Adaptation to near addition lenses - Effect of AV/A ratio and age

AIM: The primary purpose of this thesis was to evaluate the pattern of changes to accommodation and phoria when pre-presbyopic individuals perform near work for 20 minutes with +2D lenses. In addition, the thesis also investigates the effect of the accommodative vergence cross-link (AV/A) and age on binocular adaptation to addition lenses. METHODS: Accommodation was measured using the PowerRefractor (Multichannel Systems, Germany) and phoria was measured using the modified Thorington Technique. Twenty four pre-presbyopic and emmetropic individuals (11 adults and 13 children) participated in the study. All participants fixated a near target at a distance of 33 cm for 20 minutes with +2D (lens condition) and without (no lens condition) +2D addition lenses. Binocular and monocular changes in accommodation and near phoria were measured at the outset and at 3, 6, 9, 15 and 20 minute intervals. RESULTS: Effect of +2D lenses on accommodation and phoria: The emmetropic adult participants exhibited lag of accommodation under the no lens condition (binocular: 0.51 ± 0.12D; monocular: 0.64 ± 0.15D) that were eliminated (under monocular viewing) and reversed (exceeded demand by 0.51 ± 0.11 D under binocular viewing condition) with the addition of +2D lenses. The near phoria showed a significant increase towards exophoria by 6 ± 0.56 ∆D upon introduction of +2D lenses. Sustained near viewing with +2 D lenses resulted in significant reduction of the binocular focus alone (not monocular focus) after 3 minutes of binocular viewing (magnitude of reduction: 0.24D; P<0.01). The exophoria also showed a concomitant reduction after 3 minutes of fixation at the near task (Magnitude of reduction: 3.6 ± 0.6 ∆D; P<0.001). The magnitude and rate of vergence adaptation, determined using an exponential function, was found to be 4.6 ± 0.21 ∆D and 2.12 minutes respectively for the emmetropic adult participants. Effect of age on vergence adaptation: A pattern of significant reduction in phoria and binocular focus similar to the adult participants was observed in young children. Analysis of the vergence adaptation curves in the two age groups did not show any significant difference in both the magnitude as well as the rate of phoria adaptation within the age range tested (Magnitude of adaptation - Adults: 4.65 ∆D; Children: 4.51 ∆D; P > 0.05; Time constants -Adults: 2.12 minutes: Children: 1.53 minutes, P > 0.05). Effect of AV/A ratio on vergence adaptation: The stimulus (St-AV/A) and the response AV/A (R-AV/A) ratios were determined and the participants were divided into two groups (low and high AV/A ratio) under both the conditions. The result indicated that, under both testing conditions (stimulus and response AV/A), the individuals with higher AV/A ratios demonstrated greater magnitudes of vergence adaptation than those individuals with lower ratios (Magnitude of adaptation: Low St-AV/A = 4.12 ∆D; Low R-AV/A= 4.25∆D; High St-AV/A = 4.88 ∆D; High R-AV/A = 4.65∆D; P<0.05) CONCLUSIONS: Introduction of near addition lenses initiated an increase in exophoria and convergence driven accommodation. Vergence adaptation occurred after 3 minutes of binocular viewing thus reducing exophoria and convergence driven accommodation. The magnitude and completeness of phoria adaptation were seen to depend on an individuals AV/A ratio with greater magnitude and incomplete adaptation observed in participants with higher AV/A ratios. Age, within the limits of the study did not appear to influence phoria adaptation with near addition lenses

Near addition lenses as a tool to investigate vergence adaptation in myopic children

