The functional anatomy of middle-latency auditory evoked potentials: thalamocortical connections.

1. An 8 x 8-channel microelectrode array was used to map epicortical field potentials from a 4.375 x 4.375-mm2 area in the right parietotemporal neocortex of four rats. Potentials were evoked with bilaterally presented click stimuli and with electrical stimulation of the ventral and dorsal divisions of the medial geniculate body. 2. Epicortical responses to click stimuli replicated earlier findings. The responses consisted of a positive-negative biphasic waveform (P1a and N1) in the region of primary auditory cortex (area 41) and a positive monophasic waveform (P1b) in the region of secondary auditory cortex (area 36). Two potential patterns, one at the latency of the N1 and the other at the latency of the P1b, were used to represent activation of cells within areas 41 and 36. A linear combination of these patterns was sufficient to explain from 90 to 94% of the variance of the evoked potential complex at all latencies. 3. In the same animals, epicortical responses to electrical stimulation of the ventral and dorsal divisions of the medial geniculate body were also localized to areas 41 and 36, respectively. A linear combination of potential patterns from these separate stimulation conditions was sufficient to explain from 80 to 93% of the variance of the original click-evoked potential complex at all latencies. 4. These data provide functional evidence for anatomically defined topographical thalamocortical projections to primary and secondary auditory cortex. They suggest that short-latency cortical evoked potentials (10-60 ms poststimulus) are dominated by parallel thalamocortical activation of areas 41 and 36.     

Multiple neural origins of early auditory evoked potential of rat

The multiple-origin hypothesis has been often considered for an unclear neurogenesis of a characteristic wave in various evoked potentials, none of which has been verified so far. Auditory evoked potential (AEP) in the temporal cortex of rodents has typical slow positive/negative (P1/N1) biphasic waves, which are occasionally associated with an additional 2-4-ms earlier small deflection (P0/N0). Despite previous extensive efforts, P0/N0 deflection is still discussed within the multiple-origin hypothesis. In this historical perspective, we hypothesized that observable AEP is an additive mixture of mutually temporally independent signals from different origins, and that the balance of the mixture impacts on the waveform of AEP. We attempted to verify this hypothesis for the first time by independent component analysis (ICA) of epidurally densely mapped AEPs in the primary auditory cortex of rats. The mapping showed that low amplitude AEPs tended to have more P0/N0 deflections in both pentobarbital- and ketamine/xylazine-anesthesia preparations. ICA of these AEP maps suggested that AEP consisted of at least three independent components and that the deflection appeared when subcortical contribution to AEP was equal to or larger than cortical contribution. In epicranially measured evoked potentials, subcortical and cortical contributions are mixed together because distances from electrodes to cortical sources approximate distances to subcortical sources. In such conditions, e.g. in human scalp-recording experiments or routine clinical screenings, our idea is specifically worth considering for the interpretation of signals.     

经颅磁刺激诱发电位的采集与分析

经颅磁刺激(TMS)与脑电(EEG)采集系统结合进行TMS诱发电位分析是一种有效地揭示脑功能的方法。TMS诱发电位分析目前使用的主要方法有:直接波形分析、频谱分析、全局平均场幅度(GMFA)方法、最小范数估计(MNE)等。TMS诱发电位的研究正处于蓬勃发展阶段,现阶段主要应用在分析TMS对大脑的影响、探索大脑功能、研究神经连接以及分析神经类药物的作用等方面。     

Human middle latency auditory evoked magnetic fields

Summary The magnetic equivalents of SN10, Po, Na, Pa, Nb and Pb (SN10m, Pom, Nam, Pam, Nbm and Pbm) in short and middle latency auditory evoked potentials were measured with a 7-channel DC superconducting quantum interference device (SQUID). The sources of Pom, Nam, Pam, Nbm and Pbm responses were estimated to be located in the auditory cortex, while the source of SN10m was considered to be in a deeper part of the brain. In addition, the source of Pam was estimated to be in the vicinity of the moving N100m source. The source of Pbm was considered to be in a separate area, anterior to the source of Pam and N100m, which suggested that the source of Pam was located in the primary auditory cortex, while the source of Pbm was located in the secondary auditory cortex. The source of N100m was considered to spread from the primary auditory cortex to the secondary auditory cortex.     

