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A Hobson

Publications and source records attributed to A Hobson.

At least 19 recordsLinked to original sources
PubMed2006

Assessing the temporal reproducibility of human esophageal motor-evoked potentials to transcranial magnetic stimulation.

BACKGROUND: Although the electrophysiological properties and reproducibility of somatic limb motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are well characterized, little is known about the reproducibility of MEPs for viscerosomatic structures such as the esophagus. AIM: To determine the temporal reproducibility of esophageal MEPs to TMS. METHODS: MEPs to TMS were recorded from the proximal esophagus, using a swallowed catheter housing a pair of electrodes, in eight healthy subjects at five stimulus intensities (SI) (motor threshold [MT] to 20% above MT). For each SI, 20 consecutive TMS stimuli at 5-second intervals were delivered over a single scalp site (dominant hemisphere at site exhibiting MT at lowest SI) and repeated 40 and 80 minutes thereafter. MEP amplitudes and latencies were measured, and means were sequentially calculated for each SI and then log-transformed. The repeatability coefficients (RC) for the three time points were calculated across each set of 20 stimuli and presented as an exponential ratio. RESULTS: Best RC (amplitude/latency) were achieved at 120% SI relative to MT, being 1.8/1.2 (optimal = 1.0). For lower intensities of 115%, 110%, 105%, and 100% SI, the RC were 2.1/1.2, 2.1/1.1, 2.4/1.2, and 2.6/1.4, respectively. For all SI, the greatest reductions in RC occurred over the first 10 stimuli, with little additional gain beyond this number. CONCLUSIONS: Latencies of esophageal MEP to TMS across intensities are highly reproducible, whereas amplitudes are more stimulus intensity-dependent, being most reliable and reproducible at the highest stimulus strengths. SIGNIFICANCE: Using careful parameters, TMS can be used reliably in future studies of viscerosomatic structures, although the size of the response variability needs to be taken into account when assessing changes in cortico-fugal activity.

Adult↗
PubMed2003

Changes in pharyngeal corticobulbar excitability and swallowing behavior after oral stimulation.

Faucial pillar (FP) stimulation is commonly used in swallowing rehabilitation, yet its physiological basis remains uncertain. We investigated the effects of intraoral FP stimulation on human corticobulbar excitability and swallowing behavior, to explore the possibility of a central mechanism for functional change. In 10 healthy subjects, corticobulbar projections to pharynx were investigated with transcranial magnetic stimulation, via intraluminal electrodes, before and up to 1 h after 10 min of electrical FP stimulation with three frequencies (0.2, 1, and 5 Hz) or sham and peripheral (median nerve) stimulation. In a second study, swallowing behavior was assessed with videofluoroscopy before and after FP stimulation. FP stimulation at 5 Hz inhibited the corticobulbar projection (-14 +/- 6%, P < 0.02) and lengthened swallow response time (+114 +/- 24%, P = 0.02). By comparison, FP stimulation at 0.2 Hz facilitated this projection (+60 +/- 28%, P < 0.04), without enhancing swallowing behavior. Neither 1-Hz, sham, nor median nerve stimulation altered excitability. Thus changes in corticobulbar excitability to FP stimulation are frequency dependent with implications for the treatment for neurogenic swallowing dysfunction.

Adult↗
PubMed2000

Cortical processing of human somatic and visceral sensation.

Somatic sensation can be localized precisely, whereas localization of visceral sensation is vague, possibly reflecting differences in the pattern of somatic and visceral input to the cerebral cortex. We used functional magnetic resonance imaging to study the cortical processing of sensation arising from the proximal (somatic) and distal (visceral) esophagus in six healthy male subjects. Esophageal stimulation was performed by phasic distension of a 2 cm balloon at 0.5 Hz. For each esophageal region, five separate 30 sec periods of nonpainful distension were alternated with five periods of similar duration without distension. Gradient echoplanar images depicting bold contrast were acquired using a 1.5 T GE scanner. Distension of the proximal esophagus was localized precisely to the upper chest and was represented in the trunk region of the left primary somatosensory cortex. In contrast, distension of the distal esophagus was perceived diffusely over the lower chest and was represented bilaterally at the junction of the primary and secondary somatosensory cortices. Different activation patterns were also observed in the anterior cingulate gyrus with the proximal esophagus being represented in the right midanterior cingulate cortex (BA 24) and the distal esophagus in the perigenual area (BA32). Differences in the activation of the dorsolateral prefrontal cortex and cerebellum were also observed for the two esophageal regions. These findings suggest that cortical specialization in the sensory-discriminative, affective, and cognitive areas of the cortex accounts for the perceptual differences observed between the two sensory modalities.

