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PubMed1996

The Impact of Environmental Variation on Demographic Convergence of Leslie Matrix Population Models: An Assessment Using Lyapunov Characteristic Exponents

In a constant environment, the rate of convergence of a density-independent Leslie matrix model to stable age distribution is determined by the damping ratio (the ratio of the absolute magnitudes of the first and second eigenvalues of the projection matrix). In a stochastic environment, the difference between the first two Lyapunov exponents is known to be analogous to the logarithm of the damping ratio, but there has been no systematic investigation of the consequences of enviromnental variation on convergence rates. In this study, the Lyapunov spectrum has been calculated for a wide variety of density-independent projection matrices subject to random variations in vital rates. This allows the impact of these random variations on convergence rates to be assessed. For rapidly convergent life histories, stochastic variation leads to a decrease in convergence rate. For life histories which are slow to converge, stochastic variation speeds up convergence. These effects are, however, relatively minor, and the value of the damping ratio for the mean matrix is a good predictor of the damping ratio in a stochastic environment. Consequently, when only an approximate indication of convergence rates is needed, the damping ratio for the mean projection matrix gives a very good guide. Detailed calculations of the Lyapunov spectrum would only be necessary to make comparisons between similar life histories or if very precise information on convergence rates were needed.

Journal Article↗
PubMed1987

Dorsal horn (convergent) neurones in the intact anaesthetized arthritic rat. I. Segmental excitatory influences.

Recordings were made from dorsal horn neurones in intact anaesthetized rats rendered polyarthritic by s.c. injection into the base of the tail, of Mycobacterium butyricum suspended in oil; the experiments were carried out during the acute phase of the illness (3-4 weeks post inoculation) during which hyperaesthesia occurred. The majority (60.8%) of the neurones studied had properties close to those of corresponding groups of units in healthy rats. These 'typical' neurones could be subdivided into convergent (13.2%), non-noxious (34.4%) and proprioceptive (13.2%) units. By contrast, and in agreement with a previous study in the unanaesthetized spinal arthritic rat, the segmental electrophysiological characteristics of the remaining large proportion of neurones were changed both in terms of the size and distribution of their excitatory receptive fields and their responsiveness to peripheral stimuli; these were designated as 'atypical' neurones. According to their electrophysiological properties, these neurones were differentiated as atypical convergent (27.8%) and atypical non-noxious (11.4%) units. The main qualitative difference between the typical and atypical neurones was that the atypical had an additional receptive field on the oedematous ipsilateral ankle and, in several cases, showed high levels of background activity with sometimes dramatic increases. By comparison with neurones recorded in healthy rats, quantitative data revealed other modifications: typical and atypical convergent neurones and atypical non-noxious neurones had larger classical excitatory receptive fields; while C-fibre responses evoked by transcutaneous electrical stimulation were facilitated in the case of typical convergent neurones, 47% of the atypical convergent neurones had no C-fibre responses, and when present (53%) the threshold for obtaining these C-fibre responses was higher with suprathreshold stimuli producing a minimal number of spikes; in these cells, gentle mechanical stimuli gave rise to high rates of firing which sometimes resulted in dramatic, long lasting after-discharges. The possibility that typical convergent, atypical convergent and atypical non-noxious neurones were derived from the homogeneous population of convergent neurones in the healthy rat is discussed; the atypical properties could be the result of a change in the characteristics of convergent neurones, resulting from arthritis.

Animals↗
PubMed2005

Assessment of convergence angles of tooth preparations for complete crowns among dental students.

