PubMed Health⌕ Search

Biomedical subjects

H G Vaughan

Publications and source records attributed to H G Vaughan.

At least 55 records · Page 3Linked to original sources
PubMed1991

Striate cortical contribution to the surface-recorded pattern-reversal VEP in the alert monkey.

The striate cortical contribution to the surface pattern-reversal visual evoked potential (VEP) was investigated in awake monkeys during performance of a visual fixation task, by examining laminar profiles of VEP, current source density (CSD) and concomitant multiunit activity (MUA) in Area 17, recorded simultaneously at incremental depths using multicontact electrodes. Stimuli were black/white bar gratings centered on the fixation point. The typical surface pattern-reversal VEP over striate cortex consists of a prominent positivity peaking at 50-70 msec (P60), followed by a large negativity peaking at approx. 80 msec (N80), and then by a late broad positivity, peaking between 120 and 150 msec (P125). P60 is often preceded by a small negativity peaking at 45-55 msec (N50), and on rare occasions a small positivity (P40) is also observed. N50 is generated primarily by current sinks in Lamina 4C. P60 arises from large current sources in the supragranular laminae. N80 and P125 appear to be composite waveforms reflecting complex contributions from local activity and from activity occurring outside of the foveal/immediate parafoveal representation in Area 17. The basic physiologic sequence elicited by patterned stimulation is similar to that elicited by diffuse luminance or by electrical stimulation, but is characterized by more prominent supra- and infragranular activation. It is consistent with the cellular and synaptic anatomy of Area 17: initial activation of the thalamorecipient subdivisions of Lamina 4C, followed by activation of mid/upper Lamina 4 and of supra- and infragranular laminae. Our results suggest the possibility of differentiating synaptic stages and cellular processes reflected in the human VEP, based on homologies with simian VEP components.

Animals↗
PubMed1991

The relationship between N2 and N400: scalp distribution, stimulus probability, and task relevance.

The N2 and N400 components of the event-related brain potential were compared along several dimensions. In one task, subjects discriminated on the basis of size, and in another task, subjects discriminated on the basis of semantic category. In one set of conditions stimuli deviated infrequently along one stimulus dimension (either in size or semantic category) and in another set of conditions stimuli deviated infrequently along two stimulus dimensions (both size and semantic category). When subjects discriminated on the basis of size, infrequent deviation in the size of the stimuli was associated with a negativity that peaked around 320 ms (N2). When subjects discriminated on the basis of semantic category, infrequent deviation in the semantic category of the stimuli was associated with a negativity that peaked around 400 ms (N400). N2 and N400 were found to be generally alike in scalp distribution, hemispheric asymmetry, and sensitivity to stimulus probability. N2 was not elicited when size changes were task irrelevant and N400 was not elicited when changes in semantic category were task irrelevant.

Adult↗
PubMed1990

Binocularity in the lateral geniculate nucleus of the alert macaque.

We examined the timing and the laminar distribution of binocular interactions and non-dominant eye effects in the lateral geniculate nucleus (LGN) of the alert macaque. Flash-evoked multiunit activity (MUA) was recorded simultaneously from multiple LGN laminae during contralateral eye, ipsilateral eye and binocular stimulation. Evidence of binocularity was noted in all laminae except 4. Inhibitory effects included (1) binocular suppression, a 15-70% reduction in the peak amplitude of MUA during binocular, compared to monocular stimulation of the dominant eye for that lamina, and (2) non-dominant suppression, a 10-45% reduction in MUA below its spontaneous level, with stimulation of the non-dominant eye. In lamina 3, the onset of this effect often preceded that of the excitatory response to stimulation of the dominant eye, and was coincident with non-dominant suppression in lamina 2, and with the dominant eye-mediated excitatory response in contralaterally innervated lamina 1. This is consistent with a parvo/magnocellular interaction. Corresponding excitatory effects, binocular and non-dominant facilitation, were also noted throughout LGN. The latter occurred both within the time frame of retinally driven activity (15-65 ms), and at longer latencies as well. The earliest response in lamina 6 of striate cortex, the origin of the corticogeniculate projections, was 30-35 ms, thus precluding a role of corticofugal modulation in the earlier effects in LGN.

