Biomedical subjects

D S Barth

Publications and source records attributed to D S Barth.

At least 37 records · Page 2Linked to original sources

PubMed1994

The anatomic organization of evoked potentials in rat parietal cortex: electrically evoked commissural responses.

1. Two 8 x 8 channel microelectrode arrays were positioned over 3.5 x 3.5 mm2 areas in homologous regions of right and left parietal cortex of four rats. Potentials were evoked by delivering epicortical electrical stimulation to each electrode on one hemisphere while mapping the commissural response from the contralateral array. Spatial distributions of the electrically evoked potential (EECP) complex were compared directly with cytochrome oxidase-stained sections of the recorded region. 2. Electrode sites most capable of eliciting a commissural EECP were arranged along a diagonal band extending medially from the rostral to caudal region of each electrode array, approximating the pattern of dysgranular cortex separating primary auditory (Te1) from primary somatosensory (Par1) cortex. Electrode sites in the rostromedial and caudolateral region were ineffectual in eliciting an EECP in either hemisphere. Stimulation sites within secondary visual cortex (Oc2L) also produced strong responses. Only weak responses were elicited from stimulation of Te1 and no EECP could be evoked when stimulating within Par1. 3. When an EECP in the maximally sensitive diagonal region was elicited, its spatial distribution was typically asymmetrical throughout the recording array; the response was largest along a diagonal region also extending medially from the rostral to caudal area of each electrode array. Thus the pattern of EECP in each hemisphere closely matched the pattern of electrically excitable regions in the contralateral hemisphere. 4. The EECP was usually heterogeneous. EECP distributions within the strongly responding diagonal area often formed two regions of maximum amplitude separated by a less active zone. Although responses in Te1 were significantly weaker than those in the adjacent dysgranular cortex, they also revealed a heterogeneous spatial distribution with multiple closely spaced maxima. Only responses in Oc2L appeared consistently homogeneous, with a single maximum representing the EECP. 5. These results provide functional evidence supporting a model of parietal cortex in which there are two basic types of recipient regions, densely granular regions, which are the termination sites of specific thalamocortical fibers, and dysgranular or agranular regions, which receive both ipsilateral and contralateral projections. The functional parceling of rodent parietal cortex on the basis of the spatial and temporal distribution of the epicortical evoked potential complex may be superimposed onto the anatomic parceling into granular and dysgranular zones. Implications for stages of sensory information processing are discussed.

Animals↗

PubMed1993

Binaural vs. monaural auditory evoked potentials in rat neocortex.

High spatial resolution epicortical recording techniques and numerical modeling were used to investigate laterality effects on the middle latency auditory evoked potential (MAEP) complex. Our data confirm previous reports that auditory stimulus laterality has a consistent effect on the amplitude, timing, and spatial distribution of the MAEP complex. The earliest temporal components (P1a, P1b and N1) show the greatest sensitivity, and are absent during ipsilateral stimulation. The later positive slow wave (P2) is present at the same amplitude during all stimulation conditions. Generation of the P2 appears to be independent of prior activation of areas 36 and 41 reflected in the early components, suggesting its generation by a more diffuse thalamocortical pathway, possibly from the medial division of the medial geniculate. Serial vs. parallel activation of rodent auditory cortex is discussed in the context of laterality-sensitive MAEP components.

Animals↗

PubMed1993

The neurophysiological basis of epileptiform magnetic fields and localization of neocortical sources.

In this review, we present animal studies that evaluate the appropriateness of the dipole model as a physical representation of neocortical sources produced by evoked and spontaneous epileptiform activity in neocortex. Three-dimensional electrical measurements of cellular currents in rat sensory neocortex are compared to associated extracranial magnetic fields. Comparisons are performed for the direct cortical response evoked by electrical stimulation of the cortical surface, and for evoked and spontaneous interictal and ictal discharge of the penicillin focus in the same animal preparation. These data support the hypothesis that evoked and epileptiform magnetic fields result from intradendritic currents oriented perpendicular to the cortical surface. Furthermore, magnetic fields can be detected from epileptic foci smaller than 3 x 3 mm2. Results from animal studies are then used as an empirical foundation for physical models with which to interpret noninvasive neuromagnetic recordings of epileptic discharge in human focal seizure disorders. Examples are presented in which the spatial and temporal distribution of extracranial magnetic fields produced by interictal events in humans may be adequately represented by multiple dipolar sources.

