brain regions provided the best goodness-of-fit across both groups. In healthy subjects, the P50 ratio score correlated positively with the hippocampal dipole moment ratio, whereas a significant association with the DLPFC dipole moment ratio was observed in schizophrenia patients. In each instance, the neural structure was found to account for unique variance in explaining the P50 ratio, along with some suggestion of DLPFC involvement in healthy subjects.

Sequential temporo-fronto-temporal activation during monitoring of the auditory environment for temporal patterns

Subjects detected rarely occurring shifts between two simple tone-patterns, in a paradigm that dissociated the effects of rarity from those of pitch, habituation, and attention. Whole-head magnetoencephalography suggested that rare attended pattern-shifts evoked activity first in the superior temporal plane (sTp, peak ~100 ms), then superior temporal sulcus (sTs, peak ~130 ms), then posteroventral prefrontal (pvpF, peak ~230 ms), and anterior temporal cortices (aT, peak ~370 ms). Activity was more prominent in the right hemisphere. After subtracting the effects of nonshift tones (balanced for pitch and habituation status), weak but consistent differential effects of pattern-shifts began in aT at 90-130 ms, spread to sTs and sTp at ∼130 ms, then pvpF, and finally returned to aT. Cingulate activity resembled prefrontal. Responses to pattern shifts were greatly attenuated when the same stimuli were ignored, suggesting that the initial superior temporal activity reflected an attention-related mismatch negativity. The prefrontal activity at ~230 ms corresponded in latency and task correlates with simultaneously recorded event-related potential components N2b and P3a; the subsequent temporal activity corresponded to the P3b. These results were confirmed in sensors specific for frontal or temporal cortex, and thus are independent of the inverse method used. Overall, these results suggest that auditory working memory for temporal patterns begins with detection of the pattern change by an interaction of anterior and superior temporal structures, followed by identification of the event and its consequences led by posteroventral prefrontal and cingulate cortices, and finally, definitive encoding of the event in anterior temporal areas.

A complementary analytic approach to examining medial temporal lobe sources using magnetoencephalography

Neuropsychological and neuroimaging findings reveal that the hippocampus is important for recognition memory. However, it is unclear when and whether the hippocampus contributes differentially to recognition of previously studied items (old) versus novel items (new), or contributes to a general processing requirement that is necessary for recognition of both types of information. To address this issue, we examined the temporal dynamics and spectral frequency underlying hippocampal activity during recognition of old/new complex scenes using magnetoencephalography (MEG). In order to provide converging evidence to existing literature in support of the potential of MEG to localize the hippocampus, we reconstructed brain source activity using the beamformer method and analyzed three types of processing-related signal changes by applying three different analysis methods: (1) Synthetic aperture magnetometry (SAM) revealed event related and non-event-related spectral power changes; (2) Inter-trial coherence (ITC) revealed time-locked changes in neural synchrony; and (3) Event-related SAM (ER-SAM) revealed averaged event-related responses over time. Hippocampal activity was evident for both old and new information within the theta frequency band and during the first 250 ms following stimulus onset. The early onset of hippocampal responses suggests that general comparison processes related to recognition of new/old information may occur obligatorily.

Ketamine effects on CNS responses assessed with MEG/EEG in a passive auditory sensory-gating paradigm: an attempt for modelling some symptoms of psychosis in man

Disturbances in integrative function have been consistentLy described in psychotic disorder; for instance, prepulse inhibition of the startle reflex (startle-PPI) which is a marker of sensory gating, is deficient in persons with schizophrenia. The N-methyl-D-aspartate antagonist ketamine produces in control subjects a spectrum of neurobehavioural symptoms like encountered in schizophrenia, and disrupts startle-PPI in animals. In the present study, we investigated in 12 healthy subjects whether ketamine would reduce sensory-gating in auditory responses at doses which produce psychotic symptoms. In a double-blind, crossover design loading doses of 0.024, 0.081 and 0.27 mg/kg or saline were employed, followed by maintenance infusion for 120 min. A passive paradigm has been developed which consisted in tone bursts, preceded or not by a (near-threshold) click at intervals of 100 ms or 500 ms. Brain electromagnetic activity imaging of the responses to sound stimuli has been carried out by way of a 148-channel magnetoencephalography-system. Actual evoked response amplitudes and underlying equivalent current dipole strengths have been compared to multi-electrode evoked potentials from the scalp. A click stimulus is capable to inhibit test responses under placebo at the 100 ms interval. During maintenance infusion of ketamine at steady-state (for >30 min) after 0.27 mg/kg, no such amplitude changes were observed anymore (p <0.05) and under these circumstances significant increases in Brief Psychiatric Rating scale and Scale for the Assessment of Negative Symptoms scores were evidenced (p < 0.001). Intermediate effects have been observed when the dose was lowered to 0.081 mg/kg. The present results have shown that ketamine may induce a psychotic-like clinical state associated with gating deficits in healthy subjects.

