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PubMed2003

Depth-related visually evoked potentials by dynamic random-dot stereograms in humans: negative correlation between the peaks elicited by convergent and divergent disparities.

Binocular visually evoked potentials (VEP) were recorded from the left and right occipital cortices of right-handed subjects in response to convergent and divergent stimuli, each having six disparity levels, using dynamic random-dot stereograms (DRDS). The VEP recorded consisted of a negative peak (N300) and a positive peak (P500) within intervals of 200-400 ms and 400-600 ms, respectively, with respect to the stimulus onset. For convergent disparities, the relationships between the amplitude of N300 and the degree of disparity showed convexity towards the disparity axis, whereas the same relationships displayed concavity for P500. For divergent disparities, on the other hand, the amplitude and the degree of disparity relationships showed concavity towards the disparity axis for N300, in contrast to those obtained by convergent disparities. Although the disparity profile of P500 displayed concavity in the left hemisphere, its right hemisphere counterpart turned out to be bi-modal in behaviour, indicating a relative loss of disparity sensitivity in the mid-disparity range. The significant negative correlation between the N300 and P500 behaviour in response to both stimulus modalities suggests that the activity of the N300 centre is effective in changing the synchronization level of the cell population comprising the P500 centre. The significant negative correlations between the profiles of the N300 wave in response to convergent DRDS and its divergent DRDS counterpart in both hemispheres, and between the profiles of the convergent and divergent P500 waves in the left hemisphere imply that the N300 and the P500 foci are sensitive to both the magnitude and the direction of the disparity. The latency differences between the two response modalities revealed that the N300 wave in response to convergent DRDS always led the N300 wave elicited by divergent DRDS, indicating that convergent disparities are processed faster than the divergent disparities.

Adolescent↗
PubMed2000

Baroreceptive and somatosensory convergent thalamic neurons project to the posterior insular cortex in the rat.

Connectivity between the rat posterior insula and the ventrobasal thalamus has been demonstrated anatomically. Neurons convergent for baroreceptor and nociceptive input have also been identified in the homologous anterior insula of the primate. Whether similar convergent cells exist in the ventrobasal thalamus was investigated in 30 urethane anesthetized male Sprague--Dawley rats. Six classes of cells were identified in the right ventrobasal thalamus: (a) 83/159 (52%) baroreceptive and nociceptive convergent units; (b) 2/159 (1%) convergent cells responding to baroreceptor activation and light touch; (c) 44/159 (28%) purely nociceptive units; (d)10/159 (6%) purely baroreceptive units; (e) 1/159 (0.6%) cells responding to brush alone and (f) 19/159 (12%) unresponsive units. Of the viscerosomatic convergent cells, 66/85 (78%) were situated in the ventroposterolateral nucleus (VPL), 6/85 (7%) in the ventroposterolateral parvicellular nucleus (VPLpc), and 13/85 (15%) in the ventroposteromedial nucleus (VPM). Fifteen right ventrobasal thalamic units were antidromically activated and 34 units orthodromically activated by right posterior insular microstimulation. Cobalt injection into the right ventrobasal thalamus blocked the right insular response to baroreceptor activation by >70%. These data indicate: (a) baroreceptive and somatosensory nociceptive convergent units exist in the ventrobasal thalamus; (b) thalamic convergent neurons project directly to the ipsilateral posterior insula and receive reciprocal insulothalamic projections; and (c) a significant proportion of baroreceptor input relays to the posterior insula through the ipsilateral ventrobasal thalamus.

Animals↗
PubMed2002

The timing and laminar profile of converging inputs to multisensory areas of the macaque neocortex.

Two fundamental requirements for multisensory integration are convergence of unisensory (e.g. visual and auditory) inputs and temporal alignment of the neural responses to convergent inputs. We investigated the anatomic mechanisms of multisensory convergence by examining three areas in which convergence occurs, posterior auditory association cortex, superior temporal polysensory area (STP) and ventral intraparietal sulcus area (VIP). The first of these was recently shown to be a site of multisensory convergence and the latter two are more well known as 'classic' multisensory regions. In each case, we focused on defining the laminar profile of response to the unisensory inputs. This information is useful because two major types of connection, feedforward and feedback, have characteristic differences in laminar termination patterns, which manifest physiologically. In the same multisensory convergence areas we also examined the timing of the unisensory inputs using the same standardized stimuli across all recordings. Our findings indicate that: (1) like somatosensory input [J. Neurophysiol., 85 (2001) 1322], visual input is available at very early stages of auditory processing, (2) convergence occurs through feedback, as well as feedforward anatomical projections and (3) input timing may be an asset, as well as a constraint in multisensory processing.

