Download the Free Unbound MEDLINE PubMed App to your smartphone or tablet.
Available for iPhone, iPad, iPod touch, and Android.
Vision Res [journal]
- Psychophysical contrast calibration. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 30.
Electronic displays and computer systems offer numerous advantages for clinical vision testing. Laboratory and clinical measurements of various functions and in particular of (letter) contrast sensitivity require accurately calibrated display contrast. In the laboratory this is achieved using expensive light meters. We developed and evaluated a novel method that uses only psychophysical responses of a person with normal vision to calibrate the luminance contrast of displays for experimental and clinical applications. Our method combines psychophysical techniques (1) for detection (and thus elimination or reduction) of display saturating non-linearities; (2) for luminance (gamma function) estimation and linearization without use of a photometer; and (3) to measure without a photometer the luminance ratios of the display's three color channels that are used in a bit-stealing procedure to expand the luminance resolution of the display. Using a photometer we verified that the calibration achieved with this procedure is accurate for both LCD and CRT displays enabling testing of letter contrast sensitivity to 0.5%. Our visual calibration procedure enables clinical, internet and home implementation and calibration verification of electronic contrast testing.
- The effects of spatial offset, temporal offset and image speed on sensitivity to global motion in human amblyopia. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 27.:59-65.
The presence of a general global motion processing deficit in amblyopia is now well established, although its severity may depend on image speed and amblyopia type, but its underlying cause(s) is still largely indeterminate. To address this issue and to characterize further the nature of the global motion perception deficit in human amblyopia, the effects of varying spatial offset (jump size-Δs) and temporal offset (delay between positional updates-Δt) in discriminating global motion for a range of speeds (1.5, 3 and 9°/s) in both amblyopic and normal vision were evaluated. For normal adult observers (NE) and the non-amblyopic eye (FE) motion coherence thresholds measured when Δt was varied were significantly higher than those when Δs was varied. Furthermore when Δt was varied, thresholds rose significantly as the speed of image motion decreased for both NEs and FEs. AE thresholds were higher overall than the other eyes and appeared independent of both the method used to create movement and speed. These results suggest that the spatial and temporal limits underlying the perception of global motion are different. In addition degrading the smoothness of motion has comparatively little effect on the motion mechanisms driven by the AE, suggesting that the internal noise associated with encoding motion direction is relatively high.
- Measurement and prediction of subjective gradations of images in presence of monochromatic aberrations. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 23.:52-58.
The three objectives of this study were (i) to explore the effect of various levels of aberrations on subjective vision by scoring images, (ii) to compare subjective scores obtained with real optics and simulated images and (iii) to test the ability of image quality metrics to predict these scores. In a first experiment, 14 subjects evaluated the quality of images degraded by 0.05, 0.1, 0.2, 0.4 and 0.8μm of defocus, astigmatism, trefoil, coma, spherical aberration (SA4) and secondary spherical aberration (SA6) by putting a mark on a 5-items continuous grading scale. The desired aberration was introduced either by a deformable mirror or by displaying a simulated image. In the second experiment, 5 of the previous subjects evaluated the quality of through-focus images in presence of SA4, SA6 and combinations of SA4 and SA6. Both experiments were performed with an artificial pupil of 6mm diameter. The addition of increasing amounts of aberrations reduced the subjective grading of the targets, with SA6, SA4 and defocus being the most degrading aberrations. The correlation between the results obtained with the AO device and with simulated images gave a r(2) of 0.95. Combinations of 0.4μm of SA4 and 0.2μm of SA6 of opposite signs induced a bimodal through-focus image score curve. We were able to anticipate the subjective gradation of subject's vision thanks to image quality metrics (r(2)=0.92). Image quality score shows similar results as that obtained by objective image quality metrics, which provides a useful tool for optical designers and practitioners.
- Perceived segmentation of center from surround by only illusory contours causes chromatic lateral inhibition. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 23.:66-70.