Adult↗
PubMed2000

Comparison of cortical potentials evoked by mechanical and electrical stimulation of the rectum.

Patients with irritable bowel syndrome have heightened perception of gut sensation. The mechanisms responsible for this remain unknown, due to current poor knowledge of the central processing of gut sensation. Cortical evoked potentials (CEPs) have been recorded following both electrical rectal stimulation (ERS) and mechanical rectal stimulation (MRS). Because of the lack of a direct comparison of these two methods, their robustness for future clinical use remains unknown. The aim of our study was to compare the characteristics of CEPs following ERS and MRS. CEPs were recorded from the vertex in 14 healthy volunteers following ERS with bipolar ring electrodes, and MRS by repeated rectal distension. CEPs were recorded in all subjects following electrical stimulation, but only in 11 subjects following mechanical stimulation. In comparison with electrical stimulation, mechanical stimulation produced CEPs with a smaller amplitude and longer latency. However, the morphology of CEPs following electrical and mechanical rectal stimulation was similar, with no difference in the interpeak latencies. In conclusion, we have demonstrated that electrical rectal stimulation is a more reliable stimulus for recording CEPs. The similarity of the morphology and interpeak latencies of the CEPs suggests that both stimuli are activating a similar network of cortical neurones.

Adult↗
PubMed1999

Viremia in neonatal herpes simplex virus infections.

BACKGROUND: Polymerase chain reaction assays of the peripheral blood mononuclear cells (PBMC) and plasma may facilitate the diagnosis of neonatal herpes simplex virus (HSV). METHODS: Assays for HSV DNA were submitted from at least 1 specimen site (PBMC, plasma or cerebrospinal fluid) in 11 consecutive cases of neonatal HSV infection. RESULTS: HSV DNA was detected by PCR in the PBMC of 6 of 10 infants tested (60%), the plasma of 4 of 6 tested (67%) and the cerebrospinal fluid of 4 of 11 tested (36%). CONCLUSIONS: HSV viremia is more frequent than previously appreciated, and detection of HSV DNA in PBMC and plasma is a useful diagnostic tool, particularly in infants without skin lesions.

DNA, Viral↗
PubMed1999

Laterality effects of human pudendal nerve stimulation on corticoanal pathways: evidence for functional asymmetry.

BACKGROUND: Although motor and sensory pathways to the human external anal sphincter are bilateral, a unilateral pudendal neuropathy may still disrupt anal continence. Anal continence can, however, be preserved despite unilateral pudendal damage, and so to explain those differing observations, we postulated that pudendal innervation might be asymmetric. AIMS: To explore the individual effects of right and left pudendal nerve stimulation on the corticofugal pathways to the human external anal sphincter and thus assess evidence for functional asymmetric pelvic innervation. METHODS: In eight healthy subjects, anal sphincter electromyographic responses, evoked to transcranial magnetic stimulation of the motor cortex, were recorded 5-500 msec after digital transrectal electrical conditioning stimuli applied to each pudendal nerve. RESULTS: Right or left pudendal nerve stimulation evoked anal responses of similar latencies but asymmetric amplitudes in six subjects: dominant responses (>50% contralateral side) from the right pudendal in four subjects and from the left in two. Cortical stimulation also evoked anal responses with amplitude 448 (121) microV and latency 20.9 (1.1) msec. When cortical stimulation was preceded by pudendal nerve stimulation, the cortical responses were facilitated at interstimulus intervals of 5-20 msec. Dominant pudendal nerve stimulation induced greater facilitation of the cortically evoked responses than the non-dominant nerve. CONCLUSIONS: Cortical pathways to the external anal sphincter are facilitated by pudendal nerve conditioning, in an asymmetric manner. This functional asymmetry may explain the presence and absence of anal incontinence after unilateral pudendal nerve injury.

