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PubMed1996

Convergence demands by spectacle magnifiers.

A general equation, c delta = k1b + k2sF, for finding the binocular convergence demands by spectacle magnifiers to view images at any distance is presented. Factor k1 in the equation yields the accommodative demand to view the image; factor k2 determines the actual reduction in convergence demand provided by the vendors' incorporation of base-in prism. When magnifiers from virtual images at finite distances, such as at the least distance of distinct vision or 25 cm, the interpupiliary distance (b), the separation between the lenses and the eyes (d), and the distance between the optical centers of the lenses (s) are basic quantities, according to this equation. The fundamental datum that the vendors should specify is the distance (s) between the optical centers of the lenses, rather than base-in prism. The specification of base-in prism is unrellable when images are formed at finite distances and the frame PD is not equal to the distance IPD. When the image is formed at infinity, that is when the angular magnification M = F/4, the convergence demand by spectacle magnifiers only depends on the separation between the optical centers of the lenses and the lens power, that is, c delta = sF. It is independent of the interpupillary distance (b) and the separation between the lenses and the eyes (d). We also present an equation, to find the disparity of the accommodative/convergence relation caused by spectacle magnifiers. Knowing the demands on convergence and accommodation, the practitioner can probably evaluate the potential for successful adaptation to spectacle magnifiers from routine measurements of positive and negative relative convergence and accommodation.

Accommodation, Ocular↗
PubMed2001

Three-dimensional binocular kinematics of torsional vestibular nystagmus during convergence on head-fixed targets in humans.

When a human subject is oscillated about the nasooccipital axis and fixes upon targets along the horizontal head-fixed meridian, angular eye velocity includes a vertical component that increases with the horizontal eccentricity of the line-of-sight. This vertical eye movement component is necessary to prevent retinal slip. We asked whether fixation on a near head-fixed target during the same torsional vestibular stimulation would lead to differences of vertical eye movements between the right and the left eye, as the directions of the two lines-of-sight are not parallel during convergence. Healthy human subjects (n = 6) were oscillated (0.3 Hz, +/-30 degrees) about the nasooccipital axis on a three-dimensional motor-driven turntable. Binocular movements were recorded using the dual search coil technique. A head-fixed laser dot was presented 1.4 m (far head-fixed target) or 0.25 m (near head-fixed target) in front of the right eye. We found highly significant (P < 0.01) correlations (R binocular = 0.8, monocular = 0.59) between the convergence angle and the difference of the vertical eye velocity between the two eyes. The slope of the fitted linear regression between the two parameters (s = 0.45) was close to the theoretical slope necessary to prevent vertical retinal slippage (predicted s = 0.5). Covering the left eye did not significantly change the slope (s = 0.52). In addition, there was a marked gain reduction (approximately 35%) of the torsional vestibuloocular reflex (VOR) between viewing the far and the near targets, confirming earlier results by others. There was no difference in torsional gain reduction between the two eyes. Lenses of +3 dpt positioned in front of both eyes to decrease the amount of accommodation did not further change the gain of the torsional VOR. In conclusion, ocular convergence on a near head-fixed target during torsional vestibular stimulation leads to deviations in vertical angular velocity between the two eyes necessary to prevent vertical double vision. The vertical deviation velocity is mainly linked to the amount of convergence, since it also occurs during monocular viewing of the near head-fixed target. This suggests that convergence during vestibular stimulation automatically leads to an alignment of binocular rotation axes with the visual axes independent of retinal slip.

Adult↗
PubMed2002

Evolutionary convergence in nervous systems: insights from comparative phylogenetic studies.

