Periodic letter strokes within a word affect fixation disparity during reading

We investigated the way in which binocular coordination in reading is affected by the spatial structure of text. Vergence eye movements were measured (EyeLink II) in 32 observers while they read 120 single German sentences (Potsdam Sentence Corpus) silently for comprehension. The similarity in shape between the neighboring strokes of component letters, as measured by the first peak in the horizontal auto-correlation of the images of the words, was found to be associated with (i) a smaller minimum fixation disparity (i.e. vergence error) during fixation; (ii) a longer time to reach this minimum disparity and (iii) a longer overall fixation duration. The results were obtained only for binocular reading: no effects of auto-correlation could be observed for monocular reading. The findings help to explain the longer reading times reported for words and fonts with high auto-correlation and may also begin to provide a causal link between poor binocular control and reading difficulties. © ARVO

Assessment of Dual-Mode and Switched-Channel Models with Experimental Vergence Responses

Controversy exists in the literature regarding the basic neural control structure that mediates convergence responses. This study constructed and simulated two models, the switched-channel feedback model and the dual-mode model consisting of preprogrammed with feedback control. Models were constructed and compared to experimental data. The stimuli consisted of 2 deg and 4 deg vergence steps. Both closed- and open-loop settings were utilized. After parameter adjustment, both models could accurately simulate step responses from subjects having a range of response dynamics. The model with a preprogrammed element required less parameter modification when stimulus amplitude changed. Both models could accurately simulate some attributes of vergence; however, neither model could represent the modifications commonly observed within the transient portion of the vergence response

The effect of vergence vision training on binocularly normal subjects

Vergence is the disjunctive (inward or outward) movement of the eyes that is stimulated by retinal disparity (difference of where an image is projected to the retina and the fovea). A recent randomized clinical trial showed the efficacy of vision therapy for children with the binocular dysfunction known as convergence insufficiency is 73%. However, it is unknown whether binocularly normal persons will have any significant change to their vergence ocular motor system if they participate in vision training sessions. A total of ten (n = 10) binocularly normal persons participated in this study (18 to 28 years of age). A haploscope with an integrated infrared video-based eye tracking system manufactured by ISCAN presented vergence stimuli to the subject to record eye movement data before and after 12 hours of vision therapy with a custom LabVIEW program. Vision therapy entailed a random walk of 2° and 4° steps at near and far space along with ramps ranging from 1° to 20° of total vergence angular rotation. All processing and statistical analyses were conducted in MATLAB. All subjects experienced a significant decrease in time to peak velocity (p\u3c0.05). However, each subject’s peak velocity values were subject dependent and either increased, decreased, or maintained at the same level after training. The peak velocity of responses to 2° steps began to approach a more critically damped linear control system after vision training supporting an improvement in the accuracy of responses. Data support that even in binocularly normal control subjects; vision therapy improves vergence eye movements quantified as significant improvements in the time to fuse the new target and significantly more accurate responses compared to each subject’s baseline measurements

Relationship between threshold and suprathreshold perception of position and stereoscopic depth.

We seek to determine the relationship between threshold and suprathreshold perception for position offset and stereoscopic depth perception under conditions that elevate their respective thresholds. Two threshold-elevating conditions were used: (1) increasing the interline gap and (2) dioptric blur. Although increasing the interline gap increases position (Vernier) offset and stereoscopic disparity thresholds substantially, the perception of suprathreshold position offset and stereoscopic depth remains unchanged. Perception of suprathreshold position offset also remains unchanged when the Vernier threshold is elevated by dioptric blur. We show that such normalization of suprathreshold position offset can be attributed to the topographical-map-based encoding of position. On the other hand, dioptric blur increases the stereoscopic disparity thresholds and reduces the perceived suprathreshold stereoscopic depth, which can be accounted for by a disparity-computation model in which the activities of absolute disparity encoders are multiplied by a Gaussian weighting function that is centered on the horopter. Overall, the statement equal suprathreshold perception occurs in threshold-elevated and unelevated conditions when the stimuli are equally above their corresponding thresholds describes the results better than the statement suprathreshold stimuli are perceived as equal when they are equal multiples of their respective threshold values