Animals↗

PubMed1993

Anatomic organization of evoked potentials in rat parietotemporal cortex: somatosensory and auditory responses.

1. Two 8 x 8-channel microelectrode arrays were used to map epicortical field potentials from a 3.5 x 3.5-mm2 area in homologous regions of right and left parietotemporal cortex of four rats. Potentials were evoked with bilaterally presented click stimuli and with bilateral tactile stimulation of the 25 major vibrissae. The spatial distribution of temporal components of the somatosensory evoked potential (SEP) and auditory evoked potential (AEP) complex were compared directly with cytochrome oxidase-stained sections of the recorded region. 2. Epicortical responses in both hemispheres to bilateral vibrissal stimuli consisted of a biphasic sharp wave (P1a-N1) constrained to the vibrissa/barrel granular region of primary somatosensory cortex (SmI). A slightly later sharp positive wave (P1b) was localized to secondary somatosensory cortex (SmII) and to perigranular cortex medial to the vibrissa/barrel field. The SEP complex ended with a biphasic slow wave (P2-N2). The P2 was centered on SmI and spread to dysgranular lateral cortex, caudal to but excluding SmII. The N2 was centered on SmII and spread to dysgranular cortex caudal to but excluding SmI. 3. The anatomic organization of the AEP in many ways approximated that of the SEP in the same animals. The timing and morphology of the AEP were nearly identical to the SEP. The AEP consisted of a P1a-N1 sharp wave constrained to the estimated region of primary auditory cortex (AI) in the lateral parietotemporal region, a later P1b localized to secondary auditory cortex (AII), and subsequent slow waves (P2 and N2) that were centered on AI and AII, respectively, and spread to dysgranular regions overlapping the distributions of the P2 and N2 of the SEP complex. 4. These data suggest that the basic neural generators for the SEP and AEP in parietotemporal cortex are quite similar, and provide evidence for the functional anatomy of each temporal component of the sensory evoked potential complex. It is concluded that the early fast waves of the SEP and AEP are modality specific and may represent the parallel activation of primary and secondary sensory cortex through established parallel afferent projections from lateral and medial thalamic nuclei. The later slow waves of the SEP and AEP appear to selectively involve primary and secondary sensory cortex but are more widely distributed, possibly reflecting a less modality-specific level of information processing in dysgranular cortex.

Acoustic Stimulation↗

PubMed1992

Topographical analysis of epileptiform potentials in rat somatosensory cortex: the interictal to ictal transition.

Large quantities of penicillin were applied to the face and forelimb region of rat somatosensory cortex, producing an epileptic focus with both electrographic and behavioral signs of seizures that regularly repeated over a period of several minutes. Epicortical potentials were recorded simultaneously from a 64 channel micro-electrode array (8 x 8 platinum electrodes) with inter-electrode distances of 0.5 mm, covering a 3.5 x 3.5 mm2 area centered on the penicillin injection site. Cluster analysis was used to classify successive epileptiform discharges into interictal, transitional, and ictal groups. Principal components analysis (PCA) was used to extract fundamental waveforms producing the spike complex in each group, and to estimate the locations and spatial extent of neuronal populations participating in epileptiform discharge. During all states of epileptic excitability, it was possible to account for over 90% of the variance in the epicortical potential waveforms using a model with only two spatially overlapping populations of cells. The location and spatial extent of the populations remained unchanged by the transition to seizures; the interictal and ictal states were distinguished only by changes in the timing and amplitude of potentials in the two putative neuronal populations. The present model, using only two stationary neuronal populations to reproduce all spatiotemporal patterns in the neocortical epileptogenic focus, is compared to models proposed by others in which epileptic discharge is thought to propagate sequentially through adjacent cortex. It is concluded that the initiation, maintenance, and termination of seizures in neocortex relies on mechanisms that are not necessarily reflected in changes in spatiotemporal interactions among epicortically recorded cell groups within the focus. These mechanisms may be distinguished from those responsible for the spread of seizures within neocortex.

Animals↗

PubMed1992

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.

Acoustic Stimulation↗

PubMed1991

The functional anatomy of middle latency auditory evoked potentials.