Action Potentials↗
PubMed1990

Tonotopic features of speech-evoked activity in primate auditory cortex.

To further clarify the neural mechanisms underlying the cortical encoding of speech sounds, we have recorded multiple unit activity (MUA) in the primary auditory cortex (A1) and thalamocortical (TC) radiations of an awake monkey to 3 consonant-vowel syllables, /da/, /ba/ and /ta/, that vary in their consonant place of articulation and voice onset time (VOT). In addition, we have examined the responses to the syllables' isolated formants and formant pairs. Response features are related to the cortical tonotopic organization, as determined by examining the responses to selected pure tones. MUA patterns that differentially reflect the spectral characteristics of the steady-state formant frequencies and formant transition onset frequencies underlying consonant place of articulation occur at sites with similarly differentiated tone responses. Whereas the detailed spectral characteristics of the speech sounds are reflected in low frequency cortical regions, both low and high frequency areas generate responses that reflect their temporal characteristics of fundamental frequency and VOT. Formant interactions modulate the responses to the whole syllables. These interactions may sharpen response differences that reflect consonant place of articulation. Response features noted in A1 also occur in TC fibers. Thus, differences in the encoding of speech sounds between the thalamic and cortical levels may include further opportunities for formant interactions within auditory cortex. One effect could be to heighten response contrast between complex stimuli with subtle acoustical differences.

Acoustic Stimulation↗
PubMed1990

Laminar analysis of bicuculline-induced epileptiform activity in area 17 of the awake macaque.

We investigated the laminar profile of visually evoked paroxysmal discharge following bicuculline administration in area 17 of the alert macaque. Paroxysmal discharge was focussed in lamina 3, with lesser concomitants in infragranular laminae. There were no marked effects on the flash-evoked laminar activation sequence. The neocortex of the alert macaque studied with the present techniques may provide a model for evaluating basic mechanisms of epilepsy, and for linking these to the macroscopic phenomena seen in humans.

Animals↗
PubMed1990

Inhibitory processes in the flash evoked potential of the monkey.

Flash visual evoked potentials (VEP) and concurrent multiple unit activity (MUA) were recorded from closely spaced intracortical sites in unanesthetized monkeys before and after intracortical injection of the GABAA antagonist bicuculline. Laminar VEP profiles were subjected to current source density (CSD) analysis to localize the transmembrane current flows contributing to the generation of the field potentials. Before bicuculline, the first large VEP component, N40, was generated principally within the parvocellular thalamorecipient sublamina 4Cb. After bicuculline injection, the current sinks associated with N40 spread throughout lamina 4, consistent with a release of intracortical inhibition mediated by GABA. A subsequent component, P65, believed to represent recurrent inhibitory activity within 4Cb, was greatly diminished in size after bicuculline injection. The laminar pattern of current sources and sinks coincident with this component was more complicated after bicuculline, reflecting the summation of current flows associated with disinhibited lamina 4 activity. Bicuculline also altered the responses of neuronal elements in laminae 3 and 5, evidenced by large increases in MUA in these laminae that began approximately 50 msec after stimulation. Finally, bicuculline diminished the degree of intracortical ocular dominance, implicating GABAergic mechanisms in the maintenance and refinement of ocular input segregation within cortical columns.

Animals↗
PubMed1990

Differentiation of negative event-related potentials in an auditory discrimination task.

Several different negative potentials elicited in auditory perceptual tasks make spatially and temporally overlapping contributions to the scalp-recorded event-related potential. Frequent non-target tones in a 2-stimulus oddball pitch discrimination task, when compared with the same stimuli in ignore or simple reaction time conditions, elicit a negative deflection with two peaks (NA1 and NA2) differing in their latency and topography from the exogenous N92-P156 deflections. Oddball tones, when compared with the frequent ones, elicit mismatch negativity (MMN) in both ignore and discrimination conditions; MMN displays a frontocentral-posterolateral polarity inversion. In the discrimination condition, MMN is followed by N2 and P3b; the former has a more central amplitude maximum than MMN, and no posterolateral polarity inversion. When the pitch discrimination task was made more difficult, there was no effect on NA1 or NA2, but the latency of MMN, N2, P3b, and reaction time all increased in parallel. It is hypothesized that MMN reflects the outcome of an automatic mismatch detection process, and that the subsequent processing of targets is related to the event of mismatch detection.

