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Biomedical subjects

D A Robinson

Publications and source records attributed to D A Robinson.

At least 19 recordsLinked to original sources
PubMed1992

A neural network model of the vestibulo-ocular reflex using a local synaptic learning rule.

Vertebrates use the vestibulo-ocular reflex to maintain clear vision during head movements. This reflex requires eye-velocity commands from the semicircular canals to be integrated (mathematically) to produce eye-position commands for the extraocular muscles. This is accomplished by a neural network in the caudal pons. A model of this network is proposed using positive feedback via lateral inhibition. The model has been adapted to a learning network. We have developed a synaptic learning rule using only local information to make the model more physiological.

Algorithms↗
PubMed1992

Safety and efficacy of fire ant venom in the diagnosis of fire ant allergy.

Thirty-three adult patients who had had systemic allergic reactions to fire ant stings and 33 insect-nonallergic control subjects were skin tested with single lots of Solenopsis invicta (Sol i) fire ant venom (IFAV) and two commercially available imported fire ant whole body extracts (IFA WBEs). All three extracts were analyzed for protein concentration. Sol i II and Sol i III concentrations were each assayed by means of two ELISAs with complementary monoclonal antibodies, one species specific and one cross-reactive. Radioallergosorbent test (RAST) to IFAV and both IFA WBEs was performed on sera from all study subjects. Both IFA WBEs contained high concentrations of fire ant body proteins. Sol i II and III concentrations each varied twofold between the two IFA WBE preparations. Patients were generally more reactive to IFAV than IFA WBE by skin testing and RAST. IFAV RAST appeared to be a more sensitive assay than IFA WBE RAST. No adverse reactions occurred to skin testing with IFAV, but intradermal testing with higher concentrations of IFA WBE caused delayed large local reactions in 16 of 30 (53%) control subjects. These reactions were attributed to the large amounts of extraneous body proteins in IFA WBE. These results (1) demonstrate that skin testing with IFAV is safe, (2) indicate that IFAV is more potent than IFA WBE, and (3) suggest that IFAV may be the superior reagent for diagnosis of fire ant allergy.

Adult↗
PubMed1992

Context-specific adaptation of the gain of the vestibulo-ocular reflex in humans.

Previous experiments show that altered visual feedback can change VOR gain. Such changes also presumably occur when eyeglasses are donned and doffed, or when bifocals are worn. In these cases, a nonvisual cue accompanies the required gain adjustment (frames on/off for eyeglasses, looking up/down for bifocals). We set out to show that a subject can establish two VOR gains, and to determine if one of the associated nonvisual cues alone is sufficient to subsequently determine which gain to employ. Each of three subjects sat in a rotating chair inside an OKN drum during 2 hours of sinusoidal rotation at 0.2 Hz, 30 degrees/s peak. For 10 minutes the chair and drum counterrotated , driving VOR gain toward 1.7, while subjects looked up 20 degrees. Chair and drum were then coupled for 10 minutes, driving gain toward zero, during which subjects looked down 20 degrees. This sequence was repeated for 2 hours. Immediately thereafter, VOR gains were measured while subjects looked alternately up and down, using 20 degrees to 40 degrees step rotations. A fixation target, presented before and after each step, provided accurate gain determination by measuring the size of the re-fixation saccade. Results show a consistent reduced VOR gain looking downward (average 6%) and increased gain looking upward (average 6%) and increased gain looking upward (average 8%). We conclude that humans can adjust their VOR gain dependent on a situational context; we speculate that this context can take many forms.

Adaptation, Ocular↗
PubMed1991

Failure of the oculomotor neural integrator from a discrete midline lesion between the abducens nuclei in the monkey.

Recent anatomical studies indicate that axons of neurons in the vestibular nuclei, projecting to the contralateral abducens nuclei, cross the midline at the abducens level. These axons then give off collaterals to the contralateral vestibular and prepositus nuclei that may be important for the neural integrator that converts eye-velocity to eye-position signals. We disrupted a subset of these commissural projections by making a small midline lesion between the abducens nuclei in a monkey. The vestibulo-ocular reflex and saccades were still present post-lesion, indicating that premotor drive was intact, but the lesion produced severe post-saccadic drift, indicating failure of the neural integrator. We conclude that commissural projections crossing at the abducens level may be important for oculomotor integration.

Abducens Nerve↗
PubMed1991

A learning network model of the neural integrator of the oculomotor system.

