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Biomedical subjects

V L Towle

Publications and source records attributed to V L Towle.

11 recordsLinked to original sources
PubMed1992

Hypoglycemic thresholds for cognitive dysfunction in IDDM.

Fourteen poorly controlled insulin-dependent diabetes mellitus (IDDM) patients (HbA1c 11 +/- 0.5%) with a mean +/- SE duration of disease of 15 +/- 2 yr were studied to evaluate the hypoglycemic threshold for cognitive dysfunction under insulin-induced hypoglycemia. The P300 event-related potential, a measure of cognitive function, and reaction time (RT) in response to visual stimuli under euglycemic conditions and at plasma glucose concentrations of 3.5 and 2.5 mM (63 and 45 mg/dl, respectively) during a constant insulin infusion were recorded. Baseline P300 latency was similar to that of a nondiabetic control group, but baseline RT was greater in the IDDM group. There was no increase in P300 latency or RT under euglycemic clamp conditions or at a plasma glucose level of 3.5 mM (63 mg/dl). However, when plasma glucose was lowered to 2.5 mM (45 mg/dl), there was an increase in P300 latency and a prolongation of RT. As plasma glucose returned to baseline, P300 latency and RT remained prolonged. After administration of intravenous glucose and a meal, P300 latency and RT returned to baseline. P140, an event-related potential reflecting sensory processes, was not altered. Because P300 latency changes paralleled RT changes, hypoglycemia appears to slow decision-making processes in IDDM. This study revealed that 1) baseline P300 latency is not elevated in poorly controlled IDDM patients, suggesting no cumulative cognitive dysfunction.

Blood Glucose↗
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

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↗
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

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↗
PubMedDate not supplied

Three-dimensional human pattern visual evoked potentials. I. Normal subjects.

Pattern visual evoked potentials (PVEPs) were obtained from 30 normal adult volunteers, recording from both a conventional horizontal occipital array and three orthogonal bipolar antipodal channels approximating the three dimensions of space. Central and eccentric fixation of 60' checks and central fixation of 30' checks under binocular and monocular viewing conditions was employed. The three antipodal wave forms were displayed as a single 3-D Lissajous trajectory which contained four apices, corresponding to P40 (apex A), N70 (apex B), P100 (apex C) and N125 (apex D). The 3-D evoked potentials depicted the dynamic nature of the human PVEP in terms of changes in the 3-D voltage-voltage-voltage plots of the recordings. The orientation of the A-B, B-C and C-D curvilinear segments reflected the stimulating condition (central fixation vs. right vs. left hemi-field stimulation) for all subjects with more accuracy than did the wave forms from the conventional array. Spherical statistical methods are described for quantifying and evaluating 3-D evoked potential recordings.

Adult↗
PubMedDate not supplied

Three-dimensional human pattern visual evoked potentials. II. Multiple sclerosis patients.

Pattern visual evoked potentials were obtained from 46 patients with definite relapsing/remitting multiple sclerosis, using both a conventional 5-channel occipital array and a 3-D recording technique consisting of three bipolar derivations approximating the three dimensions of space. These three orthogonal wave forms were displayed as a 3-D Lissajous trajectory for each subject. Two of the 15 patients with completely normal conventional pattern VEPs had abnormalities of the orientation of the B-C curvilinear segment of the 3-D pattern VEPs. Delays in the first major occipital positive component (P100) were evident using both techniques; the correlation between P100 latency and the latency of the corresponding trajectory apex was r = 0.99 (P less than 0.01). Post-chiasmal MRI abnormalities were associated with 3-D VEP orientation abnormalities. Three-dimensional pattern VEPs are moderately more sensitive than conventional pattern VEPs at detecting dysfunction posterior to the optic chiasm in demyelinating disease and do not require the use of eccentric fixation to do so.

Adult↗
PubMedDate not supplied

Tetrahedral recording of 3-D BAEPs: evidence for the centered dipole model.

Three-dimensional brain-stem auditory evoked potentials (3-D BAEPs) were recorded from 12 normal subjects using a new tetrahedral montage, as well as two other bipolar montages previously described for 3-channel Lissajous' trajectories (3-CLTs). Mean responses, as well as between-subject and within-subject variability were described. A mathematical transformation was applied to the recorded trajectories to render them in a common canonical form to test the assumption that the BAEP conforms to a centrally generated dipolar field. Apex, segment, and plane orientations were measured for each trajectory, and discrepancies between montages were evaluated to judge the adequacy of the centered dipole model. For the vector means of apices, segments, and planes, median angles of discrepancy between montages ranged from 10 to 23 degrees. These results support the validity of a centered dipole model for the BAEP and affirm the rationale for employing the 3-channel recording technique. Among the montages studied, the tetrahedron provided maximum economy by using fewer electrodes, avoided certain problematic recording sites, and produced less variable data.

Acoustic Stimulation↗