Over the past 20 years, cladistic analyses have revolutionized our understanding of brain evolution by demonstrating that many structures, some of which had previously been assumed to be homologous, have evolved many times independently. These and other studies demonstrate that evolutionary convergence in brain anatomy and function is widespread. Although there are relatively few neuroethological studies in which brain and behavior have be studied within an evolutionary framework, three relatively well studied cases are reviewed here: electric communication among gymnotiform and mormyriform fishes, prey capture among frogs, and sound localization among owls. These three examples reveal similar patterns of brain evolution. First, it is clear that novel abilities have evolved many times independently in taxa whose common ancestors lack these abilities. Second, it is apparent that small changes in neural pathways can lead to dramatic changes in an organism's abilities. Brain evolution at this small scale is quite common. The behavioral importance of small scale changes on one hand, and the pervasiveness of convergent evolution on the other, have several implications for understanding brain evolution. First, similar abilities may be conferred by convergent rather than homologous circuits, even among closely related species. Furthermore, closely related species may use the same information in different ways, or they may use different means to obtain the same information. One reason that convergence is so common in the biological world may be that the evolutionary appearance of novel functions is associated with constraints, for example in the algorithms used for a given neural computation. Convergence in functional organization may thus reveal basic design features of neural circuits in species that possess unique evolutionary histories but use similar algorithms to solve basic computational problems.

Animals↗
PubMedDate not supplied

Decline of vertical gaze and convergence with aging.

BACKGROUND: Disturbance of vertical eye movement and ocular convergence is often observed in elderly people, but little is known about its frequency. OBJECTIVE: The purpose of this study was to investigate age-associated changes in vertical eye movement and convergence in healthy elderly people, using a digital video camera system. METHODS: We analyzed vertical eye movements and convergence in 113 neurologically normal elderly subjects (mean age 70 years) in comparison with 20 healthy young controls (mean age 32 years). The range of vertical eye movement was analyzed quantitatively and convergence was analyzed qualitatively. RESULTS: In the elderly subjects, the angle of vertical gaze decreased with advancing age and it was significantly smaller than that of the younger subjects. The mean angle of upward gaze was significantly smaller than that of downward gaze for both young and elderly subjects. Upward gaze impairment became apparent in subjects in their 70s, and downward gaze impairment in subjects in their 60s. Disturbance in convergence also increased with advancing age, and was found in 40.7% of the elderly subjects. CONCLUSION: These findings indicate that the mechanisms of age-related change are different for upward and downward vertical gaze. Digital video camera monitoring was useful for assessing and monitoring eye movements.

Adult↗
PubMed2003

Effects of TMS over the right prefrontal cortex on latency of saccades and convergence.

PURPOSE: The prefrontal cortex (PFC) is known to inhibit unwanted saccades through its connections to the superior colliculus (SC). Indeed, transcranial magnetic stimulation (TMS) of the PFC decreases saccade latency by increasing the rate of express saccades. This study examined whether a similar phenomenon exists for vergence. METHODS: In a gap paradigm, six healthy subjects were asked to look at LEDs placed in a horizontal plane and to make lateral saccades, pure convergence along the median plane, and combined eye movements. Eye movements were recorded binocularly. TMS was applied over the right (r)PFC synchronously with the onset of the target. In a control condition, TMS was applied over the motor cortex (MC). RESULTS: TMS over the MC had no effect on the latency of any type of eye movements. In contrast, TMS over the rPFC (1) decreased significantly (P = 0.00367) the latency of contralateral pure saccades, (2) had no effect on the latency of pure convergence, (3) and caused a mild decrease in the latency of both the saccadic and the convergence components of combined eye movements, and the effect was bilateral. Decreased latencies were mainly due to an increase of the rate of express movements. CONCLUSIONS: The inhibition exerted by PFC over SC and preventing express movements from occurring is presumably a saccade-specific mechanism. When the saccade is combined with convergence, the express triggering can be transferred to a certain extent to the convergence.

Adult↗
PubMedDate not supplied

Suppression of vestibular nystagmus by forced convergence in normal human subjects.