OBJECTIVES: Convergence angles for complete crown preparations have been recommended at 4-12 degrees . However, practitioners have difficulty meeting these recommendations. This study measured and compared the convergence angles of tooth preparations for complete crowns prepared at three Colleges of Dentistry: the University of Tanta, Egypt, King Abdulaziz University, Saudi Arabia, and The Ohio State University, Columbus, Ohio. METHODS: The convergence angles of 499 tooth preparations for complete crowns were evaluated. These comprised of a random sample of 262 teeth prepared by third-year dental students at the University of Tanta on extracted molars under normal preclinical conditions, 37 preparations on molar teeth on typodonts done by first-year dental students. The Ohio State University and 200 molar typodont complete crown preparations done by fourth-year dental students at King Abdulaziz University, Saudi Arabia. The bucco-lingual and mesio-distal convergence angles of each preparation were measured with a goniometer microscope. Mean convergence angles were calculated, and differences between groups were tested for statistical significance with analysis of variance (ANOVA) at 5% level of confidence. RESULTS: Convergence angle measurements were significantly different between the groups and the dimensions (P<0.001). The greatest convergence value (19.8+/-10.0) was for bucco-lingual measurements prepared by Egyptian dental students. The smallest convergence value (14.1+/-3.8) was for mesio-distal measurements prepared by Saudi dental students. CONCLUSIONS: This study emphasized the difference between what is taught at dental schools (what is theoretically possible) and the school results of actual practice. There was a considerable disparity between the convergent angles recorded in this study and the ideal configurations recommended in fixed prosthodontic textbooks and the dental literature.

Crowns↗
PubMed2003

Convergence defects in patients with temporomandibular disorders.

The aim of this study is to show the presence of a correlation between ocular convergence defects (OCD) and temporomandibular disorders (TMD) among a group of adult subjects. The group studied was made up of 48 subjects (12 males and 36 females). The average age was 35 with a range of 19-45 years of age. The subjects presented with TMD and muscular pain and/or dysfunction. Forty-eight subjects with TMD for the case study were matched by gender and age to 48 control subjects seeking routine dental care (control group). All the subjects were examined by the same orthoptist who classified the ocular convergence degree using two tests. The first test evaluated the distances (in centimeters) of the convergence near point (3-4 cm: normal; 5-7 cm: sufficient; > 7 cm: insufficient). The second test assessed the fusional convergence using a Berens prism test (> 25 diopters: normal; between 18-25: sufficient; < 18 diopters: insufficient). In the TMD group, 36 subjects (75%) showed a compromise of convergence: 13 (36%) were classified in the 5-7 degree range and 23 (48%) in the > 7 cm degree range. The Berens test showed ten subjects (28%) in the group < 18D and 26 (72%) in the group 18-25D. The control-group presented ten (21%) subjects with compromise of convergence: three classified in the group < 18D and seven in the group 18-25D. The TMD subjects presented a higher statistical percentage (p < 0.0001) of ocular convergence defects. The TMD patients also reported a strong association referred to specific signs and symptoms, i.e., limited maximal opening or myofascial pain. There were some subjective reports also of headaches and torcicollis (neck stiffness) which appeared significantly more frequently in subjects with a compromise of convergence. The study showed a much higher prevalence of ocular convergence defects in patients with head, neck, and shoulder pain.

Adult↗
PubMed2003

Optimizer convergence and local minima errors and their clinical importance.

Two of the errors common in the inverse treatment planning optimization have been investigated. The first error is the optimizer convergence error, which appears because of non-perfect convergence to the global or local solution, usually caused by a non-zero stopping criterion. The second error is the local minima error, which occurs when the objective function is not convex and/or the feasible solution space is not convex. The magnitude of the errors, their relative importance in comparison to other errors as well as their clinical significance in terms of tumour control probability (TCP) and normal tissue complication probability (NTCP) were investigated. Two inherently different optimizers, a stochastic simulated annealing and deterministic gradient method were compared on a clinical example. It was found that for typical optimization the optimizer convergence errors are rather small, especially compared to other convergence errors, e.g., convergence errors due to inaccuracy of the current dose calculation algorithms. This indicates that stopping criteria could often be relaxed leading into optimization speed-ups. The local minima errors were also found to be relatively small and typically in the range of the dose calculation convergence errors. Even for the cases where significantly higher objective function scores were obtained the local minima errors were not significantly higher. Clinical evaluation of the optimizer convergence error showed good correlation between the convergence of the clinical TCP or NTCP measures and convergence of the physical dose distribution. On the other hand, the local minima errors resulted in significantly different TCP or NTCP values (up to a factor of 2) indicating clinical importance of the local minima produced by physical optimization.