Certain premotor neurons of the oculomotor system fire at a rate proportional to desired eye velocity. Their output is integrated by a network of neurons to supply an eye position command to the motoneurons of the extraocular muscles. This network, known as the neural integrator, is calibrated during infancy and then maintained through development and trauma with remarkable precision. We have modeled this system with a self-organizing neural network that learns to integrate vestibular velocity commands to generate appropriate eye movements. It learns by using current eye movement on any given trial to calculate the amount of retinal image slip and this is used as the error signal. The synaptic weights are then changed using a straight-forward algorithm that is independent of the network configuration and does not necessitate backwards propagation of information. Minimization of the error in this fashion causes the network to develop multiple positive feedback loops that enable it to integrate a push-pull signal without integrating the background rate on which it rides. The network is also capable of recovering from various lesions and of generating more complicated signals to simulate induced post-saccadic drift and compensation for eye muscle mechanics.

Algorithms↗
PubMed1991

Benefits of a multidisciplinary pulmonary rehabilitation program. Improvements are independent of lung function.

We evaluated the conditions of 33 patients who completed an outpatient pulmonary rehabilitation program to determine what types of improvements occurred, and whether these changes were related to the baseline degree of ventilatory impairment, to determine whether rehabilitation was beneficial to patients, regardless of the degree of underlying lung dysfunction. Endurance measurements, including sustained submaximal performance on a cycle ergometer and the 12-minute walk distance (1,349 +/- 625 feet to 1,700 +/- 670 feet) increased significantly (p less than 0.01), as did multiple educational and subjective parameters. Maximal exercise performance on a graded cycle test improved very little, with a decline in the ventilatory equivalent for oxygen consumption (VE/VO2) being the only significant change (48.2 +/- 28.3 L/ml to 36.6 +/- 8.7 L/ml). Of the observed changes, only one endurance measurement, the sustained submaximal exercise performance, correlated with FEV1 (r = 0.5, p less than 0.01), but only if it was expressed as an absolute number (liters) and not as percent predicted. Lung function did not correlate with changes in the 12-minute walk distance, in maximal exercise performance on the cycle ergometer or with changes in educational and subjective parameters. We conclude that because the magnitude of change in both physiologic and psychologic parameters was not directly related to lung function, the benefits of rehabilitation can extend to all patients with chronic lung disease, regardless of the severity of preexisting pulmonary dysfunction.

Aged↗
PubMed1990

Distributed parallel processing in the vertical vestibulo-ocular reflex: learning networks compared to tensor theory.

The vestibulo-ocular reflex (VOR) is capable of producing compensatory eye movements in three dimensions. It utilizes the head rotational velocity signals from the semicircular canals to control the contractions of the extraocular muscles. Since canal and muscle coordinate frames are not orthogonal and differ from one another, a sensorimotor transformation must be produced by the VOR neural network. Tensor theory has been used to construct a linear transformation that can model the three-dimensional behavior of the VOR. But tensor theory does not take the distributed, redundant nature of the VOR neural network into account. It suggests that the neurons subserving the VOR, such as vestibular nucleus neurons, should have specific sensitivity-vectors. Actual data, however, are not in accord. Data from the cat show that the sensitivity-vectors of vestibular nucleus neurons, rather than aligning with any specific vectors, are dispersed widely. As an alternative to tensor theory, we modeled the vertical VOR as a three-layered neural network programmed using the back-propagation learning algorithm. Units in mature networks had divergent sensitivity-vectors which resembled those of actual vestibular nucleus neurons in the cat. This similarity suggests that the VOR sensorimotor transformation may be represented redundantly rather than uniquely. The results demonstrate how vestibular nucleus neurons can encode the VOR sensorimotor transformation in a distributed manner.

Animals↗
PubMed1990

Precision of Larsen grading of radiographs in assessing progression of rheumatoid arthritis in individual patients.

A study was designed to evaluate observer variation in the assessment of radiographic deterioration of individual patients using the Larsen grading system. Radiographs of hands and feet of 52 patients were assessed by three observers. Each patient had paired films taken one year apart which were assessed together for change in score. To assess within-observer variation each set of films was read twice by all observers. The average progression was 11.6 (SD 9.0). Analysis of the source of variation showed the single observer replication SD to be 3.7 but that for different observers to be 5.5. This may be interpreted as indicating that to achieve 95% confidence of detecting a true change an increase in Larsen score of 8 is required if the same observer assesses or up to 11 if a different observer assesses.

Arthritis, Rheumatoid↗
PubMed1990

Retinal image motion alone does not control disconjugate postsaccadic eye drift.