Acoustic Stimulation↗
PubMed2003

When are phylogenetic analyses misled by convergence? A case study in Texas cave salamanders.

Convergence, i.e., similarity between organisms that is not the direct result of shared phylogenetic history (and that may instead result from independent adaptations to similar environments), is a fundamental issue that lies at the interface of systematics and evolutionary biology. Although convergence is often cited as an important problem in morphological phylogenetics, there have been few well-documented examples of strongly supported and misleading phylogenetic estimates that result from adaptive convergence in morphology. In this article, we propose criteria that can be used to infer whether or not a phylogenetic analysis has been misled by convergence. We then apply these criteria in a study of central Texas cave salamanders (genus Eurycea). Morphological characters (apparently related to cave-dwelling habitat use) support a clade uniting the species E. rathbuni and E. tridentifera, whereas mitochondrial DNA sequences and allozyme data show that these two species are not closely related. We suggest that a likely explanation for the paucity of examples of strongly misleading morphological convergence is that the conditions under which adaptive convergence is most likely to produce strongly misleading results are limited. Specifically, convergence is most likely to be problematic in groups (such as the central Texas Eurycea) in which most species are morphologically very similar and some of the species have invaded and adapted to a novel selective environment.

Adaptation, Biological↗
PubMed2003

Globally convergent image reconstruction for emission tomography using relaxed ordered subsets algorithms.

We present two types of globally convergent relaxed ordered subsets (OS) algorithms for penalized-likelihood image reconstruction in emission tomography: modified block sequential regularized expectation-maximization (BSREM) and relaxed OS separable paraboloidal surrogates (OS-SPS). The global convergence proof of the existing BSREM (De Pierro and Yamagishi, 2001) required a few a posteriori assumptions. By modifying the scaling functions of BSREM, we are able to prove the convergence of the modified BSREM under realistic assumptions. Our modification also makes stepsize selection more convenient. In addition, we introduce relaxation into the OS-SPS algorithm (Erdoğan and Fessler, 1999) that otherwise would converge to a limit cycle. We prove the global convergence of diagonally scaled incremental gradient methods of which the relaxed OS-SPS is a special case; main results of the proofs are from (Nedić and Bertsekas, 2001) and (Correa and Lemaréchal, 1993). Simulation results showed that both new algorithms achieve global convergence yet retain the fast initial convergence speed of conventional unrelaxed ordered subsets algorithms.

Algorithms↗
PubMed2006

Convergent incremental optimization transfer algorithms: application to tomography.

No convergent ordered subsets (OS) type image reconstruction algorithms for transmission tomography have been proposed to date. In contrast, in emission tomography, there are two known families of convergent OS algorithms: methods that use relaxation parameters, and methods based on the incremental expectation-maximization (EM) approach. This paper generalizes the incremental EM approach by introducing a general framework, "incremental optimization transfer." The proposed algorithms accelerate convergence speeds and ensure global convergence without requiring relaxation parameters. The general optimization transfer framework allows the use of a very broad family of surrogate functions, enabling the development of new algorithms. This paper provides the first convergent OS-type algorithm for (nonconcave) penalized-likelihood (PL) transmission image reconstruction by using separable paraboloidal surrogates (SPS) which yield closed-form maximization steps. We found it is very effective to achieve fast convergence rates by starting with an OS algorithm with a large number of subsets and switching to the new "transmission incremental optimization transfer (TRIOT)" algorithm. Results show that TRIOT is faster in increasing the PL objective than nonincremental ordinary SPS and even OS-SPS yet is convergent.

Algorithms↗
PubMed1992

Experimental studies to define the geometry of the flow convergence region. Laser Doppler particle tracking and color Doppler imaging.

