Magnetic sources of the M50 response are localized to frontal cortex

Revisiting the relationship between neural correlates of sensory gating and self-reported sensory gating inventory: An MEG investigation

BACKGROUND

Accumulated evidence has revealed that bilateral superior temporal gyrus (STG), inferior frontal gyrus (IFG), and inferior parietal lobule (IPL) are involved in the processes of sensory gating (SG). However, it remains unknown which neural correlate(s) of SG specifically reflect individuals' perceptual experiences, as measured by the Sensory Gating Inventory (SGI). Thus, this study aims to investigate the relationship of SGI with cortical SG-related regions. Furthermore, we examine whether SG hemispheric asymmetry exists, which is still an inconclusive issue.

METHODS

Twenty-two healthy young adults performed the auditory paired-stimulus paradigm during magnetoencephalographic recordings. SG of M50 and M100 was measured as ratios (S2/S1) and differences (S1-S2). They were also evaluated with SGI, which factored into three categories of Perceptual Modulation, Distractibility, and Over-Inclusion. SG in the STG, IFG, and IPL were compared between left and right hemispheres, and were used to determine the relationship with SGI.

RESULTS

Only M100 SG differences (S1-S2) of the right IFG were significantly correlated with scores of Perceptual Modulation (partial r = -0.392, p = 0.040) and total SGI scores (partial r = -0.387, p = 0.041). However, we did not find significant lateralization of M50 SG and M100 SG in any studying region.

CONCLUSIONS

The individual's perceptual experience is specifically related to electrophysiological SG function of the right IFG.

Neuromagnetic evidence of abnormal automatic inhibitory function in subjective memory complaint

Subjective memory complaint (SMC), a self-perceived worsening in memory capacity concurrent with normal performance on standardized cognitive assessments, is considered a risk factor for the development of Alzheimer's disease (AD). Deficient sensory gating (SG), referring to the lack of automatic inhibition of neural responses to the second identical stimulus, has been documented in prodromal and incident AD patients. However, it remains unknown whether the cognitively normal elderly with SMC demonstrate alterations of SG function compared with those without SMC. A total of 19 healthy controls (HC) and 16 SMC subjects were included in the present study. Neural responses to the auditory paired-stimulus paradigm were recorded by the magnetoencephalography and analyzed by the distributed source imaging method of minimum norm estimate. The SG of M50 and M100 components were measured using the amplitude ratio of the second response over the first response at the cortical level. Compared to HC, subjects with SMC showed significantly increased M50 SG ratios in the inferior parietal lobule (IPL). Furthermore, M50 SG ratios in the right IPL yielded an acceptable discriminative ability to distinguish SMC from HC. However, we did not find a significant association between SG ratios and cognitive function requiring inhibitory control either in the HC or SMC group. In conclusion, although SMC subjects have intact cognitive functioning revealed by objective neuropsychological tests, their deficits in automatic inhibitory function could be detected through neurophysiological recordings. Our results suggest that altered brain function occurs in SMC prior to the obvious decline of cognitive performance.

Dysfunction of Inferior Parietal Lobule During Sensory Gating in Patients With Amnestic Mild Cognitive Impairment

Patients with amnestic mild cognitive impairment (aMCI) demonstrate significant cognitive deficits, especially in the memory aspect. The memory deficiency might be attributed to the difficulties in the inhibitory function to suppress redundant stimuli. Sensory gating (SG) refers to the attenuation of neural responses to the second identical stimulus in a paired-click paradigm, in which auditory stimuli are delivered in pairs with inter-stimulus intervals (ISI) of 500 ms and inter-pair intervals of 6-8 s. It is considered as an electrophysiological signal to reflect the brain's automatic response to gate out repetitive sensory inputs. However, there has been no study systematically investigating SG function in aMCI patients. Thus, the present study used magnetoencephalography (MEG) to record neuromagnetic responses to a paired-click paradigm in 23 healthy controls (HC) and 26 aMCI patients. The Stimulus 2/Stimulus 1 (S2/S1) amplitude ratio was used to represent the SG function. Compared to HC, aMCI patients showed M50 SG deficits in the left inferior frontal gyrus (IFG) and right inferior parietal lobule (IPL). M100 SG defects were also observed in the right IPL. Based on the ROIs showing significant between-group SG differences, we found that a more deficient M50 SG function in the right IPL was associated with poorer performance in the immediate recall of Logic Memory (LM), Chinese Version Verbal Learning Test (CVVLT) and Digit Span Backward (DSB) Test. Furthermore, the M50 SG ratios of the right IPL together with the neuropsychological performance of LM and CVVLT demonstrated very good accuracy in the discrimination of aMCI from HC. In conclusion, compared to HC, aMCI patients showed a significant SG deficit in the right IPL, which was correlated with the auditory short-term memory function. We suggest the combination of SG in the right IPL, LM and CVVLT to be sensitive indicators to differentiate aMCI patients from HC.

