Age and sex effects on paired-pulse suppression and prepulse inhibition of auditory evoked potentials.

Responses to a sensory stimulus are inhibited by a preceding stimulus; if the two stimuli are identical, paired-pulse suppression (PPS) occurs; if the preceding stimulus is too weak to reliably elicit the target response, prepulse inhibition (PPI) occurs. PPS and PPI represent excitability changes in neural circuits induced by the first stimulus, but involve different mechanisms and are impaired in different diseases, e.g., impaired PPS in schizophrenia and Alzheimer's disease and impaired PPI in schizophrenia and movement disorders. Therefore, these measures provide information on several inhibitory mechanisms that may have roles in clinical conditions. In the present study, PPS and PPI of the auditory change-related cortical response were examined to establish normative data on healthy subjects (35 females and 32 males, aged 19-70 years). We also investigated the effects of age and sex on PPS and PPI to clarify whether these variables need to be considered as biases. The test response was elicited by an abrupt increase in sound pressure in a continuous sound and was recorded by electroencephalography. In the PPS experiment, the two change stimuli to elicit the cortical response were a 15-dB increase from the background of 65 dB separated by 600 ms. In the PPI experiment, the prepulse and test stimuli were 2- and 10-dB increases, respectively, with an interval of 50 ms. The results obtained showed that sex exerted similar effects on the two measures, with females having stronger test responses and weaker inhibition. On the other hand, age exerted different effects: aging correlated with stronger test responses and weaker inhibition in the PPS experiment, but had no effects in the PPI experiment. The present results suggest age and sex biases in addition to normative data on PPS and PPI of auditory change-related potentials. PPS and PPI, as well as other similar paradigms, such as P50 gating, may have different and common mechanisms. Collectively, they may provide insights into the pathophysiologies of diseases with impaired inhibitory function.

Pre-emptive ice pack cryotherapy for reducing pain caused by long-acting deltoid injectable antipsychotic treatment: A single-center open-label study.

PURPOSE: This empirical study aims to investigate the efficacy of pre-emptive cryotherapy in reducing pain that is caused by the deltoid intramuscular (IM) injection of long-acting injectable (LAI) antipsychotics in clinical settings. PATIENTS AND METHODS: This study included 29 outpatients receiving LAI antipsychotic treatment. The evaluations of pain during (1) the usual procedure (control), (2) pre-emptive use of ice pack cryotherapy (pre-cooling), and (3) pre-emptive use of a room-temperature ice pack (pre-touching) were conducted using a numerical rating scale (NRS) for comparison. All patients were administered with LAI antipsychotics via deltoid IM. Furthermore, the results of the Positive and Negative Symptom Scale (PANSS), clinical global impressions (CGI) scale, and Global Assessment of Functioning (GAF) scale that were administered during the control procedure were evaluated. RESULTS: The median NRS pain scores during the IM injection of LAI antipsychotics were 4.0 (3.0-5.0), 2.0 (1.0-3.0), and 3.0 (2.5-6.0) for the control, pre-cooling, and pre-touching conditions, indicating a significant difference (p = 6.0 × 10-6). The NRS pain scores for the pre-cooling condition were significantly lower than those for the control and pre-touching conditions (p = 2.5 × 10-5 and 6.7 × 10-5, respectively). No significant correlation was observed between the NRS pain scores for the control condition and the PANSS, CGI scale, or GAF scale scores. Furthermore, no adverse events were recorded during the study period. CONCLUSION: Pain during the deltoid IM injection of LAI antipsychotics was found to be reduced by pre-emptive skin cooling. To date, this is the first study to confirm the effectiveness of pre-emptive cryotherapy for relieving such pain in clinical situations.

Preoperative Motor Function Associated with Short-Term Gain of Health-Related Quality of Life after Surgery for Lumbar Degenerative Disease: A Pilot Prospective Cohort Study in Japan.