The color flow convergence method for calculating volume flow through regurgitant or forward flow restrictive orifices has gained significant interest and a number of in vitro studies have suggested that this method is accurate, even in pulsatile models. Clinical application of the method over a wide range of conditions will require improved understanding of the effect of orifice size, flow geometry, and flow rate on the flow convergence geometry. In this study, we performed laser particle tracking investigations to allow visualization of streamlines into stenotic orifices. These streamlines are theoretically perpendicular to the isovelocity surfaces used for flow convergence calculations. We compared those observations to color flow map, flow convergence images obtained with a Toshiba 160A for orifices 5 to 15 mm 2 with flow rates of 1.5 to 9.7 L/min. Our results show that for large orifices, low flow rates, and/or low pressure gradients, more oblique streamlines in the velocity of the orifice correspond to nonhemispherical, but more elliptical, flow convergence geometries. This can be corrected for by using lower Nyquist limits and calculating flow convergence at greater distances from the orifice. Under high flow and high gradient conditions, increased Nyquist limits and shorter aliasing radii are more suitable. Our studies yield insights into flow convergence geometry and yield corrective procedures to improve volume flow calculation.

Blood Flow Velocity↗
PubMed1996

Convergent carotid sinus nerve and superior laryngeal nerve afferent inputs to neurons in the NTS.

A population of 43 neurons in the nucleus of the solitary tract (NTS) was identified in pentobarbital sodium anesthetized, paralyzed, and artificially ventilated cats that received convergent inputs from carotid sinus nerve (CSN) and superior laryngeal nerve (SLN) afferents. In 21 neurons, electrical stimulation of the CSN and SLN each evoked an excitatory postsynaptic potential (EPSP; mean onset latency +/- SE of CSN-evoked input = 7.2 +/- 0.8 ms, range 2.1-14.1 ms; of SLN-evoked input = 10.3 +/- 2.1 ms, range 2.8-46.8 ms). In 22 neurons, electrical stimulation of either the CSN or SLN evoked an EPSP/ inhibitory postsynaptic potential (IPSP) sequence (CSN-evoked input = 6.7 +/- 0.6 ms, range 2.1-12.2 ms; SLN-evoked input = 8.4 +/- 0.8 ms, range 3.0-19.4 ms). Spatial interactions (facilitation, summation, occlusion) and time-dependent inhibitory interactions were observed between the convergent inputs. Natural stimulation of specific receptors indicated that 14 cells received a convergent excitatory input from carotid sinus baroreceptors and laryngeal mechanoreceptors, 10 received a convergent excitatory input from carotid body chemoreceptors and laryngeal mechanoreceptors, and 5 received a convergent excitatory input from baroreceptors, chemoreceptors, and laryngeal mechanoreceptors. The interactions and various patterns of convergence suggest a significant integration of convergent inputs from disparate afferent sources by these neurons.

Afferent Pathways↗
PubMed2002

Neural tube closure requires Dishevelled-dependent convergent extension of the midline.

In Xenopus, Dishevelled (Xdsh) signaling is required for both neural tube closure and neural convergent extension, but the connection between these two morphogenetic processes remains unclear. Indeed normal neurulation requires several different cell polarity decisions, any of which may require Xdsh signaling. In this paper we address two issues: (1) which aspects of normal neurulation require Xdsh function; and (2) what role convergent extension plays in the closure of the neural tube. We show that Xdsh signaling is not required for neural fold elevation, medial movement or fusion. Disruption of Xdsh signaling therefore provides a specific tool for uncoupling convergent extension from other processes of neurulation. Using disruption of Xdsh signaling, we demonstrate that convergent extension is crucial to tube closure. Targeted injection revealed that Xdsh function was required specifically in the midline for normal neural tube closure. We suggest that the inherent movement of the neural folds can accomplish only a finite amount of medial progress and that convergent extension of the midline is necessary to reduce the distance between the nascent neural folds, allowing them to meet and fuse. Similar results with Xenopus strabismus implicate the planar cell polarity (PCP) signaling cascade in neural convergent extension and tube closure. Together, these data demonstrate that PCP-mediated convergent extension movements are crucial to proper vertebrate neurulation.