Algorithms↗
PubMed2005

Convergence analysis of a deterministic discrete time system of Oja's PCA learning algorithm.

The convergence of Oja's principal component analysis (PCA) learning algorithms is a difficult topic for direct study and analysis. Traditionally, the convergence of these algorithms is indirectly analyzed via certain deterministic continuous time (DCT) systems. Such a method will require the learning rate to converge to zero, which is not a reasonable requirement to impose in many practical applications. Recently, deterministic discrete time (DDT) systems have been proposed instead to indirectly interpret the dynamics of the learning algorithms. Unlike DCT systems, DDT systems allow learning rates to be constant (which can be a nonzero). This paper will provide some important results relating to the convergence of a DDT system of Oja's PCA learning algorithm. It has the following contributions: 1) A number of invariant sets are obtained, based on which we can show that any trajectory starting from a point in the invariant set will remain in the set forever. Thus, the nondivergence of the trajectories is guaranteed. 2) The convergence of the DDT system is analyzed rigorously. It is proven, in the paper, that almost all trajectories of the system starting from points in an invariant set will converge exponentially to the unit eigenvector associated with the largest eigenvalue of the correlation matrix. In addition, exponential convergence rate are obtained, providing useful guidelines for the selection of fast convergence learning rate. 3) Since the trajectories may diverge, the careful choice of initial vectors is an important issue. This paper suggests to use the domain of unit hyper sphere as initial vectors to guarantee convergence. 4) Simulation results will be furnished to illustrate the theoretical results achieved.

Algorithms↗
PubMed1995

The velocity distribution in the aortic annulus at different times during systole is mainly determined by the pattern of flow convergence in the left ventricular outflow tract--an experimental study using Doppler colour flow mapping.

An experimental study in 10 open chest normal pigs (body weight: 24 +/- 1 kg) was carried out to explore the relationship between the velocity distribution in the aortic annulus and the pattern of flow convergence in the left ventricular outflow tract. The cross-sectional velocity profiles in the aortic annulus were constructed by using Doppler colour flow mapping with a previously validated time-interpolation method. The pattern of flow convergence in the left ventricular outflow tract was quantified by measuring the colour flow areas on the anterior and posterior sides of the central axis of the aortic annulus, and calculating their difference. The dynamic changes of the velocity distribution, the pattern of flow convergence and the septal angle throughout systole were observed. The velocity distribution in the aortic annulus changed according to the pattern of flow convergence in the left ventricular outflow tract. During early systole, the pattern of flow convergence was most asymmetrical. With the central longitudinal axis of the aortic annulus as a reference, the main part of the converging flow was along the anterior wall of the left ventricular outflow tract. Consequently, the velocity profile in the aortic annulus was most skewed during the early systole, with the highest velocity along the anterior wall. Towards late systole, the pattern of flow convergence became more and more symmetrical, and the velocity distribution in the aortic annulus gradually became flat. A significant correlation was found between the extent of asymmetry of the pattern of flow convergence in the left ventricular outflow tract and the extent of skewness of the velocity distribution in the aortic annulus (r = 0.69, P < 0.001). Throughout systole, septal angle increased very slightly (from 35 +/- 3 to 38 +/- 3 degrees, P < 0.01). The pattern of flow convergence in the left ventricular outflow tract is a major determinant of the velocity distribution in the aortic annulus in pigs.

Animals↗
PubMed1992

Convergence of visceral and somatic inputs onto subnucleus reticularis dorsalis neurones in the rat medulla.