BACKGROUND: It has been suggested that the principal mechanism of nystagmus suppression in the nystagmus blockage syndrome is either adduction of the eye or convergence. We examined this issue using the nystagmus of the vestibulo-ocular reflex (VOR) as a model. METHODS: A motorized, computer-controlled rotary chair was used to produce VOR in darkness, using either sinusoidal or velocity step stimulation. Left eye position was monitored and horizontal slow-phase eye velocity was calculated. Subjects were cued to converge or perform other gaze tasks. RESULTS: Convergence suppressed nystagmus. With sinusoidal stimulation, nystagmus was nearly extinguished in extreme lateroversion, probably due to mechanical tethering of the eye. However, VOR gain suppression of 47% during convergence was observed even when the monitored eye was close to primary position. With velocity step stimulation, nystagmus was nearly extinguished at moderate angles of adduction. CONCLUSIONS: Convergence is sufficient to suppress nystagmus, without vision and without regard to whether the eye is adducted.

Adult↗
PubMed1990

Effects of stimulus distance on measurements of dark convergence.

Dark convergence is often measured psychophysically by aligning two dichoptic stimuli, which are briefly flashed in a dark surround. In the present study, the effect of different stimulus parameters on dark convergence measurements was investigated. Chief among these was the effect of viewing distance (50, 100 and 500 cm) of the stimulus. Measured dark convergence was biased towards the actual viewing distance of the stimuli; this effect was stronger with bar stimuli than with point-and-line stimuli. Measured dark convergence depended on the viewing distance to a lesser extent if bar stimuli were observed through pin-holes (open accommodative feedback loop). The subject's knowledge of the viewing distance had a small but significant effect; the size of the nonius bars had none. To reduce artefacts due to the stimulus distance on psychophysical measurement of dark convergence, point-and-line stimuli should be used at a viewing distance of about 1 m. In this case, the effect of stimulus distance can be about 0.25 meter angle (ma), averaged over subjects, but it may be as large as 0.5 ms in some individuals.

Adolescent↗
PubMed1989

Methyldopa-related convergence insufficiency.

A case report of convergence insufficiency related to the use of methyldopa is presented. Methyldopa is a systemic antihypertensive agent. It has sympatholytic properties which lower blood pressure by decreasing peripheral vascular resistance. The proposed effects of methyldopa on binocular vision are reduced accommodative convergence and increased demand on fusional convergence. These effects would create a susceptibility to convergence insufficiency. To my knowledge, an association between methyldopa and convergence insufficiency has not been previously reported.

Accommodation, Ocular↗
PubMed1992

The cellular basis of the convergence and extension of the Xenopus neural plate.

There is great interest in the patterning and morphogenesis of the vertebrate nervous system, but the morphogenetic movements involved in early neural development and their underlying cellular mechanisms are poorly understood. This paper describes the cellular basis of the early neural morphogenesis of Xenopus laevis. The results have important implications for neural induction. Mapping the fate map of the midneurula (Eagleson and Harris: J. Neurobiol. 21:427-440, 1990) back to the early gastrula with time-lapse video recording demonstrates that the prospective hindbrain and spinal cord are initially very wide and very short, and thus at the beginning of gastrulation all their precursor cells lie within a few cell diameters of the inducing mesoderm. In the midgastrula, the prospective hindbrain and spinal cord undergo very strong convergence and extension movements in two phases: In the first phase they primarily undergo thinning in the radial direction and lengthening (extension) in the animal-vegetal direction, and the second phase is characterized primarily by mediolateral narrowing (convergence) and anterior-posterior lengthening (extension). These movements also occur in sandwich explants of the gastrula, thus demonstrating the local autonomy of the forces producing them. Tracing cell movements with fluorescein dextran-labeled cells in embryos or explants shows that the initial thinning and extension occurs by radial intercalation of deep cells to form fewer layers of greater area, all of which is expressed as increased length. The subsequent convergence and extension occurs by mediolateral intercalation of deep cells to form a longer, narrower array. These results establish that a similar if not identical sequence of radial and mediolateral cell intercalations underlie convergence and extension of the neural and the mesoderm tissues (Wilson and Keller: Development, 112:289-300, 1991). Moreover, these results establish that radial and mediolateral intercalation are the principal neural cell behaviors induced by the planar signals emanating from the dorsal involuting marginal zone (the Spemann organizer) in the early gastrula (Keller et al: Develop. Dynamics, 193: 218-234, 1992). Radial and mediolateral intercalation are induced among the 5 to 7 rows of cells comprising the prospective hindbrain and spinal cord, thus producing the massive convergence and extension movements that narrow and elongate these regions of the nervous system in the late gastrula. A more general significance of these results is that neural induction is best analyzed and understood in terms of the dynamics of the morphogenetic processes involved.