Adult↗
PubMed1998

Natal origins of migratory monarch butterflies at wintering colonies in Mexico: new isotopic evidence.

Each year, millions of monarch butterflies from eastern North America migrate to overwinter in 10-13 discrete colonies located in the Oyamel forests of central Mexico. For decades efforts to track monarch migration have relied on observations and tag-recapture methods, culminating with the discovery of the wintering colonies in 1975. Monarch tag returns from Mexico, however, are few and primarily from two accessible colonies, and therefore tag-recapture techniques have not quantified natal origins or distinctiveness among monarch populations at wintering sites. Such information would be invaluable in the conservation of the monarch and its migration phenomenon since the wintering sites currently are threatened by habitat alteration. Here we show that stable hydrogen (deltaD) and carbon (delta13C) isotope ratios of wintering monarchs can be used to evaluate natal origins on the summer breeding range. Stable-hydrogen and carbon isotopic values of 597 wintering monarchs from 13 wintering roost sites were compared with isotopic patterns measured in individuals at natal sites across their breeding range over a single migration cycle. We determined that all monarch wintering colonies were composed of individuals originating mainly from the Midwest, United States, thereby providing evidence for a panmictic model of wintering colony composition. However, two colonies showed more northerly origins, suggesting possible priority colonies for conservation efforts.

Journal Article↗
PubMed1998

Sensorimotor modulation of human cortical swallowing pathways.

1. Transcranial magnetic stimulation over motor areas of cerebral cortex in man can activate short latency bilateral cortical projections to the pharynx and oesophagus. In the present paper we investigate the interaction between pathways from each hemisphere and explore how activity in these pathways is modulated by afferent feedback from the face, pharynx and oesophagus. 2. Comparison of unilateral and bilateral stimulation (using interstimulus intervals (ISIs) of 1, 5 or 10 ms between shocks) showed spatial summation of responses from each hemisphere at an ISI of 1 ms, indicating that cortical efferents project onto a shared population of target neurones. Such summation was not evident at ISIs of 5 or 10 ms. There was little evidence for transcallosal inhibition of responses from each hemisphere, as described for limb muscles. 3. Single stimuli applied to the vagus nerve in the neck or the supraorbital nerve, which alone produce intermediate (onset 20-30 ms) and long (50-70 ms) latency reflex responses in the pharynx and oesophagus, were used to condition the cortical responses. Compared with rest, responses evoked by cortical stimulation were facilitated when they were timed to coincide with the late part of the reflex. The onset latency was reduced during both parts of the reflex response. No facilitation was observed with subthreshold reflex stimuli. 4. Single electrical stimuli applied to the pharynx or oesophagus had no effect on the response to cortical stimulation. However, trains of stimuli at frequencies varying from 0.2 to 10 Hz decreased the latency of the cortically evoked responses without consistently influencing their amplitudes. The effect was site specific: pharyngeal stimulation shortened both pharyngeal and oesophageal response latencies, whereas oesophageal stimulation shortened only the oesophageal response latencies. 5. Cortical swallowing motor pathways from each hemisphere interact and their excitability is modulated in a site-specific manner by sensory input. The latter may produce a mixture of excitation and inhibition at both brainstem and cortical levels.

Adult↗
PubMed1998

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.

Adult↗
PubMed1998

Spinal and pudendal nerve modulation of human corticoanal motor pathways.

We investigated the effects of lumbosacral and pudendal nerve stimulation on the corticofugal pathways to the human external anal sphincter. In 11 healthy subjects, anal sphincter electromyographic responses, evoked to transcranial magnetic stimulation of the motor cortex, were recorded 5-500 ms after lumbosacral root or pudendal nerve stimulation. Lumbosacral and pudendal nerve stimulation alone evoked responses with amplitudes of 293 +/- 73 and 401 +/- 153 microV and latencies of 3.2 +/- 0.2 and 2.2 +/- 0.2 ms, respectively. Cortical stimulation also evoked responses with amplitudes of 351 +/- 104 microV and latencies of 20.9 +/- 1.1 ms. When lumbosacral or pudendal nerve stimulation preceded cortical stimulation, the cortically evoked responses were facilitated (P < 0.01), with the effect appearing greatest at 5-20 ms after both lumbosacral and pudendal excitation and at 50-100 ms after lumbosacral excitation alone. Our results demonstrate that cortical pathways to the external anal sphincter are facilitated by prior lumbosacral and pudendal nerve stimulation, indicating that sensorimotor interactions are important in the central neural control of sphincter function.

