Sensory gating in the human hippocampal and rhinal regions: regional differences.

To further explore the roles of medial temporal structures in mediating sensory gating of incoming irrelevant or redundant auditory input, twenty-seven patients with intractable epilepsy with depth electrodes implanted in the medial temporal lobe for presurgery evaluation underwent evoked response recording to auditory paired-stimuli (S1-S2). Seventeen subjects were diagnosed with left medial temporal lobe epilepsy (MTLE) and 10 with right MTLE. Only data from the nonlesion side were included. Twenty-three records from rhinal and anterior hippocampal regions, and 21 from posterior hippocampal regions were included in the analysis. The rhinal region had two prominent components (a negativity peaking around 200 ms followed by a positivity peaking around 400 ms). Both the anterior and posterior hippocampal regions exhibited a dominant negative potential peaking around 400 ms. These components were all composed predominantly of slower frequencies. In contrast, a negativity in the posterior hippocampus at around 100 ms was composed of slow and fast frequencies. All components but the early rhinal negativity were attenuated by stimulus repetition. This is the first report documenting that different regions of the medial temporal area are differentially involved in the processing of auditory input, most likely reflecting separate steps of processing. The data support the need for further exploration of the contribution of these regions to sensory gating. This information helps to increase our understanding of this basic but important and complex function.

Generators of the intracranial P50 response in auditory sensory gating.

Clarification of the cortical mechanisms underlying auditory sensory gating may advance our understanding of brain dysfunctions associated with schizophrenia. To this end, data from nine epilepsy patients who participated in an auditory paired-click paradigm during pre-surgical evaluation and had grids of electrodes covering temporal and frontal lobe were analyzed. A distributed source localization approach was applied to the intracranial P50 response and the Gating Difference Wave obtained by subtracting the response to the second stimuli from the response to the first stimuli. Source reconstruction of the P50 showed that the main generators of the response were localized in the temporal lobes. The analysis also suggested that the maximum neuronal activity contributing to the amplitude reduction in the P50 time range (phenomenon of auditory sensory gating) is localized at the frontal lobe. Present findings suggest that while the temporal lobe is the main generator of the P50 component, the frontal lobe seems to be a substantial contributor to the process of sensory gating as observed from scalp recordings.

Sensory gating of auditory evoked and induced gamma band activity in intracranial recordings.

Oscillatory activity in the gamma band range (30-50 Hz) and its functional relation to auditory evoked potentials (AEPs) is yet poorly understood. In the current study, we capitalized on the advantage of intracranial recordings and studied gamma band activity (GBA) in an auditory sensory gating experiment. Recordings were obtained from the lateral surface of the temporal lobe in 34 epileptic patients undergoing presurgical evaluation. Two kinds of activity were differentiated: evoked (phase locked) and induced (not phase locked) GBA. In 18 patients, an intracranial P50 was observed. At electrodes with maximal P50, evoked GBA occurred with a similar peak latency as the P50. However, the intensities of P50 and evoked GBA were only modestly correlated, suggesting that the intracranial P50 does not represent a subset of evoked GBA. The peak frequency of the intracranial evoked GBA was on average relatively low (approximately 25 Hz) and is, therefore, probably not equivalent to extracranially recorded GBA which has normally a peak frequency of approximately 40 Hz. Induced GBA was detected in 10 subjects, nearly exclusively in the region of the superior temporal lobe. The induced GBA was increased after stimulation for several hundred milliseconds and encompassed frequencies up to 200 Hz. Single-trial analysis revealed that induced GBA occurred in relatively short bursts (mostly <<100 ms), indicating that the duration of the induced GBA in the averages originates from summation effects. Both types of gamma band activity showed a clear attenuation with stimulus repetition.

Sensory gating in the human hippocampal and rhinal regions.