Analysis of magnetic source localization of P300 using the multiple signal classification algorithm

The authors studied the localization of P300 magnetic sources using the multiple signal classification (MUSIC) algorithm. Six healthy subjects (aged 24-34 years old) were investigated with 148-channel whole-head type magnetencephalography using an auditory oddball paradigm in passive mode. The authors also compared six stimulus combinations in order to find the optimal stimulus parameters for P300 magnetic field (P300m) in passive mode. Bilateral MUSIC peaks were located on the mesial temporal, superior temporal and parietal lobes. Interestingly, all MUSIC peaks in these regions emerged earlier in the right hemisphere than in the left hemisphere, suggesting that the right hemisphere has predominance over the left in the processing activity associated with P300m. There were no significant differences among the six stimulus combinations in evoking those P300m sources. The results of the present study suggest that the MUSIC algorithm could be a useful tool for analysis of the time-course of P300m.

Dynamic neural activity recorded from human amygdala during fear conditioning using magnetoencephalography

Magnetoencephalography (MEG) was used to record the dynamics of amygdala neuronal population activity during fear conditioning in human participants. Activation during conditioning training was compared to habituation and extinction sessions. Conditioned stimuli (CS) were visually presented geometric figures, and unconditioned stimuli (US) were aversive white-noise bursts. The CS+ was paired with the US on 50% of presentations and the CS- was never paired. The precise temporal resolution of MEG allowed us to address the issue of whether the amygdala responds to the onset or offset of the CS+, and/or the expectation of the initiation or offset of the an omitted auditory US. Fear conditioning elicited differential amygdala activation for the unpaired CS+ compared to the CS-, extinction and habituation. This was especially robust in the right hemisphere at CS onset. The strongest peaks of amygdala activity occurred at an average of 270 ms in the right and 306 ms in the left hemisphere following unpaired CS+ onset, and following offset at 21 ms in the left and 161 ms in the right (corresponding to an interval of 108 ms and 248 ms after the anticipated onset of the US, respectively). However, the earliest peaks in this epoch preceded US onset in most subjects. Thus, the activity dynamics suggest that the amygdala both differentially responds to stimuli and anticipates the arrival of stimuli based on prior learning of contingencies. The amygdala also shows stimulus omission-related activation that could potentially provide feedback about experienced stimulus contingencies to modify future responding during learning and extinction.

Age-related effects on superior temporal gyrus activity during an auditory oddball task

We used magnetoencephalography in combination with magnetic resonance imaging to investigate the effects of aging on the temporal dynamics of activity localized to several brain regions during an auditory oddball task. The most interesting effects were noted in the superior temporal gyrus as follows: (1) responses were generally stronger to rare than to frequent tones throughout the entire 600-ms time interval, and (2) increases in the amplitude of the 40-ms peak and the latency of the maximum late response were evident in the elderly. Although superior temporal gyrus activity has traditionally been associated with early sensory processing, these results suggest that superior temporal gyrus activity is also important for later decision-related processing.