This study aimed to estimate the relationship between preoperative motor function and short-term recovery of health-related quality of life after lumbar surgery in patients with lumbar degenerative disease. This prospective cohort study involved 50 patients with lumbar degenerative disease at a general hospital in Japan. The primary outcome was the achievement of minimal clinically important difference (MCID) for EuroQOL 5 dimensions (EQ-5D) at discharge. Preoperative demographic, medication, surgical, and physical function data were collected. Logistic regression analysis was performed using the achievement of MCID for EQ-5D as the dependent variable and preoperative characteristics, including the Five Times Sit to Stand test (FTSTS), Oswestry Disability Index (ODI), and Self-rating Depression Scale (SDS), as the independent variables. The logistic regression analysis showed that Model 1 had a moderate predictive accuracy (Nagelkerke R2: 0.20; Hosmer-Lemeshow test: p = 0.19; predictive accuracy: 70.0%). Among the independent variables in the logistic regression model, the FTSTS was the only independent variable related to the achievement of MCID for EQ-5D at discharge (odds ratio: 0.03; 95% CI: 1.79 × 10-3, 0.18). Our results highlighted the importance of baseline motor function in the postoperative recovery of health-related quality of life in individuals with lumbar degenerative disease.

The 40-Hz auditory steady-state response enhanced by beta-band subharmonics.

The 40-Hz auditory steady-state response (ASSR) has received special attention as an index of gamma oscillations owing to its association with various neuropsychiatric disorders including schizophrenia. When a periodic stimulus is presented, oscillatory responses are often elicited not only at the stimulus frequency, but also at its harmonic frequencies. However, little is known about the effect of 40-Hz subharmonic stimuli on the activity of the 40-Hz ASSR. In the present magnetoencephalography study, we focused on the nature of oscillation harmonics and examined oscillations in a wide frequency range using a time-frequency analysis during the 6.67-, 8-, 10-, 13.3-, 20-, and 40-Hz auditory stimuli in 23 healthy subjects. The results suggested that the 40-Hz ASSR represents activation of a specific circuit tuned to this frequency. Particularly, oscillations elicited by 13.3- and 20-Hz stimuli exhibited significant enhancement at 40 Hz without changing those at the stimulus frequency. In addition, it was found that there was a non-linear response to stimulation in the beta band. We also demonstrated that the inhibition of beta to low-gamma oscillations by the 40-Hz circuit contributed to the violation of the rule that harmonic oscillations gradually decrease at higher frequencies. These findings can advance our understanding of oscillatory abnormalities in patients with schizophrenia in the future.

Cortical activity during the wind-up of flexion reflex and pain: a magnetoencephalographic study using time-frequency analysis.

Wind-up is a nociceptive-specific phenomenon in which pain sensations are facilitated, in a frequency-dependent manner, by the repeated application of noxious stimuli of constant intensity, with invariant tactile sensations. Thus, cortical activities during wind-up could be an alteration associated with pain potentiation. We aimed to investigate somatosensory-evoked cortical responses and induced brain oscillations during wind-up by recording magnetoencephalograms. Wind-up was produced by the application of 11 consecutive electrical stimuli to the sural nerve, repeated at a frequency of 1 Hz without varying the intensity. The augmentation of flexion reflexes and pain rating scores were measured simultaneously as an index of wind-up. In the time-frequency analyses, the γ-band late event-related synchronization and the ß-band event-related desynchronization were observed in the primary somatosensory region and the bilateral operculo-insular region, respectively. Repetitive exposure to the stimuli enhanced these activities, along with an increase in the flexion reflex magnitude. The evoked cortical activity reflected novelty, with no alteration to these repetitive stimuli. Observed oscillations enhanced by repetitive stimulation at a constant intensity could reflect a pain mechanism associated with wind-up.

Mechanisms of Short- and Long-Latency Sensory Suppression: Magnetoencephalography Study.