A comparative magnetoencephalographic study of cortical activations evoked by noxious and innocuous somatosensory stimulations

We recorded somatosensory-evoked magnetic fields and potentials produced by painful intra-epidermal stimulation (ES) and non-painful transcutaneous electrical stimulation (TS) applied to the left hand in 12 healthy volunteers to compare cortical responses to noxious and innocuous somatosensory stimulations. Our results revealed that cortical processing following noxious and innocuous stimulations was strikingly similar except that the former was delayed approximately 60 ms relative to the latter, which was well explained by a difference in peripheral conduction velocity mediating noxious (Adelta fiber) and innocuous (Abeta fiber) inputs. The first cortical activity evoked by both ES and TS was in the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulated side. The following activities were in the bilateral secondary somatosensory cortex (SII), insular cortex, cingulate cortex, anterior medial temporal area and ipsilateral SI. The source locations did not differ between the two stimulus modalities except that the dipole for insular activity following ES was located more anterior to that following TS. Both ES and TS evoked vertex potentials consisting of a negativity followed by a positivity at a latency of 202 and 304 ms, and 134 and 243 ms, respectively. The time course of the vertex potential corresponded to that of the activity of the medial temporal area. Our results suggested that cortical processing was similar between noxious and innocuous stimulation in SI and SII, but different in insular cortex. Our data also implied that activities in the amygdala/hippocampal formation represented common effects of noxious and tactile stimulations.     

Changes of motor cortical excitability in human subjects from wakefulness to early stages of sleep: a combined transcranial magnetic stimulation and electroencephalographic study

The effect of sleep on human motor cortical excitability was investigated by evaluating the latency and amplitude of motor evoked potentials in ten subjects using transcranial magnetic stimulation. Motor evoked potentials and electroencephalographic data were recorded simultaneously and analyzed. Recordings were performed before, during and after a sleep period. A significant decrease in motor evoked potentials amplitude and a slight change in motor evoked potentials latency were noted in the recordings during the different sleep stages with a return to baseline values on awakening. A decrease in motor cortical excitability is suggested as explaining the effect of sleep.     

Mechanisms underlying state dependent surface-evoked response patterns

Cortical evoked response potentials (ERPs) display a rich set of waveforms that are both context and state dependent. However, the mechanisms that underlie state dependent ERP patterns are unclear. Determining those mechanisms through analysis of single trial ERP waveform signatures may provide insight into the regulation of cortical column state and the roles that sleep plays in cortical function. We implanted rats with electroencephalogram (EEG) and electromyogram (EMG) electrodes to record ERPs and to assess sleep/wake states continuously during 1–2 s random auditory clicks. Individual cortical auditory ERPs were sorted into one of eight behavioral states, and fell into three categories based on amplitude and latency characteristics. ERPs within waking and rapid eye movement (REM) sleep were predominantly low amplitude and short latency. Approximately 50% of ERPs during light quiet sleep (quiet sleep 1 and quiet sleep 2) exhibited low amplitude, short latency responses, and the remaining ERPs had high amplitude, long latency responses. This distribution was characteristic of EEG fluctuations during low frequency delta waves. Significantly more individual ERPs showed very low amplitudes during deep quiet sleep (quiet sleep 3 and quiet sleep 4), resulting in a lower average ERP. These results support the hypothesis that evoked response amplitudes and waveform patterns follow specific EEG patterns. Since evoked response characteristics distribute differently across states, they could aid our understanding of sleep mechanisms through state-related and local neural signaling.     