Acoustic Stimulation↗
PubMed1989

Timing and distribution of flash-evoked activity in the lateral geniculate nucleus of the alert monkey.

Simultaneous recording of activity from multiple cortical laminae in alert monkeys, using multichannel electrodes, has been used to identify the intracranial generators of surface-recorded, visually evoked potentials (VEP) to stroboscopic flash. Beyond their clinical implications, these results offer an unique view of the timing and sequence of cortical visual processing in the alert monkey, including the somewhat surprising findings of an extremely short-latency response in lamina IVA, a contra- over ipsilateral latency advantage throughout lamina IV, and the lack of a consistent flash-evoked response in the major cortical recipient of the magnocellular system, lamina IVCa. The present study used similar techniques to examine flash-evoked activity in LGN and in optic tract, both to elucidate the role of the subcortical pathways in establishing this pattern, and to provide a parallel, detailed view of the timing of visual activity in LGN and optic tract in the alert monkey. Flash-evoked responses are robust in both parvo- and magnocellular laminae, but these responses differ along several dimensions: (1) parvocellular multiunit activity (MUA) is 1/4 to 1/2 the amplitude of magnocellular MUA; (2) oscillatory activity is higher in frequency and shorter in duration in parvo- than in magnocellular responses; (3) inhibitory processes appear less prominent and diverse in parvo- than in magnocellular activity; (4) mean onset latencies of MUA are longer in parvo- than in magnocellular laminae, but there is extensive overlap in these distributions. Latencies encountered in ipsilateral lamina 3, and at laminar borders dorsal to 3, group more clearly with those of the magnocellular laminae than with those of the other parvocellular laminae. As a result, in the parvocellular division as a whole, the average latency to ipsilateral stimulation is shorter than that to contralateral stimulation. The optic tract exhibits a dorsal-to-ventral progression of onset latency and oscillation frequency consistent with a dorsal/ventral segregation of the inputs to parvo- and magnocellular layers. Comparison of optic tract and LGN data reveals that while many LGN response characteristics are initiated in the retina, significant modification of retinal output occurs at LGN. The techniques used here permit a particularly sensitive and reliable assessment of the timing and distribution of visual responses in the optic tract and LGN of alert monkeys. Our data support the view that in the alert monkey, the surface-VEP to passive, binocular flash primarily reflects activation of parvocellular thalamorecipient laminae of Area 17.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Cortical responses to speech sounds and their formants in normal infants: maturational sequence and spatiotemporal analysis.

Cortical auditory evoked potentials (AEPs) to the synthesized syllables [da[ and [ta[ and to the isolated 3 formants of [da[ were obtained from 32 normal infants studied at monthly intervals from birth through 3 months and at 6 months of age. A bilateral array of 16 electrodes referenced to the mid-occiput permitted a topographic analysis of the cortical AEPs at selected latencies. A differential maturational sequence was seen: a predominantly negative cortical AEP wave form became positive, first over the frontocentral region (around term), and then over the temporal region (at 1-2 months). The timing of these electrophysiological changes coincides with a differential anatomical maturational sequence in the auditory cortex, as myelination and synaptogenesis are more advanced in primary than secondary auditory areas at term. All infants in this study followed this developmental sequence. However, there was no systematic effect of the center frequency of the formant stimuli on the maturational level of their respective cortical AEPs, suggesting a relative maturational equivalence in those regions of auditory cortex responding to stimuli across the frequency range present in human speech. In term infants, an initial midline positivity and bitemporal negativities were asynchronous in their peak and offset latencies. suggesting independent generators for each of these components. In infants from 3 to 6 months of age, cortical AEP wave forms consisted of 2 initial positive peaks followed by a negative peak. While the wave forms were similar over midline and lateral scalp, spatiotemporal analysis revealed differences in the latency of onset, duration and in the spatial extent of these components, again suggesting that 2 bilateral, temporally overlapping generators contribute to the cortical AEP. No systematic topographic difference was observed in the cortical AEPs elicited by each of the 3 formants, which differed in center frequencies.

Auditory Cortex↗
PubMed1988

Effects of the amount of stimulus information processed on negative event-related potentials.