Prepulse inhibition (PPI) is sensory suppression whose mechanism (i.e., whether PPI originates from specific inhibitory mechanisms) remains unclear. In this study, we applied the combination of short-latency PPI and long-latency paired pulse suppression in 17 healthy subjects using magnetoencephalography to investigate the mechanisms of sensory suppression. Repeats of a 25-ms pure tone without a blank at 800 Hz and 70 dB were used for a total duration of 1600 ms. To elicit change-related cortical responses, the sound pressure of two consecutive tones in this series at 1300 ms was increased to 80 dB (Test). For the conditioning stimuli, the sound pressure was increased to 73 dB at 1250 ms (Pre 1) and 80 dB at 700 ms (Pre 2). Six stimuli were randomly presented as follows: (1) Test alone, (2) Pre 1 alone, (3) Pre 1 + Test, (4) Pre 2 + Test, (5) Pre 2 + Pre 1, and (6) Pre 2 + Pre 1 + Test. The inhibitory effects of the conditioning stimuli were evaluated using N100m/P200m components. The results showed that both Pre 1 and Pre 2 significantly suppressed the Test response. Moreover, the inhibitory effects of Pre 1 and Pre 2 were additive. However, when both prepulses were present, Pre 2 significantly suppressed the Pre 1 response, suggesting that the Pre 1 response amplitude was not a determining factor for the degree of suppression. These results suggested that the suppression originated from a specific inhibitory circuit independent of the excitatory pathway.

Relationship of loudness-dependent auditory evoked potentials with change-related cortical responses.

Previous studies have suggested that change-related cortical responses are phenomena similar to the onset response and could be applied to the loudness dependence of auditory evoked potential (LDAEP) paradigm. In the present study, we examined the relationship between LDAEP and the change-related response using electroencephalography findings in 50 healthy subjects. There were five conditions (55, 65, 75, 85, and 95 dB) for LDAEP and five similar conditions (abrupt sound pressure increase from 70 to 75, 80, 85, 90, and 95 dB) for the change-related response. Both the onset and abrupt sound pressure increase evoked a triphasic response with peaks at approximately 50 (P50), 100 (N100), and 200 (P200) ms. We calculated the peak-to-peak amplitudes for P50/N100 and N100/P200. Medians and slopes for P50/N100 and N100/P200 amplitudes were calculated and compared between the two measures. Results revealed a significant correlation for both the slope and median for P50/N100 (r = 0.36, 0.37, p = 1.0 × 10-2, 7.9 × 10-3), N100/P200 (r = 0.40, 0.34, p = 4.0 × 10-3, 1.6 × 10-2), and P50/N100/P200 (r = 0.36, 0.35, p = 1.0 × 10-2, 1.3 × 10-2). These results suggested that the change-related response and LDAEP shared generation mechanisms at least partially.

A Minimally Invasive Method for Observing Wind-Up of Flexion Reflex in Humans: Comparison of Electrical and Magnetic Stimulation.

Wind-up like pain or temporal summation of pain is a phenomenon in which pain sensation is increased in a frequency-dependent manner by applying repeated noxious stimuli of uniform intensity. Temporal summation in humans has been studied by observing the increase in pain or flexion reflex by repetitive electrical or thermal stimulations. Nonetheless, because the measurement is accompanied by severe pain, a minimally invasive method is desirable. Gradual augmentation of flexion reflex and pain induced by repetitive stimulation of the sural nerve was observed using three stimulation methods-namely, bipolar electrical, magnetic, and monopolar electrical stimulation, with 11 healthy male subjects in each group. The effects of frequency, intensity, and number of repetitive stimuli on the increase in the magnitude of flexion reflex and pain rating were compared among the three methods. The reflex was measured using electromyography (EMG) from the short head of the biceps femoris. All three methods produced a frequency- and intensity-dependent progressive increase in reflex and pain; pain scores were significantly lower for magnetic and monopolar stimulations than for bipolar stimulation (P < 0.05). The slope of increase in the reflex was steep during the first 4-6 stimuli but became gentler thereafter. In the initial phase, an increase in the reflex during the time before signals of C-fibers arrived at the spinal cord was observed in experiments using high-frequency stimulation, suggesting that wind-up was caused by inputs of A-fibers without the involvement of C-fibers. Magnetic and monopolar stimulations are minimally invasive and useful methods for observing the wind-up of the flexion reflex in humans. Monopolar stimulation is convenient because it does not require special equipment. There is at least a partial mechanism underlying the wind-up of the flexion reflex that does not require C-fibers.