The neural origins of middle latency auditory evoked potentials (MAEP) were studied in rat cortex. MAEP were mapped from the cortical surface with a high spatial resolution electrode array. Spatiotemporal analysis, based on multivariate statistical methods, was then used to relate putative neural generators of the MAEP complex to established cytoarchitectural anatomy. These data indicate that the MAEP waveform reflects systematic asynchronous activation of both primary and secondary auditory cortex during the processing of simple click stimuli.

Acoustic Stimulation↗

PubMed1991

The electrophysiological basis of epileptiform magnetic fields in neocortex: spontaneous ictal phenomena.

In a previous report (Barth, D.S. and Di, S., Brain Research, 530 (1990) 35-39), electrical measurements of epileptiform cellular currents produced by physiologically evoked interictal penicillin spikes in rat somatosensory cortex were directly compared to the extracranial magnetic fields these currents generate. The present study uses the same methodology to extend these observations to spontaneous interictal and ictal phenomena in rat auditory cortex, and provides a more realistic empirical foundation for physical models with which to interpret non-invasive neuromagnetic recordings of human focal seizures. These data indicate that seizure foci under 1 x 1 cm2 in cortical surface area are capable of producing magnetic fields that may be recorded at extracranial distances similar to those used in humans. Furthermore, physical models based on the dipole approximation appear to be appropriate for the interpretation of ictal magnetic field phenomena in neocortex.

Animals↗

PubMed1991

Topographic analysis of field potentials in rat vibrissa/barrel cortex.

An 8 x 8 multichannel microelectrode array was used to simultaneously record epicortical field potentials, evoked by displacement of contralateral vibrissae, from a 4 x 4 mm2 area of vibrissa/barrel cortex in 4 rats. The epicortical responses began with early positive (P1) and negative (N1) sharp waves, followed by slower positive (P2) and negative (N2) waves. The potential complex systematically shifted location with vibrissa stimulated, in accordance with the known somatotopic anatomy of vibrissa/barrel cortex. Topographical distributions of potentials at the P1, N1, P2 and N2 peaks were approximately concentric, but had distinct spatial extents, suggesting that they were generated by different but overlapping neuronal subpopulations. We propose that the SEP in the vibrissa/barrel cortex is produced by both sequential and parallel processing of somatosensory information, and that all components of the epicortical SEP are generated only in primary somatosensory cortex of the rat. Applications and weaknesses of topographic analysis methods are discussed.

Animals↗

PubMed1991

Neuromagnetic investigation of somatotopy of human hand somatosensory cortex.

In order to investigate functional topography of human hand somatosensory cortex we recorded somatosensory evoked fields (SEFs) on MEG during the first 40 ms after stimulation of median nerve, ulnar nerve, and the 5 digits. We applied dipole modeling to determine the three-dimensional cortical representations of different peripheral receptive fields. Median nerve and ulnar nerve SEFs exhibited the previously described N20 and P30 components with a magnetic field pattern emerging from the head superior and re-entering the head inferior for the N20 component; the magnetic field pattern of the P30 component was of reversed orientation. Reversals of field direction were oriented along the anterior-posterior axis. SEFs during digit stimulation showed analogous N22 and P32 components and similar magnetic field patterns. Reversals of field direction showed a shift from lateral inferior to medial superior for thumb to little finger. Dipole modeling yielded good fits at these peak latencies accounting for an average of 83% of the data variance. The cortical digit representations were arranged in an orderly somatotopic way from lateral inferior to medial superior in the sequence thumb, index finger, middle finger, ring finger, and little finger. Median nerve cortical representation was lateral inferior to that of ulnar nerve. Isofield maps and dipole locations for these components are consistent with neuronal activity in the posterior bank of central fissure corresponding to area 3b. We conclude that SEFs recorded on MEG in conjunction with source localization techniques are useful to investigate functional topography of human hand somatosensory cortex non-invasively.

Brain Mapping↗

PubMed1991

Empirical comparison of the MEG and EEG: animal models of the direct cortical response and epileptiform activity in neocortex.

This review directly addresses the appropriateness of the dipole model as a physical representation of neocortical sources produced by evoked and spontaneous epileptiform activity in neocortex. Three dimensional electrical measurements of cellular currents in rat sensory neocortex are compared to the extracranial magnetic fields these currents produce. Comparisons are performed for the direct cortical response (DCR) evoked by electrical stimulation of the cortical surface, and for evoked and spontaneous interictal and ictal discharge of the penicillin focus in the same animal preparation. Our data support the hypothesis that evoked and epileptiform magnetic fields result from intradendritic currents oriented perpendicular to the cortical surface. Furthermore, magnetic fields can be detected from epileptic foci smaller than 3 x 3 mm2. This work provides an empirical foundation for physical models with which to interpret noninvasive neuromagnetic recordings of epileptic discharge in human focal seizure disorders. The dipole approximation appears to be appropriate for the interpretation of magnetic field phenomena in neocortex.