Animals↗
PubMed2003

Sorting and convergence of primary olfactory axons are independent of the olfactory bulb.

Primary olfactory axons expressing the same odorant receptor gene sort out and converge to fixed sites in the olfactory bulb. We examined the guidance of axons expressing the P2 odorant receptor when they were challenged with different cellular environments in vivo. In the mutant extratoes mouse, the olfactory bulb is lacking and is replaced by a fibrocellular mass. In these animals, primary olfactory axons form glomerular-like loci despite the absence of normal postsynaptic targets. P2 axons are able to sort out from other axons in this fibrocellular mass and converge to form loci of like axons. The sites of these loci along mediolateral and ventrodorsal axes were highly variable. Similar convergence was observed for larger subpopulations of axons expressing the same cell surface carbohydrates. The sorting out and convergence of like axons also occurred during regeneration following bulbectomy. Olfactory axon behaviour in these models demonstrates that sorting and convergence of axons are independent of the target, which instead provides distinct topographic cues for guidance.

Animals↗
PubMed2005

Initiation of convergence and extension movements of lateral mesoderm during zebrafish gastrulation.

Embryonic morphogenesis is accomplished by cellular movements, rearrangements, and cell fate inductions. Vertebrate gastrulation entails morphogenetic processes that generate three germ layers, endoderm, mesoderm, and ectoderm, shaped into head, trunk, and tail. To understand how cell migration mechanistically contributes to tissue shaping during gastrulation, we examined migration of lateral mesoderm in the zebrafish. Our results illustrate that cell behaviors, different from mediolaterally oriented cell intercalation, also promote convergence and extension (C&E). During early gastrulation, upon internalization, individually migrating mesendodermal cells contribute to the elongation of the mesoderm by moving animally, without dorsal movement. Convergence toward dorsal starts later, by 70% epiboly (7.7 hpf). Depending on location along the Animal-Vegetal axis, an animal or vegetal bias is added to the dorsalward movement, so that paths fan out and the lateral mesoderm both converges and extends. Onset of convergence is independent of noncanonical Wnt signaling but is delayed when Stat3 signaling is compromised. To understand which aspects of motility are controlled by guidance cues, we measured turning behavior of lateral mesodermal cells. We show that cells exhibit directional preference, directionally-regulated speed, and turn toward dorsal when off-course. We estimate that ectoderm could supply from a fraction to all the dorsalward displacement seen in mesoderm cells. Using mathematical modeling, we demonstrate that directional preference is sufficient to account for mesoderm convergence and extension, and that, at minimum, two sources of guidance cues could orient cell paths realistically if located in the dorsal midline.

Animals↗
PubMed1992

On the convergent cell movements of gastrulation in Fundulus.

Mainly because of its transparency, the Fundulus gastrula constitutes ideal material for direct study of morphogenetic cell movements in vivo. Marking studies show that deep cells of the germ ring converge toward and enter the embryonic shield, where they undergo extension. Those close to the shield move faster. Analysis of videotapes reveals that all deep cells of the dorsal germ ring move toward the shield. But none moves in a direct line. All meander considerably. Germ ring cells nearer the shield move toward it at a higher net rate than those farther away because they meander less. This suggests that exogenous factors promote their directionality. Cells in the prospective yolk sac adjacent to the germ ring also show net convergence, but they meander more. Directional forces are apparently stronger in the germ ring. Converging deep cells move both by filolamellipodia and, less frequently, by blebs. However, there is very little individual cell movement; all cells are almost always in adhesive contact with other cells in moving cell clusters. Clusters vary constantly in size, continually aggregating with other cells and other clusters and splitting. Filolamellipodial cells show contact inhibition of cell movement. Nevertheless, they move and do so directionally, presumably in part because, as members of cell clusters, much of their movement is passive. They also show intercalation or invasive activity, but, consistent with their contact-inhibiting properties, only when neighboring cells separate and provide free space. Cells moving by blebbing locomotion are non-contact inhibiting and intercalate readily. Cell division continues during convergence. Although this temporarily arrests their movement, the daughter cells soon join in the mass convergent movement.