Accommodation and vergence are two interacting ocular motor systems that function to maintain clear and single vision across a wide range of distances. Sustained fixation results in the adaptation of these ocular motor systems and has been widely investigated in adults but not in children. Moreover, limited reports have measured adaptation to disparities induced by ophthalmic lenses. This thesis used near addition lenses as a means to investigate binocular adaptation in children. The specific aims of this thesis were three-fold. First, the thesis aimed to gain insight into the mechanism of changes to accommodation and vergence during binocular adaptation in children. The second objective was to determine the role of vergence-bias category (eso/exo/normals) on adaptation. Lastly, this thesis evaluated the influence of myopia on binocular adaptation. Thirty- eight myopic and 38 emmetropic children between 7-14 years of age were examined for the purpose of this thesis. A series of studies were performed to evaluate adaptation using varying demands for accommodation and vergence, stimulated by binocular fixation at near (33 cm), through the addition of +2D and -2D over corrective lenses (closed loop accommodation) and using 10 base-out prisms (open-loop accommodation at 4M). In each closed-loop condition, measures of binocular and monocular accommodation (PowerRefractor, Multichannel systems) and near phoria (modified Thorington technique) were recorded at frequent intervals when children binocularly fixated a high contrast near target (33 cm) for 20 min. For the open-loop condition (obtained using 0.5 mm pinhole pupils), binocular accommodation and tonic vergence (distance heterophoria through pinhole pupils) were determined at frequent intervals when binocular fixation was sustained at 4M for 20 min. For all conditions, tonic accommodation was measured before and after the near task to measure accommodative adaptation. The results of this thesis make three major contributions to the literature. First, it outlines that the addition of +2D and -2D lenses alters both accommodation and near phoria during sustained binocular fixation, which can be explained based on the models of accommodation and vergence. Second, it shows that the direction of phoria influences the pattern of binocular vs. monocular accommodation in closed-loop conditions and alters the degree of vergence adaptation in both closed and open-loop accommodation. These changes have been primarily attributed to the varying demands on fusional vergence. Lastly, this thesis demonstrates that myopic children show reduced vergence adaptation when fusional convergence was initiated through plus adds or base-out prisms but not when fusional divergence was initiated through minus addition lenses. Further, myopic children also showed variations in other ocular motor parameters such as higher accommodative lags, greater variability of accommodative response, larger accommodative after-effects, and higher AV/A ratios compared to emmetropes. Consistent with the models of accommodation and vergence, the thesis highlights that it is necessary to measure changes to both accommodation and vergence when evaluating the response of the ocular motor system. The direction of phoria and type of refractive error play a significant role in determining binocular adaptation in children. Future studies should differentiate these parameters when evaluating adaptation of the ocular motor system

Binocular adaptation to near addition lenses in emmetropic adults

The final publication is available at Elsevier via https://doi.org/10.1016/j.visres.2008.02.015 © 2008. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Near addition lenses are prescribed to pre-presbyopic individuals for treatment of binocular motor problems such as convergence excess and to control the progression of myopia. To date, no investigation has looked at the complete sequence of binocular motor responses during a period of near work with +2 D lenses. This investigation evaluated changes to accommodation and vergence responses when young adults sustained fixation at 33 cm with +2 D addition lenses. In addition, the effect of the accommodative vergence cross-link (AV/A) on the magnitude and the completeness of binocular adaptation to these lenses were evaluated. The results showed that +2 D lenses initiate an increase in exophoria and convergence driven accommodation. The degree of the initial induced phoria was dependant upon the magnitude of the AV/A ratio. Vergence adaptation occurred after 3 min of near fixation and reduced the exophoria and convergence driven accommodation. The magnitude of vergence adaptation was dependant upon the size of the induced phoria and hence the AV/A ratio. The completeness of adaptation was seen to vary inversely with induced exophoria and thus the AV/A ratio.This study was supported by grants from Canada Foundation for Innovation; NSERC Canada (E.L.I., W.R.B.); CLLRnet (W.R.B.), CRC and PREA (E.L.I.)

Effect of near adds on the variability of accommodative response in myopic children

This is the peer reviewed version of the following article: Sreenivasan, V., Irving, E. L., & Bobier, W. R. (2011). Effect of near adds on the variability of accommodative response in myopic children. Ophthalmic and Physiological Optics, 31(2), 145–154, which has been published in final form at https://doi.org/10.1111/j.1475-1313.2010.00818.x. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.Purpose: Higher variability of accommodative response (VAR) has been reported in myopes and speculated to be a possible risk factor for the progression of myopia. We investigated whether near adds of +2D and −2D were capable of altering accommodative variability and also determined the influence of near phoria and viewing condition (binocular vs monocular) on the VAR in myopic and emmetropic children. Methods: Twenty-seven myopic and 25 emmetropic children between 7 and 14 years were examined. All children were classified into ‘normophores’ (0 to 4 exo), exophores (>6 exo) or esophores (>2 eso) based on their near phoria. Binocular and monocular steady-state measures of accommodation were obtained for 5 s using a PowerRefractor (Multichannel Co) while children fixated a high contrast target (33 cm) with distance correction, and then with +2D add and −2D add over the corrective lenses. The variation in accommodative responses (VAR) was defined as the standard deviation of the accommodative response during the 5 s period. Results: Myopic children showed higher VAR through their distance spectacle corrections compared to emmetropes (emmetropes = 0.23 ± 0.03D, myopes = 0.37 ± 0.07D, p 0.9). Introduction of a −2D add significantly increased the VAR in both refractive groups; however, myopes showed greater VAR compared to emmetropes (emmetropes = 0.39 ± 0.03D, myopes = 0.53 ± 0.07D, p < 0.001). Near phoria or binocular viewing did not alter the magnitude of fluctuations in either refractive group. VAR significantly correlated with the monocular accommodative error in both refractive groups (emmetropes r2 = 0.34; p < 0.0001; myopes: r2 = 0.35; p < 0.001). Pupil size, while varying with add type, did not confound the VAR. Conclusions: The near steady state accommodative response of young myopes shows greater variability than non-myopes. This difference is maintained when accommodative responses are increased beyond the vergence plane using − 2D adds. However, accommodative fluctuations were reduced to emmetropic levels when the stimulus to accommodation is reduced using a +2D add. The resulting VAR through adds appear to follow that expected from variations in accommodative demands and hence properties of the accommodative controller. Vergence postures (eso and exo phoria) do not appear to influence the VAR with and without near adds.This study was supported by grants from Canada Foundation for Innovation; NSERC Canada (WRB, ELI); CRC (ELI), COETF (VS,WRB,ELI); AOF Ezell fellowship sponsored by Bausch & Lomb (VS)