P50, N100, and P200 Auditory Sensory Gating Deficits in Schizophrenia Patients

BACKGROUND

Sensory gating describes neurological processes of filtering out redundant or unnecessary stimuli during information processing, and sensory gating deficits may contribute to the symptoms of schizophrenia. Among the three components of auditory event-related potentials reflecting sensory gating, P50 implies pre-attentional filtering of sensory information and N100/P200 reflects attention triggering and allocation processes. Although diminished P50 gating has been extensively documented in patients with schizophrenia, previous studies on N100 were inconclusive, and P200 has been rarely examined. This study aimed to investigate whether patients with schizophrenia have P50, N100, and P200 gating deficits compared with control subjects.

METHODS

Control subjects and clinically stable schizophrenia patients were recruited. The mid-latency auditory evoked responses, comprising P50, N100, and P200, were measured using the auditory-paired click paradigm without manipulation of attention. Sensory gating parameters included S1 amplitude, S2 amplitude, amplitude difference (S1-S2), and gating ratio (S2/S1). We also evaluated schizophrenia patients with PANSS to be correlated with sensory gating indices.

RESULTS

One hundred four patients and 102 control subjects were examined. Compared to the control group, schizophrenia patients had significant sensory gating deficits in P50, N100, and P200, reflected by larger gating ratios and smaller amplitude differences. Further analysis revealed that the S2 amplitude of P50 was larger, while the S1 amplitude of N100/P200 was smaller, in schizophrenia patients than in the controls. We found no correlations between sensory gating indices and schizophrenia positive or negative symptom clusters. However, we found a negative correlation between the P200 S2 amplitude and Bell's emotional discomfort factor/Wallwork's depressed factor.

CONCLUSION

Till date, this study has the largest sample size to analyze P50, N100, and P200 collectively by adopting the passive auditory paired-click paradigm without distractors. With covariates controlled for possible confounds, such as age, education, smoking amount and retained pairs, we found that schizophrenia patients had significant sensory gating deficits in P50-N100-P200. The schizophrenia patients had demonstrated a unique pattern of sensory gating deficits, including repetition suppression deficits in P50 and stimulus registration deficits in N100/200. These results suggest that sensory gating is a pervasive cognitive abnormality in schizophrenia patients that is not limited to the pre-attentive phase of information processing. Since P200 exhibited a large effect size and did not require additional time during recruitment, future studies of P50-N100-P200 collectively are highly recommended.

Attention modulates topology and dynamics of auditory sensory gating

This work challenges the widely accepted model of sensory gating as a preattention inhibitory process by investigating whether attention directed at the second tone (S2) within a paired-click paradigm could affect gating at the cortical level. We utilized magnetoencephalography, magnetic resonance imaging and spatio-temporal source localization to compare the cortical dynamics underlying gating responses across two conditions (passive and attention) in 19 healthy subjects. Source localization results reaffirmed the existence of a fast processing pathway between the prefrontal cortex (PFC) and bilateral superior temporal gyri (STG) that underlies the auditory gating process. STG source dynamics comprised two gating sub-components, Mb1 and Mb2, both of which showed significant gating suppression (>51%). The attention directed to the S2 tone changed the gating network topology by switching the prefrontal generator from a dorsolateral location, which was active in the passive condition (18/19), to a medial location, active in the attention condition (19/19). Enhanced responses to the attended stimulus caused a significant reduction in gating suppression in both STG gating components (>50%). Our results demonstrate that attention not only modulates sensory gating dynamics, but also exerts topological rerouting of information processing within the PFC. The present data, suggesting that the cortical levels of early sensory processing are subject to top-down influences, change the current view of gating as a purely automatic bottom-up process.

Focus on the pedunculopontine nucleus. Consensus review from the May 2018 brainstem society meeting in Washington, DC, USA

The pedunculopontine nucleus (PPN) is located in the mesopontine tegmentum and is best delimited by a group of large cholinergic neurons adjacent to the decussation of the superior cerebellar peduncle. This part of the brain, populated by many other neuronal groups, is a crossroads for many important functions. Good evidence relates the PPN to control of reflex reactions, sleep-wake cycles, posture and gait. However, the precise role of the PPN in all these functions has been controversial and there still are uncertainties in the functional anatomy and physiology of the nucleus. It is difficult to grasp the extent of the influence of the PPN, not only because of its varied functions and projections, but also because of the controversies arising from them. One controversy is its relationship to the mesencephalic locomotor region (MLR). In this regard, the PPN has become a new target for deep brain stimulation (DBS) for the treatment of parkinsonian gait disorders, including freezing of gait. This review is intended to indicate what is currently known, shed some light on the controversies that have arisen, and to provide a framework for future research.