Animals↗
PubMed1993

Entropy and convergence in dynamics and demography.

Demographic dynamics is formally equivalent to the dynamics of a Markov chain, as is true of some nonlinear dynamical systems. Convergence to demographic equilibrium can be studied in terms of convergence in the Markov chain. Tuljapurkar (1982) showed that population entropy (Kolmogorov-Sinai entropy) provides information on the rate of this convergence. This paper begins by considering finite state Markov chains, providing elementary proofs of the relationship between convergence rate and entropy, and discusses in detail the uses and limitations of entropy as a convergence measure; these results also apply to Markovian dynamical systems. Next, new qualitative and quantitative arguments are used to discuss the demographic meaning of entropy. An exact relationship is established giving population entropy in terms of the eigenvalues of the Leslie matrix characteristic equation. Finally, the significance of imprimitive and periodic limits is discussed in relation to population entropy.

Demography↗
PubMed1993

Convergence of somatic and visual afferent impulses in the Wulst of pigeon.

In 79 pigeons, unit discharges of Wulst neurons responding to bilateral stimulation of N. radialis and N. sciaticus and to bilateral light flashes were recorded. Of 178 such neurons, 55 showed multisensory convergence of various types: 40 of these were somatic convergent neurons, characterized as 3 types; three were binocular convergent cells; and twelve were bimodal (visual-somatosensory) convergent neurons. Of these neurons, the convergent action was either inhibitory or excitatory. These results suggest that the Wulst may function as an important centre for integrating visual and somatic sensory information.

Afferent Pathways↗
PubMed1982

Convergence of sensory inputs upon projection neurons of somatosensory cortex.

Cortico-cortical neurons and pyramidal tract neurons of the cat were tested for convergent inputs from forelimb afferents. Neurons were recorded in cortical areas 1, 2, and 3a. Consideration was given to both suprathreshold and subthreshold inputs evoked by electrical stimulation of forelimb nerves. Individual cortico-cortical neurons and also pyramidal tract neurons were characterized by convergence of multiple somatosensory inputs from different regions of skin, from several muscle groups, and between group I deep afferents and low threshold cutaneous afferents. Certain patterns of afferent input varied with cytoarchitectonic area. There was, however, no difference between area 3a and areas 1-2 in the incidence of cross-modality convergence in the form of input from cutaneous and also deep nerves. Many of the inputs were subthreshold. Arguments are presented that these inputs, though subthreshold, must be considered for a role in cortical information processing. The convergent nature of the sensory inputs is discussed in relation to the proposed specificities of cortical columns. The patterns of afferent inputs reaching cortico-cortical neurons seem to be appropriate for them to have a role in the formation of sensory fields of motor cortex neurons. PT neurons of somatosensory cortex have possible roles as modifiers of ascending sensory systems, however, the convergent input which these PT neurons receive argues against a simple relationship between the modality of peripheral stimuli influencing them and the modality of the ascending tract neurons under their descending control.

Afferent Pathways↗
PubMed1989

Convergence and interaction of neck and macular vestibular inputs on locus coeruleus and subcoeruleus neurons.

