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N A Shaw

Publications and source records attributed to N A Shaw.

At least 19 recordsLinked to original sources
PubMed1992

Auditory potentials elicited by the grass photic stimulator in the rat.

Flash visual evoked potentials were recorded from the rat to determine whether the Grass photic stimulator could generate an auditory component of the waveform. When the click associated with each lamp flash remained unmuffled, the primary response of the visual cortex (P30) was preceded by an earlier positivity with a latency between 6-8 ms. When the photic stimulator lamp was subsequently insulated in order to deaden the sound of the click, the P30 potential remained intact while the earlier response disappeared. It is concluded that the photic stimulator is capable of eliciting a distinct auditory potential of probable collicular origin.

Animals↗
PubMed1992

The effects of low-pass filtering on the flash visual evoked potential of the albino rat.

Flash visual evoked potentials (FVEPs) were recorded from the rat in order to determine the effects of low-pass filtering on the wave form. The low-frequency (high pass) filter remained fixed at 3.2 Hz while the setting of the high-frequency (low-pass) filter was progressively raised from 32 Hz to 3.2 kHz. The amplitude of the primary cortical potential (P30) steadily increased while its latency decreased until asymptotic values were recorded with a low-pass cut-off of 320 Hz. Thereafter, there was little additional change in wave form. It is concluded that a bandpass of 3.2-320 Hz is optimal to record the primary cortical response of the FVEP, and this is consistent with the theory that the P30 potential is generated by comparatively slow post-synaptic activity. In a second experiment the effects of low-pass filtering were examined on the later and more labile secondary components of the FVEP wave form. These were found to be less responsive to low-pass filtering than the early components and assumed a near optimal configuration when the high-frequency cut-off was raised to 80 Hz. The high-frequency filter setting which is most appropriate to record the primary component of the FVEP therefore appears to be more than adequate also to record the secondary responses. It is also shown that the same principles of low-pass filtering on the FVEP will apply irrespective of whether the subject is awake or anaesthetised.

Anesthesia↗
PubMed1992

The effects of low pass filtering on central somatosensory conduction time.

Central Somatosensory conduction time (CSCT) in the rat was measured by simultaneously recording the cortical somatosensory evoked potential (SEP) and cervical SEP and then subtracting the peak latency of the cervical response from that of the primary cortical response. The low frequency (high pass) filter of the recording system was kept fixed at 3.2 Hz but the high frequency cut-off was progressively raised from 32 to 16 kHz to examine the effects of low pass filtering on the two waveforms from which CSCT is derived. With a bandpass of 3.2-32 Hz, no activity could be reliably identified in either the cortical or cervical traces. With subsequent rises in the high frequency filter, the amplitude of both potentials increased with a concomitant decrease in their latencies. Stable values were obtained with a bandpass of 3.2-3.2 kHz and there was little additional change in waveform configuration. In contrast to the waveforms, low pass filtering had only minimal effects on CSCT and near constant values (in the range 3.6-4.1 ms) were obtained regardless of the setting of the low pass filter. I concluded that low pass filtering has a largely uniform effect on somatosensory activity generated in both primary cortical and cervico-medullary locations.

Anesthesia↗
PubMed1992

Age-dependent changes in central somatosensory conduction time.

The effects of aging on central somatosensory conduction time (CSCT) were examined using 125 neurologically healthy subjects aged 10-79 years. There was a mean overall increase in CSCT during the age span studied of 0.5 ms. However, the principal increase in CSCT did not appear to start until the end of the fourth decade, after which there was a steady incremental increase. There was little evidence of an abrupt stepwise increase in CSCT at any age. Age-dependent changes in asymmetry (right-left difference in CSCT) remained approximately constant until the end of the sixth decade. From the age of 60 years onwards, there was a small mean increase in asymmetry of 0.1 ms. Subjects were divided into six age groups and the upper limit of normal for both CSCT and asymmetry was defined for each.

Adolescent↗
PubMed1992

Pattern visual evoked potential luminance and multiple sclerosis.

Pattern visual evoked potentials (PVEPs) were recorded from 111 patients classified as having possible, probable or definite multiple sclerosis. Patients were stimulated with a checkerboard pattern using high and low luminances in order to test the hypothesis that an attenuated pattern luminance increases the detection rate of PVEP abnormalities. With increasing certainty of diagnosis, there was a concomitant increase in the incidence of PVEP abnormalities. However, there was no evidence that stimulating with a lower luminance pattern enhanced the sensitivity of the test. The same findings were also apparent when the patient data was analyzed according to the presence or absence of a history of optic neuritis or other visual symptoms. It is concluded that, within the luminance limits used in this study, the role of varied luminance in detecting demyelinating lesions in the optic nerves using the PVEP is minimal, although there was some limited evidence that a high level of luminance may be more appropriate than a low level.