Suppression of Low-Frequency Gamma Oscillations by Activation of 40-Hz Oscillation.

Gamma oscillations have received considerable attention owing to their association with cognitive function and various neuropsychiatric disorders. However, interactions of gamma oscillations at different frequency bands in humans remain unclear. In the present magnetoencephalographic study, brain oscillations in a wide frequency range were examined using a time-frequency analysis during the 20-, 30-, 40-, and 50-Hz auditory stimuli in 21 healthy subjects. First, dipoles for auditory steady-state response (ASSR) were estimated and interaction among oscillations at 10-60 Hz was examined using the source strength waveforms. Results showed the suppression of ongoing low-gamma oscillations at approximately 30 Hz during stimulation at 40 Hz. Second, multi-dipole analyses suggested that the main dipole for ASSR and dipoles for suppressed low-frequency gamma oscillations were distinct. Third, an all-sensor analysis was performed to clarify the distribution of the 40-Hz ASSR and suppression of low-frequency gamma oscillations. Notably, the area of suppression surrounded the center of the 40-Hz ASSR and showed a trend of extending to the vertex, indicating that different groups of neurons were responsible for these two gamma oscillations and that the 40-Hz oscillation circuit have specific inhibitory innervation to the low-gamma circuit.

Mechanisms of Long-Latency Paired Pulse Suppression: MEG Study.

Paired pulse suppression is an electrophysiological method used to evaluate sensory suppression and often applied to patients with psychiatric disorders. However, it remains unclear whether the suppression comes from specific inhibitory mechanisms, refractoriness, or fatigue. In the present study, to investigate mechanisms of suppression induced by an auditory paired pulse paradigm in 19 healthy subjects, magnetoencephalography was employed. The control stimulus was a train of 25-ms pure tones of 65 dB SPL for 2500 ms. In order to evoke a test response, the sound pressure of two consecutive tones at 2200 ms in the control sound was increased to 80 dB (Test stimulus). Similar sound pressure changes were also inserted at 1000 (CS2) and 1600 (CS1) ms as conditioning stimuli. Four stimulus conditions were used; (1) Test alone, (2) Test + CS1, (3) Test + CS1 + CS2, and (4) Test + CS2, with the four sound stimuli randomly presented and cortical responses averaged at least 100 times for each condition. The baseline-to-peak and peak-to-peak amplitudes of the P50m, N100m, and P200m components of the test response were compared among the four conditions. In addition, the response to CS1 was compared between conditions (2) and (3). The results showed significant test response suppression by CS1. While the response to CS1 was significantly suppressed when CS2 was present, it did not affect suppression of the test response by CS1. It was thus suggested that the amplitude of the response to a conditioning stimulus is not a factor to determine the inhibitory effects of the test response, indicating that suppression is due to an external influence on the excitatory pathway.

Conditioned Pain Modulation: Comparison of the Effects on Nociceptive and Non-nociceptive Blink Reflex.