Does COMT val158met polymorphism influence P50 sensory gating, eye tracking or saccadic inhibition dysfunctions in schizophrenia?

Three electrophysiological endophenotypes are routinely studied in schizophrenia (SCZ): smooth pursuit eye movement (SPEM) dysfunction, deficits in P50 auditory-evoked potential inhibition, and saccadic inhibition deficits. The current study aimed to investigate the relationship between the COMT val158met polymorphism and these three endophenotypes. One hundred four SCZ patients (DSM-IV-R criteria) and 89 healthy controls were included in this study. P50 auditory-evoked potential inhibition, antisaccade paradigm and SPEM were analyzed. All individuals were genotyped for the COMT val158met. SCZ patients showed a higher rate of deficits measured by the SPEM, antisaccade and P50 inhibition paradigms without association with COMT val158met. However, in our control group, we have found an association between the Val polymorphism and the smoking status. More importantly, we have found a higher accuracy of saccades during the predictive pursuit task associated to the Met polymorphism in controls but not in SCZ patients who were receiving antidopaminergic medications. This result is in line with the hypothesis of the relationship between the Met polymorphism of the COMT gene, a higher level of dopamine in the prefrontal cortex and the role of the fronto-cerebellar loop in smooth predictive pursuit.

The 10 Hz Frequency: A Fulcrum For Transitional Brain States

A 10 Hz rhythm is present in the occipital cortex when the eyes are closed (alpha waves), in the precentral cortex at rest (mu rhythm), in the superior and middle temporal lobe (tau rhythm), in the inferior olive (projection to cerebellar cortex), and in physiological tremor (underlying all voluntary movement). These are all considered resting rhythms in the waking brain which are "replaced" by higher frequency activity with sensorimotor stimulation. That is, the 10 Hz frequency fulcrum is replaced on the one hand by lower frequencies during sleep, or on the other hand by higher frequencies during volition and cognition. The 10 Hz frequency fulcrum is proposed as the natural frequency of the brain during quiet waking, but is replaced by higher frequencies capable of permitting more complex functions, or by lower frequencies during sleep and inactivity. At the center of the transition shifts to and from the resting rhythm is the reticular activating system, a phylogenetically preserved area of the brain essential for preconscious awareness.

Arousal and the control of perception and movement

Recent discoveries on the nature of the activity generated by the reticular activating system (RAS) suggest that arousal is much more involved in perception and movement than previously thought. The RAS is not simply an amorphous, unspecific region but rather a distinct group of nuclei with specific cell and transmitter types that control waking and modulate such processes as perception and movement. Thus, disturbances in the RAS will affect a number of neurological disorders. The discovery of gamma band activity in the RAS determined that high threshold calcium channels are responsible for generating gamma band activity in the RAS. Results showing that waking is mediated by CaMKII modulation of P/Q-type channels and REM sleep is modulated by cAMP/PK modulation of N-type channels points to different intracellular pathways influencing each state. Few studies address these important breakthroughs. Novel findings also show that the same primate RAS neurons exhibiting activity in relation to arousal are also involved in locomotion. Moreover, deep brain stimulation of this region, specifically the pedunculopontine nucleus (PPN DBS), in Parkinson's disease has salutary effects on movement, sleep, and cognition. Gamma oscillations appear to participate in sensory perception, problem solving, and memory, and coherence at these frequencies may occur at cortical or thalamocortical levels. However, rather than participating in the temporal binding of sensory events, gamma band activity generated in the RAS may help stabilize coherence related to arousal, providing a stable activation state during waking, and relay such activation to the cortex. Continuous sensory input will thus induce gamma band activity in the RAS to participate in the processes of preconscious awareness, and provide the essential stream of information for the formulation of many of our perceptions and actions. Such a role has received little attention but promises to help understand and treat a number of neurological disorders.

Arousal, motor control, and parkinson's disease

This review highlights the most important discovery in the reticular activating system (RAS) in the last 10 years, the manifestation of gamma (γ) band activity in cells of the RAS, especially in the pedunculopontine nucleus (PPN), which is in charge of the high frequency states of waking and rapid eye movement sleep. This discovery is critical to understanding the modulation of movement by the RAS and how it sets the background over which we generate voluntary and triggered movements. The presence of γ band activity in the RAS is proposed to participate in the process of preconscious awareness, and provide the essential stream of information for the formulation of many of our actions. Early findings using stimulation of this region to induce arousal, and also to elicit stepping, are placed in this context. This finding also helps explain the novel use of PPN deep brain stimulation for the treatment of Parkinson's disease, although considerable work remains to be done.

