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Biomedical subjects

V L Towle

Publications and source records attributed to V L Towle.

At least 37 records · Page 2Linked to original sources
PubMed1990

Hypoglycemic thresholds for cognitive dysfunction in humans.

Nineteen healthy adult volunteers were studied to define the nature of and threshold for the cognitive dysfunction that occurs during insulin-induced hypoglycemia. The P300 cerebral event-related potential is an electrophysiological correlate of cognitive decision-making processes that can be measured in response to either an auditory or visual stimulus. P300 and reaction time (RT) were recorded from a visual stimulus under euglycemic conditions and at plasma glucose concentrations of 3.3 and 2.6 mM during insulin infusion in 10 subjects. Reducing plasma glucose levels to 3.3 mM was not associated with an increase in either the latency or amplitude of the P300 component or a change in RT. However, further lowering of plasma glucose to 2.6 mM resulted in an increase in the latency of P300 and a prolongation in RT. Similar changes were seen for the auditory P300 in experiments performed on 9 additional subjects in which both auditory and visual stimuli were presented. The prolongation of P300 did not correct immediately when plasma glucose was raised to basal levels with intravenous glucose but returned to normal 45-75 min later, after ingestion of a carbohydrate-containing meal. Analysis of another event-related potential, P140 (a measure of the sensory processes), showed no change in response to hypoglycemia. Prolongation of RT paralleled the prolongation of P300 latency, suggesting that motor processes were not altered. Therefore, hypoglycemia appears to induce abnormalities in decision-making processes.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1989

Hemi-field pattern visual evoked potentials: a comparison of display and analysis techniques.

Three methods for analyzing the spatial organization of visual evoked potentials were compared. Pattern reversal visual evoked potentials were obtained from a single subject under three viewing conditions: stimulation of the left, right, and both visual fields. The scalp distribution of the VEP to 1 deg checks was displayed using three recording and analysis techniques: a conventional horizontal occipital array of electrodes, topographic mapping, and 3-dimensional evoked potentials. All three techniques revealed "paradoxical" lateralization of P100. The relative merits of each technique are discussed.

Brain Mapping↗
PubMed1989

Electrophysiologic studies on locked-in patients: heterogeneity of findings.

Somatosensory evoked potentials, brain-stem auditory evoked potentials and electroencephalograms were obtained from 9 patients with the diagnosis of 'locked-in' syndrome. No pattern of evoked potential abnormality was specific to this syndrome, with findings ranging from bilaterally normal to unilaterally or bilaterally absent. The evoked potential studies complemented radiographic findings in defining the extent of the lesion and revealed that a portion of the pontine tegmentum was usually involved. Pathology from 2 patients corroborated the findings of the evoked potential studies. The value of evoked potential studies of patients with locked-in syndrome is to provide early objective evidence of brain-stem involvement independent of the clinical examination, EEG and radiographic studies.

Adult↗
PubMed1989

Vector analysis of three-dimensional evoked potentials: eccentric dipoles.

The quantitative description of three-dimensional cerebral evoked potentials is extended to include eccentric dipolar sources. Eccentricity-related distortions in dipole orientation and magnitude are assessed. The use of nonstandard montages, the prediction of topographic surface maps, dynamic analysis, and theoretical mechanisms of planar segment formation are discussed.

Brain↗
PubMed1986

Clinical studies with an aldose reductase inhibitor in the autonomic and somatic neuropathies of diabetes.