Adolescent↗
PubMed1992

A possible collicular component of the auditory evoked potential and its relationship to brainstem and cerebellar auditory potentials.

Auditory evoked potentials were recorded from the rat using skull screw electrodes inserted over the inferior colliculus and the cerebellum. In addition, brainstem auditory evoked potentials (BAEPs) were also recorded. The response recorded from over the inferior colliculus consisted of a slow positive potential with one of two possible peak latencies. The mean latency of the earlier potential was 5.6 ms and that of the later potential was 6.4 ms. A hypothetical generator for the first collicular potential is the termination of the lateral lemniscus in the ventrolateral inferior colliculus, while the later collicular potential could have its origins within the brachium of the inferior colliculus. None of the principal nor minor BAEP waves corresponded to either of the collicular responses. Nor did the trough of negativity between BAEP waves IV and V which is often thought to reflect activity generated within the midbrain. The potential recorded over the cerebellum also consisted of a slow positivity but with a slightly sharper contour than that of the collicular response. The mean latency of the cerebellar potential was 4.9 ms. As there was no temporal relationship between collicular and cerebellar potentials, the present study provided no support for the theory that cerebellar auditory potentials are artefactual and simply far field reflections of activity generated in the inferior colliculus. Judging by the timing of the BAEP waves, it is also concluded that the afferent volley most likely projects to the cerebellum via a collateral pathway branching off the caudal part of the lateral lemniscus.

Acoustics↗
PubMed1991

Cloning of high-affinity methionine transport genes from Salmonella typhimurium.

At least one of the genes encoding a methionine transport system in Salmonella typhimurium has been cloned on a 15 kbp fragment in the lambda 1059 vector, and the region responsible for the transport activity was subcloned in a set of plasmids with inserts ranging from 1.4 to 4.2 kbp in the multicopy plasmid pUC8. Two proteins of Mr 34 and 40 kDa were expressed from the larger inserts.

Biological Transport↗
PubMed1991

A comparison between electroencephalography and somatosensory evoked potentials for outcome prediction following severe head injury.

The value of somatosensory evoked potentials (SEPs) for the prediction of outcome following severe head injury (HI) is established. The role of the electroencephalogram (EEG) in this setting is uncertain. In this comparative study, SEPs and EEGs were recorded within 3 days of severe HI in 90 patients, and the results related to outcome at 6 months. Patients with an isoelectric EEG or an EEG with repeated isoelectric intervals died. Reactivity of the EEG to external stimulation tended to be associated with favorable outcome. Grading of the EEGs on the basis of frequency composition otherwise provided no prognostic information. The presence of SEP scalp potentials bilaterally predicted favorable outcome, particularly if the central conduction times were normal. Conversely, the absence of one of both scalp potentials was associated with unfavorable outcome. EEGs thus provided useful prognostic information in only a minority of patients. By comparison, SEPs allowed prediction of both favorable and unfavorable outcomes in a much larger number of patients, and were therefore prognostically superior.

Adolescent↗
PubMed1991

A possible thalamic component of the auditory evoked potential in the rat.

An attempt is described to identify a thalamic component of the auditory evoked potential in the rat. Auditory potentials were recorded simultaneously from skull locations over the thalamus and the primary auditory cortex. From over the thalamus a slow positive response was recorded with a mean peak latency of 7.3 ms. This preceded the primary cortical response by 1.2 ms. It is concluded that this potential is a more likely candidate for a thalamic response than either a late high frequency component of the brainstem auditory evoked potential or else one of the early components of the middle latency auditory evoked potential.

Animals↗
PubMed1990

Central auditory conduction time in the rat.

Central conduction time is the time for an afferent volley to traverse the central pathways of a sensory system. In the present study, central auditory conduction time (CACT) was calculated for the rat, the first such formal measurement in any animal. Brainstem auditory evoked potentials (BAEPs) were recorded simultaneously with the primary response of the auditory cortex (P1). The latency of wave II of the BAEP, which arises in the cochlear nucleus, was subtracted from that of P1. This yielded a mean CACT of 6.6 ms. The results confirm a previous theoretical estimate that CACT in the rat is at least twice as long as central somatosensory conduction time.