Estudo da disparidade de fixação numa população clínica

Dissertação de mestrado em Optometria AvançadaO uso exagerado de novas tecnologias, implica um esforço excessivo/contínuo da acomodação e vergência na visão ao perto. Uma vez que não há consenso entre os investigadores sobre a correlação da Disparidade de Fixação (DF) com as heteroforias dissociadas, na visão ao perto (VP), é importante realizar um estudo que determine realmente a significância da sua medição. Em vários estudos comprovou-se que a DF é um bom diagnóstico de disfunções de visão binocular, expondo ser melhor do que a medição do valor da heteroforia dissociada. Em muitos casos a DF é a causa da astenopia e outros sintomas não detetados com outros exames. Assim sendo, analisou-se uma população clínica de 55 indivíduos, dos quais 36 do sexo feminino e 19 do sexo masculino. As idades estão compreendidas entre os 9 e 68 anos. As medidas foram obtidas no gabinete de optometria da Ótica O Palácio. Numa primeira fase os pacientes realizaram o questionário CISS modificado, avaliação da refração e realização dos testes de visão binocular. Após a análise dos dados obtidos, verificou-se que um individuo emetrope tem maior associação com orto disparidade e menor associação com exo disparidade; no caso de um hipermetrope há maior associação com orto disparidade. Tal como a heteroforia dissociada aumenta com o aumento da idade. Não se constatou correlação entre a DF e indivíduos assintomáticos e sintomáticos, isto significa que não houve qualquer correlação entre o valor dos sintomas e o valor da DF. Também não foi possível estabelecer nenhuma associação estatisticamente significativa do sentido da DF, isto é orto disparidade, endo disparidade e exo disparidade, com os indivíduos sintomáticos.The exaggerated use of new technologies implies an excessive/continuous effort of accommodation and vergence at near vision. And, since there is no consensus among researchers about the correlation of fixation disparity with heterophoria in near vision, it is important to carry out a study that really determines the significance of its measurement. Several studies have shown that fixation disparity is a good diagnosis of binocular vision dysfunctions, even reporting that it is better than measuring the value of heterophoria. In many cases, fixation disparity is the cause of asthenopia and other symptoms not detected by other tests. Therefore, a clinical population of 55 individuals was analyzed, of which 36 were female and 19 were male. The ages ranged from 9 to 68 years old. The measurements were obtained at the optometry office of Ótica O Palácio. In the first phase, the patients were submitted to the modified CISS questionnaire, refraction evaluation and binocular vision tests. After analyzing the data obtained, it was found that an emmetrop individual has a greater association with ortho disparity and less association with exo disparity; in the case of a farsighted person, there is a greater association with ortho disparity. Just as dissociated heterophoria increases with increasing age. There was no correlation between DF and asymptomatic and symptomatic individuals, which means that there was no correlation between the value of symptoms and the value of DF. It was also not possible to establish any statistically significant association of the sense of FD, that is, ortho disparity, endo disparity and exo disparity, with symptomatic individuals

Eksperimentāls verģences atbildes un fiksācijas disparitātes pētījums

Anotācija. Darba mērķis ir izpētīt verģences atbildes dinamiskās īpašības un pārbaudīt, cik lielā mērā šīs īpašības kopā ar citiem fizioloģiskiem mehānismiem ir saistītas ar fiksācijas disparitātes izveidošanos. Mērījumi veikti subjektīvi, izmantojot nonius metodi, un objektīvi, izmantojot videookulogrāfu EyeLink II. Verģences atbildes asimetrijas individuālās atšķirības spēj izskaidrot ne vairāk kā 50 % no fiksācijas disparitātēm. Fiksācijas disparitātes izveides process ir saistīts arī ar tumsas verģenci un metodes kļūdu, kā arī ar akomodācijas darbību. Līdz ar to var secināt, ka klīniski nozīmīga fiksācijas disparitāte, kas noteikta, izmantojot subjektīvas metodes, parādās atšķirīgu fizioloģisku procesu un metodes kļūdas kombinācijas rezultātā. Atslēgas vārdi: Fiksācijas disparitāte, verģence, tumsas verģence, heteroforijaAbstract. The aim of the present study is to investigate dynamic properties of vergence response and to test to what extent they can be used to determine the origin of fixation disparity together with other underlying physiological mechanisms. Measurements were taken subjectively using nonius technique and objectively using the video-based EyeLink II. The individual differences in the asymmetry of vergence dynamics in convergent and divergent direction were able to explain at most 50% of the inter-individual variance in fixation disparity. The origin of fixation disparity is related also to the dark vergence and nonius bias, as well as accommodation. Accordingly, we can conclude that the clinically relevant subjective fixation disparity originates from distinct physiological sources and from the nonius bias as an artifact of the nonius method. Keywords: Fixation disparity, vergence, dark vergence, heterophoria