Adaptor Proteins, Signal Transducing↗
PubMed1998

Using coupling with the Gibbs sampler to assess convergence in animal models.

The coupling method was investigated as a method to assess the convergence of the Gibbs sampler when drawing marginal inferences with an animal model. This method is based on the output of two Markov chains with different starting values but the same conditional deviates. The coupling method shows that the Gibbs sampler has an exponential convergence when the variance components are assumed to be known. All the variables in the model have the same rate of convergence, and it is closely related with the largest eigenvalue of a matrix derived from the coefficient matrix of the mixed model equations. Models including the variance components as unknowns showed a rate of convergence equal for all the variables, and this rate of convergence was approximately exponential. The coupling method provides an estimation of the convergence at the current iteration, and it does not require a post-Gibbs analysis as do the single chain-based methods. The coupling method is less computationally demanding than multiple chain methods, because only two chains are required to assess convergence.

Analysis of Variance↗
PubMed1992

Thermal convergence fails to predict heat tolerance limits.

Work under very hot, humid conditions may raise skin temperature (Tsk) to meet rectal temperature (Tre). This convergence has been said to produce imminent collapse and therefore to predict human tolerance limits for work in heat, especially for those wearing impermeable protective clothing. The purpose of this study was to examine the ability of subjects to continue work beyond the time when they experienced convergence. A total of 15 healthy adult volunteers participated in two protocols: A) Each of nine subjects performed four experiments wearing impermeable clothing and walking on an inclined treadmill at a metabolic rate averaging 450 W with dry bulb temperature (Tdb) 29 or 38 degrees C; B) Each of nine subjects performed eight experiments wearing heavy, semipermeable clothing under a range of conditions involving Tdb = 22-40 degrees C, relative humidity 15-88%, and work load = 200-500 W. Convergence occurred in 42 cases; in the majority (60%) the subject continued walking until Tre rose to the criterion temperature 39 degrees C, working for 10-45 min after convergence. No subject approached collapse, nor did convergence have any special effect on the rate of rise of temperature or heart rate. Thus, while convergence marks severe, time-limited heat stress, it does not accurately predict tolerance limits in highly motivated subjects. Termination of work at convergence would unnecessarily truncate heat stress exposures, depriving investigators of valuable data and industrial users of a large portion of their safe work time.

Adult↗
PubMed1992

Ocular convergence-related neuronal responses in the lateral suprasylvian area of alert cats.

Neuronal spike discharges were recorded from the lateral suprasylvian (LS) area while ocular convergence was elicited in five alert cats. Ocular convergence was elicited by presenting a visual target moving in depth. Cats were rewarded for convergence eye movement. In 9 out of 426 cells sampled in the caudal postero-medial LS area, the number of spikes was positively correlated with the peak eye velocities during ocular convergence. Significant correlation was found mostly within 400 ms preceding the moment at which the maximum velocity of ocular convergence was obtained. The result favors the hypothesis that the LS area plays an important role in the integrative control of ocular convergence.

Animals↗
PubMed2004

Convergence reduces ocular counterroll (OCR) during static roll-tilt.

When humans are roll-tilted around the naso-occipital axis, both eyes roll or tort in the opposite direction to roll-tilt, a phenomenon known as ocular counterroll (OCR). While the magnitude of OCR is primarily determined by vestibular, somatosensory, and proprioceptive input, direction of gaze also plays a major role. The aim of this study was to measure the interaction between some of these factors in the control of OCR. Videooculography was used to measure 3D eye position during maintained whole body (en bloc) static roll-tilt in darkness, while subjects fixated first on a distant (at 130 cm) and then a near (at 30 cm) head-fixed target aligned with the subject's midline. We found that while converging on the near target, human subjects displayed a significant reduction in OCR for both directions of roll-tilt--i.e. the interaction between OCR and vergence was not simple addition or subtraction of torsion induced by vergence with torsion induced by roll-tilt. To remove the possibility that the OCR reduction may be associated with the changed horizontal position of the eye in the orbit during symmetric convergence, we ran an experiment using asymmetric convergence in which the distant and near targets were aligned directly in front of one eye. We found the magnitude of OCR in this asymmetric convergence case was also reduced for near viewing by about the same amount as in the symmetric vergence condition, confirming that the convergence command rather than horizontal position of the eye underlies the OCR reduction, since there was no horizontal movement of the aligned eye in the orbit between fixation on the distant and near targets. Increasing vergence from 130 to 30 cm reduced OCR gain by around 35% on average. That reduction was equal in both eyes and occurred in both the symmetric and asymmetric convergence conditions. These results demonstrate the important role vergence plays in determining ocular counterroll during roll-tilt and may support the contention that vergence acts to reduce the conflict facing a stereopsis-generating mechanism.