Although conditioned pain modulation (CPM) is considered to represent descending pain inhibitory mechanisms triggered by noxious stimuli applied to a remote area, there have been no previous studies comparing CPM between pain and tactile systems. In this study, we compared CPM between the two systems objectively using blink reflexes. Intra-epidermal electrical stimulation (IES) and transcutaneous electrical stimulation (TS) were applied to the right skin area over the supraorbital foramen to evoke a nociceptive or a non-nociceptive blink reflex, respectively, in 15 healthy males. In the test session, IES or TS were applied six times and subjects reported the intensity of each stimulus on a numerical rating scale (NRS). Blink reflexes were measured using electromyography (R2). The first and second sessions were control sessions, while in the third session, the left hand was immersed in cold water at 10 °C as a conditioning stimulus. The magnitude of the R2 blink and NRS scores were compared among the sessions by 2-way ANOVA. Both the NRS score and nociceptive R2 were significantly decreased in the third session for IES, with a significant correlation between the two variables; whereas, TS-induced non-nociceptive R2 did not change among the sessions. Although the conditioning stimulus decreased the NRS score for TS, the CPM effect was significantly smaller than that for IES (p = 0.002). The present findings suggest the presence of a pain-specific CPM effect to a heterotopic noxious stimulus.

The Auditory Steady-State Response: Electrophysiological Index for Sensory Processing Dysfunction in Psychiatric Disorders.

Sensory processing is disrupted in several psychiatric disorders, including schizophrenia, bipolar disorder, and autism spectrum disorder. In this review, we focus on the electrophysiological auditory steady-state response (ASSR) driven by high-frequency stimulus trains as an index for disease-associated sensory processing deficits. The ASSR amplitude is suppressed within the gamma band (≥30 Hz) among these patients, suggesting an imbalance between GABAergic and N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission. The reduced power and synchronization of the 40-Hz ASSR are robust in patients with schizophrenia. In recent years, similar ASSR deficits at gamma frequencies have also been reported in patients with bipolar disorder and autism spectrum disorder. We summarize ASSR abnormalities in each of these psychiatric disorders and suggest that the observed commonalities reflect shared pathophysiological mechanisms. We reviewed studies on phase resetting in which a salient sensory stimulus affects ASSR. Phase resetting induces the reduction of both the amplitude and phase of ASSR. Moreover, phase resetting is also affected by rare auditory stimulus patterns or superimposed stimuli of other modalities. Thus, sensory memory and multisensory integration can be investigated using phase resetting of ASSR. Here, we propose that ASSR amplitude, phase, and resetting responses are sensitive indices for investigating sensory processing dysfunction in psychiatric disorders.

Test-retest reliability of paired pulse suppression paradigm using auditory change-related response.

BACKGROUND: Sensory suppression is an important brain function for appropriate processing of information and is known to be impaired in patients with various types of mental illness. Long latency suppression which is a paradigm using change-related cortical response with repeated paired pulses embedded in a train of conditioning pulses is a factor used to measure sensory suppression. NEW METHOD: The present study assessed the test-retest reliability of long-latency suppression in latency, amplitude, and suppression rate of the P50, N100, and P200 components of auditory evoked potentials in 35 healthy adults. The sound stimulus was repeats of a 25-ms pure tone at 65 dB and 2000 ms in total duration, during which the sound pressure level was increased to 80 dB twice at 1100 ms and 1700 ms. Measurements were performed twice and the validity of the findings was evaluated using intra-class correlations. RESULTS: The results showed high intra-class correlation (ICC) values (>0.7) for the amplitude of all components, except for P50 (0.44), while latency also showed high ICC values (>0.66), except for P50 (0.20). In addition, the suppression rate showed good reproducibility for the N100-P200 component (0.60). COMPARISON WITH EXISTING METHOD: The method can be performed with a short inspection time of approximately 5 min and provides high ICC values. In addition, it may reflect suppression mechanisms different from those relating to existing methods. CONCLUSION: These results support the use of long latency suppression as a biomarker in clinical settings.

Test-retest reliability of prepulse inhibition paradigm using auditory evoked potentials.