Animals↗

PubMed1991

Functional anatomy of human hand sensorimotor cortex from spatiotemporal analysis of electrocorticography.

We measured chronic electrocorticography (ECoG) of sensorimotor cortex during contralateral median nerve stimulation in 6 patients with partial seizures evaluated for surgery. We analyzed the spatiotemporal structure of the somatosensory evoked response (SER) using multiple source modeling to investigate functional anatomy of its neuronal sources. Two dipole sources in postcentral gyrus explained the large majority of the first 60 msec of the SER, indicating a subregion of hand somatosensory cortex generating this activity. The source locations agreed with normal functional anatomy from cortical stimulations, intraoperative photographs, and postoperative neurological examinations after focal excisions. The time patterns of both sources were biphasic like the previously described N20-P30 and P25-N35 peaks. The spatiotemporal patterns of both sources overlapped. Spatiotemporal analysis with multiple dipole sources appears useful to determine the number, locations, and spatiotemporal field patterns of cortical regions active during peripheral somatosensory stimulation and reveals simplicity in the macroscopic functional anatomy of dynamic human sensorimotor cortex.

Adolescent↗

PubMed1991

Spatiotemporal modeling of cerebral evoked magnetic fields to median nerve stimulation.

We measured somatosensory evoked magnetic fields during median nerve stimulation in 6 normal subjects. We applied multiple dipole models to study the spatiotemporal structure of early somatosensory evoked magnetic fields (SEFs), as well as the number, 3-dimensional location and time activity of their underlying neuronal sources. Two dipole sources were necessary to model the first 40 msec of SEFs explaining 85% of the data variance. Source 1 was located deeper than source 2, showed primarily a tangential orientation, and accounted for a larger part of the variance; source 2 showed no consistent orientation across subjects. Both sources showed biphasic time activities corresponding to the previously described N20-P30 and P25-N35 components. Spatiotemporal modeling could identify sources which could not be modeled consistently above noise by single moving dipoles (P25 component), revealed small latency differences of the two sources in some subjects suggesting parallel activation of these sources, and allowed separation of sources overlapping considerably both in space and time. We conclude that spatiotemporal modeling of SEFs may be useful to study functional anatomy of human sensorimotor cortex non-invasively.

Electric Stimulation↗

PubMed1991

Laminar excitability cycles in neocortex.

1. Laminar field potentials produced by paired electrocortical stimuli were recorded with a linear microelectrode array inserted perpendicular to the surface of rat somatosensory cortex. Current source-density (CSD) distributions of the direct cortical response (DCR) were computed from the potential profiles. Principal component analysis (PCA) was used to estimate the time course of evoked transmembrane currents of putative pyramidal cell populations in the supragranular and infragranular layers. 2. Both supra- and infragranular cells displayed an initial period after the conditioning stimulus in which test stimuli produced subnormal evoked response amplitudes. This was followed in both layers by a long period of supernormal then subnormal responses and a second period of supernormal responses. 3. The main laminar difference encountered was a general shortening of all phases of the excitability cycle in the supragranular cells. 4. Excitability cycles in the supra- and infragranular layers closely followed the morphology of average evoked responses to the conditioning stimulus alone. These results and physiological support to the validity of lamina-specific evoked response waveforms derived from combined CSD and PCA analysis of extracellular potential measurements. 5. The relationship between evoked potential amplitude changes and cortical excitability is discussed.

Animals↗

PubMed1991

Localization of partial epilepsy using magnetic and electric measurements.