Adult↗
PubMed2005

Target spatial frequency determines the response to conflicting defocus- and convergence-driven accommodative stimuli.

Asthenopia, or visual fatigue, is a frequent complaint from observers of stereoscopic three-dimensional displays. It has been proposed that asthenopia is a consequence of anomalous oculomotor responses generated by conflict between accommodative and convergence stimuli. The hypothesis was examined by measuring accommodation and convergence continuously with a Shin-Nippon SRW5000 infrared autorefractor and a limbus tracking device. Subjects viewed a high contrast Maltese Cross target at three levels of Gaussian filter target blur under conditions of relatively low- and high-conflict between accommodation and convergence stimuli, the latter inducing the sensation of stereopsis. Under the low-conflict conditions accommodation was stable, but convergence-driven accommodation was dominant when the target was extremely blurred. Under the high-conflict conditions the role of convergence-driven accommodation increased systematically with the degree of target blur. It is proposed that defocus-driven accommodation becomes weak when the target comprises low spatial frequency components. Large accommodative overshoots to step stimuli that are not blurred or only mildly blurred were consistently observed and are attributed to the initial accommodative response being convergence-driven. Whereas the possibility that high-conflict conditions are a cause of asthenopia has been previously reported, this is the first evidence that they specifically affect accommodative responses while viewing stereoscopic displays.

Accommodation, Ocular↗
PubMed2001

Plasticity of convergence-dependent variations of cyclovergence with vertical gaze.

Binocular alignment of foveal images is facilitated by cross-couplings of vergence eye movements with distance and direction of gaze. These couplings reduce horizontal, vertical and cyclodisparities at the fovea without using feedback from retinal image disparity. Horizontal vergence is coupled with accommodation. Vertical vergence that aligns tertiary targets in asymmetric convergence is thought to be coupled with convergence and horizontal gaze. Cyclovergence aligns the horizontal retinal meridians during gaze elevation in symmetrical convergence and is coupled with convergence and vertical gaze. The latter vergence-dependent changes of cyclovergence have been described in terms of the orientation of Listing's plane and have been referred to as the binocular extension of Listing's law. Can these couplings be modified? Plasticity has been demonstrated previously for two of the three dimensions of vergence (horizontal and vertical). The current study demonstrates that convergence-dependent changes of the orientation of Listing's plane can be adapted to either exaggerate or to reduce the cyclovergence that normally facilitates alignment of the horizontal meridians of the retinas with one another during gaze elevation in symmetrical convergence. The adaptability of cyclovergence demonstrates a neural mechanism that, in conjunction with the passive forces determined by biomechanical properties of the orbit, could play an active role in implementing Listing's extended law and provide a means for calibrating binocular eye alignment in three dimensions.

Accommodation, Ocular↗
PubMed2003

Suppression of the human vestibulo-ocular reflex by visual fixation or forced convergence in the dark, with a model interpretation.