Event-related potentials were recorded during simple reaction time and 3 discrimination conditions which varied in the amount of stimulus information that needed to be processed. It was found that NA became longer in duration as the amount of stimulus information that required processing was increased. Using sequential topographic mapping, it was concluded that there are at least 3 overlapping deflections that comprise NA. The experimental effect appeared to be mainly on the third deflection of NA. The problem of overlap between NA and later positive-going components, P380 and P3b, is discussed in terms of their relative latencies and scalp distributions.

Adult↗
PubMed1988

The N2 component elicited by stimulus matches and multiple targets.

This paper examines two methodological issues concerning the N2 component of human event-related potentials. The first issue concerns the circumstance that the most common way to obtain N2 in discrimination tasks is with an infrequent deviant stimulus that mismatches a frequent, standard stimulus. In these studies it is not possible to disentangle the effects of stimulus probability and stimulus mismatch on N2. In the present study it was found that, if two stimuli regularly alternate, N2 is elicited by infrequent repetitions of either stimulus. Thus, N2 is elicited by infrequent stimulus matches as well as infrequent stimulus mismatches. The second issue concerns the effect of stimulus probability on N2. Whereas previous research has established that the amplitude of N2 is inversely related to stimulus probability, the present study found that the number of possible targets in a visual discrimination task also has effects on N2 amplitude, with the overall probability of targets kept constant. Increasing the number of targets was associated with an increase in the duration of N2 and a differential enhancement of N2 at fronto-central as opposed to posterior-lateral recording sites. The latter results provide further evidence for the existence of two visual N2 components and tentative grounds for differentiating N2 from N400.

Adult↗
PubMed1988

The anatomic and physiologic bases of brain stem auditory evoked potentials.

The generators of human BAEPs, as summarized in Table 1 and Figure 12, are more complex than has often been presumed. Most peaks reflect activity in several structures, though patterns of BAEP abnormality can be correlated with abnormalities in general areas of the brain stem. The phasic peaks originate in action potentials, whereas the SN reflects postsynaptic potentials within brain-stem auditory nuclei.

Acoustic Stimulation↗
PubMed1987

Movement-related potentials within the hippocampal formation of the monkey.

A series of large amplitude potentials were recorded within and surrounding the hippocampal formation in association with repetitive, self-paced hand movements in the monkey. The earliest component precedes the onset of muscle contraction by approximately 100 ms, inverts in polarity across the pyramidal cell layer of CA3 and CA1 and is coincident with increased local multiple unit activity (MUA). Later components, generated within the hippocampal formation, follow the muscle contraction by as much as 600 ms and are not associated with net changes in MUA. The onset of voluntary movements is often associated with desynchronization of low-frequency rhythmic hippocampal EEG patterns and these changes antecede the movement. The findings strongly suggest that the hippocampal formation is involved in the initiation and control of even simple voluntary movements in the primate.

Animals↗
PubMed1987

Trimodal evoked potentials compared with magnetic resonance imaging in the diagnosis of multiple sclerosis.

Twenty-three patients with the clinical diagnosis of possible multiple sclerosis (MS) were tested with magnetic resonance imaging (MRI) and trimodal evoked potentials. Fourteen patients showed abnormalities on both MRI scans and at least one evoked potential modality (65%). Four patients had normal MRI scans but at least one abnormality on evoked potential testing (17%). One patient had normal triple evoked potentials with an abnormal MRI result. Four patients had normal results on both MRI and triple evoked potential testing; two of these patients were later found to have immunologic abnormalities in the cerebrospinal fluid consistent with the diagnosis of MS. Combined evoked potential testing was found to have a higher sensitivity than MRI in confirming a diagnosis of MS. Three patients with the clinical diagnosis of definite MS were also tested. All these patients showed abnormalities on evoked potential testing, although one patient had a normal MRI result. Of all 26 patients who were studied, 17 showed abnormal MRI results and 21 showed at least one abnormality on evoked potential testing.

Adult↗
PubMed1987

Electrophysiologic assessment of auditory pathways in high risk infants.