When a light and also its surrounding context slowly oscillate in chromaticity over time, the color appearance of the light depends on the relative phase of center and surround. The influence of the surround is generally accounted for by retinotopic center-surround organization, with the surround inhibiting signals from the center. The traditional neural account, however, cannot rule out lateral inhibition due to cortical mechanisms sensitive to object segmentation cues. Experiments here reveal that illusory contours are sufficient to separate a center from its surround. Observers adjusted the Michelson contrast of a matching disk to equal the perceived modulation depth of a central area within a surround. Both the central test and matching disk were maintained at constant luminance and modulated in-phase at 2Hz along one chromatic axis (L/(L+M) or S/(L+M)). The center was perceptually segmented from the surround by either a physical (retinotopic separation) or illusory (cortically represented) triangle contour. Segmentation of center from surround by the illusory contour strongly attenuated the perceived modulation depth for both chromatic axes. Further, the strength of attenuation was consistently greater with the illusory than the physically segmenting triangle. This cannot be accounted for by retinal center-surround antagonism; instead it points to a cortical neural representation of contours, with lateral inhibition following neural mechanisms sensitive to object segmentation cues.
- Background, an important factor in visual search. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 25.
The ability to detect an object depends on the contrast between the object and its background. Despite this, many models of visual search rely solely on the properties of target and distractors, and do not take the background into account. Yet, both target and distractors have their individual contrasts with the background. These contrasts generally differ, because the target and distractors are different in at least one feature. Therefore, background is likely to play an important role in visual search. In three experiments we manipulated the properties of the background (luminance, orientation and spatial frequency, respectively) while keeping the target and distractors constant. In the first experiment, in which target and distractors had a different luminance, changing the background luminance had an extensive effect on search times. When background luminance was in between that of the target and distractors, search times were always short. Interestingly, when the background was darker than both the target and the distractors, search times were much longer than when the background was lighter. Manipulating orientation and spatial frequency of the background, on the other hand, resulted in search times that were longest for small target-background differences. Thus, background plays an important role in search. This role depends on the individual contrast of both target and distractors with the background and the type of feature contrast (luminance, orientation or spatial frequency).
- Eye movements while reading an unspaced writing system: The case of Thai. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 19.:71-80.
Thai has an alphabetic script with a distinctive feature - it has no spaces between words. Since previous research with spaced alphabetic systems (e.g., English) has suggested that readers use spaces to guide eye movements, it is of interest to investigate what physical factors might guide Thai readers' eye movements. Here the effects of word-initial and word-final position-specific character frequency, word-boundary bigram frequency, and overall word frequency on 30 Thai adults' eye movements when reading unspaced and spaced text was investigated. Linear mixed-effects model analyses of viewing time measures (first fixation duration, single fixation duration, and gaze duration) and of landing sites were conducted. Thai readers tended to land their first fixation at or near the centre of words, just as readers of spaced texts do. A critical determinant of this was word boundary characters: higher position-specific frequency of initial and of final characters significantly facilitated landing sites closer to the word centre while word-boundary bigram frequency appeared to behave as a proxy for initial and final position-specific character frequency. It appears, therefore, that Thai readers make use of the position-specific frequencies of word boundary characters in targeting words and directing eye movements to an optimal landing site.
- Reading behavior of emmetropic schoolchildren in China. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 18.:43-51.
Knowing the actual behavior of readers will help us understand how near work influences a reader's eyes, comfort, reading efficiency, pleasure, and the ability to learn to read. We designed a methodology for reading behavior research, and investigated the reading behavior of emmetropic schoolchildren in China and factors that influence their reading. Children from grades 2 through 5 read text in an armchair, at a desk, and when reading and writing at the desk with three different font sizes. Their preferred reading distance was very near to the eyes, averaging 28.5±6.4cm in the armchair, 25.4±6.6cm at the desk and 20.6±6.5cm in the reading/writing task, and was always slightly closer for the smallest font. Second grade children averaged just a 16.3±4.1cm reading distance in the reading/writing task. Head tilt and angle of gaze were altered by reading condition and font size. Reading speed was fastest at the desk and for those with longer reading distances and, surprisingly, for the smallest font size. Reading behavior is not a fixed entity but differs with grade level and reading condition. This suggests that reading behavior can be altered through better ergonomics and text design which may reduce myopia, aesthenopia, and binocular anomalies and help children read better.