1. In anaesthetized rats, recordings were made within the medullary subnucleus reticularis dorsalis (SRD) from neurones that exhibited convergence of nociceptive inputs from the entire surface of the body. Neurones with total nociceptive convergence responded to supramaximal percutaneous electrical stimuli (2 ms duration) with early and late peaks due to the activation of A delta and C fibres, respectively, no matter which part of the body was stimulated. Neurones with partial nociceptive convergence responded to identical stimuli with an A delta peak of activation regardless of which part of the body was stimulated and with a C peak of activation from some, mainly contralateral, parts of the body. The characteristics of the responses of these neurones to graded colo-rectal distension (< or = 100 mmHg) were analysed. 2. The majority of neurones with total nociceptive convergence (n = 13 out of 16) responded to colo-rectal distension by increasing their firing rates. Although these neurones were virtually unresponsive to the lowest pressure employed (12.5 mmHg), they increased their discharges monotonically for distensions in the 26-100 mmHg range and these responses were sometimes followed by after-discharges. One of these neurones, which exhibited a high level of spontaneous activity, was inhibited during colo-rectal distension. None of the neurones with partial nociceptive convergence recorded (n = 10) ever changed its firing rate during increases of intracolonic pressure up to 100 mmHg. 3. It is concluded that neurones with total nociceptive convergence give monotonic stimulus-response relationships for colo-rectal distensions. Thus, neurones with total nociceptive convergence can encode the strength of visceral stimuli, probably within the noxious range, just as they have previously been shown to do for thermal and mechanical cutaneous stimuli. Together with previous electrophysiological and neuroanatomical findings, this study provides further evidence for the convergence of noxious inputs onto single SRD neurones. 4. It is suggested that neurones with total nociceptive convergence could be a link in spino-bulbospinal loops involved in autonomic reactions to strong visceral stimulation. In addition, SRD neurones could be an important supraspinal relay in the mechanisms of visceral pain.

Action Potentials↗
PubMed1984

Neural control of vergence eye movements: convergence and divergence neurons in midbrain.

Animals with binocular single vision use disjunctive (vergence) eye movements to align the two eyes on a visual target. Several lines of evidence suggest that conjugate and vergence eye movement commands are generated independently and combined at the medial rectus motoneurons. If this were true, then a pure vergence eye-position signal should exist. This signal would be proportional to the horizontal angle between the eyes (vergence angle), without regard to the direction of conjugate gaze. The purpose of this experiment was to identify and study neurons that carry a pure vergence signal. Extracellular unit recordings were made from midbrain and pontine sites in monkeys trained to track visual targets moving in the horizontal, vertical, and depth (or target vergence) planes. The most commonly encountered neuron that had a vergence signal was the convergence cell. These units had a firing rate that was linearly proportional to the convergence angle; their activity was unaffected by changes in conjugate gaze. Changes in convergence cell activity preceded the change in vergence angle slightly. Convergence cell activity increased for increased convergence regardless of whether the change was in response to purely accommodative or disparity cues. Divergence cells were found far less frequently. These cells were similar to convergence cells except that they decreased their firing rate for increases in convergence. The activity of divergence cells was unaffected by changes in the direction of conjugate gaze. Both convergence and divergence cells were found, intermixed, in the mesencephalic reticular formation must outside the oculomotor nucleus. Most cells with a vergence signal were found within 1-2 mm of the nucleus. These results support the view that conjugate and vergence signals are generated independently and are combined at the extraocular motoneurons. Convergence cells seem ideally suited to provide the vergence signal required by the nearby medial rectus motoneurons.

Accommodation, Ocular↗
PubMed1988

Regional expression, pattern and timing of convergence and extension during gastrulation of Xenopus laevis.