Duration estimation entails predicting when

The estimation of duration can be affected by context and surprise. Using MagnetoEncephaloGraphy (MEG), we tested whether increased neural activity during surprise and following neural suppression in two different contexts supported subjective time dilation (Eagleman and Pariyadath, 2009; Pariyadath and Eagleman, 2012). Sequences of three 300 ms frequency-modulated (FM, control) or pure tones (test) were presented and followed by a fourth FM varying in duration. In test, the last FM was perceived as significantly longer than veridical duration (Tse et al., 2004) but did not differ from the perceived duration in control. Several novel and distinct neural signatures were observed in duration estimation: first, neural suppression of standard stimuli was observed for the onset but not for the offset auditory evoked responses. Second, ramping activity increased with veridical duration in control whereas at the same latency in test, the amplitude of the midlatency response increased with the distance of deviant durations. Third, in both conditions, the amplitude of the offset auditory evoked responses accounted well for participants' performance: the longer the perceived duration, the larger the offset response. Fourth, neural duration demarcated by the peak latencies of the onset and ramping evoked activities indexed a systematic time compression that reliably predicted subjective time perception. Our findings suggest that interval timing undergoes time compression by capitalizing on the predicted offset of an auditory event.

Modulatory role of the prefrontal generator within the auditory M50 network

The amplitude variability of the M50 component of neuromagnetic responses is commonly used to explore the brain's ability to modulate its response to incoming repetitive or novel auditory stimuli, a process conceptualized as a gating mechanism. The goal of this study was to identify the spatial and temporal characteristics of the cortical sources underlying the M50 network evoked by tones in a passive oddball paradigm. Twenty elderly subjects [10 patients diagnosed with mild cognitive impairment (MCI) or probable Alzheimer disease (AD) and 10 age-matched controls] were examined using magnetoencephalographic (MEG) recordings and the multi-dipole Calibrated Start Spatio-Temporal (CSST) source localization method. We identified three cortical regions underlying the M50 network: prefrontal cortex (PF) in addition to bilateral activation of the superior temporal gyrus (STG). The cortical dynamics of the PF source within the 30-100 ms post-stimulus interval was characterized and was found to be comprised of two subcomponents, Mb1c and Mb2c. The PF source was localized for 10/10 healthy subjects, whereas 9/10 MCI/AD patients were lacking the PF source for both tone conditions. The selective activation of the PF source in healthy controls along with the inactivation of the PF region for MCI/AD patients, enabled us to examine the dynamics of this network of activity when it was functional and dysfunctional, respectively. We found significantly enhanced activity of the STG sources in response to both tone conditions for all subjects who lacked a PF source. The reported results provide novel insights into the topology and neurodynamics of the M50 auditory network, which suggest an inhibitory role of the PF source that normally suppresses activity of the STG sources.

Frequency-pattern functional tomography of magnetoencephalography data allows new approach to the study of human brain organization

A method based on a set of new theorems for the analysis of multichannel time series is described, based on precise Fourier transform and coherence analysis of the restored signals from a detailed set of frequency components. Magnetic field recordings of spontaneous and evoked activity by means of magnetic encephalography demonstrated that multichannel precise Fourier spectrum contains a very large set of harmonics with high coherence. The inverse problem can be solved with great precision based on coherent harmonics, so the technique is a promising platform of general analysis in brain imaging. The analysis method makes it possible to reconstruct sites and timing of electrical activity generated by both spontaneous and evoked brain function at different depths in the brain in the millisecond time range.

Electrophysiological aberrations associated with negative symptoms in schizophrenia

Clinical heterogeneity is a confound common to all of schizophrenia research. Deficit schizophrenia has been proposed as a homogeneous disease entity within the schizophrenia syndrome. The use of the Schedule for the Deficit Syndrome (SDS) has allowed the definition of a subgroup dominated by persistent and primary negative symptoms. While a number of studies have appeared over the years examining the electrophysiological correlates of the cluster of negative symptoms in schizophrenia, only a few studies have actually focused on the Deficit Syndrome (DS). In this chapter, electrophysiological investigations utilizing EEG, Evoked Potentials (EPs), polysomnography (PSG), or magnetoencephalography (MEG) to probe "negative symptoms," or "Deficit Syndrome" are reviewed. While this line of research is evidently in its infancy, two significant trends emerge. First, spectral EEG studies link increased slow wave activity during wakefulness to the prevalence of negative symptoms. Second, sleep studies point to an association between decrease in slow wave sleep and prevalence of negative symptoms. Several studies also indicate a relationship of negative symptoms with reduced alpha activity. A host of other abnormalities including sensory gating and P300 attenuation are less consistently reported. Three studies specifically addressed electrophysiology of the DS. Two of the three studies provided evidence suggesting that the DS may be a separate disease entity and not simply a severe form of schizophrenia.