OBJECTIVE: The objective of this work was to ascertain if sensory gating can be demonstrated within the human medial temporal lobe. METHODS: Eight patients with intractable epilepsy with depth electrodes implanted in the medial temporal lobe for pre-surgery evaluation underwent evoked response recording to auditory paired-stimuli (S1-S2). Each of the eight subjects had a diagnosis of left medial temporal lobe epilepsy (MTLE). RESULTS: Data from the non-focal right hippocampi revealed a large negative response on S1 (starting at about 190 ms and lasting for approximately 300 ms from stimulus onset). Rhinal region recordings revealed a positive response (starting at about 240 ms with a rapid incline, followed by a long-lasting decline). A significant attenuation of both responses to S2 stimuli was observed. CONCLUSIONS: Data are suggestive of an involvement of the human medial temporal lobe in the processing of simple auditory information which occurs in a time frame later than the neocortical auditory evoked components. The exact role of these anatomical structures in the sensory gating process remains to be defined. SIGNIFICANCE: This study provides the first evidence of an activation of the rhinal cortex after simple auditory stimulation and provides new evidence that the activation of the medial temporal lobe structures occurs at a later stage than that of the neocortex.

Impaired niacin sensitivity in acute first-episode but not in multi-episode schizophrenia.

Niacin (vitamin B3) flushing--a marker of altered prostaglandin signaling--is indirectly linked to the phospholipid-prostaglandin metabolism. Diminished skin flushing was repeatedly found in schizophrenia, but has not been systematically investigated at different stages of disorder as yet. We compared niacin sensitivity of 32 first-episode and 32 multi-episode patients (mainly on stable medication) with age and gender matched healthy controls. Methylnicotinate was applied in three concentrations onto the inner forearm skin. Flush response was assessed in 3 min intervals over 15 min using optical reflection spectroscopy. Whereas first-episode patients showed significantly diminished flush response as compared to controls, comparable differences were not found between multi-episode patients and controls. Comparison of niacin sensitivity at different stages of schizophrenia support the notion of altered prostaglandin signaling primarily at the onset of disorder. Longitudinal studies have to rule out possible long-term effects of neuroleptic medication.

The dipole location shift within the auditory evoked neuromagnetic field components N100m and mismatch negativity (MMNm).

OBJECTIVE: To investigate whether within the latency range of the neuromagnetic mismatch negativity (MMNm) a similar dipole shift can be observed as found in previous studies for the auditory evoked field component N100m. METHODS: For this purpose selected neuromagnetic data of 29 subjects were analysed in a time window of 15 ms before and 15 ms beyond the peak maximum of both components. In our oddball paradigm, we applied frequency, duration and intensity deviants within one block. The time course of dipole location was analysed by means of a repeated measure analysis of variance (ANOVA). RESULTS: As expected, N100m dipoles shifted significantly from superior to inferior and from posterior to anterior within their latency range. In analogy, the MMNm dipoles of duration and intensity deviants also exhibited a significant shift from posterior to anterior within their latency range. However, the MMNm dipoles of all 3 deviants did not shift significantly from superior to inferior. Concerning this direction, the ANOVA revealed a significant TIME*COMPONENT interaction between the N100m and the MMNm of duration and intensity deviants. CONCLUSIONS: The finding of a different time course of N100m and MMNm dipoles does not support the assumption that the MMN represents an amplitude- and latency-modulated auditory N100 response.

[Mismatch negativity in schizophrenia research. An indicator of early processing disorders of acoustic information]. / Die Mismatch-Negativität (MMN) in der Schizophrenieforschung. Ein Indikator früher Störungen der akustischen Informationsverarbeitung.

Mismatch negativity (MMN) represents an event-related component of the auditory evoked potentials at about 100-250 ms, evoked by discernible changes in an ongoing uniform acoustic stimulation. The current paper reviews all recently published MMN studies in the field of schizophrenia research. A reduced MMN in schizophrenic patients is found in the majority of the studies. This deficit is likely to be related to the disorder, since antipsychotic medication seems to have little influence on these results. Interestingly, a reduced MMN is also found in first-degree relatives of patients. Clear evidence for a hemispheric lateralization of the MMN reduction in schizophrenic patients is lacking. A hypofunction of the N-methyl-D-aspartate (NMDA) receptor is discussed as a possible explanation of this deficit.