Magnetic fields in the human hippocampal area evoked by a somatosensory oddball task

We recorded neuromagnetic fields evoked by a somatosensory oddball task requiring subjects to discriminate target stimuli from nontarget stimuli, which are different in stimulation intensity, with a mental count of the target stimuli. A whole head type 80-channel magnetoencephalography (MEG) system with a 50-mm baseline gradiometer array was used. Targets and nontargets were somatosensory stimuli with an electrical current intensity of twice and three times the sensory threshold, respectively. The source current locations of the evoked magnetic fields in three dimensions and the equivalent current dipole (ECD) moments were calculated by using a single dipole model, assuming the brain as a sphere. In 28 of 63 recording sessions for 7 subjects, the loci of neuronal activities were observed in the left and/or right hippocampal areas. The latency of left hippocampal activity (293 +/- 49 ms) was significantly shorter than that of the right (333 +/- 45 ms) (P = 0.027, non-paired t-test). In view of previous studies that have shown the time window of sensory integration as approximately 200-300 ms, sensory information may be transferred to hippocampal areas following sensory integration.

Dynamics of cognitive processing in the human hippocampus by neuromagnetic and neurochemical assessments

Various features of cognitive processing have been studied using event-related electric potentials and magnetoencephalography (MEG), as well as neuroimaging. Magnetic resonance spectroscopy (MRS) is a unique, noninvasive approach of measuring cellular metabolism that reflects the static metabolic state. The present study is the first to show noninvasively the dynamic neurophysiological and metabolic changes that occur during cognitive processing in vivo in the human hippocampus, as measured by MEG and spin-echo dynamic (1)H-MRS time-locked to the onset of the stimulus. The stimuli consisted of unpleasant and pleasant pictures of faces of human or primate babies. The event-related synchronization of theta activity and levels of creatine/phosphocreatine and choline-containing compounds relative to the respective level in the resting condition increased significantly, more in the right hippocampus than in the left, during the target discrimination task and also more in the right hippocampus in response to the unpleasant target picture than the pleasant one. These results suggest that excitatory postsynaptic metabolism in the hippocampus, especially in the right hippocampus, is involved in discriminative and cognitive processing of emotional information. This newly devised method combining event-related MEG with MRS can be used to noninvasively elucidate the dynamic features of neurophysiology and neurochemical metabolism and represents a promising approach toward improving our understanding of brain pathophysiology.

Differential activation in the medial temporal lobe during a sound-sequence discrimination task across age in human subjects

To elucidate the brain mechanisms to encode sequential events, event-related potentials (ERPs) were recorded during a sound-sequence discrimination task using young and middle-aged adult subjects. In the task, a series of six or 12 kinds of natural sounds were sequentially presented; 70-80% of the stimuli were presented in a fixed order (Non-target), but the remaining stimuli, in a random order (Target). The subjects were instructed to detect the Targets and press a button at the end of each Target. In a control task, the same sounds were randomly presented (Control), and they were instructed to press the button at the end of each sound. Behavioral results indicated that the young subjects learned the task faster than did the middle-aged subjects. Positive ERP waves were evoked by Targets and Non-targets in the parieto-occipital area around 300-700 ms after stimulus onset. The mean amplitudes during this period in the young subjects were larger in Target than Control conditions, and those in Target condition were larger in the young than middle-aged subjects. Furthermore, the mean amplitudes in the Target condition were significantly correlated with behavioral performance. Equivalent dipoles for the ERPs evoked by Targets were estimated in the medial temporal lobe including the hippocampal formation and parahippocampal gyrus. The results suggest that the ERPs around 300-700 ms latency are involved in sound-sequence information processing. Furthermore, decrease in amplitudes of this positivity in the middle-aged subjects suggests that age-related memory decline is associated with deficits in encoding and retrieval of unfamiliar sequence.

The longitudinal axis of the hippocampal formation: its anatomy, circuitry, and role in cognitive function

The ability to learn and access new memories requires an intact hippocampal formation, a complex three-dimensional structure that spans the anterior-posterior aspect of the temporal lobe. Historically, the transverse axis has dominated studies exploring mnemonic properties of the hippocampus, but in the last decade the importance of the long axis has been coming into focus. Anatomical and physiological findings are reviewed suggesting that the long axis functions as a circuit. Recent imaging studies investigating the long axis as a circuit are summarized, pointing to specific mechanisms that can account for how the hippocampus associates separate sensory input during memory acquisition and recall.