Adult↗
PubMed1997

Frequent genital herpes simplex virus 2 shedding in immunocompetent women. Effect of acyclovir treatment.

Reactivation of herpes simplex virus type 2 (HSV-2) occurs intermittently as perceived clinically and by viral culture. We performed a series of studies to evaluate the frequency and pattern of HSV-2 reactivation using both viral isolation and HSV PCR assay. Daily samples of genital secretions were obtained from 27 HSV-2 seropositive women; a subset of subjects obtained samples while receiving oral acyclovir 400 mg PO twice a day. HSV DNA was detected in genital swab specimens on 28% of 1,410 d compared with 8.1% of days by viral isolation. 11 of 20 women had HSV DNA detected on > 20% of days, 4 on > 50%, and 2 on > 75% of days; in contrast, none of the women shed on > 21% of days by viral isolation. The daily administration of oral acyclovir promptly reduced the frequency of HSV DNA detection by a median of 80%. Within 3-4 d of discontinuing daily acyclovir, HSV DNA again appeared in the genital area. HSV-2 shedding in the genital mucosa occurs much more frequently than previously appreciated. This frequent reactivation likely plays a role in the epidemic spread of genital herpes worldwide.

Acyclovir↗
PubMed1997

Evaluation of a quantitative competitive PCR assay for measuring herpes simplex virus DNA content in genital tract secretions.

Previous studies have shown an association between the approximate titer of herpes simplex virus (HSV) DNA in clinical specimens and the ability to isolate HSV from genital secretions. To control for variance in amplification conditions, we developed a competitive quantitative PCR (QC PCR) for the detection of HSV DNA. The assay accurately measured from 10 to 10(6) copies of HSV DNA. We compared the QC PCR with our previous semiquantitative detection method and found concordance for 61 of 63 positive specimens. We also evaluated the HSV DNA content from individual swabs of genital secretions obtained from individual sites of the genital tract (cervix, vulva, and rectum) with that from one swab with secretions from all three sites. The concordance for detecting HSV DNA was 91%; for only 4 of 143 collection days was there a > 1 log difference between the two collection methods. A single swab with secretions from all three genital sites and evaluated in a QC PCR format can accurately measure the frequency of subclinical and clinical shedding of HSV and the titer of HSV shed from the genital region. Such an approach should be very useful in the evaluation of antiviral chemotherapy for HSV.

Base Sequence↗
PubMed1997

Cranial nerve modulation of human cortical swallowing motor pathways.

Animal data indicate that cortical swallowing pathways can be modulated by cranial nerve afferent stimulation. We therefore studied the effects of human trigeminal and vagal nerve excitation on the corticofugal pathways to the oropharynx and esophagus, using electromagnetic stimulation. Unilateral stimulation of either the trigeminal or vagus nerve evoked two distinct reflex electromyographic responses in the pharynx and esophagus, an early response (latency range 19-30 ms) and a late response (latency range 42-72 ms). In the mylohyoid muscles, however, only a single response was seen (latency range 36-64 ms). Cortical stimulation also evoked electromyographic responses in the mylohyoid muscles, pharynx, and esophagus, with latencies of 8.5 +/- 0.3, 9.3 +/- 0.3, and 10.1 +/- 0.4 ms, respectively. When either trigeminal or vagus nerve stimulation preceded cortical stimulation, the cortically evoked responses were facilitated, with maximal effects at interstimulation intervals of 30-200 ms for pharynx and esophagus (P < 0.02) and at interstimulation intervals of 50-100 ms for mylohyoid muscles (P < 0.05). Our results demonstrate that stimulation of human cranial nerve afferent fibers facilitates cortical swallowing motor pathways.

Adult↗
PubMed1995

Manometric asymmetry in the anal canal in controls and patients with fecal incontinence.