Stimulus duration influences the dipole location shift within the auditory evoked field component N100m.

The equivalent source of the neuromagnetic auditory evoked field (AEF) component N100m shifts systematically within its latency range. In the current study, possible effects of stimulus duration on this shift were analysed. 15 subjects were stimulated monaurally with tones of different duration (50, 100, 200 ms) and AEFs were recorded successively over both hemispheres. Dipoles were calculated in 5-ms-steps from 15 ms before to 15 ms after the N100m peak maximum. A dipole location shift within the N100m latency from posterior to anterior and from superior to inferior was observed. The shift in anterior-posterior direction was found to be larger in the right compared to the left hemisphere. Stimulus duration significantly affected the degree of dipole shift in this direction. It was found to be shorter the shorter the stimulus.

Habituation of the auditory evoked field component N100m and its dependence on stimulus duration.

OBJECTIVES: In the current study we investigated the habituation of the neuromagnetic auditory evoked field (AEF) component N100m and its dependence on stimulus duration. METHODS: Fifteen subjects were stimulated monaurally in 3 blocks of 210 trials with 1000 Hz tones of different duration (50, 100, 200 ms) and in a single block of 70 trials with 1200 Hz tones. The order of blocks was counterbalanced within the total experiment. Tones were separated by a stimulus onset asynchrony of 2000 ms. AEF were recorded over both hemispheres by means of a 31 channel system (Philips) on two different days. The AEF were compared within and between the blocks and between the hemispheres. Additionally, the effects of block order were analyzed. RESULTS: In the course of the experiment a pronounced decrease of N100m mean global field power (MGFP) and an increase of its latency were observed. While the order of blocks clearly affected the degree of habituation, stimulus duration did not have any influence on it. Within the blocks, habituation also had an impact on dipole location in inferior-superior direction. The application of the 1200 Hz stimulus led to a slight response recovery. The change in tone pitch affected the dipole orientation, as an indicator for the tonotopic organization of the auditory cortex.

Use of remote sensing to detect soybean cyst nematode-induced plant stress.

Integrating remote sensing and geographic information systems (GIS) technologies offers tremendous opportunities for farmers to more cost effectively manage the causes of crop stress. Initial soybean cyst nematode (SCN) population densities from 995 2-x-3-m quadrats were obtained from a soybean field near Ames, Iowa, in 2000. The percentage of sunlight reflected from each quadrat was measured weekly using a multispectral radiometer beginning in mid-May and continuing through mid-September. Aerial images were obtained at heights above the field ranging from 45 to 425 m on 12 dates during the soybean growing season. This was accomplished using color film and infrared film in conjunction with a filter to measure reflectance in the near-infrared region (810 nm). Satellite images (Landsat 7) were obtained for five dates during the 2000 growing season. Maps depicting initial SCN population densities, soybean yield, soy oil, and soy protein were generated using the GIS software program ArcView. Percentage reflectance (810 nm), aerial image intensity, and satellite image intensity data then were regressed against soybean yield, soy oil, and soy protein concentrations obtained from each geospatially referenced soybean quadrat. Percentage reflectance measurements explained up to 60% of the variation in initial SCN population densities within soybean quadrats and up to 91% of the variation in soybean yield. Aerial image and satellite image intensities explained up to 80% and 47% of the variation in soybean yield, respectively. Percentage reflectance data also explained 36% and 54% of the variation in oil and protein concentrations of the harvested soybeans, respectively. These results indicate that remote sensing coupled with GIS technologies may provide new tools to detect and quantify SCN population densities and their impacts on the quantity and quality of soybean yield.

Effect of ketamine on the neuromagnetic mismatch field in healthy humans.

