158 research outputs found
Hyperopia is predominantly axial in nature
No full textPURPOSE: Myopia has been found to be predominantly axial in nature, i.e. myopic eyes have longer than normal axial lengths, with corneal radius variations having only a small influence on the magnitude of the refractive error. In this study we assess whether a similar relationship exists for hyperopia. METHODS: Biometric data were collected on 57 subjects with either emmetropic or hyperopic refractive errors ranging in magnitude from -0.37 D to +17.25 D. Our main analysis concentrated on subjects with less than +10 D of hyperopia (group 1, n = 53), as subjects with +10 D of hyperopia or more (group 2, n = 4) exhibited marked differences in their biometric characteristics. RESULTS: Analysis of group 1 data revealed a significant relationship (r2 = 0.611, p = 0.0001) between the degree of hyperopia and the measured axial lengths. A weak but statistically significant relationship (r2 = 0.128, p = 0.009) was also found between mean corneal radius measures and mean spherical refractive errors, with the mean corneal radius flattening with increasing hyperopia. In group 2, three of the four subjects exhibited much steeper corneal characteristics than predicted from the group 1 data. CONCLUSIONS: Our results suggest that hyperopia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. These results have implications for refractive surgery and visual performance in hyperopic eyes
Measuring contrast sensitivity with inappropriate optical correction
No full textSpatial frequency-selective minima (notches) in the contrast sensitivity function (CSF) because of defocus can mimic those that occur with ocular disease. We examined the influence of measurement conditions on CSF shape in simulated clinical testing. CSF notches occurred with almost all levels of defocus for all subjects. Multiple notches were found under some conditions. Notches were found with defocus as small as 0.50 D. Effects of induced astigmatism depended on the orientation of the target. Notches were apparent in defocus conditions after stimulus size and room illuminance were modified and when subjects had insufficient accommodation to compensate for hypermetropic defocus. The equivalent of notches was not noted with the Pelli–Robson chart. As defocus-induced CSF notches may be mistaken for functional loss, careful refractive correction\ud
should be conducted prior to clinical or experimental CSF measurement, even at low spatial frequencies
Effects of defocus and pupil size on human contrast sensitivity
No full textDefocus lowers the contrast sensitivity function (CSF), producing a complex function with local dips and peaks. Previously, we were able to predict the shape of the CSF with large pupils from measured transverse aberrations with hypermetropic defocus but not with myopic defocus (Atchison et al., 1998c, J. Opt. Soc. Am. A. 15, 2536). As there is no reason that myopic defocus should be more difficult to predict than hypermetropic defocus, we modified the procedure to try to improve CSF predictions with myopic defocus. Also, we extended the study to consider a range of pupil sizes. CSFs were measured for three subjects at three defocus levels (in-focus, -2D and +2D) and three pupil sizes (2 mm, 4 mm and 6 mm). Using a diffraction optics model, transverse aberration measures and in-focus CSF measures, we predicted the defocused CSFs. The predicted defocused CSFs were lower than the in-focus CSF as expected, and had complex shapes that varied with defocus and pupil size and between subjects. While a few predictions were poor, generally, the overall magnitude and shape of the defocused CSFs were well predicted and similarly so for myopic and hypermetropic defocus. Some further improvements in technique are indicated
Dynamic accommodation response in the presence\ud of astigmatism
No full textIt has been suggested that in the presence of astigmatism some individuals make cyclic changes in focus over the astigmatic interval to obtain better visual performance. The aim in the present study was to identify such cyclic accommodative behavior and to characterize the variability of the response in the presence of astigmatism.\ud
The dynamic accommodation response in the presence of induced astigmatism was recorded objectively with an infrared optometer in seven young adults. Astigmatism led directly to increased accommodative variability in certain individuals. In two of seven participants there was evidence for aperiodic cyclic accommodative responses between different portions of the astigmatic interval. However, the amplitude of these tracking\ud
responses was much smaller than the astigmatic interval
Influence of Stiles-Crawford apodization on visual acuity
No full textThe Stiles–Crawford effect (SCE) of the first kind has often been considered to be important to spatial visual performance in that it ameliorates the influence of defocus and aberrations. We investigated the influence of SCE apodization on visual acuity as a function of defocus (out to 62 D) in four subjects. We used optical\ud
filters, conjugate with the eye’s entrance pupil, that neutralized or doubled the existing SCE. With an illiterate-E task, the influence of the SCE was more noticeable for myopic defocus than for hypermetropic defocus, was generally more noticeable for high-contrast than for low-contrast letters, and increased with increase\ud
in pupil size. The greatest influence on visual acuity of neutralizing the SCE, across the subjects and range of conditions, was deterioration of 0.06 (4-mm pupil), 0.16 (6-mm pupil), and 0.29 log unit (7.6-mm pupil)
Imposed retinal image size changes--do they provide a cue to the sign of lens-induced defocus in chick?