PURPOSE: This study was designed to ascertain the existence of manometric asymmetry in the anal canal in controls and in patients with fecal incontinence. METHODS: A computerized manometric technique with an eight-channel perfusion catheter was used. Coefficient of variation (CV, a mathematical expression of the degree or magnitude of deviation of a set of data points from the mean) was developed in this study as an index of anal canal manometric asymmetry. RESULTS: At 1 cm from the anal verge, mean CV was 9.3 and 8.7 percent in asymptomatic control males and females, respectively. During a maximum voluntary squeeze effort, mean CV in control males was 9.3 percent and was not significantly different (P = 0.28) from that in control females (7.8 percent). In 14 consecutive female patients with fecal incontinence, mean CV at rest (1 cm from the anal verge) was 21 percent and was significantly higher (P < 0.01) than in control females. Similarly, during a maximum voluntary squeeze effort, mean CV in patients with fecal incontinence was 20.5 percent and was significantly higher (P < 0.01) than in the female control group (7.8 percent). CONCLUSION: It is concluded that CV, a method of expressing anal sphincter manometric asymmetry, is a useful manometric parameter in the assessment of anorectal function.

Adult↗
PubMed1995

Topographic mapping of cortical potentials evoked by distension of the human proximal and distal oesophagus.

We describe cortical potentials evoked by balloon distension of the proximal and distal oesophagus in 8 healthy right handed volunteers. Oesophageal stimulation was performed using a pump which rapidly inflated a 2 cm silicone balloon positioned either 3 cm distal to the upper oesophageal sphincter or 5 cm proximal to the lower oesophageal sphincter, at a frequency of 0.2 Hz, using inflation volumes which produced a definite but not painful sensation. Oesophageal evoked cortical potentials were recorded in all subjects with an initial negative and positive component (N1 and P1), followed by a second negative and positive component (N2 and P2) in 6 subjects. The morphology and the scalp topography of the N1 component elicited by proximal and distal oesophageal stimulation suggests activation of the primary somatosensory cortex and/or the insular. There was also evidence for hemispheric dominance for the N1 potential which was independent of handedness. The frontal emphasis of the proximal oesophageal N1 component, in contrast to the central emphasis of the distal oesophageal N1 component, suggests that different neuronal populations were activated by stimulation of the two sites. The frontal emphasis of the ensuing P1 component from both oesophageal sites suggests that it originates in a separate precentral source. The topography of the N2 components obtained by stimulation of either oesophageal site was similar to that of the N1 component, suggesting that they originate in similar areas of the cortex. The P2 component evoked by stimulation of both oesophageal sites was localised at the vertex. The inter- and intra-subject variation in the morphology of the N2 and P2 components suggests that secondary cortical processes related to cognition may be involved in their generation.

Adult↗
PubMed1994

Esophageal myoelectric responses to magnetic stimulation of the human cortex and the extracranial vagus nerve.

We describe the use of magnetic stimulation of the human motor cortex and the extracranial vagus nerve in the neck to study human esophageal electromyographic (EMG) responses. Diffuse cortical stimulation produced both an early and a late EMG response in the proximal esophagus with latencies of 10.9 +/- 0.5 and 43.4 +/- 4.3 ms, respectively. Increasing the stimulation intensity increased the amplitude and duration, but reduced the latency, of the early response (P < 0.01), whereas the late response remained unchanged. The early response, but not the late response, was facilitated when cortical stimulation was performed during valsalva maneuver. Maximal early response amplitudes were obtained by focal stimulation of either hemisphere 4-8 cm lateral to the vertex. Stimulation of the neck over the extracranial vagus nerve at the angle of the jaw also produced an early and a late EMG response in the proximal esophagus with latencies of 4.3 +/- 0.4 and 56.8 +/- 14.5 ms, respectively. Early responses were also recorded in the midesophagus and distal esophagus from either cortical or vagal stimulation, but amplitudes were smaller and rise times were longer, suggesting passive "volume conduction" from the proximal esophagus. Comparison of laryngeal, pharyngeal, and esophageal early EMG responses showed similar amplitudes and rise times, suggesting that the action potentials were generated locally at each site and that true esophageal responses were being detected.

Adult↗