Change-related auditory P50: a MEG study

Changes in continuous sounds elicit a preattentive component that peaks at around 100ms (Change-N1m) on electroencephalograms or magnetoencephalograms (MEG). Change-N1m is thought to reflect brain activity relating to the automatic detection of changes, which facilitate processes for the execution of appropriate behavior in response to new environmental events. The aim of the present MEG study was to elucidate whether a component relating to auditory changes existed earlier than N1m. Change-related cortical responses were evoked by abrupt sound movement in a train of clicks at 100Hz. Sound movement was created by inserting an interaural time delay (ITD) of 0.15, 0.25, 0.35, and 0.45ms into the right ear. Ten out of 12 participants exhibited clear change-related cortical responses earlier than Change-N1m at around 60ms (Change-P50m). The results of source analysis showed that Change-P50m originated from the superior temporal gyrus of both hemispheres and that its location did not differ significantly from dipoles for the response to the sound onset. The magnitude of Change-P50m increased and the peak latency shortened with an increase in the ITD, similar to those of Change-N1m. These results suggest that change-related cortical activity is present as early as its onset latency at around 50ms.

Hyper-arousal decreases human visual thresholds

Arousal has long been known to influence behavior and serves as an underlying component of cognition and consciousness. However, the consequences of hyper-arousal for visual perception remain unclear. The present study evaluates the impact of hyper-arousal on two aspects of visual sensitivity: visual stereoacuity and contrast thresholds. Sixty-eight participants participated in two experiments. Thirty-four participants were randomly divided into two groups in each experiment: Arousal Stimulation or Sham Control. The Arousal Stimulation group underwent a 50-second cold pressor stimulation (immersing the foot in 0-2° C water), a technique known to increase arousal. In contrast, the Sham Control group immersed their foot in room temperature water. Stereoacuity thresholds (Experiment 1) and contrast thresholds (Experiment 2) were measured before and after stimulation. The Arousal Stimulation groups demonstrated significantly lower stereoacuity and contrast thresholds following cold pressor stimulation, whereas the Sham Control groups showed no difference in thresholds. These results provide the first evidence that hyper-arousal from sensory stimulation can lower visual thresholds. Hyper-arousal's ability to decrease visual thresholds has important implications for survival, sports, and everyday life.

Neurophonetics

Neurophonetics aims at the elucidation of the brain mechanisms underlying speech communication in our species. Clinical observations in patients with speech impairments following cerebral disorders provided the initial vantage point of this research area and indicated distinct functional-neuroanatomic systems to support human speaking and listening. Subsequent approaches-considering speech production a motor skill-investigated vocal tract movements associated with spoken language by means of kinematic and electromyographic techniques-allowing, among other things, for the evaluation of computational models suggesting elementary phonological gestures or a mental syllabary as basic units of speech motor control. As concerns speech perception, the working characteristics of auditory processing were first investigated based upon psychoacoustic techniques such as dichotic listening and categorical perception designs. More recently, functional hemodynamic neuroimaging and electrophysiological methods opened the door to the delineation of multiple stages of central auditory processing related to signal detection, classification, sensory memory processes, and, finally, lexical access. Beyond the control mechanisms in a stricter sense, both speech articulation and auditory processing represent examples of 'grounded cognition'. For example, both domains cannot be restricted to text-to-speech translation processes, but are intimately interwoven with neuropsychological aspects of speech prosody, including the vocal expression of affects and the actual performance of speech acts, transforming propositional messages to 'real' utterances. Furthermore, during language acquisition, the periphery of language-i.e., hearing and speaking behavior-plays a dominant role for the construction of a language-specific mental lexicon as well as language-specific action plans for the production of a speech message. WIREs Cogn Sci 2013, 4:191-200. doi: 10.1002/wcs.1211 For further resources related to this article, please visit the WIREs website.

