Human middle-latency auditory evoked potentials: vertex and temporal components

We recorded middle-latency (20–70 msec) auditory evoked potentials (MLAEPs) to monaural and binaural clicks in 30 normal adults (ages 20–49 years) at 32 scalp locations all referred to a balanced non-cephalic reference. Our goal was to define the MLAEP components that were present at comparable latencies and comparable locations across the subject population. Group and individual data were evaluated both as topographic maps and as MLAEPs at selected electrode locations.Three major components occurred between 20 and 70 msec, two well-known peaks centered at the vertex, and one previously undefined peak focused over the posterior temporal area. Pa is a 29 msec positive peak centered at the vertex and present with both monaural and binaural stimulation, Pb is a 53 msec positive peak also centered at the vertex but seen consistently only with binaural and right ear stimulation. TP41 is a 41 msec positive peak focused over both temporal areas. TP41 has not been identified in previous MLAEP studies that concentrated on central scalp locations and/or used active reference electrode sites such as ears or mastoids.Available topographic, intracranial, pharmacologic, and lesion studies indicate that Pa, Pb and TP41 are of neural origin. Whether Pa and/or Pb are produced in Heschl's gyrus, primary auditory cortex, remains unclear. TP41 is probably produced by auditory cortex on the posterior lateral surface of the temporal lobe. It should prove of considerable value in experimental and clinical evaluation of higher level auditory function in particular and of cortical function in general.     

Short- and middle-latency auditory evoked potentials in abstinent chronic alcoholics: preliminary findings

Short- and middle-latency auditory evoked potentials (BAEPs and MAEPs) were studied in 15 chronic alcoholic patients after 1 month's abstinence and compared with those of 15 healthy controls, matching the patients pairwise for sex and age. Most of the parameters studied varied more within the alcoholic group than within the control group. The BAEP results agree with previous reports; in the alcoholic group, BAEP peak V was significantly delayed and the inter-peak intervals, III–V and I–V, were lengthened. The latencies of the MAEP components Na and Pa, on the other hand, were significantly shortened. These findings suggest that chronic abusive consumption of alcohol may bring about structural and/or neurochemical alterations at various levels in the auditory pathway.     

Relationship of transient and steady-state auditory evoked fields

Transient and steady-state auditory evoked fields (AEFs) to brief tone pips were recorded over the left hemisphere at 7 different stimulus rates (0.125–39 Hz) using a 37-channel biomagnetometer. Previous observations of transient auditory gamma band response (GBR) activity were replicated. Similar rate characteristics and equivalent dipole locations supported the suggestion that the steady-state response (SSR) at about 40 Hz represents the summation of successive overlapping (10 Hz) middle latency responses (MLRs). On the other hand, differences in equivalent dipole locations and habituation effects suggest that the magnetically recorded GBR is a separate phenomenon which occurs primarily at low stimulus rates and is unrelated to either the magnetically recorded MRL or SSR.     

Cortical localisation of magnetic fields evoked by oesophageal distension

Magnetoencephalographic source localisation techniques were used to measure oesophageal evoked magnetic fields from the cerebral cortex in 3 subjects. By using rapid balloon distension as a stimulus, a comparison of proximal and distal oesophageal cortical representation was made. The distal oesophagus was represented bilaterally in the insular cortex and SII as well as the inferior aspect of SI. The proximal oesophagus was represented unilaterally in superior and inferior SI, insular cortex and SII. Significantly, the superior portion of SI was consistently activated in subjects following stimulation of the proximal oesophagus, but similar activation was not found in response to distal stimulation. This may reflect the contribution from somatic afferent fibres in the striate muscle of the proximal segment. In conclusion, vagal afferents appear to contribute more to cortical activation following stimulation of the distal rather than the proximal oesophagus, while spinal afferents appear to be activated by both proximal and distal oesophageal stimulation.     