Binocular Adaptation to +2 D Lenses in Myopic and Emmetropic Children:

This is a non-final version of an article published in final form in Sreenivasan, V., Irving, E. L., & Bobier, W. R. (2009). Binocular Adaptation to +2 D Lenses in Myopic and Emmetropic Children: Optometry and Vision Science, 86(6), 731–740. https://doi.org/10.1097/OPX.0b013e3181a59d78Purpose. To compare vergence adaptation to +2 D addition lenses in myopic and emmetropic children and to evaluate the influence of the accommodative-vergence crosslink (AC/A ratio) on this adaptation. Methods. Nine myopic and 11 emmetropic children fixated a near target at a distance of 33 cm. Measures of binocular and monocular accommodation and phoria were obtained during a 20-min near task with and without +2 D lenses. Response AC/A ratios were determined from the experimental results. Vergence adaptation was quantified by the magnitude of reduction in phoria and the percentage of completeness (PC, return of adapted phoria to habitual level) after the near task. Results. Myopic children showed significantly higher AC/A ratios, which led to greater lens-induced exophoria and a greater demand for vergence adaptation. Both refractive groups showed significant vergence adaptation; however, myopes exhibited significantly reduced (p < 0.01) magnitudes compared with emmetropes (myopes = 3.95 ± 0.15 Δ; emmetropes = 4.41 ± 0.08 Δ). The mean PC was also significantly (p < 0.001) reduced in myopes (61.02 ± 1.57) compared with emmetropes (76.6 ± 2.10). There was a significant correlation between magnitude of adaptation and AC/A in both the refractive groups; however, myopes consistently showed reduced magnitudes compared to emmetropes. AC/A ratio influenced PC in emmetropic but not myopic children. In the accommodation system, +2 D lenses eliminated the accommodative lags observed in myopic children during natural viewing conditions. These lenses resulted in a small over-focus (−0.24 ± 0.27 D) at the onset of near work, which decreased during sustained viewing through the near add. Conclusions. Myopic children demonstrate reduced magnitude and completeness of vergence adaptation to +2 D lenses. The magnitude of vergence adaptation varied with AC/A in both refractive groups; however, the presence of myopia differentiated the amount of adaptation for all AC/A ratios. Conversely, the degree of completeness appears to be primarily associated with the type of refractive error.This work was supported in part by supported by grants from Canada Foundation for Innovation; Natural Sciences and Engineering Research Council of Canada; Canadian Language and Literacy Research Network; and Canadian Research Chair Award

Effect of heterophoria type and myopia on accommodative and vergence responses during sustained near activity in children

The final publication is available at Elsevier via https://doi.org/10.1016/j.visres.2012.01.011 © 2012. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The influence of phoria-type and myopia on changes to vergence and accommodation during prolonged near-task was examined in 53 children. Participants were classified into phoria and refractive categories based on near phoria and cycloplegic refraction respectively. Measures of near phoria, binocular (BA) and monocular accommodation (MA) were obtained before and during a 20 min task when children binocularly fixated a high-contrast target at 33 cm through best corrective lenses. Vergence adaptation and accommodative adaptation were quantified using changes to near phoria and tonic accommodation respectively. The direction and magnitude of vergence adaptation was modified by the phoria-type (p MA while esophores showed MA > BA in both refractive groups. Accommodative adaptation was higher in myopes (p = 0.010) but did not demonstrate a significant effect of phoria (p = 0.4). The influence of phoria-type on vergence adaptation and the pattern of BA vs. MA relates primarily to the varying fusional vergence demands created by the direction of phoria. The greater divergent (or less convergent) shift in vergence adaptation seen in myopes (compared to emmetropes) could be attributed to their higher accommodative adaptation. Nevertheless, the adaptive patterns observed in myopic children do not appear to explain their high response AV/A ratios identified as a risk factor for myopia development.This study was supported by grants from Canada Foundation for Innovation; NSERC Canada (WRB, ELI); CRC (ELI), COETF (VS, WRB, ELI); AOF Ezell fellowship sponsored by Bausch & Lomb (VS)

Can current models of accommodation and vergence predict accommodative behavior in myopic children?