Animals↗
PubMed1990

The effects of chloralose on the brainstem auditory evoked potential of the rat.

The effects of alpha-chloralose were examined on the brainstem auditory evoked potential of the rat. Subjects were given one of four different doses of chloralose. These were a sedative dose (50 mg/kg), an anaesthetic dose (100 mg/kg), a usually fatal dose (150 mg/kg) or an invariably fatal dose (200 mg/kg). Recordings were made 60 min later. There was no evidence of any dose-dependent increase or decrease in the amplitudes of the four principal components (waves I, II, III and IV) of the brainstem auditory potential. There were only minor dose-dependent increases in latency. These were less than 0.1 msec for waves I, II and III and just over 0.1 msec for wave IV. Wave IV was the component which was most sensitive to chloralose, although this was apparent only at a large dose. In general, the latencies showed little or no difference from those recorded previously from awake animals. The results are discussed in relation to a series of experiments, where animals were initially immobilized with chloralose in order to study the effects of a variety of pharmacological agents on the brainstem auditory potential.

Animals↗
PubMed1990

Early prediction of outcome from cerebral trauma by somatosensory evoked potentials.

The prognostic value of short-latency somatosensory evoked potentials (SEP) during the first 4 days after severe head injury was studied in a group of 100 ICU patients. There was a strong association between the presence of bilateral cortical potentials and a good recovery or moderate disability 6 months after injury. In contrast, the bilateral or unilateral absence of cortical potential was associated with severe disability, persistent vegetative state, or death in a high percentage of patients. A reliable prediction of outcomes was obtained from SEP recorded within 24 h of head injury. Predictive accuracy was not influenced by the time of recording or cumulative analysis of consecutive daily SEP over the first 4 days after injury. Short-latency SEP can provide a reliable and accurate prognosis for sedated and curarized patients, and can have an important role in the management and triage of patients with severe head injury who are undergoing intensive therapy.

Adolescent↗
PubMed1989

Epileptiform discharges in presence of continuous background activity in anoxic coma.

The EEGs of a group of patients in anoxic coma following cardiorespiratory arrest were retrospectively studied. Twelve of 70 patients had at least one EEG recorded during the acute period which consisted of epileptiform discharges in the presence of continuous diffuse background activity. Nine of the patients died while three made a generally favorable recovery. The analysis confirms the prediction of a recent revision of EEG grades in coma that this particular pattern of activity is of uncertain prognostic significance. This is in contrast to the occurrence of epileptiform discharges during discontinuous "burst suppression" activity which generally indicates a fatal outcome.

Aged↗
PubMed1988

Origin of reprint requests.

One thousand consecutive reprint requests were classified according to their countries of origin. One third came from the United States of America and over 70% originated in six countries (USA, Czechoslovakia, France, West and East Germany and Canada). When reprint requests were adjusted for population size, a different order became apparent with Czechoslovakia requesting proportionally the most, followed by Israel, East Germany, Canada, Switzerland and Belgium. Approximately half of the reprint requests came from European countries with the overall rates for western and eastern European nations being similar. There were comparatively few requests received from Third World nations. The analysis questions several of the reasons commonly given for honouring reprint requests.

Humans↗
PubMed1988

Effect of electroconvulsive shock on the slow components of the brain stem auditory evoked potential.

The effects of electroconvulsive shock on the slow components of the brain stem auditory evoked potential were investigated in the rat during the ictal and the immediate postictal periods. Shock did not significantly alter any aspect of the slow positive-negative complex which underlies the high-frequency waves of the brain stem auditory evoked potential. However, a succeeding potential of probable cortical origin was markedly reduced in amplitude and its peak latency was temporarily increased.

Animals↗
PubMed1988

Somatosensory evoked potentials following severe head injury: loss of the thalamic potential with brain death.

The thalamic component (P17) of the short-latency somatosensory evoked potential (SEP) was assessed to determine its usefulness in patients with severe head injury. Subjects were a group of patients admitted to the Auckland Hospital Critical Care Unit who subsequently died from head injury. In all instances where brain death was unequivocally established and a SEP recording made in close temporal proximity to the time of brain death the P17 potential was absent. When there was evidence of continuing brainstem activity and particularly where prolonged survival occurred following the last SEP recording the P17 potential remained intact bilaterally. This study shows that the presence or absence of the thalamic component of the short-latency SEP provides a reliable electrophysiological measure of brainstem function in patients where brain death has been suspected.

Adolescent↗