Prepulse inhibition (PPI) is a neurological phenomenon in which a weak initial stimulus reduces the level of responses to a subsequent stronger stimulus. Although acoustic startle reflexes are usually used for PPI examinations, recent studies have observed similar phenomena with event-related cortical potentials. In the present study, test-retest reliability of PPI measured using auditory change-related cortical responses was assessed in 35 healthy adults. Four sound stimuli were randomly presented at an even probability; Standard, Test alone, Prepulse alone, and Test + Prepulse. The Standard stimulus was a train of 25-ms tone pulses at 70 dB for 650 ms, while for Test alone and Prepulse alone, the sound pressure was increased to 80 dB at 350 ms and 73 dB at 300 ms, respectively. Measurements were performed twice with at least 7 days separation, and validity was evaluated using intra-class correlation (ICC) for latency, amplitude, and suppression rate of the P50, N100, and P200 components. The results showed high ICC values for the latency and amplitude of nearly all components, except for response to Prepulse alone (0.3-0.6). Furthermore, ICC for suppression rate was greater than 0.5 for the peak-to-peak amplitude. Good reproducibility for N100 and P200 components was obtained with this method. The present results support the PPI paradigm as a reliable tool for clinical measurements of inhibitory functions.

Assessment of haptic memory using somatosensory change-related cortical responses.

Haptic memory briefly retains somatosensory information for later use; however, how and which cortical areas are affected by haptic memory remain unclear. We used change-related cortical responses to investigate the relationship between the somatosensory cortex and haptic memory objectively. Electrical pulses, at 50 Hz with a duration of 500 ms, were randomly applied to the second, third, and fourth fingers of the right and left hands at an even probability every 800 ms. Each stimulus was labeled as D (preceded by a different side) or S (preceded by the same side). The D stimuli were further classified into 1D, 2D, and 3D, according to the number of different preceding stimuli. The S stimuli were similarly divided into 1S and 2S. The somatosensory-evoked magnetic fields obtained were divided into four components via a dipole analysis, and each component's amplitudes were measured using the source strength waveform. The results showed that the preceding event did not affect the amplitude of the earliest 20-30 ms response in the primary somatosensory cortex. However, in the subsequent three components, the cortical activity amplitude was largest in 3D, followed by 2D, 1D, and S. These results indicate that such modulatory effects occurred somewhere in the somatosensory processing pathway higher than Brodmann's area 3b. To the best of our knowledge, this is the first study to demonstrate the existence of haptic memory for somatosensory laterality and its impact on the somatosensory cortex using change-related cortical responses without contamination from peripheral effects.

Detecting cognitive decline in community-dwelling older adults using simple cognitive and motor performance tests.

AIM: The objective of this study was to estimate the predictive accuracy of simple cognitive and motor performance tests to detect cognitive decline (CD) in community-dwelling older adults. METHODS: In total, 102 community-dwelling older adults participated in this study. Cognitive function, gait performance and coordinated finger movements were assessed using the Montreal Cognitive Assessment, the 10-m walking test and the finger-tapping test, respectively. We classified the participants into either a CD (n = 60) or a healthy control (n = 42) group. RESULTS: Significant group differences were found in the visuospatial/executive function subscale score on the Montreal Cognitive Assessment, stride length and total finger tap count. The results of multivariate logistic regression analysis showed that visuospatial/executive function subscale score, stride length and total tap count were the significant predictors for the presence of CD (sensitivity 83.3%, specificity 82.9%, predictive accuracy 83.2%). We also constructed a decision tree model with these three variables to increase the usefulness of our model as a screening tool by assigning a cut-off value for each assessment. The sensitivity and specificity of the model were 88.1% and 85.2%, respectively, with an overall predictive accuracy of 86.4%. CONCLUSIONS: The results of the present study suggest that simple cognitive and motor performance tests have moderate-to-high discriminant validity for the presence of CD in community-dwelling older adults. Addition of such tests might lead to the more accurate detection of early cognitive decline. Geriatr Gerontol Int 2020; ••: ••-••.

Change-Related Acceleration Effects on Auditory Steady State Response.