We show with time-lapse micrography that narrowing in the circumblastoporal dimension (convergence) and lengthening in the animal-vegetal dimension (extension) of the involuting marginal zone (IMZ) and the noninvoluting marginal zone (NIMZ) are the major tissue movements driving blastopore closure and involution of the IMZ during gastrulation in the South African clawed frog, Xenopus laevis. Analysis of blastopore closure shows that the degree of convergence is uniform from dorsal to ventral sides, whereas the degree of extension is greater on the dorsal side of the gastrula. Explants of the gastrula show simultaneous convergence and extension in the dorsal IMZ and NIMZ. In both regions, convergence and extension are most pronounced at their common boundary, and decrease in both animal and vegetal directions. Convergent extension is autonomous to the IMZ and begins at stage 10.5, after the IMZ has involuted. In contrast, expression of convergent extension in the NIMZ appears to be dependent on basal contact with chordamesoderm or with itself. The degree of extension decreases progressively in lateral and ventral sectors. Isolated ventral sectors show convergence without a corresponding degree of extension, perhaps reflecting the transient convergence and thickening that occurs in this region of the intact embryo. We present a detailed mechanism of how these processes are integrated with others to produce gastrulation. The significance of the regional expression of convergence and extension in Xenopus is discussed and compared to gastrulation in other amphibians.

Animals↗
PubMed1990

Convergence in mammalian nucleus of solitary tract during development and functional differentiation of salt taste circuits.

To determine the type and extent of neural rearrangements that are made during functional differentiation of circuits for salt taste processing, we determined receptive field size and salt response characteristics of second-order taste cells in 3 age groups of sheep. Neurophysiological recordings were made from single cells in the nucleus of the solitary tract (NST) in fetal, perinatal, and postnatal sheep. Responses to NH4Cl, NaCl, and KCl were measured, and location and number of fungiform papillae in the receptive field were determined by stimulating individual papillae with anodal electrical current. The data are compared with previous, parallel measures from chorda tympani nerve afferent taste fibers to permit conclusions about convergence or divergence onto second-order cells. Receptive field size of second-order taste neurons increases during development, in contrast to the decrease in field size observed previously for chorda tympani nerve fibers during the same period. Furthermore, receptive fields of second-order cells are significantly larger than those of first-order fibers at perinatal and lamb ages, but not fetal. Thus, there is convergence of first-order taste afferents onto brain-stem neurons, and the convergence increases remarkably between fetal and perinatal periods. Associated with the increase in convergence are increased salt response frequencies relative to afferent fibers for NaCl in perinatal animals and lambs, and for KCl in lambs. The increase in frequencies occurs before NST neurons are functionally mature, as indicated by the rapid response adaptation of many cells in young animals. Convergence in NST during development apparently functions to maximize gain for processing neural responses to NaCl. In the periphery, response frequencies to NaCl are very low in fetuses, and increase progressively during development. In the NST, NaCl response frequencies are high even in fetuses, and remain high. The process of convergence onto second-order cells is accomplished with maintenance of order in afferent projections because receptive fields of NST neurons are composed of fungiform papillae that are clustered together, not dispersed over the tongue. Our quantification of taste receptive field size at 2 neural levels provides strong evidence for increasing convergence in the NST during development. Altering patterns of afferent neural input and geometry of second-order neurons may have a role in establishing convergence. The convergence has an apparently special function: to increase gain for NaCl taste sensation. Therefore, neural rearrangements during differentiation of salt taste pathways result in specific functional outcomes.

Ammonium Chloride↗
PubMed1985

The function and mechanism of convergent extension during gastrulation of Xenopus laevis.

The processes thought to function in Xenopus gastrulation include bottle cell formation, migration of cells on the roof of the blastocoel, and autonomous convergent extension of the circumblastoporal region. A review of recent and classical results shows that only the last accounts for the bulk of the tissue displacement of gastrulation, including spreading of the marginal zone toward the blastopore, involution of the marginal zone, and closure of the blastopore. Microsurgical manipulation and explantation studies, analysed by time-lapse video and cine microscopy, shows that the dorsal circumblastoporal region contains two regions which show either autonomous or semiautonomous convergent extension. The dorsal involuting marginal zone (IMZ) undergoes convergence (narrowing) and extension (lengthening) after its involution, beginning at the midgastrula stage and continuing through neurulation, such that it simultaneously extends posteriorly across the yolk plug and narrows the blastoporal circumference. Concurrently, the corresponding region of the overlying non-involuting marginal zone (NIMZ) begins a complementary convergent extension, but at a greater rate, which spreads vegetally to occupy surface area vacated by the IMZ. Tissue recombination experiments show that the deep cells of the dorsal IMZ bring about convergent extension. Labelling of small populations of these cells with a cell lineage tracer shows that convergent extension involves intercalation of deep cells to form a longer, narrower array. Direct time-lapse video and cine micrography of deep cells in cultured explants show that convergent extension involves radial and circumferential intercalation. Removal of the entire blastocoel roof of the early gastrula, including all or part of the NIMZ, shows that convergent extension of the IMZ alone can bring about its involution and blastopore closure. The role of convergent extension in gastrulation of other amphibians and other metazoans and its significance to related problems in early development are discussed.