Clinical investigations with the aldose reductase inhibitor (ARI) sorbinil in diabetic patients with neuropathy are described. Cardiac autonomic neuropathy was studied in 36 patients, in a double-blind, placebo-controlled, randomized, noncrossover trial. Patients received sorbinil (250 mg qd) or placebo over 6 weeks after a one-week baseline period. Diabetic control did not change over the study period, as indicated by unchanged glycohemoglobin. Response was assessed by expiration/inspiration (E/I) ratios on EKG during 6 c/min respiration and resting minimum heart rate, both measures of vagal function. In the sorbinil group, E/I ratios improved from 1.074 +/- 0.012 to 1.096 +/- 0.020 (P less than 0.03) with a slight decrease in the placebo group from 1.112 +/- 0.023 to 1.105 +/- 0.023 (P = NS). The difference between the week 6 and week 0 changes in each group was significant (P less than 0.01). Resting minimum heart rate decreased in the sorbinil group from 76.4 +/- 2.3 to 66.8 +/- 2.4 beats/min (P less than 0.001), with a mean change of 10 +/- 2. In the placebo group, heart rate was unchanged (77.9 +/- 3.9 to 77.5 +/- 3.3). The two sample t tests of the within-group differences were likewise significant (P less than 0.001). These changes in both E/I ratio and resting minimum heart rate are consistent with a sorbinil-related improvement in cardiac parasympathetic nerve function. Several isolated cases with apparent sorbinil-related improvement in autonomic symptoms will also be described. Studies of somatic neuropathy have previously shown improvement in nerve conduction velocities with sorbinil. In a study of 11 patients with severely painful diabetic neuropathy treated with sorbinil for 3 weeks [placebo-controlled in single-blind fashion (n = 8)], pains (as assessed on a 0 to 20 rating scale) improved from a mean score of 16 down to 8, with deterioration following drug withdrawal. Objective improvements in sensation and strength were observed in some cases. In this group of patients, statistically significant improvements in nerve conduction velocity, E/I ratios, and resting minimal heart rate, similar to those previously discussed, were also documented. Somatosensory-evoked potentials studies in the 36-patient study showed significant improvements in peripheral conduction and cortical responses. Sorbinil toxicity in 106 patients was 11.3%, with sex incidence of 7/73 males (9.6%) and 5/33 females (15.2%).(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1986

Spectral analysis of EEG during carotid endarterectomy.

Spectral analysis of the electroencephalogram (EEG) was monitored during 105 carotid endarterectomies. Seventy-eight percent of the patients showed no significant change in EEG spectral power as a result of clamping of the internal carotid artery. Two patterns of change were observed in the remaining 22% of patients: partial reduction (significant decrease of power in one or two of three frequency bands) and global reduction (significant decrease of power in all three frequency bands). High frequencies (over 10.5 Hz) changed more frequently with clamping than did low frequencies (less than 6 Hz), but reduction of high frequencies alone was tolerated with no postoperative deficits. The only non-shunted patient demonstrating global EEG reduction for the duration of carotid clamping suffered a transient hemiparesis.

Aged↗
PubMed1985

Diagnosing functional visual deficits with the P300 component of the visual evoked potential.

The visual evoked potential (VEP) is routinely used to assess visual function, though it occasionally does not reflect a patient's conscious experience. Reports of normal flash or pattern VEPs obtained from blind persons are extreme examples of this problem. The difficulty in interpreting VEPs in light of such findings can be partly overcome by obtaining a cognitive component of the evoked potential, P300. We obtained traditional visual acuity measurements, pattern-reversal VEPs, and VEPs containing P300s from three patients with clinically diagnosed functional visual deficits. The P300s were obtained in response to stimuli that the patients claimed they could not see, supporting the clinical conclusions that malingering or hysteria was involved. The P300 component can be helpful in assessing the subjective visual experience of patients suspected of having functional visual loss.

Clinical Trials as Topic↗
PubMed1983

Evoked potential and preferential looking estimates of visual acuity in pediatric patients.

Pattern visual evoked potentials (VEPs) and behavioral visual acuity measured with forced-choice preferential looking techniques (FPL) were obtained from 172 pediatric patients between the ages of 4 months and 10 years. More children younger than 2 years of age successfully completed monocular testing by pattern VEP than by FPL methods. For children older than 2 years, we were equally successful in completing each of the two tests. Of those children who completed both tests, the VEP results agreed with Snellen, Allen, and "E" acuity measures more often (66%) than did the FPL results (50%). Both tests were more sensitive than Allen single characters in detecting interocular acuity differences in children younger than 3 years of age.

Age Factors↗
PubMed1983

The visual evoked potential in glaucoma and ocular hypertension: effects of check size, field size, and stimulation rate.

In order to determine the optimum stimulus conditions for the detection of optic nerve damage due to glaucoma and ocular hypertension, checkerboard pattern reversal visual evoked potentials (VEPs) were recorded from 20 glaucoma patients, 20 ocular hypertensive patients, and 20 age-matched normals. Two check sizes (12' and 48'), two field sizes (14 degrees and 28 degrees), and two alternation rates (1.9 and 7.5 alt/sec) were used. All subjects had visual acuities of 20/40 or better in each eye and equal pupils of 2 to 5 mm diameter. The largest number of VEP abnormalities were found with large checks (48') reversing at a fast rate (7.5 alt/sec). After correcting for the effects of age, visual acuity, and pupil size, 16 of 30 eyes with glaucomatous visual field defects had abnormally long VEP latencies under this condition (beyond the 99% confidence limit of the normal subjects). Nine of 40 ocular hypertensive eyes also had abnormally long latencies. Increased pattern VEP latency was significantly correlated with both the severity and location of visual field defects and the degree of cupping and pallor of the optic disc. VEP latency was not significantly related to intraocular pressure.