Tracking the speech signal--time-locked MEG signals during perception of ultra-fast and moderately fast speech in blind and in sighted listeners

Blind people can learn to understand speech at ultra-high syllable rates (ca. 20 syllables/s), a capability associated with hemodynamic activation of the central-visual system. To further elucidate the neural mechanisms underlying this skill, magnetoencephalographic (MEG) measurements during listening to sentence utterances were cross-correlated with time courses derived from the speech signal (envelope, syllable onsets and pitch periodicity) to capture phase-locked MEG components (14 blind, 12 sighted subjects; speech rate=8 or 16 syllables/s, pre-defined source regions: auditory and visual cortex, inferior frontal gyrus). Blind individuals showed stronger phase locking in auditory cortex than sighted controls, and right-hemisphere visual cortex activity correlated with syllable onsets in case of ultra-fast speech. Furthermore, inferior-frontal MEG components time-locked to pitch periodicity displayed opposite lateralization effects in sighted (towards right hemisphere) and blind subjects (left). Thus, ultra-fast speech comprehension in blind individuals appears associated with changes in early signal-related processing mechanisms both within and outside the central-auditory terrain.

Mapping repetition suppression of the P50 evoked response to the human cerebral cortex

OBJECTIVE

The cerebral network subserving repetition suppression (RS) of the P50 auditory evoked response as observed using paired-identical-stimulus (S1-S2) paradigms is not well-described.

METHODS

We analyzed S1-S2 data from electrodes placed on the cortices of 64 epilepsy patients. We identified regions with maximal amplitude responses to S1 (i.e., stimulus registration), regions with maximal suppression of responses to S2 relative to S1 (i.e., RS), and regions with no or minimal RS 30-80 ms post stimulation.

RESULTS

Several temporal, parietal and cingulate area regions were shown to have significant initial registration activity (i.e., strong P50 response to S1). Moreover, prefrontal, cingulate, and parietal lobe regions not previously proposed to be part of the P50 habituation neural circuitry were found to exhibit significant RS.

CONCLUSIONS

The data suggest that the neural network underlying the initial phases of the RS process may include regions not previously thought to be involved like the parietal and cingulate cortexes. In addition, a significant role for the frontal lobe in mediating this function is supported.

SIGNIFICANCE

A number of regions of interest are identified through invasive recording that will allow further probing of the RS function using less invasive technology.

Functional imaging of the hemodynamic sensory gating response in schizophrenia

The cortical (auditory and prefrontal) and/or subcortical (thalamic and hippocampal) generators of abnormal electrophysiological responses during sensory gating remain actively debated in the schizophrenia literature. Functional magnetic resonance imaging has the spatial resolution for disambiguating deep or simultaneous sources but has been relatively under-utilized to investigate generators of the gating response. Thirty patients with chronic schizophrenia (SP) and 30 matched controls participated in the current experiment. Hemodynamic response functions (HRFs) for single (S1) and pairs (S1 + S2) of identical ("gating-out" redundant information) or nonidentical ("gating-in" novel information) tones were generated through deconvolution. Increased or prolonged activation for patients in conjunction with deactivation for controls was observed within auditory cortex, prefrontal cortex, and thalamus in response to single tones during the late hemodynamic response, and these group differences were not associated with clinical or cognitive symptomatology. Although patient hyperactivation to paired-tones conditions was present in several regions of interest, the effects were not statistically significant for either the gating-out or gating-in conditions. Finally, abnormalities in the postundershoot of the auditory HRF were also observed for both single and paired-tones conditions in patients. In conclusion, the amalgamation of the entire electrophysiological response to both S1 and S2 stimuli may limit hemodynamic sensitivity to paired tones during sensory gating, which may be more readily overcome by paradigms that use multiple stimuli rather than pairs. Patient hyperactivation following single tones is suggestive of deficits in basic inhibition, neurovascular abnormalities, or a combination of both factors.

Gamma band activity in the reticular activating system

This review considers recent evidence showing that cells in three regions of the reticular activating system (RAS) exhibit gamma band activity, and describes the mechanisms behind such manifestation. Specifically, we discuss how cells in the mesopontine pedunculopontine nucleus (PPN), intralaminar parafascicular nucleus (Pf), and pontine subcoeruleus nucleus dorsalis (SubCD) all fire in the beta/gamma band range when maximally activated, but no higher. The mechanisms behind this ceiling effect have been recently elucidated. We describe recent findings showing that every cell in the PPN have high-threshold, voltage-dependent P/Q-type calcium channels that are essential, while N-type calcium channels are permissive, to gamma band activity. Every cell in the Pf also showed that P/Q-type and N-type calcium channels are responsible for this activity. On the other hand, every SubCD cell exhibited sodium-dependent subthreshold oscillations. A novel mechanism for sleep–wake control based on well-known transmitter interactions, electrical coupling, and gamma band activity is described. The data presented here on inherent gamma band activity demonstrates the global nature of sleep–wake oscillation that is orchestrated by brainstem–thalamic mechanism, and questions the undue importance given to the hypothalamus for regulation of sleep–wakefulness. The discovery of gamma band activity in the RAS follows recent reports of such activity in other subcortical regions like the hippocampus and cerebellum. We hypothesize that, rather than participating in the temporal binding of sensory events as seen in the cortex, gamma band activity manifested in the RAS may help stabilize coherence related to arousal, providing a stable activation state during waking and paradoxical sleep. Most of our thoughts and actions are driven by pre-conscious processes. We speculate that continuous sensory input will induce gamma band activity in the RAS that could participate in the processes of pre-conscious awareness, and provide the essential stream of information for the formulation of many of our actions.