Dipole methods applied to brain electric and magnetic fields have made several significant advances in investigation of epilepsy and sensorimotor cortex. The magnetoencephalogram (MEG) and the MEG-electroencephalogram (EEG) combination have contributed substantially. The MEG-EEG combination has shown a mean accuracy of somatosensory evoked response (SER) localization of the central fissure similar to electrocorticography (ECoG), resolution of ambiguity in ECoG of alternative configurations of hand sensorimotor cortex, and resolution of ambiguity of the causes of the difference between EEG and ECoG map patterns. MEG has shown simple dipolar maps of the temporal lobe interictal spike, localization estimates with about 6 mm error, and spatial separation of propagating multiple sources. MEG and EEG have shown a new neocortical propagation pathway in the temporal lobe, noninvasive estimates of the area of the spike focus, and complementary detection sensitivity. Application of spatiotemporal multiple dipole modeling in the simplest field using MEG has given a realistic quantification of spike zones. MEG and EEG have shown simple dipolar patterns for seizure origin, suggesting focality of some frontal seizures, and appears to increase EEG utility. Improved accuracy may result from a combined methodology including MEG and EEG. Dipole methods have potential utility as a noninvasive diagnostic procedure in epilepsy.

Brain Mapping↗

PubMed1990

The electrophysiological basis of epileptiform magnetic fields in neocortex.

Electrical measurements of epileptiform cellular currents in a penicillin model of focal epilepsy were directly compared to the extracranial magnetic fields these currents produce. Our data support the hypothesis that epileptiform magnetic fields result from intradendritic currents oriented perpendicular to the cortical surface. Furthermore, magnetic fields could be detected from epileptic foci smaller than 3 mm2. This work provides an empirical foundation for physical models with which to interpret noninvasive neuromagnetic recordings of epileptic discharge in human focal seizure disorders.

Animals↗

PubMed1990

Laminar interactions in rat motor cortex during cyclical excitability changes of the penicillin focus.

Laminar interactions between neurons in rat motor cortex during cyclical seizure episodes in the penicillin focus were studied using a combination of current source-density (CSD) and principal component analysis (PCA), combined with computer-based physical modeling. These data suggest that all phases of cyclical seizure phenomena are produced by interactions between two distinct populations of neurons, the same neuronal circuits previously reported to give rise to the direct cortical response (DCR) and electrically evoked interictal penicillin spikes (EIIS). The first population consists of small pyramidal cells in the supragranular layer, and the second population consists of larger pyramidal cells in the infragranular layers with apical dendrites extending to the cortical surface. The supragranular cells serve as a trigger zone for initiating both spontaneous interictal spikes (IIS) and polyspike bursts (PSB) during seizures. Fast activity in the supragranular cells is typically followed by a hyperpolarizing slow wave that may be the result of Ca2+-activated K+ currents. This slow wave increases during seizures, possibly reflecting changes in extracellular Ca2+ associated with seizure onset and termination. The monophasic response of infragranular cells is similar for both IIS and PSB and consists of a large depolarizing shift followed by a rapid but partial repolarization period and a subsequent gradual repolarization period lasting several hundred milliseconds. The infragranular response is similar in polarity and morphology to the intracellularly recorded paroxysmal depolarization shift (PDS) and may indicate that these deeper neurons are mainly responsible for this phenomena in neocortex. Finally, there is a marked postictal slow oscillation between the supra- and infragranular layers. This oscillation appears first and largest in the supragranular cells and may reflect a disturbance in excitatory feedback in these cells produced by the disinhibitory effect of penicillin, a disturbance capable of pathologically synchronizing the epileptic neuronal aggregate sufficiently for activation of the spike-generating mechanism and subsequent seizures.

Animals↗

PubMed1990

Laminar analysis of extracellular field potentials in rat vibrissa/barrel cortex.

1. A 16-channel electrode array was used to record simultaneously extracellular laminar field potentials evoked by displacement of contralateral vibrissa from vibrissa/barrel cortex in five rats. Current source-density (CSD) analysis combined with principal component analysis (PCA) was used to determine the time course of laminar-specific transmembrane currents during the evoked response. 2. The potential complex consisted of biphasic fast components followed by long-lasting slow waves. It began with activity in supragranular cells consisting of a source in layers I-II and a sink in layers IV-V; this was followed by activation of the infragranular cells with a paired sink and source in layers I-IV and V-VI, respectively. The slow-wave sequences also began in the supragranular cells followed by infragranular neurons. 3. We propose that the fast components reflect sequential intralaminar depolarization processes, and the slow waves, hyper- or repolarization processes. These results suggest that a basic neuronal circuit, consisting of sequential activation of the supragranular and then the infragranular pyramidal cells, gives rise to the field potentials evoked by physiological stimulation. This is consistent with our previous studies of direct cortical responses (DCR) and pathological discharges of the penicillin focus.

Animals↗