Simultaneous measurements of microflow and evoked potentials in the somatomotor cortex of the cat brain during specific sensory activation

The behaviour of both microflow and evoked potentials was investigated in the right somatomotor cortex of the cat (anaesthetized with chloralose) during electrical stimulation of the contralateral left forepaw. Frequency, amplitude, and time of stimulation were varied. Using the local hydrogen clearance method the changes of microflow were continuously monitored in the same cortical area from which the evoked potentials were recorded.The experiments have shown that activation of the somatomotor cortex by somatic stimulation of the contralateral forepaw results in changes of microflow which clearly correlate to the side and amplitude of the primary evoked potentials. An increase in flow as well as in amplitude of the potentials depends on the stimulation parameters. The changes of microflow are limited to a small area of 1–2 mm in diameter. We conclude that a tight coupling of flow to functional activity exists in the microcirculatory range.The results were presented in part at 49. Tagung der Deutschen Physiologischen Gesellschaft, March 7–10, 1978, in Göttingen     

Effects of sleep on pain-related somatosensory evoked potentials in humans

We investigated effects of sleep on pain-related somatosensory evoked potentials (SEP) following painful electrical stimulation of the left index finger. The biggest advantage of this method is that signals ascending through both A-beta fibers relating to touch and A-delta fibers relating to pain can be recorded simultaneously. While the subject was awake, non-painful stimulation evoked early- and middle latency components, N20, P30 and N60, at the C4 electrode, and painful stimulation evoked not only early- and middle latency components at the C4 but also later pain-specific components, N130 and P240, at the Cz electrode. During sleep, N20 and P30 did not show a significant change in amplitude, N60 showed a slight but significant amplitude reduction, and N130 and P240 significantly decreased in amplitude or disappeared, as compared with those while awake. Therefore, we speculate on the mechanisms generating each component as follows; (1) N20 and P30 are the primary components generated in SI ascending through A-beta fibers. (2) N60 is the secondary component generated in SI involving cognitive function to some degree. (3) N130-P240 are the pain-specific components ascending through A-delta fibers, and closely related to cognitive function, because they were much affected by consciousness, different from the components ascending through A-beta fibers.     

Somatosensory evoked potentials following proprioceptive stimulation of finger in man

Brisk passive flexion of the proximal interphalangeal joint of the middle finger, produced by using a newly devised instrument, elicited evoked potentials on the scalp. The present study carefully excluded the possible contribution of sensory modalities other than proprioception. The initial part of cortical response was a positive deflexion at the contralateral central area (P1 at 34.6 ms after the stimulus). This was followed by a midfrontal negative wave (N1 at 44.8 ms) and a clear positivity at the contralateral centroparietal area (P2 at 48.0 ms). The evoked responses persisted in spite of the abolition of cutaneous and joint afferents of the finger caused by ischemic anesthesia, but they were lost by ischemic anesthesia of the forearm. Thus, the cortical evoked responses obtained in this study most probably reflect muscle afferent inputs. The scalp distribution of P1 suggested that its cortical generator source was different from that of the N20-P20 components of evoked potentials to electrical median nerve stimulation. Brodmann areas 2 and 3a of human brain, which are known to receive deep receptor inputs, are the most plausible generator sites for the early components of the proprioception-related evoked responses. The amplitude of P2 was related to the velocity but not to the magnitude of movement. In conclusion, the present study established a method for recording the evoked responses to the brisk passive movement of the finger joint, which mainly reflect the dynamic aspects of proprioception mediated through muscle afferent.     

Electrocortical effects of early postnatal thyroxine administration in the rat

Using cortical evoked potentials and number of dendritic spines as indicators of degree of neocortical maturation, the effects produced by an excess of thyroxine early in life were studied in rats. Before Day 20 postnatally, thyroxine caused shortened mean peak latencies, advanced the organization of the configuration of potentials, caused higher amplitude evoked responses and shortened the duration of the evoked repetitive discharges to sensory stimuli. However, from the 20th postnatal day, a subsequent retardation of the development occurred, as indicated by a prolonged mean peak latency of primary responses, persistence of an increased amplitude, and prolonged duration of the evoked repetitive discharges to sensory cues. These electrophysiological effects were associated with an initial increase followed by a decrement at later ages in the number of spines of the large cortical pyramidal cells. From present results it is suggested that thyroxine early in life would interfere with the development of the cortical and subcortical structures underlying evoked electrical activity.     