Magnetic brain activity phase-locked to the envelope, the syllable onsets, and the fundamental frequency of a perceived speech signal

During speech perception, acoustic correlates of syllable structure and pitch periodicity are directly reflected in electrophysiological brain activity. Magnetoencephalography (MEG) recordings were made while 10 participants listened to natural or formant-synthesized speech at moderately fast or ultrafast rate. Cross-correlation analysis was applied to show brain activity time-locked to the speech envelope, to an acoustic marker of syllable onsets, and to pitch periodicity. The envelope yielded a right-lateralized M100-like response, syllable onsets gave rise to M50/M100-like fields with an additional anterior M50 component, and pitch (ca. 100 Hz) elicited a neural resonance bound to a central auditory source at a latency of 30 ms. The strength of these MEG components showed differential effects of syllable rate and natural versus synthetic speech. Presumingly, such phase-locking mechanisms serve as neuronal triggers for the extraction of information-bearing elements.

Cold pressor stimulation diminishes P50 amplitude in normal subjects

The present study examined how cold pressor stimulation influences electrophysiological correlates of arousal. We measured the P50 auditory evoked response potential in two groups of subjects who immersed their foot in either cold (0-2°C) or room temperature (22-24°C) water for 50 seconds. The P50, which was recorded before and after stimulation, is sleep-state dependent and sensitive to states of arousal in clinical populations. We found a significant reduction in P50 amplitude after exposure to cold, but not room temperature water. In comparison with other studies, these results indicate that cold pressor stimulation in normal subjects may evoke a regulatory process that modulates the P50 amplitude, perhaps to preserve the integrity of sensory perception, even as autonomic and subjective aspects of arousal increase.

COMT Val108/158Met genotype modulates human sensory gating

BACKGROUND

The catechol-O-methyltransferase (COMT) Val(108/158)Met polymorphism of the dopamine system is essential for prefrontal cortex processing capacity and efficiency. In addition, dopaminergic neurotransmission is also associated with the sensory gating phenomenon protecting the cerebral cortex from information overload. It is however unclear if COMT genotype as a predictor of prefrontal efficiency modulates sensory gating on the level of the auditory cortex, i.e. the gating of the auditory evoked P50 and N100 components.

METHODS

P50 and N100 gating and COMT Val(108/158)Met genotype were determined in 282 healthy subjects of German descent carefully screened for psychiatric or neurological disorders.

RESULTS

A significant effect of the COMT genotype was observed for N100 gating (F=4.510, df=2, p=0.012) but not for P50 gating (F=0.376, df=2, p=0.687). Contrast analysis showed that Met/Met individuals had poorer N100 gating compared to Val/Met (F=-12.931, p=0.003) and the Val/Val individuals (F=-11.056, p=0.057).

CONCLUSION

The results indicate that a high prefrontal efficiency as suggested by the COMT Met/Met genotype is associated with to a poor sensory gating of the N100 component. This would fit in a model where a high prefrontal processing capacity allows a pronounced afferent input of sensory information from the auditory cortex as reflected by a poor sensory gating. The more pronounced prefrontal contribution to the N100 compared to the P50 component may explain the exclusive genotype association with the N100 sensory gating. This preliminary model should be replicated and validated in future investigations.

Evidence for a frontal cortex role in both auditory and somatosensory habituation: a MEG study