Extracellular recordings were obtained in precollicular decerebrate cats from 90 neurons located in the noradrenergic area of the dorsal pontine tegmentum, namely in the dorsal (LCd, n = 24) and the ventral part (LC alpha, n = 40) of the locus coeruleus (LC) as well as in the locus subcoeruleus (SC, n = 26). Among these units of the LC complex, 13 were coerulospinal (CS) neurons antidromically identified following stimulation of the spinal cord at T12-L1. Some of these neurons showed the main physiological characteristics of the norepinephrine (NE)-containing LC neurons, i.e., a slow and regular resting discharge and a typical biphasic response to fore- and hindpaw compression consisting of a short burst of excitation followed by a period of quiescence, due, in part at least, to recurrent and/or lateral inhibition. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotation about the longitudinal axis at 0.15 Hz, +/- 10 degrees peak amplitude. Among the 90 LC-complex neurons, 60 (66.7%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 67 (74.4%) responded to neck rotation. Convergence of macular and neck inputs was found in 52/90 (57.8%) LC-complex neurons; in these units, the gain and the sensitivity of the first harmonic of the response corresponded on the average to 0.34 +/- 0.45, SD, impulses.s-1.deg-1 and 3.55 +/- 2.82, SD, %/deg for the neck responses and to 0.23 +/- 0.29, SD, impulses.s-1.deg-1 and 3.13 +/- 3.04, SD, %/deg for the macular responses. In addition to these convergent units, 8/90 (8.9%) and 15/90 (16.7%) LC-complex units responded to selective stimulation either of macular or of neck receptors only. These units displayed a significantly lower response gain and sensitivity to animal tilt and neck rotation with respect to those obtained from convergent units. Most of the convergent LC-complex units were maximally excited by the direction of stimulus orientation, the first harmonic of responses showing an average phase lead of about +31.0 degrees with respect to neck position and +17.6 degrees with respect to animal position. Two populations of convergent neurons were observed. The first group of units (43/52, i.e., 82.7%) showed reciprocal ("out of phase") responses to the two inputs; moreover, most of these units were excited during side-down neck rotation, but inhibited during side-down animal tilt.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

The rate of convergence of a generalized stable population.

In an age-structured population that grows exponentially, each age group pi(t) at period t is asymptotically equivalent to x0t for some positive number x0. In this paper we show that the speed at which the ith age group reaches its exponential state of equilibrium can be measured by the rate at which the ratio vi(t) = pi(t)/pi(t-1) converges to x0. The age specific rate of convergence is determined by considering a quantity r satisfying [vi(t)-x0] less than or equal to rt when t is large; Ri = Inf r (over all initial populations, r satisfying the above inequality) is the R-factor used in numerical analysis to measure the rate at which the sequence vi(t) converges to x0; Si = -1n Ri is then defined as the rate of convergence to stability of the ith age group. The case of constant net maternity rates is studied in detail; in this context S0 is compared to the population entropy H, which was proposed by Tuljapurkar (1982) as a measure of the rate of convergence to stability.

Age Factors↗
PubMed2000

Convergence patterns of the posterior semicircular canal and utricular inputs in single vestibular neurons in cats.

The convergence of the posterior semicircular canal (PC) and utricular (UT) inputs in single vestibular nuclei neurons was studied intracellularly in decerebrate cats. A total of 160 vestibular neurons were orthodromically activated by selective stimulation of the PC and the UT nerve and classified according to whether or not they were antidromically activated from the spinal cord and oculomotor nuclei into vestibulospinal (VS), vestibulooculospinal (VOS), vestibuloocular (VO), and unidentified vestibular neurons. Fifty-three (33%) of 160 vestibular neurons received convergent inputs from both the PC and UT nerves. Seventy-nine (49%) vestibular neurons responded to PC inputs alone, and 28 (18%) neurons received inputs only from the UT nerve. Of 53 convergent neurons, 8 (15%) were monosynaptically excited from both nerves. Thirty-five (66%) received monosynaptic excitatory inputs from the PC nerve and polysynaptic excitatory or inhibitory inputs from the UT nerve, or vice versa. Approximately one-third of VS and VOS neurons received convergent inputs. A majority of the VS neurons descended to the spinal cord through the lateral vestibulospinal tract, while almost all the VOS neurons descended to the spinal cord through the medial vestibulospinal tract. The convergent neurons were found in all vestibular nuclei but more in the lateral nucleus and descending nucleus. The VS neurons were more numerous than VO neurons or VOS neurons.

Acceleration↗