- The Simon effect of spatial words in eye movements: Comparison of vertical and horizontal effects and of eye and finger responses. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 15.:6-14.
Spatial stimulus location information impacts on saccades: Pro-saccades (saccades towards a stimulus location) are faster than anti-saccades (saccades away from the stimulus). This is true even when the spatial location is irrelevant for the choice of the correct response (Simon effect). The results are usually ascribed to spatial sensorimotor coupling. However, with finger responses Simon effects can be observed with irrelevant spatial word meaning, too. Here we tested whether a Simon effect of spatial word meaning in saccades could be observed for words with vertical ("above" or "below") and horizontal ("left" or "right") meanings. We asked our participants to make saccades towards one of the two saccade targets depending on the color of the centrally presented spatial word, while ignoring their spatial meaning (Experiment 1 and 2a). Results are compared to a condition in which finger responses instead of saccades were required (Experiment 2b). In addition to response latency we compared the time course of vertical and horizontal effects. We found the Simon effects due to irrelevant spatial meaning of the words in both saccades and finger responses. The time course investigations revealed different patterns for vertical and horizontal effects in saccades, indicating that distinct processes may be involved in the two types of Simon effects.
- Contribution of the gradient refractive index and shape to the crystalline lens spherical aberration and astigmatism. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 15.:27-34.
The optical properties of the crystalline lens are determined by its shape and refractive index distribution. However, to date, those properties have not been measured together in the same lens, and therefore their relative contributions to optical aberrations are not fully understood. The shape, the optical path difference, and the focal length of ten porcine lenses (age around 6months) were measured in vitro using Optical Coherence Tomography and laser ray tracing. The 3D Gradient Refractive Index distribution (GRIN) was reconstructed by means of an optimization method based on genetic algorithms. The optimization method searched for the parameters of a 4-variable GRIN model that best fits the distorted posterior surface of the lens in 18 different meridians. Spherical aberration and astigmatism of the lenses were estimated using computational ray tracing, with the reconstructed GRIN lens and an equivalent homogeneous refractive index. For all lenses the posterior radius of curvature was systematically steeper than the anterior one, and the conic constant of both the anterior and posterior positive surfaces was positive. In average, the measured focal length increased with increasing pupil diameter, consistent with a crystalline lens negative spherical aberration. The refractive index of nucleus and surface was reconstructed to an average value of 1.427 and 1.364, respectively, for 633nm. The results of the GRIN reconstruction showed a wide distribution of the index in all lens samples. The GRIN shifted spherical aberration towards negative values when compared to a homogeneous index. A negative spherical aberration with GRIN was found in 8 of the 10 lenses. The presence of GRIN also produced a decrease in the total amount of lens astigmatism in most lenses, while the axis of astigmatism was only little influenced by the presence of GRIN. To our knowledge, this study is the first systematic experimental study of the relative contribution of geometry and GRIN to the aberrations in a mammal lens.
- Scene categorization at large visual eccentricities. [JOURNAL ARTICLE]
- Vision Res 2013 Apr 15.:35-42.
Studies of scene perception have shown that the visual system is particularly sensitive to global properties such as the overall layout of a scene. Such global properties cannot be computed locally, but rather require relational analysis over multiple regions. To what extent is observers' perception of scenes impaired in the far periphery? We examined the perception of global scene properties (Experiment 1) and basic-level categories (Experiment 2) presented in the periphery from 10° to 70°. Pairs of scene photographs were simultaneously presented left and right of fixation for 80ms on a panoramic screen (5m diameter) covering the whole visual field while central fixation was controlled. Observers were instructed to press a key corresponding to the spatial location left/right of a pre-defined target property or category. The results show that classification of global scene properties (e.g., naturalness, openness) as well as basic-level categorization (e.g., forests, highways), while better near the center, were accomplished with a performance highly above chance (around 70% correct) in the far periphery even at 70° eccentricity. The perception of some global properties (e.g., naturalness) was more robust in peripheral vision than others (e.g., indoor/outdoor) that required a more local analysis. The results are consistent with studies suggesting that scene gist recognition can be accomplished by the low resolution of peripheral vision.