Six normal humans experienced yaw axis steps of velocity at 120 degrees /s in the dark. During the post-rotary period, subjects either had a null-task (do nothing); an ocular motor task (forced convergence: crossing the eyes); or a visual task (fixating a head-stationary target against a background of 10 degrees light/dark bars). Tasks started at 3 s post-rotation, and lasted either 2, 5, 10, or 15 s. Ocular motor and visual tasks were tested on different days. Five repetitions of each task duration were recorded for each subject. A mean VOR gain of 0.52 was observed, which did not vary with experimental conditions. Both convergence and fixation markedly suppressed nystagmus; in fact, the VORs obtained with the two different tasks are superficially similar in appearance. However, mean null-task time-constants were 9.4 s for convergence days, but 8.4 s for fixation days, and there was a small but significant reduction in overall null-task VOR amplitude on fixation days. Also, post-convergence slow-phase velocities were slightly enhanced, while post-fixation velocities were significantly reduced. The time-constant of velocity storage was found to be 10.1 s for convergence responses and 8.2 s for fixation responses. These differences can be understood in terms of modifications in central velocity storage during visual fixation which do not occur with convergence. The mean fixation data were analyzed in the context of a VOR model well-established for monkey data. With appropriate choice of parameters, this model accurately reproduces most features of the human data. An estimate for the human cupula time-constant of 3.3 s is obtained. Compared with the monkey, fixation suppression is greater and post-fixation velocity reduction less. Retinal slip alone accounts well for this; "velocity dumping" by an integrator shunt must be slight if present at all. The model correctly represents the post-fixation VOR for all durations of fixation.

Convergence, Ocular↗
PubMed2004

Gain and movement time of convergence-accommodation in preschool children.

BACKGROUND: Convergence-accommodation is the synkinetic change in accommodation driven by vergence. A few studies have investigated the static and dynamic properties of this cross-link in adults but little is known about convergence-accommodation in children. The purpose of this study was to develop a technique for measuring convergence-accommodation and to study its dynamics (gain and movement time) in a sample of pre-school children. METHOD: Convergence-accommodation measures were examined on thiry-seven normal pre-school children (mean age = 4.0 +/- 1.31 yrs). Stimulus CA/C (sCA/C) ratios and movement time measures of convergence-accommodation were assessed using a photorefractor while subjects viewed a DOG target. Repeated measures were obtained on eight normal adults (mean age = 23 +/- 0.2 yrs). RESULTS: The mean sCA/C ratios and movement times were not significantly different between adults and children (0.10 D/Delta [0.61 D/M.A.], 743 +/- 70 ms and 0.11 D/Delta [0.50 D/M.A.], 787 +/- 216 ms). Repeated measures on adults showed a non-significant mean difference of 0.001 D/Delta. CONCLUSION: The results suggest that the possible differences in crystalline lens (plant) characteristics between children and adults do not appear to influence convergence-accommodation gain or duration.

Accommodation, Ocular↗
PubMed2005

A randomized clinical trial of vision therapy/orthoptics versus pencil pushups for the treatment of convergence insufficiency in young adults.

PURPOSE: : The purpose of this article is to compare vision therapy/orthoptics, pencil pushups, and placebo vision therapy/orthoptics as treatments for symptomatic convergence insufficiency in adults 19 to 30 years of age. METHODS: : In a randomized, multicenter clinical trial, 46 adults 19 to 30 years of age with symptomatic convergence insufficiency were randomly assigned to receive 12 weeks of office-based vision therapy/orthoptics, office-based placebo vision therapy/orthoptics, or home-based pencil pushups. The primary outcome measure was the symptom score on the Convergence Insufficiency Symptom Survey. Secondary outcome measures were the near point of convergence and positive fusional vergence at near. RESULTS: : Only patients in the vision therapy/orthoptics group demonstrated statistically and clinically significant changes in the near point of convergence (12.8 cm to 5.3 cm, p = 0.002) and positive fusional vergence at near (11.3Delta to 29.7Delta, p = 0.001). Patients in all three treatment arms demonstrated statistically significant improvement in symptoms with 42% in office-based vision therapy/orthoptics, 31% in office-based placebo vision therapy/orthoptics, and 20% in home-based pencil pushups achieving a score <21 (our predetermined criteria for elimination of symptoms) at the 12-week visit. DISCUSSION: : In this study, vision therapy/orthoptics was the only treatment that produced clinically significant improvements in the near point of convergence and positive fusional vergence. However, over half of the patients in this group (58%) were still symptomatic at the end of treatment, although their symptoms were significantly reduced. All three groups demonstrated statistically significant changes in symptoms with 42% in office-based vision therapy/orthoptics, 31% in office-based placebo vision therapy/orthoptics, and 20% in home-based pencil push-ups meeting our criteria for elimination of symptoms.

Adult↗