Extensive cochleotopic mapping of human auditory cortical fields obtained with phase-encoding FMRI

The primary sensory cortices are characterized by a topographical mapping of basic sensory features which is considered to deteriorate in higher-order areas in favor of complex sensory features. Recently, however, retinotopic maps were also discovered in the higher-order visual, parietal and prefrontal cortices. The discovery of these maps enabled the distinction between visual regions, clarified their function and hierarchical processing. Could such extension of topographical mapping to high-order processing regions apply to the auditory modality as well? This question has been studied previously in animal models but only sporadically in humans, whose anatomical and functional organization may differ from that of animals (e.g. unique verbal functions and Heschl''s gyrus curvature). Here we applied fMRI spectral analysis to investigate the cochleotopic organization of the human cerebral cortex. We found multiple mirror-symmetric novel cochleotopic maps covering most of the core and high-order human auditory cortex, including regions considered non-cochleotopic, stretching all the way to the superior temporal sulcus. These maps suggest that topographical mapping persists well beyond the auditory core and belt, and that the mirror-symmetry of topographical preferences may be a fundamental principle across sensory modalities.     

Mapping of tibial nerve evoked magnetic fields over the lower spine

Using a low-noise 49-channel dc-SQUID system spinal somatosensory evoked fields (SEF) were recorded which were generated by compound action currents evoked upon posterior tibial nerve stimulation. The SEF mapping showed the action current propagation along the sciatic nerve, lumbosacral plexus and cauda equina in parallel to simultaneously recorded electrical potentials (SEP). For a reliable intraindividual side-to-side comparison of spinal SEFs the right and left tibial nerves were stimulated in alternating order; this procedure minimizes artifactual inter-nerve SEF map differences due to eventual patient-to-sensor displacements which might occur in serial measurements. These large-area lumbar SEF mappings open up several clinical perspectives for magnetoneurography, in particular with respect to the 3D-localization of proximal conduction blocks.     

Somatosensory evoked magnetic fields following stimulation of the lip in humans

The topography of somatosensory evoked magnetic fields (SEFs) following stimulation of the upper and lower lips was investigated in 6 normal subjects. When the lateral side of the upper lip was stimulated. P20m and its counterpart, N20m, were identified in the hemisphere contralateral to the stimulated side. The equivalent current dipoles (ECDs) of N20m-P20m were considered to be located in the lip area of the primary sensory cortex (SI). Middle latency deflections (N40m-P40m, N60m-P60m, and N80m-P80m) were identified in bilateral hemispheres. Their ECDs were located in the SI in both hemispheres. Long latency deflections (P110m-N110m) were recognized in both hemispheres, and their ECDs were located inferior to the SI, in an area considered to be the secondary sensory cortex (SII). When the midline of the lip was stimulated, similar short and middle latency deflections were also identified, but SII deflections (P110m-N110m) were decreased in amplitude. When the lower lip was stimulated, the ECDs of short and middle latency deflections were located at a site in the SI inferior to or near those elicited by upper lip stimulation. The ECDs of P110m-N110m were located in an area of the SII similar to that upon stimulation of the upper lip, but their orientations were different.     

Pattern-reversal auditory evoked potential

The late auditory evoked potential (AEP) was studied in response to an alternatingly frequency-modulated complex tone. This ‘pattern-reversal’ AEP was found to be a heartier response than the conventional tone-burst evoked potential, albeit longer in latency.     

Gender differences in source location for the N100 auditory evoked magnetic field

Auditory evoked magnetic fields were recorded in response to contralateral stimulation over the right hemisphere in 6 adult males and 6 adult females. The data were fit to a model of a current-dipole source in a homogeneous sphere and 5 parameters of the dipole were computed — 3 spatial coordinates, orientation, and strength. When average values for the dipole parameters were compared between sexes, it was found that the current source for the N100_m is located more than 1 cm posterior in females and is oriented pointing more downward. These findings were replicated in separate measurements sessions. Viewing of individual magnetic resonance images did not reveal a corresponding anatomical disparity in the location of the primary auditory cortex which is assumed to produce the N100_m. Therefore, functional organization of the auditory cortex may be different for the sexes.     