1. In these experiments, postsaccadic ocular drift was induced by postsaccadic motion of the visual scene. In the most important case, the scene was moved in one eye but not the other. Six human subjects viewed the interior of a full-field hemisphere filled with a random-dot pattern. During training, eye movements were recorded by the electrooculogram. A computer detected the end of every saccade and immediately moved the pattern horizontally in the same or, in different experiments, in the opposite direction as the saccade. The pattern motion was exponential with an amplitude of 25% of the size of the antecedent saccade and a time constant of 50 ms. Before and after 3-4 h of such training, movements of both eyes were measured simultaneously by the eye coil-magnetic field method while subjects looked between stationary targets for calibration, explored the visual pattern with saccades, or made saccades in the dark to measure the effects of adaptation on postsaccadic ocular drift. The amplitude of this drift was expressed as a percentage of the size of the antecedent saccade. 2. In monocular experiments, subjects viewed the random-dot pattern with one eye. The other eye was patched. With two subjects, the pattern drifted backward in the direction opposite to the saccade; with the third, it drifted onward. The induced ocular drift was exponential, always in the direction to reduce retinal image motion, had zero latency, and persisted in the dark. After training, drift in the dark changed by 6.7% in agreement with our prior study with binocular vision, which produced a change of 6.0%. 3. In a dichoptic arrangement, one eye regarded the moveable random-dot pattern; the other, through mirrors, saw a different random-dot pattern (with similar spacing, contrast, and distance) that was stationary. These visual patterns were not fuseable and did not evoke subjective diplopia. In this case, the induced change in postsaccadic drift in the same three subjects was only 4.8%. In all cases the changes in postsaccadic drift were conjugate--they obeyed Hering's law. 4. Normal human saccades are characterized by essentially no postsaccadic drift in the abducting eye and a pronounced onward drift (approximately 4%) in the adducting eye. After training, this abduction-adduction asymmetry was preserved in the light and dark with monocular or dichoptic viewing, indicating again that all adaptive changes were conjugate. 5. When the subjects viewed the adapting stimulus after training, the zero-latency, postsaccadic drift always increased from levels in the dark.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1989

The distributed representation of vestibulo-oculomotor signals by brain-stem neurons.

The vestibuloocular reflex and other oculomotor functions are subserved by populations of neurons operating in parallel. This distributed aspect of the system's organization has been largely ignored in previous block diagram models. Neurons that transmit oculomotor signals, such as those in the vestibular nucleus (VN), actually combine the different types of signals in a diverse, seemingly random way that could not be predicted from a block diagram. We used the backpropagation learning algorithm to program distributed neural-network models of the vestibulo-oculomotor system. Networks were trained to combine vestibular, pursuit and saccadic eye velocity command signals. The model neurons in these neural networks have diverse combinations of vestibulo-oculomotor signals that are qualitatively similar to those reported for actual VN neurons in the monkey. This similarity implicates a learning mechanism as an organizing influence on the vestibulo-oculomotor system and demonstrates how VN neurons can encode vestibulo-oculomotor signals in a diverse, distributed manner.

Algorithms↗
PubMed1989

Visual cancellation of the torsional vestibulo-ocular reflex in humans.

Using the eye-coil/magnetic field method, we measured the torsional vestibulo-ocular reflex (VOR) in ten subjects during active head rotations in roll at about 0.5 Hz. In the dark, regardless of instructions or mental effort, the gains (eye velocity/head velocity) had a mean value of around 0.61. When they viewed a visual display that was stationary, gains rose to 0.72. When viewing a visual display that moved in roll with their heads, subjects could decrease their gains to a mean of 0.46. Separate experiments showed that, as expected at this frequency, the optokinetic system made only a weak contribution. It has been proposed that the horizontal VOR is cancelled by the smooth pursuit system. Since there is no torsional pursuit system, some other mechanism must be used to augment or partially cancel the torsional VOR. Attempts to show that imagination could change this gain showed only weak effects. When asked to imagine an earth-fixed scene, gains were around 0.63; when asked to imagine a subject-fixed scene, gains decreased to only 0.60. When allowed to use a tactile contribution to aid the imagination in cancelling the VOR, the gain dropped further but only to 0.57. We conclude that mental effort in the dark has little influence on the torsional VOR but vision does by a mechanism that is not optokinetic or pursuit.

Adult↗
PubMed1989

Tracheal tube biofilm as a source of bacterial colonization of the lung.

Biofilm formation in tracheal tubes, its bacterial content, and its interaction with ventilator gas flow were investigated. At least 50 mg (dry weight) of biofilm was found in 30 of 40 tracheal tubes used in intensive care patients for 2 h to 10 days. Electron microscopy showed bacteria in this layer, and quantitative studies showed that bacterial counts could reach up to 10(6)/cm of tube length. Bacteria were cultured from the patient side of 18 of 78 heat and moisture exchanger-microbiological filter units removed from ventilator circuits. Particles were shown to detach from tracheal tube luminal biofilm and were projected up to 45 cm from the tracheal tube tip. Following contamination of the tracheal tube biofilm with a patient's own gastrointestinal flora, entrainment of bacteria in the inspiratory gas flow provides a mechanism for initial and repeated lung colonization.

Bacteria↗