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PubMed2001

Variations in accommodation and convergence responses in a minimally controlled photorefractive setting.

PURPOSE: A remote haploscopic photorefractor, designed for assessment of accommodation and convergence in infants and clinical groups, was used to determine heterophoria accommodative convergence/accommodation (AC/A) ratios in normal naïve adults. These were compared with conventional clinical measures. METHODS: Twenty-one naïve subjects were used to compare occluded and unoccluded prism cover test responses with the remote haploscopic photorefractor using a text and picture target. RESULTS: Although luminance was generally low for both targets, binocular vergences were appropriate for target demand in both studies. Binocular accommodation showed greater lag for the highest target accommodative demand and the less demanding target. Occlusion not only reduced vergence response, but also frequently caused a marked reduction in accommodation, especially to the picture target. Normal mean AC/A values were found, but with wide variations between individual subjects. CONCLUSIONS: Although mean accommodation, vergence, and AC/A values were comparable with published data, we suggest that in these conditions using naïve subjects, accommodation is frequently inaccurate, especially on occlusion, without concomitant loss of vergence, at least at low light levels. Accommodative convergence may play a less important part in, and other cues contribute more to, the near reflex than has been previously suggested.

Accommodation, Ocular↗

PubMed2005

Accommodation, accommodative convergence, and response AC/A ratios before and at the onset of myopia in children.

PURPOSE: The purpose of this study was to investigate accommodation, accommodative convergence, and AC/A ratios before and at the onset of myopia in children. METHODS: Refractive error, accommodation, and phorias were measured annually over a period of 3 years in 80 6- to 18-year-old children (mean age at first visit = 11.1 years), including 26 who acquired myopia of at least -0.50 D and 54 who remained emmetropic (-0.25 to + 0.75 D). Refraction was measured by noncycloplegic distance retinoscopy. Concomitant measures of accommodation and phorias were taken for letter targets at 4.0 m and 0.33 m using the Canon R-1 open field-of-view autorefractor with an attached motorized Risley prism and Maddox rod. The accommodation and phoria measurements were used to calculate response AC/A ratios. RESULTS: Compared with children who remained emmetropic, those who became myopic had elevated response AC/A ratios at 1 and 2 years before the onset of myopia, in addition to at onset and 1 year later (t's = -2.97 to -4.04, p < 0.01 at all times). The significantly higher AC/A ratios in the children who became myopic are a result of significantly reduced accommodation. Accommodative convergence was significantly greater in myopes only at onset. CONCLUSIONS: These findings suggest that the abnormal oculomotor factors found before the onset of myopia may contribute to myopigenesis by producing hyperopic retinal defocus when a child is engaged in near-viewing tasks.

Accommodation, Ocular↗

PubMed1995

Convergence substitution for paralysed horizontal gaze.

Three patients with paralysed horizontal gaze are presented. Involuntary use of convergence to assist horizontal gaze was noted as a late feature. All patients showed (1) unilateral or bilateral horizontal gaze palsy (two patients had one and a half syndrome, the other had bilateral nuclear sixth nerve palsies), (2) adduction of both eyes on attempted gaze into the paralysed field, (3) miosis which coincided with adduction. Convergence substitution should be considered in the differential diagnosis of gaze induced strabismus.

Accommodation, Ocular↗

PubMed1992

Dissociated unilateral convergence paralysis in a patient with thalamotectal haemorrhage.

A 47 year old male was admitted in a comatose state. CT scan showed a haemorrhage in the right pulvinar thalamus descending into the right part of the lamina quadrigemina. He presented with anisocoria, prompt bilateral pupillary light reaction, and unilateral convergence paralysis contralateral to the lesion in combination with upward gaze palsy. During an observation period of two months, the convergence reaction returned to normal. MRI showed a lacunar lesion ventral to superior right colliculus. Angiography revealed an arteriovenous malformation (right posterior cerebral artery--sinus rectus) as the possible cause of the haemorrhage.

Cerebral Hemorrhage↗

PubMed1986

Pattern of convergence of the receptors of the barnacle's three ocelli onto second-order cells.