This study evaluated auditory processing in a group of 59 infants at risk for subsequent hearing and language disorders due to low birthweight and/or perinatal asphyxia. Auditory system integrity was evaluated electrophysiologically by recording the auditory brainstem response (ABR), middle latency response (MLR) and the cortical auditory evoked potential (CAEP). 63% of the babies had normal peripheral function or slight unilateral impairment; 84% had normal brainstem auditory system function; 82% showed normal MLRs; and 81% showed normal CAEPs. Fifty-three percent of the babies were normal on all tests and only 3% were deviant on all tests. The remaining infants showed diverse patterns of peripheral, brainstem and cortical abnormalities.

Auditory Cortex↗
PubMed1986

Short-latency auditory evoked potentials in the monkey. I. Wave shape and surface topography.

The components and topography of the short-latency auditory evoked potentials (SLAEPs) in the monkey were analyzed using a combination of surface and epidural electrodes. Nine positive peaks, and a slow negativity following wave 7, are identified in the SLAEP recorded at the vertex referred to the mastoid ipsilateral to a monaural stimulus. Subcomponents of waves 1 and 3 are consistently observed, and other waves are probably compound as well. Barbiturate anesthesia results in latency prolongations which are cumulative across components but does not alter the component sequence. With binaural stimulation there is simple summation of the monaural responses for the components preceding wave 7, but a binaural interaction for wave 7. The polarity and clarity of the various components depend on the recording configuration selected. No single configuration is optimal for the demonstration of all components, and the use of montages with multiple electrodes may increase the information gained from clinical SLAEP recordings in man. The monkey SLAEP components can be grouped based on similarities of their surface topographies, which suggest similar anatomic orientations of their underlying generators. Probable homologues between the components of the simian and human SLAEP, based on temporal sequence, topography, and the effects of variation of the stimulus parameters, are: Jewett's waves I through VII correspond to monkey waves 1, 3, and 5 through 9. Due to differences in the size and shape of the heads, the monkey waves 2 and 4 correspond to negativities in the human SLAEP, labeled IN and IIN after the components which they follow. Another peak, with latency between waves 7 and 8 in the monkey and V and VI in man, is only clearly seen in the mastoid-to-mastoid linkage, contralateral negative.

Animals↗
PubMed1986

Short-latency auditory evoked potentials in the monkey. II. Intracranial generators.

The generators of the short-latency auditory evoked potentials (SLAEPs) in the monkey have been defined by intracranial mapping from cochlea to auditory cortex. SLAEP components other than 1a and the slow negativity (SN) following wave 7 derive from compound action potentials propagated in subcortical auditory pathways. The component generators are complex due to the presence of two bursts of activity in the eighth nerve, to the fact that the ascending auditory fibers both synapse on and bypass specific relay nuclei, and to the differences in orientation of segments of the auditory pathways. Most SLAEP components recorded at the surface reflect the summation of activity from multiple generators. However, much of the activity seen within subcortical structures cannot be traced to the surface of the brain. Component 1a is identified with the cochlear summating potential, while 1b reflects the initial afferent volley in the distal portion of the eighth nerve. Component 2 represents the initial depolarization of the eighth nerve terminals within the ipsilateral cochlear nucleus. Component 3h reflects the second volley of activity in the distal portion of the eighth nerve and the outflow of the cochlear nucleus which decussates in the trapezoid body. Component 3v represents the initial cochlear nucleus outflow volley ascending the lateral lemniscus. Component 4 principally reflects the second volley of activity within the eighth nerve terminals, and outflow from the ipsilateral superior olivary complex ascending in that lateral lemniscus, with a possible contribution from activity in the contralateral CNC. Component 5 represents the outflow of the contralateral superior olivary complex ascending in that lateral lemniscus. Component 6 reflects another volley from the ipsilateral superior olivary complex ascending in that lateral lemniscus, as well as outflow from both inferior colliculi propagating in their brachii. The generators of component 7 are the most complex encountered, representing volleys in both lateral lemnisci, activity of the contralateral inferior colliculus, and activity in both auditory radiations. A component that follows wave 7, seen best in mastoid-to-mastoid recording linkage, represents outflow from both inferior colliculi propagating in their brachia. Components 8 and 9 principally reflect propagated action potentials in the auditory radiations bilaterally, with an additional contribution from activity of both inferior colliculi. The SN mainly represents volume-conducted postsynaptic potentials from both inferior colliculi and cochlear nuclei.

Animals↗