The final publication is available at Elsevier via https://doi.org/10.1016/j.visres.2014.05.008 © 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Investigations into the progression of myopia in children have long considered the role of accommodation as a cause and solution. Myopic children show high levels of accommodative adaptation, coupled with accommodative lag and high response AC/A (accommodative convergence per diopter of accommodation). This pattern differs from that predicted by current models of interaction between accommodation and vergence, where weakened reflex responses and a high AC/A would be associated with a low not high levels of accommodative adaptation. However, studies of young myopes were limited to only part of the accommodative vergence synkinesis and the reciprocal components of vergence adaptation and convergence accommodation were not studied in tandem. Accordingly, we test the hypothesis that the accommodative behavior of myopic children is not predicted by current models and whether that departure is explained by differences in the accommodative plant of the myopic child. Responses to incongruent stimuli (−2D, +2D adds, 10 prism diopter base-out prism) were investigated in 28 myopic and 25 non-myopic children aged 7–15 years. Subjects were divided into phoria groups – exo, ortho and eso based upon their near phoria. The school aged myopes showed high levels of accommodative adaptation but with reduced accommodation and high AC/A. This pattern is not explained by current adult models and could reflect a sluggish gain of the accommodative plant (ciliary muscle and lens), changes in near triad innervation or both. Further, vergence adaptation showed a predictable reciprocal relationship with the high accommodative adaptation, suggesting that departures from adult models were limited to accommodation not vergence behavior.This study was supported by grants from Canada Foundation for Innovation; NSERC Canada (WRB, ELI); CRC (ELI), COETF (VS, WRB, ELI); AOF Ezell fellowship sponsored by Bausch & Lomb (VS)

Eye Movements, Strabismus, Amblyopia and Neuro-Ophthalmology Vergence Adaptation to Short-Duration Stimuli in Early Childhood

PURPOSE. To investigate whether nonstrabismic typically developing young children are capable of exhibiting vergence adaptation. METHODS. Fifteen adults (19.5-35.8 years) and 34 children (2.5-7.3 years) provided usable data. None wore habitual refractive correction. Eye position and accommodation were recorded using Purkinje image eye tracking and eccentric photorefraction (MCS PowerRefractor). Vergence was measured in three conditions while the participant viewed naturalistic targets at 33 cm. Viewing was monocular for at least 60 seconds and then binocular for either 5 seconds (5-second condition), 60 seconds (60-second), or 60 seconds through a 10-pd base-out prism (prism 60-second). The right eye was then occluded again for 60 seconds and an exponential function was fit to these data to assess the impact of adaptation on alignment. RESULTS. The 63% time constant was significantly longer for the prism 60-second condition (mean ¼ 11.5 seconds) compared to both the 5-second (5.3 seconds; P ¼ 0.015) and the 60-second conditions (7.1 seconds; P ¼ 0.035), with no significant difference between children and adults (P &gt; 0.4). Correlations between the 63% time constant (prism 60-second condition) and age, refractive error, interpupillary distance (IPD), or baseline heterophoria were not significant (P &gt; 0.4). The final stable monocular alignment, measured after binocular viewing, was similar to the baseline initial alignment across all conditions and ages. CONCLUSIONS. For a limited-duration near task, 2-to 7-year-old children showed comparable levels of vergence adaptation to adults. In a typically developing visual system, where IPD and refractive error are maturing, this adaptation could help maintain eye alignment

QoS Challenges and Opportunities in Wireless Sensor/Actuator Networks

A wireless sensor/actuator network (WSAN) is a group of sensors and actuators that are geographically distributed and interconnected by wireless networks. Sensors gather information about the state of physical world. Actuators react to this information by performing appropriate actions. WSANs thus enable cyber systems to monitor and manipulate the behavior of the physical world. WSANs are growing at a tremendous pace, just like the exploding evolution of Internet. Supporting quality of service (QoS) will be of critical importance for pervasive WSANs that serve as the network infrastructure of diverse applications. To spark new research and development interests in this field, this paper examines and discusses the requirements, critical challenges, and open research issues on QoS management in WSANs. A brief overview of recent progress is given.Comment: 12 pages, 1 figure; revie