Mismatch negativity (MMN) is a component of the auditory evoked event-related potentials (ERP) that assesses automatic sound change detection and is disturbed in schizophrenic patients. Animal experimental evidence has linked the generation of MMN to the N-methyl-D-aspartate (NMDA) receptor. We investigated the neuromagnetic mismatch field (MMF) in healthy volunteers before and after intravenous application of a subanesthetic dose of the NMDA receptor antagonist ketamine (0.3 mg/kg). Ketamine had a significant influence on latency and dipole moment of the MMF, whereas the N100m latency of the standard tone was not prolonged and its dipole moment remained stable. Our results suggest that ketamine interferes with aspects of preattentive information processing and is in line with the view that disturbed NMDA receptor function may mediate the deficient auditory mismatch response in patients with schizophrenia.

Habituation of the auditory evoked field component N100m in male patients with schizophrenia.

The auditory evoked field (AEF) component N100m represents the most prominent and stable peak of the AEF, and its alterations in patients with schizophrenia are an extensive topic of neuropsychiatric research. In our current study, the degree of N100m habituation was investigated in 20 male schizophrenics and 19 healthy male controls. Participants were stimulated monaurally with 270 trials of 1000 Hz tones separated by an interstimulus interval between 800-1800 ms. The trial sample of the bilaterally recorded AEF was separated into three consecutive blocks of 90 trials and these blocks were compared with each other. The mean global field power (MGFP) of the N100m decreased on average 9.1% from the first to the third trial block, while the N100m latency was increasing. The analyses of the influence of habituation revealed a systematic change of dipole location in inferior-superior direction, mainly in the left hemisphere. This habituation effect was found to be the same for both groups. The groups also did not differ in the N100m latency increase and MGFP decrease, except for one parameter. The right-hemispheric MGFP decrease from the first to second block was found to be more pronounced in patients compared to controls. However, this difference was related to medication with clozapine. Overall, the habituation behaviour of the N100m seems to be undisturbed in schizophrenic patients.

Impaired sensory processing in male patients with schizophrenia: a magnetoencephalographic study of auditory mismatch detection.

The generation of mismatch negativity (MMN) as a component of auditory evoked event-related brain potentials has been reported previously to be severely disturbed in patients with schizophrenia. In the present study, we extended these findings to magnetoencephalography and investigated the neuromagnetic mismatch field (MMNm) in 15 male schizophrenic inpatients as compared to 16 healthy male volunteers. A standard tone of 1000 Hz and three different types of mismatch (1050-Hz tone, 5000-Hz tone, tone omission) were employed within the same paradigm, each mismatch occurring with a 10% pseudorandom probability. After correction for eye artifacts, the mean global field power of the mismatch reaction was calculated. Mismatch generation in patients with schizophrenia proved to be significantly impaired for all three conditions. This result confirms the theory of impaired auditory information processing in patients with schizophrenia at the level of the primary auditory cortex. Deficient generation of MMNm probably represents an impaired generation and/or faster decay of the sensory memory trace on the basis of disturbed sensory processing in male patients with schizophrenia.

Enlarged gamma band response of neuromagnetic auditory evoked fields in a visually impaired subject.

Under acoustic stimulation a phase-locked response in the gamma band (near 40 Hz) in the latency range between 20 and 130 ms is evoked. We report on a considerably visually impaired woman with Grönblad-Strandberg syndrome which involves degeneration at the level of retina, but has no overt central nervous component to the degeneration. The subject exhibited an extraordinarily high power in the phase-locked gamma band response (GBR) which was found to be more than three, and sometimes more than four, standard deviations above the average of a group of 25 subjects with normal vision. Furthermore, the dipoles of her mismatch reaction and M200 were found to be located posteriorly to the dipoles of the M100. Overall, both enlarged GBR and changed cortical representation could be results of cortical plasticity related to visual impairment.

[Sex specific differences in hemispheric lateralization in schizophrenia? An MEG-MRI study]. / Geschlechtsspezifische Unterschiede der Hemisphärenlateralisation bei Schizophrenien? Eine MEG-MRT-Studie.