Somatosensory evoked magnetic fields and potentials following passive toe movement in humans

The somatosensory evoked magnetic fields (SEFs) and evoked potentials (SEPs) following passive toe movement were studied in 10 normal subjects. Five main components were identified in SEFs recorded around the vertex around the foot area of the primary sensory cortex (SI). The first and second components, 1M and 2M, were identified at approximately 35 and 46 ms. Equivalent current dipoles (ECDs) of both 1M and 2M were estimated around SI in the hemisphere contralateral to the movement toe, and were probably generated in area 3a or area 2, which mainly receive inputs ascending through muscle and joint afferents. The large inter-individual difference of 1M and 2M in terms of ECD orientation was probably due to a large anatomical variance of the foot area of SI. The third and fourth components, 3M and 4M, were identified at approximately 62 ms and 87 ms, respectively. They appeared to be a single large long-duration component with two peaks. Since the 3M and 4M components were significantly larger than the 1M and 2M components in amplitude and their ECD location was significantly superior to that of 1M and 2M, we suspected that they were generated in different sites from those of 1M and 2M, probably area 3b or area 4. Four components, 1E, 2E, 3E and 4E, were identified in SEPs, which appeared to correspond to 1M, 2M, 3M and 4M, respectively. The variation observed in the scalp distribution of the primary component, 1E, could be accounted for by the variation of the orientation of ECD of the 1M component. There was a large difference in the waveform of the long-latency component (longer than 100 ms) between SEFs and SEPs. The 5E of SEPs was a large amplitude component, but the 5M of SEFs was small or absent. We speculate that this long-latency component was generated by multiple generators.     

Topography of middle-latency somatosensory evoked potentials following painful laser stimuli and non-painful electrical stimuli

The aim of this study was to compare cerebral evoked potentials following selective activation of Aβ and Aδ fibers. In 15 healthy subjects, Aβ fibers were activated by electrical stimulation of the left radial nerve at the wrist. Aδ fibers were activated by short painful radian heat pulses, applied to the dorsum of the left hand by a CO₂ laser. Evoked potentials were recorded with 15–27 scalp electrodes, evenly distributed over both hemispheres (bandpass 0.5–200 Hz). The laser-evoked potentials exhibited a component with a mean peak latency of 176 msec (N170). Its scalp topography showed a parieto-temporal maximum contralateral to the stimulus side. In contrast, the subsequent vertex negativity (N240), which appeared about 60 msec later, had a symmetrical scalp distribution. Electrically evoked potentials showed a component at 110 msec (N110), that had a topography similar to the laser-evoked N170. The topographies of the N170 and N110 suggest that they may both be generated in the secondary somatosensory cortex. There was no component in the electrically evoked potential that had a comparable interpeak latency to the following vertex potential: for N60 it was longer, for N110 it was shorter. On the other hand, in the laser-evoked potentials no component could be identified the topography of which corresponded to the primary cortical component N20 following electrical stimulation.     

The insulin-mediated modulation of visually evoked magnetic fields is reduced in obese subjects

Background

Insulin is an anorexigenic hormone that contributes to the termination of food intake in the postprandial state. An alteration in insulin action in the brain, named “cerebral insulin resistance”, is responsible for overeating and the development of obesity.

Methodology/Principal Findings

To analyze the direct effect of insulin on food-related neuronal activity we tested 10 lean and 10 obese subjects. We conducted a magnetencephalography study during a visual working memory task in both the basal state and after applying insulin or placebo spray intranasally to bypass the blood brain barrier. Food and non-food pictures were presented and subjects had to determine whether or not two consecutive pictures belonged to the same category.Intranasal insulin displayed no effect on blood glucose, insulin or C-peptide concentrations in the periphery; however, it led to an increase in the components of evoked fields related to identification and categorization of pictures (at around 170 ms post stimuli in the visual ventral stream) in lean subjects when food pictures were presented. In contrast, insulin did not modulate food-related brain activity in obese subjects.

Conclusions/Significance

We demonstrated that intranasal insulin increases the cerebral processing of food pictures in lean whereas this was absent in obese subjects. This study further substantiates the presence of a “cerebral insulin resistance” in obese subjects and might be relevant in the pathogenesis of obesity.     

Functional mapping in the human brain using high magnetic fields.