Fast decrement with stimulus repetition in ERPs generated by neuronal systems involving somatosensory SI and SII cortices: electric and magnetic evoked response recordings in humans.

The effect of stimulus repetition (short trains of stimuli with 1-s inter-stimulus intervals and 15-s inter-train intervals) on both electric and magnetic evoked responses were studied in four subjects. In addition to the later N140 and P300 deflections in electric potentials, a distinct and immediate amplitude decrement was obtained also for the earlier P50 and P100 deflections. The magnetic evoked responses also demonstrated the amplitude decrement for 50 ms (M50) and 100 ms (M100) latency deflections. The time-course and degree of amplitude decrement of the M100 magnetic response corresponded especially well to those of P100 electric deflections. The results thus show the rate effect on electric and magnetic responses at 50 and 100 ms latencies, and further suggest that the electric and magnetic responses, reflecting the activation of somatosensory SI and SII cortical areas at these latencies, respectively, are generated by related neuronal mechanisms.     

Component-specific effects of physostigmine on the cat visual evoked potential

Pattern visual evoked potentials (VEPs) were recorded from the pial surface of the cat primary visual cortex prior to and following the intravenous administration of physostigmine, an agent which blocks the enzyme responsible for the breakdown of synaptically released acetylcholine. The control VEP was composed of a small initial positive deflection (P1), a subsequent large negative wave (N1) and a second large positive wave (P2). Following physostigmine, the amplitude of P1-N1 was diminished whereas that of N1-P2 increased. These effects were long lasting and were blocked by prior treatment with scopolamine, a result consistent with mediation by a muscarinic cholinergic pathway. Waveform subtraction revealed that the physostigmine-sensitive component had a slow, negative polarity waveform while the physostigmine-insensitive component was also slow, but positive in polarity. The fundamental nature of these components remains to be assessed. Nevertheless, the results indicate that waveforms of different polarity combine algebraically to yield the conventional VEP.     

Optimal electrode placements for adequate spatial sampling of auditory evoked potentials

Summary When evoked responses are used in clinical practice and research the measures that are most commonly considered are the latencies and amplitudes of EP components as measured at a single electrode site. Our recent studies have shown that multichannel recordings yield measures such as potential field asymmetry that may be as important as component latency and amplitude. The purpose of this short technical note is to suggest that electrode placement is critical for demonstrating interesting features of the potential field topography, specifically, bilateral, homologous generator sites. The cortical imaging technique (CIT) was used to analyze the averaged responses for a group of thirty normal young adults to a repeated tone and a random oddball tone. Recordings were obtained at 28 scalp recording sites which included 20 placements from the 10–20 system and eight additional sites. Simulated cortical maps were derived for four components, the N1 frequent response and the N2a, P3, and N3, rare minus frequent responses for three different electrode arrays. These arrangements included the full 28-channel array, a 20-channel array that excluded eight additional central sites, and a 20-channel array that included the eight additonal sites and excluded peripheral sites. This study demonstrates that for these auditory paradigms, the placement of the electrodes is critically important for discriminating important features of the potential fields.     