Auditory and somatosensory responses to paired stimuli were investigated for commonality of frontal activation that may be associated with gating using magnetoencephalography (MEG). A paired stimulus paradigm for each sensory evoked study tested right and left hemispheres independently in ten normal controls. MR-FOCUSS, a current density technique, imaged simultaneously active cortical sources. Each subject showed source localization, in the primary auditory or somatosensory cortex, for the respective stimuli following both the first (S1) and second (S2) impulses. Gating ratios for the auditory M50 response, equivalent to the P50 in EEG, were 0.54+/-0.24 and 0.63+/-0.52 for the right and left hemispheres. Somatosensory gating ratios were evaluated for early and late latencies as the pulse duration elicits extended response. Early gating ratios for right and left hemispheres were 0.69+/-0.21 and 0.69+/-0.41 while late ratios were 0.81+/-0.41 and 0.80+/-0.48. Regions of activation in the frontal cortex, beyond the primary auditory or somatosensory cortex, were mapped within 25 ms of peak S1 latencies in 9/10 subjects during auditory stimulus and in 10/10 subjects for somatosensory stimulus. Similar frontal activations were mapped within 25 ms of peak S2 latencies for 75% of auditory responses and for 100% of somatosensory responses. Comparison between modalities showed similar frontal region activations for 17/20 S1 responses and for 13/20 S2 responses. MEG offers a technique for evaluating cross modality gating. The results suggest similar frontal sources are simultaneously active during auditory and somatosensory habituation.

The developmental decrease in REM sleep: the role of transmitters and electrical coupling

STUDY OBJECTIVES

This mini-review considers certain factors related to the developmental decrease in rapid eye movement (REM) sleep, which occurs in favor of additional waking time, and its relationship to developmental factors that may influence its potential role in brain development.

DESIGN

Specifically, we discuss some of the theories proposed for the occurrence of REM sleep and agree with the classic notion that REM sleep is, at the least, a mechanism that may play a role in the maturation of thalamocortical pathways. The developmental decrease in REM sleep occurs gradually from birth until close to puberty in the human, and in other mammals it is brief and coincides with eye and ear opening and the beginning of massive exogenous activation. Therefore, the purported role for REM sleep may change to involve a number of other functions with age.

MEASUREMENTS AND RESULTS

We describe recent findings showing that morphologic and physiologic properties as well as cholinergic, gamma amino-butyric acid, kainic acid, n-methyl-d-aspartic acid, noradrenergic, and serotonergic synaptic inputs to mesopontine cholinergic neurons, as well as the degree of electrical coupling between mostly noncholinergic mesopontine neurons and levels of the neuronal gap-junction protein connexin 36, change dramatically during this critical period in development. A novel mechanism for sleep-wake control based on well-known transmitter interactions, as well as electrical coupling, is described.

CONCLUSION

We hypothesize that a dysregulation of this process could result in life-long disturbances in arousal and REM sleep drive, leading to hypervigilance or hypovigilance such as that observed in a number of disorders that have a mostly postpubertal age of onset.

Long-term deficits of preterm birth: evidence for arousal and attentional disturbances

OBJECTIVE

Quantitative measures of pre-attentional, attentional and frontal lobe processes were compared to evaluate quantitative measures of these deficits in Ex-Preterm vs. Ex-Term adolescents.

METHODS

We compared 43 Ex-Preterm with 26 Ex-Term adolescents using the P50 auditory potential, the Psychomotor Vigilance Task (PVT), a reaction time (RT) test, and Near Infrared Spectroscopy (NIRS).

RESULTS

The mean amplitude (+/-SE) of the P50 amplitude was similar in the Ex-Preterm (1.8+/-1.4 microV) vs. Ex-Term adolescents (1.8+/-0.6 microV, df = 68, F = 0.05, p = 0.8), but the Ex-Preterm group showed a trimodal distribution in amplitude (High, 3.3+/-0.4 microV, df=42.25, F=19.2, p < 0.01; Medium, 1.7+/-0.1 microV, df = 39, F = 0.41, p = 0.53; Low, 0.7+/-0.1 microV, df = 40, F = 49.5, p < 0.01) suggested by statistically significant variance between populations (Kolmogorov-Kuiper test, df = 42.25, F = 5.4, p < 0.01). Mean RT was longer in Ex-Preterm (250+/-8 ms) vs. Ex-Term subjects (200+/-5 ms, df = 68, F = 18.8, p < 0.001). PVT lapses were increased in Ex-Preterm subjects, and varied inversely with P50 amplitude (Overall Mean 17+/-5 lapses, df = 67, F = 5.34, p < 0.05; Low P50 amplitude, 25+/-10, df = 40, F = 8.8, p < 0.01; Medium, 21+/-11, df = 38, F = 5.37, p < 0.05; High, 6+/-2, df = 39, F = 6.78, p < 0.01) vs. Ex-Term subjects (2+/-0.4 lapses, p < 0.01). NIRS levels did not differ statistically, but tended to correlate with P50 amplitude in the Ex-Preterm group.

CONCLUSIONS

These findings suggest differential pre-attentional, attentional and frontal lobe dysfunction in Ex-Preterm adolescents.

SIGNIFICANCE

These measures could provide a means to objectively assess differential dysregulation of arousal and attention in Ex-Preterm adolescents, allowing optimization of therapeutic designs.