Animals↗
PubMed1997

Convergence and divergence exhibit different response characteristics to symmetric stimuli.

The dynamic characteristics of horizontal convergence and divergence eye movement responses to symmetric stimuli were examined. Binocular eye movements were recorded in five, visually normal adult subjects using the infrared reflection technique for symmetric convergent and divergent blur-free, disparity-only, step stimuli of 2, 4, 8, 12, and 16 deg. The main sequence as well as other temporal parameters including latency, time-to-peak velocity, time constant, and total duration were analyzed. A number of fundamental differences in the response characteristics were found between convergence and divergence. First, the slope of the peak velocity vs amplitude curve was approximately twice as high for convergence than divergence. The results are consistent with neurophysiological findings in monkeys and most findings in humans. Second, the initial fast component for convergence exhibited a larger amplitude than for divergence. This may reflect differences in central neural gain for convergence and divergence. And, third, all temporally related components were shorter for convergence than divergence. These findings provide an overall framework for vergence control and suggest fundamental differences in neural processing delays and neural controller pathways for convergence and divergence.

Adult↗
PubMed1999

Nonlinear alteration of transient vergence dynamics after sustained convergence.

BACKGROUND: Adaptation models of the horizontal disparity vergence system assume a nonadaptable transient component. They also predict identical postadaptation dynamics during convergence and divergence movements. METHOD: To test the adaptation property of the transient component, a set of experiments were performed in which closed-loop vergence dynamics measured before and after sustained convergence were compared, primarily by comparing the peak vergence velocity, occurrence time of peak vergence velocity, and steady-state vergence posture. Vergence dynamics after durations of 30, 60, and 90 s of sustained convergence were compared with those after a control duration of 5 s. RESULTS: The peak divergence velocity was reduced by about 25% within 30 s of sustained vergence. However, the peak convergence velocity was unchanged for all the exposure durations. Additionally, for all durations, the peak divergence velocity was significantly higher than peak convergence velocity. In contrast to peak velocities, the occurrence time of peak convergence and divergence velocity did not differ significantly and remained unchanged for all durations. CONCLUSIONS: The transient component is adaptable. Furthermore, the adaptation is direction dependent and affects divergence and convergence dynamics differently, thereby suggesting involvement of separate pathways for convergence and divergence in the vergence sensorimotor control.

Accommodation, Ocular↗
PubMed2003

Nearpoint of convergence: test procedure, target selection, and normative data.

BACKGROUND: The purpose of this study was to help determine the most appropriate target to be used for the assessment of the nearpoint of convergence, normative data for the break and recovery in adults, and the diagnostic value of the red-glass modification and repetition of the nearpoint of convergence. METHODS: A total of 175 subjects with normal binocular vision and 38 subjects with convergence insufficiency were evaluated. The nearpoint of convergence was measured three ways, with an accommodative target, a penlight, and a penlight with red and green glasses. The nearpoint of convergence was also measured using a penlight for 10 repetitions. RESULTS: Results suggest a clinical cutoff value of 5 cm for the nearpoint of convergence break and 7 cm for the nearpoint of convergence recovery with either an accommodative target or a penlight with red and green glasses. CONCLUSION: This study establishes normative data for the nearpoint of convergence break and recovery in the adult population and supports the value of various test modifications when other testing is equivocal.

Accommodation, Ocular↗
PubMed2002

Missing lateral rectus force and absence of medial rectus co-contraction in ocular convergence.