Effects of spatial frequency on visual evoked magnetic fields

Psychophysical and visual evoked potential (VEP) studies have shown that spatial frequency of a visual stimulus affects contrast sensitivity and VEPs in humans. However, it is not clear whether and how the effect of spatial frequency varies among cortical areas. Considering that all visual inputs to the retina could be expressed as a sum of sinusoidal gratings of different spatial frequencies, the effect of spatial frequency must be clarified to separate the brain activity specific to each visual stimulus. In order to examine the effect of spatial frequency on different cortical areas, the present study compared cortical responses to sinusoidal gratings of seven different spatial frequencies using magnetoencephalography (MEG). MEG waveforms of twelve healthy adults in response to sinusoidal gratings of 0.3–18.1 cycles per degree were subjected to a multi-dipole analysis. As a result, the effect of spatial frequency was significant on the first peak latency and amplitude of the source activity around V1 and V2 but not on the source activity around V3 and V6, indicating that the effect of spatial frequency varies across different visual areas in the human brain. Our results also suggest that the responses in V1 and V2 that have a peak around 90 ms and that of V6 peaking around 120 ms should be separated to investigate the stimulus-specific cortical response, particularly when examining effects of spatial frequency on the response latency.     

A comparison of saccade evoked potentials recorded during reading and tracking tasks

The influence of visual processing demands on saccade-triggered evoked potentials was investigated at P3, P4 and Oz recording sites during reading and tracking tasks. To maximize the physical similarities between tasks, subjects tracked a series of lights that flashed in a stereotypic reading pattern behind a page of text; eye movements recorded during reading initiated the light sequence. In the first experiment, a significant decrease observed in the latency of the major positive peak recorded from Oz during tracking was attributed to the smaller amplitude of tracking, relative to reading, saccades. To confirm this interpretation, the experiment was repeated with modification to the light display. As anticipated, equating saccade amplitudes across tasks eliminated waveform differences in the second experiment. Although peak latencies and amplitudes were not influenced reliably by visual processing demands, tracking potentials exhibited a negative DC shift relative to reading waveforms that was significant at 174 msec at the Oz site. These data suggest that the saccade-triggered evoked potential components generally are insensitive to task differences within the visual modality when visual configuration and eye movement parameters are controlled.     

Magnetosensory evoked potentials: consistent nonlinear phenomena

Electromagnetic fields (EMFs) having strengths typically found in the general environment can alter brain activity, but the reported effects have been inconsistent. We theorized that the problem arose from the use of linear methods for analyzing what were actually nonlinear phenomena, and therefore studied whether the nonlinear signal-processing technique known as recurrence quantification analysis (RQA) could be employed as the basis of a reliable method for demonstrating consistent changes in brain activity. Our primary purpose was to develop such a method for observing the occurrence of evoked potentials in individual subjects exposed to magnetic fields (2G, 30 and 60 Hz). After all conditions that affected the analysis of the EEG were specified in advance, we detected magnetosensory evoked potentials (MEPs) in all 15 subjects (P<0.05 in each experiment). The MEPs, which occurred within the predicted latency interval of 109-504 ms, were independent of the frequency and the direction of the field, and were not detected using the traditional linear method of analysis, time averaging. When the results obtained within subjects were averaged across subjects, the evoked potentials could not be detected, indicating how real nonlinear phenomena can be averaged away when the incorrect method of analysis is used. Recurrence quantification analysis, but not linear analysis, permitted consistent demonstration of MEPs. The use of nonlinear analysis might also resolve apparent inconsistencies in other kinds of brain studies.     

The P300 component in sleep

The present investigation utilized the P300 component of the auditory evoked potential as an index of information processing (discrimination) in sleep. Auditory evoked potentials were recorded to target and nontarget stimuli during sleep stages 3/4, 2 and REM under two probability conditions. Corresponding "nontone" waveforms were generated in each sleep stage, representing averaged EEG activity with no tones presented. Target P300 amplitude was higher than both corresponding "nontone" targets and tone nontargets. Probability did not affect the target-nontarget relationship. Latency of P300 increased and amplitude decreased from wakefulness through sleep; however, neither amplitude nor latency differed among sleep stages. Amplitude and latency of N200 increased during sleep. While N200 amplitude was highest in Stage 3-4, N200 latency did not differ among sleep stages. These findings suggest that the P300 recorded in sleep indexes similar cognitive processes as the P300 recorded in wakefulness. That P300 as well as N200 latency increased in sleep suggests that processes indexed by these components may slow during sleep.