No full textBACKGROUND: Young chicks can adjust their eye growth to compensate for both imposed hyperopia and myopia (using negative and positive spectacle lenses); the rate of eye elongation increases in the former and slows in the latter case. This emmetropizing behavior implies that the eye can distinguish the sign and magnitude of defocus, although the identity of the cue(s) involved is unknown. As the spectacle lenses used in these studies generally introduce significant retinal image size differences that are in opposite directions for negative and positive lenses (minification vs. magnification), we asked whether retinal image size might provide the required sign information. METHODS: This question was addressed by manipulating retinal image size while keeping lens power constant. We also investigated the effect of eliminating other potential cues, accommodation and chromatic aberration, under these conditions. Three negative "size" lenses of approximately -11 D optical power were used, with 2 of the lenses producing magnification rather than minification as typical of negative lenses (i.e. +1.9% and +6.9% compared to -2.9%). The lenses were fitted monocularly to 7-day-old chicks, which were subsequently measured at 9 and 11 days of age (refractive error and axial dimensions). The same lens-wearing schedule was applied to two other groups of chicks that had monocular ciliary nerve section surgery to prevent accommodation 2 days posthatching; one of these groups was reared under monochromatic yellow light instead of white light. RESULTS: Near-perfect refractive compensation was seen by the end of the treatment period with all three lenses, for all three treatment groups, and there was also little difference in the rate of compensation among the various groups. In all cases, the typical responses of axial (mainly vitreous chamber) elongation and myopia were observed. CONCLUSIONS: That manipulations to retinal image size, which either decrease or reverse the usual effects of negative lenses, did not disrupt compensation to the imposed hyperopic defocus, even in the absence of accommodation and chromatic aberration cues, argues against imposed retinal image size changes being the directional cue to defocus in experimental emmetropization
Quantitative analysis of chloroplast protein targeting
No full textThis thesis presents the first use of the Partition of Unity Method in quantifying
the spatio-temporal dynamics of a fluorescent protein targeted to
the chloroplast twin-arginine translocation pathway.
The fluorescence loss in photobleaching technique is applied in a modified
fashion to the measurement of substrate mobilities in the chloroplast
stroma. Our in vivo results address the two suggested protein targeting
mechanisms of membrane-binding before lateral movement to the translocon
and direct binding to the translocon.
A high performance computing C/C++ implementation of the Partition
of Unity Method is used to perform simulations of fluoresence loss
in photobleaching and allow a compelling comparison to photobleaching
data series. The implementation is both mesh-free and particle-less
The value of training accommodative facility for ball sports
No full textWe address the issue of training accommodative facility as a means of improving performance in ball sports. Presenting examples from the game of baseball, we show that dynamic accommodation is unlikely to play an important role in ball skills. The movement speed of the ball combined with the last possible time at which visual information can be converted to motor action means that accommodation cannot provide useful information on the time to contact of the ball. In addition, we question whether altering accommodation to provide a clear retinal image of an approaching target is necessary for most ball sport skills
Seeing Speech: A Pronunciation Toolkit for Indigenous Language Teaching and Learning
No full textPronunciation can present a serious challenge for language teachers and learners (e.g., Munro & Derwing 2015). In the context of Indigenous languages in particular, this can be compounded by a number of factors, including small numbers of speakers and teachers, a paucity of pedagogical resources and clear descriptions of sound systems, and the pressures faced by heritage learners to authentically preserve their ancestral language (Carpenter 1997; Hinton 2011; Hinton & Ahlers 1999). Latent speakers may be inhibited from speaking by perceived concerns over their pronunciation, particularly in the presence of elders (Basham & Fatham 2008), and other learners may face similar social and linguistic challenges. Despite these hurdles, pronunciation is considered by many to be an important aspect of Indigenous language learning, and one which requires creative community-oriented solutions (AUTHOR & Kell 2015; Carpenter 1997). Towards this end, we have developed a pronunciationlearning toolthat incorporates ultrasound technology, giving learners a visual aid to help them learn to articulate challenging or unfamiliar sounds, for example “back of the mouth” consonants (e.g. /k/ vs. /q/). Ultrasound is used to create videos of a model speaker’s tongue movements during speech, which are then overlaid on videos of an external profile view of the model’s head to create ultrasound-enhanced pronunciation videos for individual words or sounds (Abel et al. 2015). A key advantage of these videos is that they allow learners direct access to the articulatory shapes and movements that are involved in pronouncing challenging words or sounds; learners are able see how speech is produced rather than just hear and try to mimic it. Although ultrasound-enhanced videos were originally developed for commonly taught languages such as Japanese and French, there has been widespread interest from Indigenous communities in Western Canada to develop their own customized videos. To date, we have partnered with communities in Alberta and British Columbia to develop videos for four languages: SENĆOŦEN, Secwepemc, Halq’emeylem, and Blackfoot. Community-driven and capacity-building, these projects involved training community members in how to produce customized ultrasound-enhanced videos using our toolkit. The resulting videos will be featured in our presentation, along with demonstrations of how and why to use ultrasound in pronunciation teaching. Our goal is to show that the ultrasound-enhanced videos can help to address some of the challenges of pronunciation learning in Indigenous languages by giving learners a new way to understand pronunciation that focuses on seeing speech. References Abel, J., B. Allen, S. Burton, M. Kazama, M. Noguchi, A. Tsuda, N. Yamane, & AUTHOR. 2015. Ultrasound-Enhanced Multimodal Approaches to Pronunciation Teaching and Learning. Canadian Acoustics 43 (3), 130-131. Basham, C. and A. Fathman. 2008. The latent speaker: Attaining adult fluency in an endangered language. International Journal of Bilingual Education and Bilingualism, 11: 577-97. AUTHOR and S. Kell. Pronunciation in the context of language revitalization. Paper presented at ICLDC 4, 2015. Carpenter, V. 1997. Teaching Children to "Unlearn" the Sounds of English. In Teaching Indigenous Languages, ed. by Jon Reyhner. Flagstaff, AZ: Northern Arizona University, pp. 31-39. Hinton, L. 2011. Language revitalization and language pedagogy: New teaching and learning strategies. Language and Education 25(4): 307-318, Hinton, L. and J. Ahlers. 1999. The issue of “authenticity” in California language restoration. Anthropology & Education Quarterly, 30: 56-67. Munro, M. J. & Derwing, T. M. 2015. A prospectus for pronunciation research in the 21st century: A point of view. Journal of Second Language Pronunciation 1(1): 11-42
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