For a given position of the eye in the orbit, most abducens motoneurons (LRMNs) fire at higher rates in converged gaze than when convergence is relaxed, implying that lateral rectus (LR) muscle force will be higher for a given eye position in convergence. If medial rectus (MR) muscle force balances LR force, it too would be higher in convergence, that is, LRMN recording studies predict horizontal rectus co-contraction in convergence. Three trained rhesus monkeys with binocular eye coils and custom muscle force transducers (MFTs) on LR and MR of one eye alternately fixated near (approximately 7 cm) and far (200 cm) targets with vergence movements of 20-30 degrees. Tonic muscle forces were also measured during conjugate fixation of far targets over a 30 x 30 degrees field. MFT characteristics and effects on oculomotility were assessed. Contrary to predictions, we found small (<1 g) decreases in both LR and MR forces in convergence, for those gaze positions that were used in the brain stem recording studies. This missing LR force paradox (higher LRMN firing rates in convergence but lower LR forces) suggests that motoneurons or muscle fibers contribute differently to oculorotary forces in converged and unconverged states, violating the final common path hypothesis. The absence of MR co-contraction is consistent with, and supports, the missing LR force finding. Resolution of the missing LR force paradox might involve nonlinear interactions among muscle fibers, mechanical specialization of muscle fibers and other articulations of the peripheral oculomotor apparatus, or extranuclear contributions to muscle innervation.

Animals↗
PubMed1998

A technique for determining convergence in human subjects undergoing rotational acceleration using a binocular eye-tracking system.

BACKGROUND: This report describes the measurement of convergent eye movements and calculation of the spatial convergence point from the angular eye position data. These measurements were made in the dark while the subject experienced inertial motion aboard a centrifuge. This was an exploratory experiment with the goal of evaluating the eyes' convergence in the dark, and to see if this convergence point is dependent on inertial motion. METHODS: The subject was rotated in the dark on NAMRL's Coriolis Acceleration Platform in Pensacola, FL. The pupil positions were tracked by two helmet-mounted infrared cameras connected to a computer-controlled data acquisition system. We used the position data to calculate the angles through which the eyes rotated, and then applied trigonometric principles to construct the line of sight for each eye for any instant in time. The intersection of these two lines of sight was the convergence point. RESULTS: With the binocular eye-tracking system, we could accurately determine an accelerating subject's convergence point to within 10%, if the point was less than 1.5 m away from the subject. At convergence distances greater than 1.5 m, the angular movements of the two eyes became so small that determining a convergence point was difficult.

Acceleration↗
PubMed1994

Microtubule converging centers and reorganization of the interphase cytoskeleton and the mitotic spindle in higher plant Haemanthus.

We analyzed the distribution and orientation of transitory microtubule structures, microtubule converging centers, during interphase and mitosis in endosperm of the higher plant Haemanthus. In interphase the pointed tips of microtubule converging centers are associated with the nuclear envelope. Their orientation gradually reverses during prophase, and the tips tend to point away from the nucleus. From prometaphase through early telophase, microtubule converging centers are present predominantly in the cytoplasm at the polar region. They are either "free" or associated with chromosomes or microtubule bundles. In late telophase, pointed tips of microtubule converging centers are again associated with the reconstructed nuclear envelope and, additionally, they often appear in the phragmoplast area. The orientation of microtubule converging centers seems to be directly correlated to the previously determined microtubule polarity, with the converging tip being minus and the diverging one, plus. Elevated temperature (35 degrees-37 degrees C) enhances the number of microtubule converging centers in the cytoplasm and at the nuclear envelope. This is especially pronounced during the telophase-interphase transition and in some interphase cells, indicating temperature and stage dependence. Our data imply that microtubule converging centers bind together MT minus ends and, thus, control the predominant direction of elongation and shortening of microtubule arrays. We argue that these configurations are instrumental during the reorganization of interphase cytoskeleton and mitotic spindle in Haemanthus endosperm.

Cell Cycle↗