The giant barnacle, Balanus nubilus, has three simple eyes, two lateral and one median. We have studied the convergence of the receptors of these ocelli by recording from the second-order cell (I-cell) and from the receptors' terminals. The I-cell's responses to illumination of the median and lateral eyes are similar in shape, dynamic range, and reversal potential, but the response to lateral input has a longer latency, a slower rise time, and a smaller amplitude. These differences primarily reflect the different voltage changes in the terminals of the decrementally conducting median and lateral receptors. Simultaneous recordings from the terminals of median and lateral receptors showed that the responses to light recorded in lateral terminals had a longer latency, a slower rise time, and smaller amplitude than signals in median terminals. The differences in the I-cell's responses to median and lateral input were essentially eliminated by stimulating the median and lateral ocellar nerves with extracellular suction electrodes positioned at equal distances from the receptors' terminals. The similarity of the I-cell's responses to median and lateral input suggests that lateral photoreceptors, like median receptors, contact the I-cell directly. No evidence was found for interaction between median and lateral receptors. Simultaneous fills of median and lateral receptors with cobalt showed minimal overlap between their terminal arbors. No voltage change was detected in the second receptor when the voltage in the first was changed with current pulses or when action potentials were elicited in the presence of tetraethylammonium ions. The absence of a detectable response in the terminals of one eye's receptors when the receptors of the other eye were stimulated with current or light suggests that there is no feedback from the I-cell to the receptors. Simultaneous illumination of the median and lateral eyes produced responses in the I-cell expected from two independent inputs. The first synaptic stage of the visual pathway in the barnacle is thus unusually simple, consisting of a small number of electrically isolated photoreceptors converging upon the same pair of second-order cells with no feedback interaction.

Animals↗

PubMed2002

Convergences in the modular and areal organization of the forebrain of mammals: implications for the reconstruction of forebrain evolution.

Early efforts to reconstruct the course of the evolution of the human brain relied on comparing the brains of a few related mammals with brains at successively higher levels of complexity. This Clark or ladder of levels approach is now seen as having limited usefulness in that species are not easily assigned to levels, and extant mammals are now recognized as mosaics of primitive and derived features. In addition, direction of change does not necessarily proceed from simple to complex, small to large, or diffuse to differentiated. A modern cladistic approach reconstructs the brains of ancestors by identifying brain characters within and across phylogenetic groups (clades), and uses parsimony or likelihood to infer direction of change and distinguish ancestral features from independently evolved convergences. Unfortunately, an idealized cladistic approach is often difficult to realize because characters may be hard to identify and validate, key species may be unavailable for study, and broadly based comparative studies can be costly, poorly funded, and labor intensive. Thus, many investigators pursue a truncated approach that is superficially Clark-like but conceptually cladistic. A truncated approach that relies on the extensive study of a few species may compensate for weaknesses by including niche-matched species that offer the opportunity to estimate the likelihood of similar brain features evolving as convergent adaptations. Because inferences about the brains of the primate ancestor are often made from the brains of tree shrews, we compare the brains of squirrel-like tree shrews with the brains of diurnal squirrels, and suggest that many of the primate-like features of the visual system of tree shrews arose independently of those in primates.

Animals↗

PubMed2002

Evolutionary convergence and shared computational principles in the auditory system.

Precise temporal coding is a hallmark of the auditory system. Selective pressures to improve accuracy or encode more rapid changes have produced a suite of convergent physiological and morphological features that contribute to temporal coding. Comparative studies of temporal coding also point to shared computational strategies, and suggest how selection acts to improve coding. Both the avian cochlear nucleus angularis and the mammalian cochlear nuclei have heterogeneous cell populations, and similar responses to sound. These shared characteristics may represent convergent responses to similar selective pressures to encode features of airborne sound.

Animals↗

PubMed2006

Effects of orthoptic treatment on the CA/C and AC/A ratios in convergence insufficiency.

PURPOSE: To evaluate the effect of orthoptic treatment on the AC/A (A, accommodation; C, convergence) and CA/C ratios in subjects with convergence insufficiency (CI). METHODS: The change in AC/A and CA/C ratios after a 12-week period of home-based orthoptic treatment was examined in 10 subjects (mean age, 25.4+/-4.1 years [SD]). Both the AC/A and CA/C ratios were measured by using gradient response methods. For the AC/A ratio, the gradient phoria method was used, and for the CA/C ratio the prism-induced change in accommodation was measured with a refractometer. RESULTS: No change in the AC/A and CA/C ratios (P>0.05) were found after orthoptic treatment. However, improvements were found (P<0.05) in the fast and slow vergence mechanisms. CONCLUSIONS: Despite improvements in the fast and slow vergence mechanisms no change was found in the AC/A and CA/C ratios after orthoptic treatment in CI subjects. This finding is unexpected in light of the present understanding of CI, and an alternative theory is proposed.

Accommodation, Ocular↗

PubMed2005

Yellow filters can improve magnocellular function: motion sensitivity, convergence, accommodation, and reading.

The magnocellular system plays an important role in visual motion processing, controlling vergence eye movements, and in reading. Yellow filters may boost magnocellular activity by eliminating inhibitory blue input to this pathway. It was found that wearing yellow filters increased motion sensitivity, convergence, and accommodation in many children with reading difficulties, both immediately and after three months using the filters. Motion sensitivity was not increased using control neutral density filters. Moreover, reading-impaired children showed significant gains in reading ability after three months wearing the filters compared with those who had used a placebo. It was concluded that yellow filters can improve magnocellular function permanently. Hence, they should be considered as an alternative to corrective lenses, prisms, or exercises for treating poor convergence and accommodation, and also as an aid for children with reading problems.