In this magnetoencephalography study the issue of hemispheric lateralisation in patients with schizophrenia was addressed using acoustically evoked neuromagnetic fields. The characteristics of dipoles in the superior temporal gyrus, the primary auditory cortex, were calculated. In contrast to other studies, alterations did not concern the localisation, but rather the orientation of dipoles. Of pathophysiological interest was that the dipoles abnormalities were found left-hemispherically in male (p = 0.02) and right-hemispherically in female patients with schizophrenia (p = 0.01) when compared to controls. The findings suggest gender-specific alterations of hemispheric lateralisation in schizophrenia.

Hemispheric differences in frequency dependent dipole orientation of the human auditory evoked field component N100m.

Auditory evoked fields (AEF) of 19 healthy male subjects were recorded bilaterally with a Philips 31 -channel biomagnetometer, using two conditions of stimulation (1000 vs. 5000 Hz tones). The N100m latency was characterized by a single moving dipole for each condition and hemisphere using a boundary element model (BEM) as volume conductor. While the right hemispheric dipole orientations and locations did not change with respect to condition, the left hemispheric dipoles differed significantly between the 1000 and 5000 Hz tones, especially in dipole orientation. The left hemispheric dipoles were orientated on average 10.8 degrees more vertically for the 5000 Hz condition. This result points to interhemispheric differences on the level of sensory processing.

Quantification and rejection of ocular artifacts in auditory evoked fields in schizophrenics.

RESULTS: In a magnetoencephalographic investigation of the auditory evoked field (AEF) in 17 schizophrenics and 17 controls, 37% of the schizophrenics and 12% of the controls showed eye artifacts in every second trial or even more frequently. In the uncorrected average fields, the ratio between the power of artifacts and the power of the magnetoencephalogram (MEG) exceeded the value of 0.1 for 48% of the schizophrenics and for 29% of the controls. Ocular artifacts biased the locations of equivalent current dipoles of the M100 component towards deeper positions. A regression algorithm for the correction of ocular artifacts in raw data and an identification technique of ocular artifacts based on the topography of transmission coefficients is described. CONCLUSIONS: A linear dependence of ocular artifacts in AEF on the electrooculogram (EOG) was confirmed. Possible errors introduced by the correction are discussed. Transmission coefficients should be calculated for several individual trials with the same type of artifact. Errors due to evoked potentials in the EOG were found to be comparable in amplitude to noise in the AEF. Examples of transmission coefficients from the EOG to the MEG are given.

Reliability of dipole localization for the movement-evoked field component MEF I.

The movement-evoked field I (MEF I) component is the largest and most stable neuromagnetic component accompanying self-paced movements. In order to use MEG for studying dynamic changes in the cortical organization of movements, data about the reliability and variability of these neuromagnetic components for individual subjects must be established during different sessions. For this aim, three male subjects were requested to perform self-paced flexions of their index finger and thumb in repeated sessions while the MEG was recorded by a 31 channel system. The MEF I was identified for each session and a single equivalent dipole was calculated for this component. The dipole localizations of the various sessions were compared. The standard deviation of the localization for all persons and all values amounts to 4.0-5.2 mm for the three spatial dimensions. Our data suggest that the spatial distance between two single focal sources fitted to the MEF I must be greater than 14 mm to be interpreted as distinct. However, the neuromagnetic field structure and the resulting dipole localization of the MEF I component are quite stable and could be used for the evaluation of cortical plasticity.

Internal consistency of dipole localizations for the human movement-evoked magnetic field component 1 (MEF 1).

The present magnetoencephalographic study was conducted in order to assess the accuracy of dipole localizations for the movement-evoked field component 1 (MEF 1). Three male subjects were requested to perform self-paced flexions of their index finger and thumb in repeated sessions of 60 trials while the neuromagnetic field was recorded by a 31 channel system. Single moving dipole localizations were performed for the MEF 1. The error within single sessions was calculated by split-half reliability and window-homogeneity in a total of 61 sessions. The mean spatial deviation between both halves amounted to 3.8 mm. The window-homogeneity was found to be 2 mm deviation/10 ms.