Rapid detection of sensory changes is important for survival. We have previously used change-related cortical responses to study the change detection system and found that the generation of a change-related response was based on sensory memory and comparison processes. However, it remains unclear whether change-related cortical responses reflect processing speed. In the present study, we simultaneously recorded the auditory steady-state response (ASSR) and change-related response using magnetoencephalography to investigate the acceleration effects of sensory change events. Overall, 13 healthy human subjects (four females and nine males) completed an oddball paradigm with a sudden change in sound pressure used as the test stimulus, i.e., the control stimulus was a train of 25-ms pure tones at 75 dB for 1,200 ms, whereas the 29th sound at 700 ms of the test stimulus was replaced with a 90-dB tone. Thereafter, we compared the latency of ASSR among four probabilities of test stimulus (0, 25, 75, and 100%). For both the control and test stimulus, stronger effects of acceleration on ASSR were observed when the stimulus was rarer. This finding indicates that ASSR and change-related cortical response depend on physical changes as well as sensory memory and comparison processes. ASSR was modulated without changes in peripheral inputs, and brain areas higher than the primary cortex could be involved in exerting acceleration effects. Furthermore, the reduced latency of ASSR clearly indicated that a new sensory event increased the speed of ongoing sensory processing. Therefore, changes in the latency of ASSR are a sensitive index of accelerated processing.

Properties of echoic memory revealed by auditory-evoked magnetic fields.

We used auditory-evoked magnetic fields to investigate the properties of echoic memory. The sound stimulus was a repeated 1-ms click at 100 Hz for 500 ms, presented every 800 ms. The phase of the sound was shifted by inserting an interaural time delay of 0.49 ms to each side. Therefore, there were two sounds, lateralized to the left and right. According to the preceding sound, each sound was labeled as D (preceded by a different sound) or S (by the same sound). The D sounds were further grouped into 1D, 2D, and 3D, according to the number of preceding different sounds. The S sounds were similarly grouped to 1S and 2S. The results showed that the preceding event significantly affected the amplitude of the cortical response; although there was no difference between 1S and 2S, the amplitudes for D sounds were greater than those for S sounds. Most importantly, there was a significant amplitude difference between 1S and 1D. These results suggested that sensory memory was formed by a single sound, and was immediately replaced by new information. The constantly-updating nature of sensory memory is considered to enable it to act as a real-time monitor for new information.

Suppression of Somatosensory Evoked Cortical Responses by Noxious Stimuli.

Paired-pulse suppression refers to attenuation of neural activity in response to a second stimulus and has a pivotal role in inhibition of redundant sensory inputs. Previous studies have suggested that cortical responses to a somatosensory stimulus are modulated not only by a preceding same stimulus, but also by stimulus from a different submodality. Using magnetoencephalography, we examined somatosensory suppression induced by three different conditioning stimuli. The test stimulus was a train of electrical pulses to the dorsum of the left hand at 100 Hz lasting 1500 ms. For the pulse train, the intensity of the stimulus was abruptly increased at 1200 ms. Cortical responses to the abrupt intensity change were recorded and used as the test response. Conditioning stimuli were presented at 600 ms as pure tones, either innocuous or noxious electrical stimulation to the right foot. Four stimulus conditions were used: (1) Test alone, (2) Test + auditory stimulus, (3) Test + somatosensory stimulus, and (4) Test + nociceptive stimulus. Our results showed that the amplitude of the test response was significantly smaller for conditions (3) and (4) in the secondary somatosensory cortex contralateral (cSII) and ipsilateral (iSII) to the stimulated side as compared to the response to condition (1), whereas the amplitude of the response in the primary somatosensory cortex did not differ among the conditions. The auditory stimulus did not have effects on somatosensory change-related response. These findings show that somatosensory suppression was induced by not only a conditioning stimulus of the same somatosensory submodality and the same cutaneous site to the test stimulus, but also by that of a different submodality in a remote area.