Accommodation, Ocular↗

PubMed2004

Relationship between mandibular deviation and ocular convergence.

Recent studies have confirmed the relationship between head posture, mandibular position and visual focusing. A case-controlled study was conducted to assess the occurrence of ocular convergence defects between subjects with functional mandibular latero-deviation and healthy subjects in pediatric age. Sixty subjects (the study group) presented mandibular latero-deviation classified as functional according to the use of a clinical examination and frontal and basal tele-radiography. Sixty subjects without functional mandibular laterodeviation (control group) were selected randomly from all subjects seeking pediatric dental care and matched by gender and age to study group. All one hundred and twenty subjects were submitted to orthoptic tests performed by the same operator. These results seemed to confirm that in mandibular latero-deviation subjects ocular convergence defects occurred in greater frequency than in controls underlining the importance of role of pediatric dentistry among interdisciplinary cooperation.

Case-Control Studies↗

PubMedDate not supplied

Convergence insufficiency in brain-injured patients.

Disorders of eye movements are relatively common in brain-injured patients. Some of these disturbances are caused by direct trauma to the orbital content, cranial nerves and other brain areas. Convergence, which is a part of the near-vision complex and depends on the integrative function of the cortical and subcortical areas, is commonly affected by traumatic brain injury. Intact vergence needs the integrative function of brain structures associated with acquisition of information on one side and intact mesencephalic function, including function of a vergence integrator, and their interconnections. Clinical investigation of vergence function in 26 traumatically brain-injured patients examined as early as possible after trauma revealed disturbances in over a third of the examinees. A follow-up of 72 patients some 3 years after injury revealed vergence insufficiency in 42%. Convergence insufficiency was associated with longer periods of coma (p less than 0.001), presence of cognitive disturbances (p less than 0.005) and patients' failing to find work in the open market (p less than 0.01). It is suggested that the presence of disturbed vergence several years after trauma, and its close association with prolonged coma and cognitive disturbances, is an expression of permanent damage to mesencephalic and cortical brain structures and can serve as an important aid in evaluation of these patients. This paper describes the close relationship between vergence state and over-all rehabilitation outcome as evaluated by patients' occupational status.

Adult↗

PubMed1994

Recovery of fusional convergence after systematic practice.

Fusional convergence was trained systematically with orthoptic devices in three patients with a severe deficit in convergent fusion resulting from vascular (one case) or traumatic (two cases) brain damage. Within a single-subject baseline design two of the three patients showed a significant, and the third patient a moderate, recovery of fusional range. All patients showed a significant improvement in visual acuity for objects close by, improved reading performance and an increase in stereo acuity (two cases). The results suggest a considerable potential for recovery of oculomotor functions in brain-damaged patients when appropriate treatment methods are applied.

Accommodation, Ocular↗

PubMed1989

The roles of convergence and apparent distance in depth constancy with motion parallax.

The question of whether motion parallax is calibrated by convergence or by apparent distance for depth perception was addressed in three experiments. In Experiment 1, a random dot parallactic display was viewed monocularly at a distance of 80 cm, and the convergence angles were set for distances of 40, 60, and 80 cm. Averaged apparent depth was not different across conditions. In Experiment 2, a display consisting of one surface showing dollar bills and one surface showing random dots was viewed monocularly at a distance of 80 cm. It was presented at two different apparent distances, which were manipulated by varying the size of the dollar bills. In one condition, normally sized dollar bills were presented, and in another condition, the size was reduced by 30%. The averaged apparent depth associated with the small-bill display was larger than the depth associated with the normally sized bill display. In Experiment 3, a random dot display was viewed monocularly at 120 cm. In the primary condition, the random dot display was viewed with an induction screen at 80 cm, and it was moved from side to side such that it appeared stationary and close to the plane of the induction screen. In a comparison condition, the display was viewed without the induction screen and was moving from side to side at 120 cm. In another comparison condition, the display was again viewed without the induction screen but was stationary at 120 cm. Observers adjusted the extent of motion parallax so that apparent depth was 1 cm. The mean extent of parallax was larger in the primary conditio.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗

PubMedDate not supplied

Unilateral surgery for exotropia with convergence weakness.