An avidly pursued new dimension in magnetic resonance imaging (MRI) research is the acquisition of physiological and biochemical information non-invasively using the nuclear spins of the water molecules in the human body. In this trial, a recent and unique accomplishment was the introduction of the ability to map human brain function non-invasively. Today, functional images with subcentimetre resolution of the entire human brain can be generated in single subjects and in data acquisition times of several minutes using 1.5 tesla (T) MRI scanners that are often used in hospitals for clinical purposes. However, there have been accomplishments beyond this type of imaging using significantly higher magnetic fields such as 4 T. Efforts for developing high magnetic field human brain imaging and functional mapping using MRI (fMRI) were undertaken at about the same time. It has been demonstrated that high magnetic fields result in improved contrast and, more importantly, in elevated sensitivity to capillary level changes coupled to neuronal activity in the blood oxygenation level dependent (BOLD) contrast mechanism used in fMRI. These advantages have been used to generate, for example, high resolution functional maps of ocular dominance columns, retinotopy within the small lateral geniculate nucleus, true single-trial fMRI and early negative signal changes in the temporal evolution of the BOLD signal. So far these have not been duplicated or have been observed as significantly weaker effects at much lower field strengths. Some of these high-field advantages and accomplishments are reviewed in this paper.     

Late pain-related magnetic fields and electric potentials evoked by intracutaneous electric finger stimulation

Surface magnetic and electric recordings were used to localize the sources of late pain-related magnetic fields and electric potentials, evoked by painful intracutaneous electric finger stimulation. We find that the source of the P90m component of the evoked magnetic field lies in the finger area of the primary somatosensory cortex; the sources of the N150m and P250m are found to reside in the frontal operculum. These findings are unexpected from the evoked electric potential data, which suggest a central location for these sources. We also note that the interpretation of the electric data was confounded by the presence of an alpha-like oscillation, which overlapped many components of the evoked potential.     

Characterizing auditory receptive fields

In this issue of Neuron, two papers by Atencio et al. and Nagel and Doupe adapt new computational methods to map the spectrotemporal response fields of neurons in the auditory cortex. The papers take different but complementary approaches to apply theoretical techniques to classical methods of receptive field mapping and, in doing so, provide exciting new insights into the way in which sounds are processed in the auditory cortex.     

Logarithmic display of auditory evoked potentials

Auditory evoked potentials (AEP) can be simultaneously recorded on-line as a succession of 11 waves, through a single input channel of a mini-computer. Since the response waves differ widely in frequency, a computing routine has been developed to display the whole response pattern in a single picture. Based upon a non-linear samples reduction of the digitized response, this routine allows a logarithmic transformation of the time axis. The method improves the identification of the AEP components and provides an objective estimate of the central auditory pathway for both neurophysiological and neuroclinical studies.     

Dantrolene modulates the influence of steady magnetic fields on hippocampal evoked potentials in vitro

Direct current-generated magnetic fields (2-3 mT, 20-min exposure) exerted biphasic effects on the population spike recorded from hippocampal slices. The initial decrease in the potential, observed during exposure of the slices to magnetic fields was followed by a recovery/amplification phase, which began after terminating the magnetic field action. During that phase the population spike exceeded the amplitude observed before application of the magnetic fields. The pattern of magnetic fields influence was not affected either by (+)-5-methyl-10,11-dihydro-5H-dibenzo (a,d) cyclohepten-5, 10-imine maleate (MK801), or by D,L,-2amino-5phosphonovalerate (APV), a noncompetitive and competitive NMDA receptor antagonist, respectively. The rising phase of the potential, however, was eliminated by dantrolene, an inhibitor of intracellular Ca(2 +) channels. This suggests that intracellular calcium channels participate in the mechanism of the influence of the direct current magnetic fields on the function of the hippocampal tissue.     

Multiple frequency steady-state evoked magnetic field mapping of digit representation in primary somatosensory cortex

Magnetic source imaging of multiple frequency steady-state somatosensory evoked responses was examined using a 151-channel magnetoencephalography (MEG) system and a dual-channel electrical stimulator. Somatotopy of digit representation was studied in healthy subjects and effects of injury-related cortical plasticity in patients with unilateral transections of the median or the ulnar nerve. Dipole source locations exhibited somatotopic order with overlap between neighboring digits. In two of three nerve injury patients evidence for cortical reorganization was found. The location of sources related to digits neighboring deafferented digits was changed and their dipole moments were enlarged by comparsion with the sources related to contralateral homologue control digits. As a basis for magnetic source imaging, the recording of multiple frequency somatosensory steady-state evoked responses may be a viable and time saving alternative to the recording of transient evoked responses.