Objective markers of drug effects on brain function from recordings of scalp potential in healthy volunteers

In order to stress the importance of P300 responses in drug development, we describe the spatiotemporal characteristics of this objective, evoked event-related potential. These brain activations reflect mnemonic function, in which limbic structures play a role. It is demonstrated that a pharmacological challenge concerning, for example, the cholinergic system in young healthy volunteers induces modifications in P300 reminiscent of the aging brain. We use this type of observation to build a model in which it can be verified whether the deterioration can be counteracted by treatment with “cognition-enhancing” drugs. If we accept the extrapolation of the pharmacological effects to symptomatology, scalp potential analysis offers an appropriate tool for the study of drug interactions in early proof-of-concept models.

People with absolute pitch process tones with producing P300

We recorded a P300 component of event-related potentials associated with auditory oddball tasks in nine absolute pitch (AP) possessors and seven non-AP possessors. The previous studies demonstrated that AP possessors did not appear to employ working memory during auditory oddball tasks because they have a fixed tonal template in their memories. However, the present findings showed that the AP possessors exhibited similar P300 as the non-AP possessors and did update the tonal context in the auditory oddball tasks. This result suggests that the AP possessors do not always refer to the fixed tonal template in their memories when executing the oddball tasks and they employ working memory properly according to the difficulty of the auditory tasks.

Cortical potentials related to assessment of pain intensity with visual analogue scale (VAS)

OBJECTIVES

To elucidate brain mechanisms underlying the psychophysical processes to measure pain intensity, pain-related somatosensory evoked potentials (pain SEPs) following painful CO(2) laser stimulation were studied while employing a task to measure intensity of pain on a visual analogue scale (VAS).

METHODS

In 12 healthy subjects, 3 kinds of CO(2) laser stimuli, different in intensity as determined by irradiation duration of 40, 60 and 80ms, were randomly delivered to the left hand dorsum at an irregular interval of 4-6s. The subject was requested to assess the intensity of each pain stimulus and point to the VAS scale by moving a pointer held with the right hand according to the subjective feeling of pain sensation (pain intensity assessment (PIA) condition). For the control condition, the subject moved the pointer to the midpoint of the VAS line irrespective of the pain intensity (control motor task condition). Electroencephalograms were recorded from 21 scalp electrodes, referenced to the linked earlobes, and were averaged time-locked to the stimulus onset for each stimulus duration as well as for each task condition.

RESULTS

The VAS scores were 2.8+/-0.5/10 for the stimulus of 40ms duration, 4.8+/-0.8/10 for 60ms and 6.1+/-0.9/10 for 80ms, and showed a highly significant positive correlation with the stimulus duration. Following the early components of pain SEPs which were affected by stimulus duration but not modulated by task conditions, a surface-positive peak at latency of 612-642ms was identified exclusively under the PIA condition regardless of the stimulus intensity and was called 'intensity assessment-related potential (IAP)'. The IAP was maximal at the midline parietal area and symmetrically distributed over the scalp. Neither latency nor amplitude of the IAP was significantly different among the 3 different stimulus intensities.

CONCLUSIONS

IAP is an event-related potential (ERP) associated with assessment of pain intensity but not influenced by pain intensity itself. From its scalp distribution, it can be assumed that the assessment of pain intensity involves multiple areas in both hemispheres.

The temporal profile of interactions between sensory information from both hands in the secondary somatosensory cortex

OBJECTIVE

To elucidate the temporal profile of interactions between sensory information from both hands in the somatosensory cortex.

METHODS

Somatosensory evoked fields (SEFs), generated by stimulation applied to the right index finger after a preceding stimulation to the left index finger, were recorded using a whole head-type magnetoencephalography (MEG). The paired electrical stimuli were applied with a stimulation onset asynchrony (SOA) of 50, 100, 200, 300, or 400 ms.

RESULTS

The mean SEF intensities in the primary somatosensory area (SI) of five subjects, which were evoked approximately 40 ms after the latter of the paired stimuli, were not significantly smaller than that evoked in the control condition when only the right finger was stimulated. In contrast, SEFs in the secondary somatosensory area (SII), generated approximately 100 ms after the stimuli, were suppressed when the paired stimuli were applied at an SOA of 100 ms (P<0.05, t test). In addition, SEFs at approximately 150 ms after the stimuli were significantly suppressed at SOAs of 50, 100 (P<0.05), 200, and 300 ms (P<0.1).