We prospectively studied 14 patients whom we treated surgically for exotropia (XT) with convergence weakness measuring at least 8 delta for distance (6 m) that increased at least 8 delta at near (1/3 m). Surgery involved lateral rectus recessions and medial rectus strengthenings (advancements or resections) to eliminate distance and near deviations and simultaneously collapse near-distance differences. The medial rectus was strengthened more than the lateral rectus was recessed; amounts were based on the near deviation. Minimum follow-up was 6 months after surgery. Preoperatively, the mean XT at distance was 18.3 delta (range, 8 to 35 delta) and at near 30.1 delta (range, 16 to 50 delta). At latest follow-up evaluations the mean distance deviation was 0.1 delta XT and at near 1.8 delta XT. The near-distance differences were reduced by a mean of 10.2 delta; the mean postoperative difference was 1.7 delta. Unilateral surgery for XT with convergence weakness biased to medial rectus strengthening and geared to near deviation can successfully collapse the near-distance differences while satisfactorily aligning both distance and near fixation. This surgery has low risk of creating long-term postoperative esodeviations at distance.

Adolescent↗

PubMed1999

An extension of an accommodation and convergence model of emmetropization to include the effects of illumination intensity.

The Flitcroft (1998) emmetropization model incorporates a classical model of accommodation and convergence with blur-driven feedback control of eye growth. We have modified this model by incorporating the effects of illumination (with or without extensive near work) on accommodation, vergence, pupil diameter and emmetropization. In addition to replicating Flitcroft's results, we show that (1) decreased illumination and (2) a low convergence accommodation/convergence (CAC) ratio exacerbate the progression of near-work-induced myopia. Our model further indicates that prescription of negative lenses, under these conditions, augments the advancement of myopia.

Accommodation, Ocular↗

PubMed1999

Rod monochromatism and blue cone monochromatism: pupillary, accommodative and convergence reactions to darkness.

PURPOSE: To elucidate the mechanism of paradoxical pupillary constriction to darkness (PPCD) and the clinical characteristics facilitating this phenomenon. METHODS: Six rod monochromats, three blue cone monochromats, with three obligate BCM carriers, and ten age matched controls were studied. Pupillary responses, refractions and eye positions were measured with an infrared refractometer, with and without background room lighting while being simultaneously recorded on VHS (infrared) video tape from onset of darkness. RESULTS: Only rod monochromats displayed typical paradoxical pupillary responses. Blue cone monochromats and obligate BCM carriers showed reduced pupillary contraction, compared to controls, but no paradoxical pupillary reaction. Changes neither in accommodation nor convergence were found during paradoxical pupillary constriction to darkness in our rod monochromats. CONCLUSIONS: Paradoxical pupillary constrictions to darkness were seen in rod monochromats but were found not to be age or gender related. This constriction was accompanied neither by accommodation nor convergence changes. No significant differences in pupillary responses to darkness were observed in BCMs nor their carriers, although pupillary dilation to darkness seemed slightly impaired as compared to normals.

Accommodation, Ocular↗

PubMed1990

[The use of the panel D-15 test in estimating the convergence points for deuteranopia].

In order to study the deuteranopic convergence points we compared the patterns of the simulated panel D-15 test with the actual data from 73 cases of deuteranopia. It was found that 39 of the 73 cases matched one of the four simulation patterns perfectly while another 25 cases showed similar patterns. Of the 64 cases, 9 cases matched the simulated pattern obtained when the x chromaticity coordinates were between 1.1810 and 1.4319, 42 cases when they were between 1.4320 and 2.2588, 7 cases when they were between 2.2589 and 3.6255 and 6 cases when they were between 3.6256 and 4.6572. These results confirm that there are significant individual differences in the convergence points of deuteranopes and support the theory that the confusion lines meet at a specific point rather than running parallel.

Adolescent↗

PubMed1996

Gain changes in the accommodative convergence cross-link.

The output of the accommodative vergence cross-link, the AC/A ratio, was measured before and after subjects viewed through a telestereoscope which increased their interocular separation (pd) approximately fourfold. The initial paradigm was designed to force an increase in the AC/A ratio in that subjects alternately fixated targets set at differing distances. In this paradigm, the varying vergence demands could not be met by a constant increase in phasic or tonic vergence responses. In a second paradigm, only one target was viewed at a fixed position from the eye. Now the constant vergence demand could be met by a set increase in phasic and/or tonic vergence. A significant increase in the AC/A ratio was found in the two-target study but not in the single-target study. Increases in the accommodative vergence gain did show considerable individual differences. Tonic vergence changes were interpolated from the data. This parameter increased significantly in both cases but more so when two targets were alternately fixated. This finding is consistent with current near triad models which predict that accommodative convergence input acts along with disparity vergence input to increase tonic levels of convergence. When AC/A ratios were calculated using clinical measures of the phoria at 6 and 0.4 m, increases were found in the two-target study and to a lesser degree in the single-target study. This clinical method of AC/A measure appears to have been confounded by changes in tonic vergence.

Accommodation, Ocular↗