Adult↗
PubMed1981

Age-related changes in the latency of the visual evoked potential: influence of check size.

Pattern reversal visual evoked potentials were recorded from visually normal adults between 10 and 80 years of age. Two check sizes, 12 and 48 min, were used. The results showed that the latency of the first major positive component, P1, increased with age for both check sizes. This rate of increase was nearly twice as fast for 12 min checks. This difference is most likely a reflection of the differential effects of aging processes on the various spatial frequency channels in the human visual system.

Adolescent↗
PubMed1977

An objective indicant of binocular vision in humans: size-specific interocular suppression of visual evoked potentials.

Evoked cortical potentials (VEPs) to grid patterns flashed to one eye were suppressed in amplitude when grid patterns were continuously presented to the other eye. The degree of interocular suppression of VEPs was influenced by the stereoacuity of the subjects. VEPs were obtained to each of two grid sizes flashed to one eye (individual squares subtending 15 and 60 min of arc) and changes in amplitude of these VEPs were considered as a function of four stimuli continuously presented to the other eye (diffuse light, 15, 30, and 60 min of arc squares in grids). Interocular suppression of VEPs was greater (a) when the continuously presented grid was of high (38.00 mL) as compared to low (00.38 mL) intensity, (b) when the continuous and flashed grids were of the same as compared to different sizes, and (c) in six subjects who had good as compared to six subjects who had poor binocularity. Eleven of the twleve subjects could be classified correctly as having good or poor binocularity on the basis of statistically significant interocular suppression of VEPs. The results were interpreted in terms of centrally located binocular neurons responsive to specific grid sizes or spatial frequencies and the decreased functioning of such neurons in subjects with poor binocularity.

Evoked Potentials↗
PubMed1977

Objective determination of human visual acuity: pattern evoked potentials.

The natural visual acuities of 15 adult persons were predicted on the basis of changes in visual evoked potentials (VEP's) to flashed patterns of various sized dots. An objective method was used to quantify the VEP's, based on the minimum-sized stimulus that would elicit a pattern VEP- the VEP pattern threshold. This measure was highly correlated with recognition and resolution measures of perceptual visual acuity (r's as high as 0.89). The regression equation between the VEP measures and predicted perceptual measures of acuity enabled the objective estimation of perceptual acuity to within +/- 0.29 decimal units.

Adult↗
PubMedDate not supplied

Functional mapping of human motor cortical activation with conventional MR imaging at 1.5 T.

A conventional 1.5-T magnetic resonance (MR) imager was used to detect signal intensity changes on T2*-weighted images of human motor and sensory cortices during performance of hand and tongue movements. Narrow receiver bandwidths were used to improve the signal-to-noise ratio. Protocols consisting of baseline, motor task, rest, and second motor task periods were performed by nine volunteers. Two-dimensional cross correlation was applied to correct in-plane translation and rotation of the head during the imaging session before the control images were subtracted from the task images. Measurements obtained during finger movement tasks indicated a 3%-8% increase in signal intensity near the contralateral central sulcus and smaller ipsilateral signal intensity increases. Bilateral signal intensity increases were also observed during tongue movement studies. A retrospective image registration technique was used to map the signal changes onto conventional anatomic images, which were used to create integrated three-dimensional models of brain structure and function. These integrated images showed that the highest signal intensity due to hand movement was near the putative central sulcus.

Adult↗
PubMedDate not supplied

Three-dimensional human somatosensory evoked potentials.

Median nerve somatosensory evoked potentials were recorded from 30 normal adults using conventional scalp derivations and an orthogonal bipolar surface electrode montage. This allowed the determination of the spatial orientation of the hypothetical centrally located equivalent dipole derived from the evoked response recorded in 3-dimensional voltage space. The 3-dimensional voltage trajectory describing changes in equivalent dipole orientation and magnitude revealed 4 major apices between 5 and 25 msec, 3 of which corresponded to the traditional P14, N20 and P25 peaks. A fourth apex at 17 msec was not as evident in the conventional recordings and signaled a transition from a vertical P14-N18 generator process to a horizontal N20 generator process. The normal within- and between-subject variability of trajectory apices, segments and planes are described, along with the theoretical and practical implications of this recording technique.

Adult↗