CONCLUSION

Within a time window of approximately 300 ms, sensory information from the left finger significantly affected the SEFs generated by sensory inputs from the right finger. This time window may be required for the integration of sensory input.

Theta oscillations index human hippocampal activation during a working memory task

Working memory (WM) is the ability to retain and associate information over brief time intervals. Functional imaging studies demonstrate that WM is mediated by a distributed network including frontal and posterior cortices, hippocampus, and cerebellum. In rodents, the presentation of stimuli in a WM task is followed by a reset of the phase of hippocampal theta. In this paper we report the observation of a similar phenomenon in normal human subjects. Neuromagnetic responses were recorded during presentation of a set of digits and a subsequent probe of the retained items. All stimuli were presented with a fixed temporal pattern. We observed phase reset of approximately 7 Hz theta in left hippocampus approximately 120 ms after probe stimuli, whereas reset of theta in right hippocampus was visible approximately 80 ms prior to these anticipated stimuli. The duration of stimulus-locked theta increased with memory load, with a limiting value of approximately 600 ms for 5-7 retained items. We suggest that, as in rats, stimulus-locked theta may index involvement of human hippocampal networks in the cognitive processing of sensory input. The anticipatory phase reset of theta indicates involvement of hippocampus in right hemisphere and cerebellar timing networks. Hippocampal structures are essential for orientation to perturbations in the sensory scene, a function that requires use of a context established by a constellation of stimuli. We suggest that the initiation and maintenance of stimulus-locked hippocampal theta observed here may facilitate processing of potentially salient and/or novel input with respect to a context established by the contents of WM.

Interactive processing of sensory input and motor output in the human hippocampus

Recent studies of visuomotor integration suggest that the motor system may be intimately involved in the detection of salient features of the sensory scene. The final stages of sensory processing occur in hippocampal structures. We measured human neuromagnetic responses during motor reaction to an auditory cue embedded in high-speed multimodal stimulation. Our results demonstrate that large-scale cognitive networks may recruit additional resources from the hippocampus during sensorimotor integration. Hippocampal activity from 300 msec before to 200 msec after cued movements was enhanced significantly over that observed during self-paced movements. The dominant hippocampal activity appeared equally synchronized to both sensory input and motor output, consistent with timing by an intrinsic mechanism, possibly provided by ongoing theta oscillations

Cortical visuomotor integration during eye pursuit and eye-finger pursuit

To elucidate cortical mechanisms of visuomotor integration, we recorded whole-scalp neuromagnetic signals from six normal volunteers while they were viewing a black dot moving linearly at the speed of 4 degrees /sec within a virtual rectangle. The dot changed its direction randomly once every 0.3-2 sec. The subject either (1) fixated a cross in the center of the screen (eye fixation task), (2) followed the moving dot with the eyes (eye pursuit task), or (3) followed the dot with both the eyes and the right index finger (eye-finger pursuit task). Prominent magnetic signals, triggered by the changes of the direction of the dot, were seen in all conditions, but they were clearly enhanced by the tasks and were strongest during the eye-finger pursuit task and over the anterior inferior parietal lobule (aIPL). Source modeling indicated activation of aIPL [Brodmann's area (BA) 40], the posterosuperior parietal lobule (SPL; BA 7), the dorsolateral frontal cortex (DLF; BA 6), and the occipital cortex (BA 18/19). The activation first peaked in the occipital areas, then in the aIPL and DLF, and some 50 msec later in the SPL. Our results suggest that all these areas are involved in visuomotor transformation, with aIPL playing a crucial role in this process.

Modality-specific subregions in human inferior parietal lobule: a magnetoencephalographic study during cognitive tasks

The inferior parietal lobule (IPL) has been considered to be a multimodal sensory association area. Both event-related potentials and magnetic responses have examined the relationships between IPL and cognitive processing. However, there have been no studies clarifying the functional subregions in IPL. We studied the event-related magnetic response during conventional auditory and visual oddball paradigms. We were able to distinguish non-invasively modality-specific subregions in IPL. The subregion in IPL activated by auditory target stimuli was located more anterior and superior than that responding to visual target stimuli on each hemisphere. The data suggests that modality-specific subregions in the IPL are differentially activated by auditory or visual stimuli.